U.S. patent application number 11/114466 was filed with the patent office on 2005-09-15 for inductor capacitor emi filter for human implant applications.
Invention is credited to Brendel, Richard L., Frysz, Christine A., Hussein, Haytham, Stevenson, Robert A..
Application Number | 20050201039 11/114466 |
Document ID | / |
Family ID | 33494114 |
Filed Date | 2005-09-15 |
United States Patent
Application |
20050201039 |
Kind Code |
A1 |
Stevenson, Robert A. ; et
al. |
September 15, 2005 |
Inductor capacitor EMI filter for human implant applications
Abstract
A feedthrough terminal assembly for an active implantable
medical device includes a conductive ferrule conductively coupled
to a housing of the medical device, a feedthrough capacitor
conductively coupled to the ferrule, an inductor closely associated
with the capacitor in non-conductive relation, and a conductive
terminal pin extending through the capacitor and the inductor. The
terminal pin extends through the inductor in non-conductive
relation and is conductively coupled to active electrode plates of
the capacitor. In one preferred form, the terminal pin is wound
about the inductor. Additionally, the inductor may be maintained in
close association with the capacitor without forming a direct
physical attachment therebetween.
Inventors: |
Stevenson, Robert A.;
(Canyon Country, CA) ; Frysz, Christine A.;
(Marriottsville, MD) ; Hussein, Haytham;
(Woodstock, MD) ; Brendel, Richard L.; (Carson
City, NV) |
Correspondence
Address: |
KELLY LOWRY & KELLEY, LLP
6320 CANOGA AVENUE
SUITE 1650
WOODLAND HILLS
CA
91367
US
|
Family ID: |
33494114 |
Appl. No.: |
11/114466 |
Filed: |
April 25, 2005 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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11114466 |
Apr 25, 2005 |
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10825900 |
Apr 15, 2004 |
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60473228 |
May 23, 2003 |
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60508426 |
Oct 2, 2003 |
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Current U.S.
Class: |
361/302 |
Current CPC
Class: |
H03H 2001/0042 20130101;
A61N 1/3754 20130101; H03H 1/0007 20130101 |
Class at
Publication: |
361/302 |
International
Class: |
H01G 004/35 |
Claims
1-172. (canceled)
173. A feedthrough terminal assembly, comprising: a conductive
ferrule; a feedthrough capacitor having first and second sets of
electrode plates; an inductor with having a surface bonded to an
opposing surface of the capacitor to create a laminate structure;
and a conductive terminal pin extending through the capacitor and
conductively coupled to the first set of electrode plates, and
extending through the inductor in non-conductive relation
thereto.
174. The assembly of claim 173, wherein the terminal pin is wound
around the inductor and adjacent portions of the wound terminal pin
are electrically insulated from one another.
175. The assembly of claim 174, wherein the adjacent portions of
the wound terminal pin are encased in a non-conductive
material.
176. The assembly of claim 175, wherein the adjacent portions of
the wound terminal pin are encased within a non-conductive
sleeve.
177. The assembly of claim 173, wherein the inductor includes a
notch for receiving the wound terminal pin.
178. The assembly of claim 177, including a ramp formed in the
notch.
179. The assembly of claim 177, wherein the inductor includes
multiple notches therein.
180. The assembly of claim 179, wherein each notch accommodates a
separate terminal pin therein.
181. The assembly of claim 177, wherein the notch includes multiple
slots for receiving corresponding multiple turns of the terminal
pin.
182. The assembly of claim 177, wherein the notch comprises
contoured corners for accommodating the terminal pin.
183. The assembly of claim 173, including means for maintaining the
inductor in close association with the capacitor without forming a
direct physical attachment therebetween.
184. The assembly of claim 183, wherein the maintaining means
comprises a lock associated with the terminal pin.
185. The assembly of claim 184, wherein the lock comprises a
mechanical lock.
186. The assembly of claim 184, wherein the lock comprises a
deformation in the terminal pin.
187. The assembly of claim 184, wherein the lock comprises a cured
polymer.
188. The assembly of claim 183, wherein the maintaining means
comprises a wire bond pad attached to the terminal pin.
189. The assembly of claim 188, including a non-conductive
substrate disposed between the wire bond pad and the inductor.
190-231. (canceled)
232. The assembly of claim 173, wherein the inductor is bonded to
the capacitor with a non-conductive material.
233. The assembly of claim 232, wherein the non-conductive material
comprises polyimide.
234. The assembly of claim 232, wherein the non-conductive material
comprises a washer disposed between the inductor and the
capacitor.
235. The assembly of claim 234, wherein the washer is comprised of
polyimide.
236. The assembly of claim 234, wherein the washer comprises a
thin-film, adhesive-backed washer.
237. The assembly of claim 173, wherein the inductor comprises a
high permeability ferrite material.
238. The assembly of claim 237, wherein the inductor comprises a
material selected from cobalt zinc ferrite, nickel zinc ferrite,
manganese zinc ferrite, powdered iron, or molypermalloy.
239. The assembly of claim 173, including a conformal coating over
the inductor.
240. The assembly of claim 239, wherein the conformal coating
comprises Paralyne.
241. The assembly of claim 240, wherein the conformal coating
comprises Paralyne C, D, E, or N.
242. The assembly of claim 173, including an insulator disposed
between the inductor and the terminal pin.
243. The assembly of claim 242, wherein the insulator comprises a
non-conductive polymer.
244. The assembly of claim 243, wherein the non-conductive polymer
comprises an epoxy, a thermal-setting non-conductive adhesive, a
non-conductive polyimide, or a silicone material.
245. The assembly of claim 173, including a second inductor through
which the terminal pin extends in non-conductive relation.
246. The assembly of claim 245, wherein the inductors are disposed
adjacent to one another.
247. The assembly of claim 246, comprising at least one additional
inductor stacked onto another one of the inductors.
248. The assembly of claim 246, wherein the inductors are each
comprised of materials having different physical or electrical
properties.
249. The assembly of claim 246, wherein the inductors are each
comprised of materials having the same physical or electrical
properties.
250. The assembly of claim 173, wherein the capacitor and the
inductor are housed within the ferrule.
251. The assembly of claim 250, including an insulative cap
disposed over the inductor opposite the capacitor.
252. The assembly of claim 247, wherein the inductors are disposed
on opposite sides of the capacitor.
253. The assembly of claim 252, wherein at least one of the
inductors is disposed on a body fluid side of the feedthrough
terminal assembly.
254. The assembly of claim 247, wherein the second inductor is
disposed adjacent to the ferrule.
255. The assembly of claim 247, wherein the inductors are disposed
adjacent to opposing surfaces of the capacitor.
256. The assembly of claim 255, wherein the inductors are bonded to
the capacitor.
257. The assembly of claim 255, wherein the capacitor and the
inductors are disposed within and conductively isolated from the
ferrule.
258. The assembly of claim 173, wherein the capacitor is disposed
on a body fluid side of the feedthrough terminal assembly.
259. The assembly of claim 173, wherein the feedthrough capacitor
comprises first and second feedthrough capacitors associated with
the inductor in non-conductive relation.
260. The assembly of claim 259, wherein the first and second
feedthrough capacitors are disposed adjacent to opposing surfaces
of the inductor.
261. The assembly of claim 260, wherein the capacitors are bonded
to opposite surfaces of the inductor.
262. The assembly of claim 260, wherein each capacitor is
internally grounded.
263. The assembly of claim 260, wherein the first and second
capacitors each include a first set of electrode plates
conductively coupled to the terminal pin, and a second set of
electrode plates conductively coupled to the ferrule.
264. The assembly of claim 263, wherein the first capacitor
comprises an externally grounded capacitor, and the second
capacitor comprises an internally grounded capacitor, the
feedthrough terminal assembly further including a conductive
material extending through both the first and second feedthrough
capacitors to conductively couple the second set of electrode
plates of the second capacitor with the second set of electrode
plates of the first capacitor.
265. The assembly of claim 264,-wherein the first and second
feedthrough capacitors are disposed adjacent to opposing surfaces
of the inductor.
266. The assembly of claim 264, wherein the conductive material
comprises a thermal setting conductive adhesive, a solder, or a
solder paste.
267. The assembly of claim 264, wherein the conductive material
comprises a conductive pin.
268. The assembly of claim 267, wherein the conductive pin
comprises a nail head pin.
269. The assembly of claim 264, wherein the conductive pin
comprises a pin attached to an underlying hermetic insulator.
270. The assembly of claim 173, including an hermetic insulator
disposed between the terminal pin and the ferrule, wherein the
capacitor is disposed adjacent to the hermetic insulator.
271. The assembly of claim 270, wherein the inductor and the
capacitor each include an aperture through which a leak detection
gas can be detected.
272. The assembly of claim 173, wherein the capacitor's second set
of electrode plates are externally grounded to the ferrule.
273. The assembly of claim 173, wherein the capacitor's second set
of electrode plates are internally grounded to the ferrule.
Description
RELATED APPLICATION
[0001] This application claims priority to U.S. Provisional Patent
Application Ser. No. 60/473,228, filed May 23, 2003 and U.S.
Provisional Patent Application Ser. No. 60/508,426, filed Oct. 2,
2003.
BACKGROUND OF THE INVENTION
[0002] This invention relates generally to feedthrough capacitor
terminal pin subassemblies and related methods of construction,
particularly of the type used in implantable medical devices such
as cardiac pacemakers, implantable defibrillators, cochlear
implants, and the like. Such terminal pin subassemblies form EMI
filters designed to decouple and shield undesirable electromagnetic
interference (EMI) signals from an associated device. Specifically,
the present invention relates to an improved EMI filter that
includes an inductive element, making the EMI filter a two element
(2-pole) or three element (3-pole) device, or even higher order
device. Feedthrough terminal assemblies are generally well known
for connecting electrical signals through the housing or case of an
electronic instrument. For example, in implantable medical devices,
such as cardiac pacemakers, defibrillators, or the like, the
terminal pin assembly comprises one or more conductive terminal
pins supported by an insulator structure for feedthrough passage
from the exterior to the interior of the medical device. Many
different insulator structures and related mounting methods are
known for use in medical devices wherein the insulator structure
provides a hermetic seal to prevent entry of body fluids into the
housing of the medical device. In a cardiac pacemaker, for example,
the feedthrough terminal pins are typically connected to one or
more lead wires within the case to conduct pacing pulses to cardiac
tissue and/or detect or sense cardiac rhythms.
[0003] However, the lead wires can also effectively act as an
antenna and thus tend to collect stray electromagnetic interference
(EMI) signals for transmission into the interior of the medical
device. Studies conducted by the United States Food and Drug
Administration, Mt. Sinai Medical Center in Miami and other
researchers have demonstrated that stray EMI, such as that caused
by cellular phones, can seriously disrupt the proper operation of
the pacemaker. It has been well documented that pacemaker
inhibition, asynchronous pacing and missed beats can occur. All of
these situations can be dangerous or life threatening for a
pacemaker-dependant patient.
[0004] In prior devices, such as those shown in U.S. Pat. Nos.
5,333,095 and 4,424,551 (the contents of which are incorporated
herein), the hermetic terminal pin subassembly has been combined in
various ways with a ceramic feedthrough capacitor filter to
decouple electromagnetic interference (EMI) signals into the
housing of the medical device. FIG. 1 is a cross-sectional view of
the feedthrough terminal assembly disclosed is U.S. Pat. No.
5,333,095. Within the drawings herein, functionally equivalent
elements of structure shown in the drawings will be referred to by
the same reference number irrespective of the embodiment shown. The
assembly 10 includes a conductive ferrule 12 which is conductively
connected to a housing or casing 14 of a human implantable device,
such as a cardiac pacemaker, an implantable defibrillator, or a
cochlear implant or the like. The assembly 10 includes a
feedthrough capacitor 16 having a grounding portion 24 which is
conductively coupled to the ferrule 12. At least one terminal pin
or lead wire 18 extends through the ferrule 12, in non-conductive
relation, and through the capacitor 16 in conductive relation.
Typically, an alumina insulator 20 is disposed between the terminal
pin 18 and the ferrule 12 or other conductive substrate through
which the terminal pin 18 passes through in non-conductive
relation. The capacitor 16 may be bonded to the insulator 20 or
separated from the insulator 20 thereby forming an air gap
depending on the assembly method used. Typically, the outside
diameter metallization 24 of the capacitor 16 is installed in
conductive relation with the conductive substrate or ferrule 12 so
that the ground electrodes of feedthrough capacitor 16 are properly
grounded. An alternative arrangement is shown in U.S. Pat, No.
5,905,627, the contents of which are incorporated herein.
[0005] FIG. 2 illustrates the uni-polar monolithic ceramic
feedthrough capacitor 16 of FIG. 1, which is typical in the prior
art described by the U.S. Pat. Nos. 5,333,095 and 4,424,551 patents
and many others. Both inside diameter and outside diameters 22 and
24 are metallized using a conductive termination which puts the
respective electrode plate sets in parallel. The feedthrough
capacitor is designed to have the lead wire 18 pass through the
center of it. The lead wire or terminal pin 18 is conductively
coupled to the inner diameter metallization 22 so as to be
conductively coupled to a first set of active electrodes 26. A
second set of ground electrodes 28 are conductively coupled to the
outer diameter metallization 24 for grounding to the conductive
substrate or ferrule 12.
[0006] FIG. 3 is the schematic diagram of the feedthrough capacitor
of FIG. 2. As shown, feedthrough capacitors are three terminal
devices which offer broadband performance and are best modeled by
transmission line equations. Feedthrough capacitors are novel in
that they act like broadband transmission lines and have very low
inductance properties. This means that they can provide effective
EMI filtering immunity over very broad frequency ranges. They do
this by de-coupling high frequency noise and shunting it to the
overall titanium or stainless shield housing 14 of the implantable
medical device. This is in contrast to rectangular monolithic chip
capacitors and other two terminal capacitors which have a
substantial amount of series inductance. Two terminal capacitors
tend to self resonate at very low frequency and thus make very poor
EMI filters, particularly for high frequencies such as cell phones,
microwave ovens, radars and other emitters.
[0007] FIGS. 4 and 5 illustrate another type of capacitor 16, which
is a multi-hole micro-planar array quad-polar feedthrough
capacitor. This has essentially the same properties as the
previously described uni-polar feedthrough capacitor illustrated in
FIGS. 2 and 3, and can accommodate multiple terminal pins
therethrough. FIG. 6 is the schematic drawing of the quad-polar
capacitor of FIGS. 4 and 5.
[0008] FIG. 7 describes the capacitor reactance equation and
illustrates how the capacitor reactance varies in ohms vs.
frequency for an ideal capacitor. At DC, capacitors look like open
circuits (in other words, like they are not there). At high
frequencies, well-designed capacitors tend to look like a very low
reactance in ohms (or short circuit). In this way, capacitors are
frequency selective components and can be used to short out or
bypass undesirable high frequencies thereby acting as low pass
filter devices.
[0009] In the past few years, a number of new devices have been
introduced to the active implantable medical device market. These
include implantable cardioverter defibrillators, which not only
offer high voltage shock therapy to the heart, but also provide
monitoring, anti-tachycardia pacing and conventional atrial and
ventricular pacing. Very recently introduced are congestive heart
failure devices, also known on the market as biventricular
pacemakers. All of these new devices have a need for an increased
number of lead wires to be implanted within the heart or outside
the vasculature of the heart. This has greatly complicated the loop
coupling and antennae coupling areas for EMI induction. This also
means that more lead wires must ingress and egress the implantable
medical device. Accordingly, it is now common for 8-pin, 12-pin or
even 16-pin devices to be present in the marketplace, all of which
have unique filtering needs.
[0010] There have also been new developments in sensor technology.
Lead based sensors are under investigation as well as new telemetry
methods. The Federal Communications Commission has recently opened
up higher frequency telemetry channels (402 MHz) to meet the
demands for more bandwidth on the part of physicians (better access
to stored data, recovery of historical cardiac waveforms, etc.).
Most modern pacemakers and implantable defibrillators store a
substantial amount of data and can download cardiac waveforms for
later investigation by the physician.
[0011] There has also been an increase in the number of emitters
generally in the marketplace. An example of this is the new Blue
Tooth System, which is rapidly gaining acceptance. Blue Tooth is a
method of interconnecting computers and the peripheral devices in a
wireless manner. This also increases the number of digital signals
to which an implantable device patient is exposed. Accordingly,
there is an ever-increasing need for better EMI immunity of
implantable medical devices over wider frequency ranges.
[0012] As mentioned, there has been a substantial amount of
research into the interaction of implantable medical devices with
cellular phones, theft detectors and other emitters. This research
is ongoing today, particularly in the area of cardiac pacemakers
and ICDs. Recently, high-gain cellular telephone amplifiers
combined with high-gain antennas have become available in consumer
markets. This creates a concern because the single element EMI
filters presently designed into pacemakers and ICDs are based on
research when cellular telephone maximum output power was limited
to 0.3 or 0.6 watts. When a cellular phone is combined with these
new amplifiers and high-gain antennas, the output power increases
by a factor of 20 to 30 dB. This is equivalent to a 23.8-watt cell
phone.
[0013] Prior art EMI filters for medical implant applications have
generally consisted of single pole devices consisting of a single
feedthrough capacitor element on each lead wire. It is possible to
increase the amount of attenuation of a single element feedthrough
capacitor by raising the capacitance value. This also desirably
lowers the frequency at which the capacitor starts to become
effective. This is known as the feedthrough capacitor's 3 dB cutoff
point. Unfortunately, raising the capacitance also has a number of
undesirable side effects. First of all, too much capacitance can
start loading down the output of an implantable medical device
thereby degrading its operation. Too much capacitance can also be a
problem in that excess energy dissipation can occur as the
capacitor must be charged and discharged during cardiac pacing or
digital signal processing in a hearing device.
[0014] In an EMI filter design of a low pass filter, a single
element filter consisting of a feedthrough capacitor increases in
attenuation at 20 dB per decade. This is a consequence of the
mathematics of computing the capacitive reactance as described in
FIG. 7 and its behavior as a low pass filter circuit. The
capacitive reactance X.sub.c in ohms varies inversely as the
capacitance value and also inversely with frequency.
[0015] An inductor per-forms the opposite function in that the
inductive reactance X.sub.L in ohms, as shown in FIG. 8, varies
directly with the frequency and the inductance in microhenries.
This formula is applicable not only to multi-turn toroids, but
single turn ferrite beads as well. The inductive reactance X.sub.L
is the opposite of capacitance reactance X.sub.c in that inductive
reactance increases with increasing frequency. As illustrated,
inductive reactance is zero ohms at DC and goes up to a very high
value at high frequency.
[0016] Therefore, when placed in series with a line, inductance can
raise the impedance of the line thereby also acting as a low pass
filter. Common prior art EMI filter circuits are shown in FIG. 9
consisting of single element feedthrough capacitors "C", "double
element L.sub.1" and "reverse L.sub.2" filters, which combine an
inductor and a capacitor, and other elements or other
configurations including "PI" and "T" configurations. The commonly
used prior art filter circuit for medical implant applications has
been the "C" circuit or feedthrough capacitor. All of the cited
patent references are based on a single element feedthrough
capacitors bonded directly to or in close proximity to the hermetic
terminal of an implantable medical device. However, using
inductance in combination with a feedthrough capacitor increases
the filter's effectiveness.
[0017] Of particular interest are the graphs shown in FIG. 10. The
horizontal or X axis is frequency in MHz and the vertical or Y axis
is the filtering efficiency measured as insertion loss in dB. For a
one component feedthrough capacitor filter "C", the insertion loss
increases with frequency at a slope of 20 dB per decade. However,
when one adds an inductive component this makes the low pass filter
into a two-element "L" filter. A two element filter like an "L"
filter goes up at a slope of 40 dB per decade. This means that its
filtering effectiveness at high frequency is much greater than a
single element filter. If one were to add inductors on both sides
of the capacitor, it would become a three component filter, which
would increase at 60 dB per decade and so on.
[0018] A single element feedthrough capacitor is limited to an
attenuation increase of 20 dB per decade. This is a linear function
on semi log paper in the region that is well above the 3 dB cutoff
point. In other words, for a single element feedthrough capacitor
filter that offers 20 dB of attenuation at 10 MHz, that same filter
would offer 40 dB at 100 MHz which is one frequency decade above.
If one were to take the same feedthrough capacitor and combine with
it an inductor element, thereby making it into an L section filter,
this now becomes a 2-element filter. A 2-element filter will
increase its attenuation effectivity by 40 dB per decade. Using the
example as previously illustrated, if an L section filter, which is
well above cutoff, exhibits 20 dB of attenuation at 10 MHz, it will
exhibit 60 dB of attenuation at 100 MHz which is a very dramatic
increase in filtering effectivity.
[0019] This is uniquely advantageous in an implantable medical
device in that one can greatly increase the amount of attenuation
of the EMI filter in frequency ranges at 1 MHz and above where many
problem emitters transmit. For example, in the 22 and 72 MHz
frequency ranges, hand held or chest strap transmitters are
commonly used to control model airplanes, model helicopters and
remote control boats. These sophisticated devices produce powerful
digitally controlled signals which can be in very close proximity
to an implanted medical device. Accordingly, a two element EMI
filter can be designed such that it offers very low attenuation in
the cardiac sensing and telemetry ranges of the implantable medical
device, but increases the attenuation curve very steeply above
these frequencies. Accordingly, there is a need to provide
multi-element filters for implantable medical devices.
[0020] As described herein, adding inductance in series with
pacemaker or implantable defibrillator leads is dramatically
effective. It has been found that the input impedance Z.sub.IN in
pacemaker biological signal sensing circuits is relatively high at
low frequencies (Z.sub.IN above 10,000 ohms) but can be quite low
and, parasitically variable at high frequencies (Z.sub.IN well
below 5). It is a novel feature of the present invention that the
addition of inductive element to the feedthrough capacitor raises
and stabilizes the input impedance of the active implantable
medical device (AIMD), particularly at these certain parasitic
frequencies. In a two element "L" filter, it is important that the
inductor element be placed on the side of the capacitor toward the
internal electronic circuitry of the AIMD. By thereby raising and
stabilizing the AIMD input impedance, the feedthrough capacitor,
which is oriented toward the body fluid side, first intercepts and
thereby becomes much more effective in bypassing high frequency EMI
signals to the overall equipotential shield or housing of the AIMD.
This shunting of undesirable signals prevents EMI signals from
entering into the AIMD housing where they could interfere with
proper AIMD circuit and therapy functions.
[0021] Exemplary ferrite beads and wire-wound inductors 30-34 are
illustrated in FIGS. 11-15. FIG. 15 illustrates placing multiple
turns of wire 36 through a ferrite or iron-core inductor element
34. This is highly efficient because the inductance of the
component goes up as the square of the number of turns. In other
words, if one were to place a single turn or a straight lead wire
36 through the ferrite bead element or ferrite core 32, this would
be defined as one turn (FIGS. 13 and 14). However, if one were to
place additional turns, the inductance would go up as the square of
the number of turns. FIG. 15 illustrates a three-turn inductor as
counted by three passes of the wire 36 through the center hole of
the toroidal inductor core 34. This would have 9 times the
inductance of the device as shown in FIG. 13, which has one pass of
wire 36 through the center hole. The toroidal inductor material can
be made of ferrite, powdered iron, molypermalloy or various other
materials which affect inductive properties.
[0022] Another major trend affecting active implantable medical
devices is the ever-increasing need for smaller size devices. Just
a few years ago, implantable cardioverter defibrillators (ICD's)
were over 100 cubic centimeters in volume. Today, ICDs are being
designed below 30 cubic centimeters. Thus, the size of all
components within the active medical device must be as small as
possible. Therefore, it is not practical to add inductive or
ferrite elements if they are to take up additional space inside the
implantable medical device.
[0023] Typical values for filter feedthrough capacitors used in
medical implant applications range from 390 picofarads all the way
up to 9000 picofarads. The average feedthrough capacitor, however,
is not very volumetrically efficient. Since only a few electrode
plates are required to reach the desired capacitance value (due to
the high dielectric constant), typical feedthrough capacitors used
in medical implantable devices incorporate a number of blank cover
sheets. A typical ceramic feedthrough capacitor used in an active
implantable medical device would have a thickness between 0.040 and
0.050 inches. Of that, only about 1/3 to 1/2 of the total height is
actually used to provide capacitance. The rest is used to provide
mechanical strength.
[0024] Implantable medical device hermetic terminals also pose
another unique problem for providing substantial inductance in EMI
filters. This comes from the nature of providing a hermetic seal to
protect against intrusion of body fluids. A typical multi-turn
inductor as described in many prior art applications (and as
illustrated herein as FIG. 15) can be held loosely in one's hands.
One can grasp a length of wire 36 and pass it back and forth
through the center forming a multi turn inductor 34, as shown in
FIG. 15. There are also a number of automatic winding machines that
are readily available in the art. However, in an implantable
medical device hermetic terminal, the lead wire is solidly captured
at one end by the nature of the hermetic terminal (usually by a
gold braze or the like). The capacitor must be mounted to the
hermetic terminal in accordance with one of the many prior art
references. A dilemma exists in how to make multiple turns with a
bonded ferrite or a bonded ferrite slab.
[0025] Accordingly, there is a need to provide multi-element
filters for implantable medical devices such that the EMI filter is
designed to offer a very low attenuation in the cardiac sensing and
telemetry ranges of the implantable medical device, but increase
the attenuation curve very steeply above these frequencies to take
into account the EMI produced by environmental emitters. Such
filters should be volumetrically efficient so as to be the smallest
possible size while having sufficient mechanical strength. Such
filters should also be able to be hermetically sealed to protect
against intrusion of body fluids into the implantable medical
device. The present invention fulfills these needs and provides
other related advantages.
SUMMARY OF THE INVENTION
[0026] The present invention resides in a feedthrough terminal
assembly which advantageously incorporates an inductor in the
feedthrough capacitor assembly. Incorporating inductors in
accordance with the present invention renders the EMI filter a two
element (two-pole) or three element (three-pole) device and
improves the EMI filter over wider frequency ranges. In particular,
the filtering efficiency measured as insertion loss (dB) is greatly
improved. Such assemblies are particularly suitable for human
implantable device applications, such as cardiac pacemakers,
implantable defibrillators, cochlear implants and the like.
[0027] Broadly, the invention comprises a feedthrough terminal
assembly that includes a conductive ferrule, a feedthrough
capacitor, and an inductor closely associated with the capacitor in
non-conductive relation. The feedthrough capacitor includes first
and second sets of electrode plates. The second set of electrode
plates are conductively coupled to the ferrule. A conductive
terminal pin extends through the capacitor such that it is
conductively coupled to the first set of electrode plates, and
through the inductor in non-conductive relation.
[0028] Preferably, the feedthrough terminal assembly is configured
for use in an active implantable medical device. Under such
circumstances, the conductive ferrule is conductively coupled to a
housing for the active implantable medical device. Typically, such
devices comprise a cardiac pacemaker, an implantable defibrillator,
a cochlear implant, a neurostimulator, a drug pump, a ventricular
assist device, a gastric pacemaker, an implantable sensing system,
or a prosthetic device.
[0029] In some embodiments, the inductor is bonded directly to the
capacitor utilizing a non-conductive polyimide, glass, ceramic
bonding material, epoxy, silicone, or a thermal plastic supportive
tape adhesive.
[0030] The inductor typically comprises a high permeability ferrite
material. Such a material may be selected from scintered alloys of
cobalt zinc ferrite, nickel zinc ferrite, manganese zinc ferrite,
powdered iron, or molypermally.
[0031] A conformal coating is typically provided over the inductor.
In the preferred embodiment, the coating disclosed comprises
Paralyne. Further, an insulator is typically disposed between the
inductor and the terminal pin. The insulator may comprise an epoxy,
a thermal-setting non-conductive adhesive, a non-conductive
polyimide, or a silicone material.
[0032] In an alternative embodiment, a second inductor is provided
through which the terminal pin extends in non-conductive relation.
The first and second inductors may be disposed adjacent to one
another or on opposite sides of the capacitor. In this regard, at
least one additional inductor may be stacked onto another one of
the inductors, and such inductors may each be comprised of
materials having different physical and electrical properties.
Alternatively, the inductors may each be comprised of materials
having the same physical properties. Further, the capacitor and the
inductor may be housed within the ferrule, and an insulative cap
may be disposed over the inductor opposite the capacitor.
[0033] When the inductors are disposed on opposite sides of the
capacitor, various configurations are possible. In one, at least
one of the inductors may be disposed on a body fluid side of the
feedthrough terminal assembly. Further, the second inductor may be
disposed adjacent to the ferrule. Alternatively, the inductors may
be bonded to opposing surfaces of the capacitor. In an illustrated
embodiment wherein a pair of inductors are disposed on opposite
sides of the capacitor, the capacitor and the inductors are
disposed within and conductively isolated from the ferrule.
[0034] In another illustrated embodiment, first and second
feedthrough capacitors are associated with the inductor in
non-conductive relation. The first and second feedthrough
capacitors may be disposed on opposing surfaces of the inductor
and, further, each capacitor may be internally grounded. The first
and second capacitors each include a first set of electrode plates
conductively coupled to the terminal pin, and a second set of
electrode plates conductively coupled to the ferrule. The first
capacitor comprises an internally and externally grounded
capacitor, and the second capacitor comprises an internally
grounded capacitor. The feedthrough terminal assembly further
includes a conductive material extending through both the first and
second feedthrough capacitors to conductively couple the second set
of electrode plates to the second capacitor with the second set of
electrode plates of the first capacitor. The conductive material
may comprise a thermal setting conductive adhesive, a solder or a
solder paste. Alternatively, the conductive material may comprise a
conductive pin. Moreover, the conductive pin may comprise a nail
head pin or a pin attached to an underlying hermetic insulator.
[0035] The hermetic insulator is typically disposed between the
terminal pin and the ferrule, and the capacitor is typically
disposed adjacent to the hermetic insulator.
[0036] In another illustrated embodiment, the inductor includes an
aperture aligned with an aperture of the capacitor through which a
leak detection gas can be detected.
[0037] The capacitor's second set of electrode plates may be
externally grounded to the ferrule, or, alternatively, internally
grounded to a ground pin which is conductively coupled to the
ferrule.
[0038] The terminal pin may be wound about the inductor to form
multiple turns. In this case, adjacent portions of the wound
terminal pin are electrically insulated from one another. The
adjacent portions of the wound terminal pin are encased in a
non-conductive material such as a non-conductive sleeve.
[0039] The inductor may include a notch for receiving the wound
terminal pin. The notch may include a ramp for accommodating the
terminal pin, and further the inductor may include multiple
notches, each for accommodating a separate terminal pin therein. In
some embodiments, the notch includes multiple slots for receiving
corresponding multiple turns of the terminal pin. The notch may
further comprise contoured corners for accommodating the terminal
pin.
[0040] In several embodiments, means are illustrated for
maintaining the conductor in close association with a capacitor
without forming a direct physical attachment therebetween. The
inductor maintaining means comprises a lock between the terminal
pin and the inductor. The lock typically comprises a mechanical
lock such as a swage, a clamp or an epoxy. The lock may,
alternatively, simply comprise a deformation in the terminal
pin.
[0041] The inductor maintaining means may further comprise a wire
bond pad attached to the terminal pin. When a wire bond pad is
provided, it may or may not be physically attached to the
underlying structure of the hermetic terminal assembly apart from
the terminal pin itself.
[0042] Other features and advantages of the present invention will
become apparent from the following more detailed description, taken
in conjunction with the accompanying drawings, which illustrate, by
way of example, the principles of the invention.
BRIEF DESCRIPTION OF THE DRAWINGS
[0043] The accompanying drawings illustrate the invention. In such
drawings:
[0044] FIG. 1 is a cross-sectional view of a surface prior art
mounted discoidal capacitor in an EMI filter assembly;
[0045] FIG. 2 is a partially sectioned prior art uni-polar
discoidal feedthrough capacitor of FIG. 1;
[0046] FIG. 3 is a schematic drawing of the feedthrough capacitor
of FIG. 2;
[0047] FIG. 4 is a perspective view of a prior art quad-polar
feedthrough capacitor;
[0048] FIG. 5 is a cross-sectional view taken on the line 5-5 in
FIG. 4;
[0049] FIG. 6 is a schematic drawing of the quad-polar capacitor of
FIG. 4;
[0050] FIG. 7 describes the capacitor reactance equation and
illustrates how the capacitor reactance varies in ohms vs.
frequency for an ideal capacitor;
[0051] FIG. 8 illustrates the equation for the inductive
reactance;
[0052] FIG. 9 illustrates schematic diagrams of common EMI filter
circuits;
[0053] FIG. 10 is a comparison chart of insertion loss vs. number
of components in a low pass EMI filter;
[0054] FIG. 11 is a perspective view of a prior art ferrite slab
toroidal inductor;
[0055] FIG. 12 is a cross-sectional view taken generally along line
12-12, of FIG. 11;
[0056] FIG. 13 is a perspective view of a prior art toroidal
ferrite inductor with one turn or a single pin going through the
center thereof;
[0057] FIG. 14 is a cross sectional view of the toroid of FIG. 13
taken generally along line 14-14;
[0058] FIG. 15 is a perspective view of a prior art toroidal
inductor with multiple lead wire turns;
[0059] FIG. 16 is a chart giving the mechanical properties of a
thermal plastic polyimide supportive tape adhesive which can be
used in accordance with the present invention;
[0060] FIG. 17 is a cross-sectional view of an EMI filter embodying
the present invention;
[0061] FIG. 18 is an enlarged view of the area 18 taken from FIG.
17, illustrating an alternative embodiment;
[0062] FIG. 19 is a schematic drawing of the EMI filter of FIG.
17;
[0063] FIG. 20 is a perspective view of the ferrite slab inductor
46 of FIG. 17;
[0064] FIG. 21 is a cross-sectional view of an EMI filter assembly
embodying the present invention, illustrating multiple inductors 46
and 46' in stacked or laminated relationship;
[0065] FIG. 22 is a schematic drawing of the EMI filter assembly of
FIG. 21;
[0066] FIG. 23 is an exploded perspective view of the laminated
inductors of FIG. 21;
[0067] FIG. 24 is a cross-sectional view illustrating placement of
the ceramic capacitor and inductor completely inside of a
surrounding ferrule;
[0068] FIG. 25 is an electrical schematic drawing of the
two-element inductor capacitor EMI filter of FIG. 24;
[0069] FIG. 26 illustrates an exploded perspective view of a five
pole or penta polar capacitor assembly that is internally grounded
embodying the present invention;
[0070] FIG. 27 is a cross-sectional view of an EMI filtered
hermetic terminal assembly modified by shortening the alumina
insulator thereof to provide a convenient bonding surface to
install a second ferrite bead 46' on the body fluid side of the
assembly;
[0071] FIG. 28 illustrates the second ferrite slab of FIG. 27;
[0072] FIG. 29 is a schematic drawing of the filtered hermetic
terminal assembly of FIG. 27;
[0073] FIG. 30 is a cross-sectional view of an EMI filtered
assembly having a ceramic capacitor disposed on the body fluid side
and an inductor bonded to an internal insulator;
[0074] FIG. 31 is a cross-sectional view of an EMI filtered
assembly embodying the present invention having inductors co-bonded
to opposing surfaces of a ceramic capacitor;
[0075] FIG. 32 is an electrical schematic drawing of the EMI filter
terminal assembly of FIG. 31;
[0076] FIG. 33 is a cross-sectional view of a PI filter assembly
embodying the present invention;
[0077] FIG. 34 is an electrical schematic drawing of the EMI filter
of FIG. 33;
[0078] FIG. 35 is a cross-sectional view illustrating a novel PI
section filter incorporating capacitors combining both external and
internal ground technologies;
[0079] FIG. 36 is an electrical schematic view of the terminal of
FIG. 35;
[0080] FIG. 37 is one possible top plan view of the assembly of
FIG. 35;
[0081] FIG. 38 is a top plan view of another possible configuration
of the assembly of FIG. 35;
[0082] FIG. 39 is a cross-sectional view illustrating another Pi
filter assembly incorporating hybrid capacitors similar to FIG.
35;
[0083] FIG. 40 is a cross-sectional view illustrating yet another
novel PI filter assembly incorporating hybrid capacitors;
[0084] FIG. 41 is a perspective view of the bottom capacitor of
FIGS. 35, 39 and 40;
[0085] FIG. 42 is a cross-sectional view through the capacitor of
FIG. 41 taken generally along line 42-42;
[0086] FIG. 43 is a cross-sectional view through the capacitor of
FIG. 42 taken generally along line 43-43, illustrating the
arrangement of active electrode plates;
[0087] FIG. 44 is a cross-sectional view through the capacitor of
FIG. 42 taken generally along line 44-44, showing the configuration
of the ground electrode plates;
[0088] FIG. 45 is a perspective view of the ferrite inductor of
FIG. 35;
[0089] FIG. 46 is a cross-sectional view of the inductor of FIG. 45
taken generally along line 46-46;
[0090] FIG. 47 is a perspective view of the upper capacitor of
FIGS. 35, 39 and 40;
[0091] FIG. 48 is a cross-sectional view of the capacitor shown in
FIG. 47 taken generally along line 48-48;
[0092] FIG. 49 is a sectional view of the capacitor of FIG. 48
taken generally along line 49-49, illustrating the arrangement of
the active electrode plates;
[0093] FIG. 50 is a cross-sectional view of the capacitor of FIG.
48 taken generally along line 50-50, showing the configuration of
the ground electrode plates;
[0094] FIG. 51 is a perspective view of an internally grounded
three-element PI circuit hermetic terminal embodying the present
invention;
[0095] FIG. 52 is a sectional view taken generally along line 52-52
of FIG. 51;
[0096] FIG. 53 is a family of performance curves illustrating the
advantages of adding the inductor filter elements of the present
invention;
[0097] FIG. 54 is a perspective view illustrating an alternative
embodiment of a ceramic capacitor and inductor mounted to a
hermetic terminal and having a center hole therethrough, which
allows for ready passage of a gas during hermetic seal testing;
[0098] FIG. 55 is a cross-sectional view of the assembly of FIG. 54
taken generally along line 55-55, showing the inductor bonded to
the capacitor with the aligned center hole for helium leak
detection;
[0099] FIG. 56 illustrates an internally grounded tri-polar
capacitor;
[0100] FIG. 57 is a cross-sectional view taken generally along line
57-57 of FIG. 56;
[0101] FIG. 58 is a plan view of an inline multi-polar EMI filter
with a grounded pin;
[0102] FIG. 59 is a cross-sectional view taken generally along line
59-59 of FIG. 58;
[0103] FIG. 60 is a schematic diagram of the EMI filter assembly of
FIGS. 58 and 59;
[0104] FIG. 61 is a top plan view of a multi-polar EMI filter with
a grounded pin, similar to FIG. 58;
[0105] FIG. 62 is a cross-sectional view taken generally along line
62-62 of FIG. 61, illustrating the use of an inductor slab instead
of individual inductor beads;
[0106] FIG. 63 is a perspective view of a novel inductor having a
notch in accordance with a preferred embodiment of the present
invention;
[0107] FIG. 64 is a cross-sectional view taken generally along the
line 64-64 of FIG. 63;
[0108] FIG. 65 is a view similar to FIG. 64, incorporating a ramp
for facilitating feed of a multiple turn lead wire through the
center hole of the ferrite inductor;
[0109] FIG. 66 is an electrical schematic drawing of the ferrite
bead of FIG. 63;
[0110] FIG. 67 is a sectional view similar to FIG. 17, but
employing the novel ferrite bead of FIG. 63;
[0111] FIG. 68 illustrates the schematic diagram of the EMI
filtered terminal assembly of FIG. 67;
[0112] FIG. 69 is an enlarged fragmented perspective view of a
portion of the terminal lead shown in FIG. 67, illustrating that a
portion of an insulator is removed from the lead as it extends
upwardly through the capacitor;
[0113] FIG. 70 is a perspective view of a uni-polar ferrite slab
designed with a novel slot arrangement;
[0114] FIG. 71 is a cross-sectional view taken generally along the
line 71-71 of FIG. 70;
[0115] FIG. 72 is a cross-sectional view illustrating a uni-polar
feedthrough capacitor utilizing the ferrite slab of FIG. 70;
[0116] FIG. 73 is a fragmented perspective view of a novel two-turn
uni-polar inductor embodying the present invention;
[0117] FIG. 74 is a perspective view of a uni-polar ferrite slab
with four slots;
[0118] FIG. 75 is a perspective view illustrating the novel
four-turn uni-polar ferrite of FIG. 74 mounted to a hermetic
terminal and assembled;
[0119] FIG. 76 is a perspective view of an inline quad-polar
ferrite bead having four slots in accordance with the present
invention;
[0120] FIG. 77 is a perspective view illustrating the mounting of
the inline quad-polar ferrite bead of FIG. 76 to a hermetic
terminal;
[0121] FIG. 78 is the schematic drawing of the quad-polar "L"
section filter shown in FIG. 77;
[0122] FIG. 79 is a perspective view of a ferrite slab embodying
the present invention and having novel slots so that an additional
turn can be added making the unit into a two-turn inductor;
[0123] FIG. 80 is a perspective view of a quad-polar feedthrough
filter terminal assembly wherein the inductor slab is loosely
seated on top of the capacitor without any bonding material;
[0124] FIG. 81 is a sectional view taken generally along the line
81-81 of FIG. 80;
[0125] FIG. 82 is a perspective view of a quad-polar feedthrough
filter terminal assembly similar to that illustrated in FIGS. 80
and 81, illustrating another embodiment thereof;
[0126] FIG. 83 is a sectional view taken generally along the line
83-83 of FIG. 82;
[0127] FIG. 84 is a sectional view similar to that illustrated in
FIG. 17, illustrating an L-shaped wire bond pad attached using
bonding insulating material to the inductor slab;
[0128] FIG. 85 is a perspective view of the L-shaped wire bond pad
of FIG. 84;
[0129] FIG. 86 is an exploded perspective view of an octapolar
(plus a grounded lead) feedthrough filter terminal assembly
embodying the present invention;
[0130] FIG. 87 is a perspective view of the feedthrough terminal
assembly of FIG. 86; and
[0131] FIG. 88 is an enlarged cross-sectional view taken generally
along the line 88-88 of FIG. 87.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0132] As shown in the accompanying drawings for purposes of
illustration, the present invention resides in an EMI filter
feedthrough terminal assembly which incorporates an inductive
element in order to increase attenuation of EMI as the frequency of
the EMI increases. The invention is particularly suited for use in
human implantable medical devices, as described above.
[0133] With reference now to FIG. 17, an EMI filter feedthrough
terminal assembly 36 embodying the present invention is
illustrated. Similar to typical implantable device application
assemblies, the assembly 36 includes a conductive substrate in the
form of a ferrule 12 which is conductively coupled to the housing
or casing 14 of an implantable medical device using a laser weld,
braze 38 or other appropriate conductive connection as is
well-known in the art. A feedthrough capacitor 40 is conductively
coupled to the ferrule 12 using a conductive thermal setting
material, braze, solder, etc. 42'. A lead wire or terminal pin 18
extends through apertures formed in the ferrule 12 and capacitor
40. Active electrodes 26 of the capacitor 40 are conductively
coupled to the terminal pin 18, by solder, conductive thermal
setting material, braze 42 or other means that are well-known in
the art. Ground electrodes 28 of the capacitor 40 are conductively
coupled to the ferrule 12, in this instance between outer
metallization 24 of the capacitor 40 and its conductive connection
42' to the ferrule 12. An insulator 20, such as an alumina ceramic,
is disposed between the conductive ferrule 12 and the terminal pin
18 so that the terminal pin 18 is in non-conductive relation
thereto. The terminal pin 18 may be adhered or otherwise fixed to
the insulator 20 by means of gold braze 44 or a glass compression
or fusion seal or the like.
[0134] The present invention advantageously incorporates an
inductor 46 into the assembly 36. The ferrite slab inductor 46 is
co-bonded to the capacitor 40 so as to be in non-conductive
relationship therewith. The capacitor element 40 is schematically
oriented towards the body fluid side and the inductor element 46 is
desirably oriented toward the inside of the implantable medical
device 14. The reason it is desirable to have the feedthrough
capacitor C oriented towards the body fluid side from an electrical
circuit point of view is that the cardiac lead wire system
represents a fairly stable source impedance. Studies indicate that
the source impedance of implanted lead wires tend to be around 80
ohms. This does vary somewhat with frequency, but this is a
reliable average. On the other hand, the input impedance of a
cardiac pacemaker or other implantable medical device is highly
variable with frequency. At low frequencies the input impedance of
a cardiac pacemaker tends to be relatively high, on the order of 10
Kohms or more. However, as the frequency increases, the input
impedance of the cardiac pacemaker can vary dramatically. At very
high frequencies above 20 MHz, the AIMD input impedance (Z.sub.IN)
can shift due to parasitic resonances and coupling between stray
capacitance and stray inductance of circuit traces and other
components. Accordingly, at certain frequencies the input impedance
of the pacemaker might be hundreds of ohms and at a nearby or
adjacent frequency the input impedance could plummet drastically to
less than 2 ohms. A feature of the inductor L as described in the
present invention is that the inductive element stabilizes the
input impedance of the cardiac pacemaker. By using the inductor
element properties, that is, both its inductive reactance and
resistive properties to raise and stabilize the input impedance of
the cardiac pacemaker, the feedthrough capacitor C becomes much
more effective as a bypass element. In other words, when EMI is
induced on the cardiac lead wires, that EMI comes from a source
impedance of approximately 80 ohms. It then encounters the
feedthrough capacitor C which represents a very low impedance to
ground. The inductive element L also blocks the EMI from getting
into the input circuits of the implantable medical device because,
by representing a relatively high impedance, the EMI is desirably
shunted to ground through the feedthrough capacitor C.
[0135] With reference to FIG. 20 and FIG. 17, the terminal pin 18
extends through an aperture 48 of the inductor 46. The space
between the lead wire 18 and the inside diameter of the inductor 46
defines an air gap 49. This air gap is desirable in that there is
no electrical connection at all required between the inductor 46
and the lead wire 18. In fact, it is preferable that the inductor
46 be maintained in insulative relationship with all of the
surrounding elements, including lead wire 18, the ceramic capacitor
40 and the ferrule 12. In a low voltage device, the air gap 49 does
not present a problem. However, in high voltage devices such as
implantable cardioverter defibrillators, air gap 49 needs to be
controlled.
[0136] The aperture 48 is aligned with apertures in the capacitor
40 and ferrule 12. As can be seen in the schematic diagram FIG. 19,
the assembly 36 becomes a two element "L" circuit EMI filter. As
shown in FIG. 10, this has the desired effect of greatly increasing
the insertion loss or filtering efficiencies throughout the
frequency range. Whereas a single component "C" filter, such as
that illustrated in FIG. 1, has an insertion loss slope of 20 dB
per decade, the two component "L" filter circuit of FIG. 17 has a
40 dB per decade slope, which is highly desirable.
[0137] Comparing the assemblies 10 and 36 of FIGS. 1 and 17, it
will be appreciated that the volumetric efficiency of the capacitor
40 in the invention is enhanced as the co-bonding of the inductor
element 46 creates a monolithic structure which has sufficient
height for mechanical strength of handling and construction.
Referring now back to FIG. 1, one can observe the height of the
typical capacitor 16 illustrated. Now referring to FIG. 17, one can
see the composite structure consisting of the thinner capacitor 40
and the co-bonded ferrite slab 46, which composite structure has
approximately the same height as the original capacitor 16 shown in
FIG. 1. This is because the internal electrode plates of the
capacitor 16 of FIG. 1 are very efficient and do not require the
entire height of the ceramic capacitor 16. Cover sheets or layers
are typically added on the top and bottom of the capacitor 16 as
shown in FIG. 1, to increase its structural integrity. Another way
of saying this is that it is really not possible to build ceramic
feedthrough capacitors that are too thin. That is, if they are
designed below 0.030 inch in thickness, warpage and cracking during
sintering become major factors (this is known in the industry as
the potato chip effect). Accordingly, cover sheets are built up to
strengthen the ceramic capacitor. In the structure shown in FIG.
17, the co-bonding of the ferrite inductor provides the required
strength. Accordingly, the capacitor 40 can be made much
thinner.
[0138] With continuing reference to FIG. 17, the insertion of the
lead wire or terminal pin 18 directly through the inductive element
46 creates a single turn inductor. As shown in FIG. 10, this single
turn increases the attenuation rate of the assembly 36 from 20
dB/decade to 40 dB/decade. The inductor 46 capacitor 40
combination, as illustrated in FIG. 17, is desirably on the inside
of the ferrule 12. That is on the inside of the pacemaker or
implantable medical device housing 14 that is protected from body
fluids by the hermetic seal 20. In general, the electronic
components of an active implantable medical device are preferably
placed inside the hermetic terminal to protect them from the
corrosive and conductive effects of body fluid intrusion.
[0139] In FIG. 17, one can see that there is an air gap 49 between
the lead wire 18 and the inside diameter of the ferrite slab 46.
This is not a problem in a low voltage application such as for an
implantable cardiac pacemaker. However, in a high voltage
application such as that of an implantable cardioverter
defibrillator, this air gap 49 can present a problem. That is
because micro-coulomb or arc type discharges can occur in the high
voltage field generated around the lead wire 18 and the inside
diameter of the ferrite slab 46. This can occur even though the
ferrite slab 46 has been conformally coated with a material such as
Paralyne or equivalent insulating materials. The high voltage field
that surrounds lead wire 18 tends to relax into the air space
surrounding it. The presence of the inductor slab 46 tends to
concentrate these equipotential lines of force which can result in
the aforementioned micro-coulomb discharges. These would appear
during high voltage testing of the device as sudden interruptions
in the charging current of the capacitor. This is a particularly
undesirable situation in a component for human implant applications
because if such discharge occurs in an area of high electric field
stress, it could lead to a catastrophic breakdown or avalanche of
the device. FIG. 18 illustrates this same air gap 49 which has been
back filled with an insulating material 51. This insulating
material can be a polymer including an epoxy, a thermal-setting
non-conductive adhesive, a non-conductive polyimide, a silicone, a
glass, a ceramic or any combinations of the above. It is desirable
that the filled material be free of voids or air holes. The
presence of the filling material 51 puts a high dielectric strength
material into the previously mentioned air gap 49. This prevents
the formation of micro coulomb discharges or arcing.
[0140] The inductor 46 is typically in the form of a ferrite slab,
as illustrated in FIG. 20. Ferrite beads and slabs are typically
formed during a powder pressing and sintering manufacturing process
(extrusion or machining techniques can also be used). Proprietary
powders, including powdered iron, manganese zinc ferrite, nickel
zinc ferrite, cobalt zinc ferrite, etc. are formed into the beads
or slabs of the final toroidal inductor configuration. The inductor
46 may be comprised of other materials such as a molypermalloy
material or other high permeability ferrite material. There are
commercially available ferrite materials that have both high
permeability and high resistivity properties, making them ideal for
medical implant EMI filter applications.
[0141] Ferrites are hard ceramic materials which can abrade wire
insulation films during winding. The inductor slab 46 is ordinarily
tumbled so that sharp edges are rounded. However, if a higher level
of insulation protection is desired, a smooth insulative conformal
coating can be provided. This coating should be soft to prevent
stressing and cracking the core upon curing or during any
temperature cycling or temperatures due to bonding. The coating
should have a low coefficient of friction and withstand normal
environments. Therefore, in an embodiment of the invention, such
ferrite bead or ferrite slab 46 is coated with suitable insulation
materials such as Paralyne C, Paralyne D, Paralyne E or Paralyne N
or other suitable conformal coating material. A conformal coating
material also desirably increases the electrical insulation
resistance of the inductor 46 to a very high value (within the
Megohm or Gigohm range). Accordingly, the conformal coating will
also serve to prevent premature battery drain of the implantable
medical device.
[0142] There are a number of materials that are ideal for
co-bonding the ceramic capacitor 40 to the ferrite bead or the
ferrite slab 46. In this regard it is important to note that there
is actual reference to two bonds. First, there is the bond between
the conformal coating to the ferrite slab 46. Second, there is the
bond between the conformal coatings, such as Paralyne or the like,
and the adhesive material 50. Therefore, it is also important that
the conformal coating be well adhered to the ferrite material
itself.
[0143] It should be noted that these conformal coatings are
typically quite thin. A typical Paralyne coating thickness would be
0.001 to 0.005 inches. Coatings that are excessively thick can be
problematic in that they would mismatch the coefficient of
expansion of the underlying ferrite material. Because the coatings
are so thin, they are generally not shown in any of the drawings.
In some of the embodiments that are depicted in the figures herein,
it would be possible to use a ferrite inductor without a conformal
coating. However, in all of the preferred embodiments, a conformal
coating such as a Paralyne coating is incorporated, but not
shown.
[0144] FIG. 16 illustrates the properties of a thermal plastic
polyimide supportive tape adhesive 50 or 50' which can be used as
shown in FIG. 17 to co-bond the inductor 46 to the capacitor 40.
This tape adhesive 50 or 50' is ideal for bonding the capacitor 40
to the ferrite slab inductor 46. This material has unique
properties and it can be di-cut or laser-cut to any desired shape
with a variety of through holes. It adheres well to the ceramic
capacitor 40, alumina 20, inductor conformal coating, and other
surrounding materials, thereby providing a convenient bonding
methodology. There are a number of suitable alternative materials
described as follows: co-curing 3M one and two part epoxies, Master
Bond one or two part epoxies, glasses approved for implantable
devices, all ceramics approved for implantable body devices and all
non-conductive polymers including polyimides. The important feature
is that these materials when bonded and cured are capable of
handling the shear stresses that occur in a laminated beam
structure as the beam deflects. For example, if the beam deflects
downward, the bottom fibers of the beam tend to elongate. The
center or neutral access to the beam is where the maximum shear
stresses occur. This is where the bending stresses are zero.
Accordingly, in order to raise the moment of inertia (I) of the
beam, a co-bonding material is required which is capable of
handling these substantial shear stresses. Fortunately, the unique
geometry of the inductor slab co-bonded to the ceramic capacitor
provides ample surface area between the two mating surfaces.
Accordingly, a variety of materials are available which can handle
the shear stresses that develop in this composite structure.
[0145] Referring to the FIG. 19 schematic diagram, we can see that
the inductor slab 46 has both an inductive property L and series
resistance property R.sub.L. It is a property of ferrite materials
that both the inductance and the resistive properties vary with
frequency. In general the inductance tends to be higher at low
frequency and goes down with elevating frequency. On the other hand
R.sub.L tends to be a very low number at lower frequencies and
tends to get higher in its ohmic value at higher frequencies. This
is particularly desirable in an implantable medical device where
biologic signals at very low frequencies are being detected by
pacemaker sense circuitry. It is a feature of the present invention
that R.sub.L be quite low at biologic frequencies so that sensing
such frequencies is not impaired. At higher frequencies, R.sub.L
acts dramatically to increase the EMI filter performance of the L
section filter as shown on the schematic diagram in FIG. 19. The
way an L section filter works is that EMI is shunted to ground
through the feedthrough capacitor 40. However, if the impedance of
the cardiac pacemaker is relatively low, the inductive reactance
X.sub.L and the resistance of the ferrite slab R.sub.L both act to
raise the input impedance of the implantable medical device. This
makes the operation of the feedthrough capacitor assembly 36 much
more effective. In other words, the attenuation of the EMI filter
capacitor assembly 36 is dramatically improved as both L and
R.sub.L go up. Therefore, it is a feature of the present invention
that inductor slab 46 have two desirable properties including the
property of inductance and high frequency resistance R.sub.L To
maximize the inductance and the resistance of the ferrite slab 46,
it is desirable that the inside diameter hole 48 of the ferrite
slab 46 fit relatively tightly to the feedthrough lead wire 18. As
mentioned, it is desired to have the inductance L and the resistive
property R.sub.L to be as high as practicable. This can be done by
increasing the overall height of the ferrite bead 46. However,
practical manufacturing restrictions exist. These restrictions are
based on the fact that, in general, the ferrite slab material 46 is
created by a pressed and sintered powder system. The powder is
pressed into a die with a central pin which is later extracted
after sintering at high temperature. If the ferrite slab 46 gets
too thick, it becomes virtually impossible to extract the fixture
pin after sintering of the ferrite material into a hard
structure.
[0146] Another factor that limits the height of the ferrite slab 46
is the amount of physical space that is available inside of the
implantable medical device. It is extremely important that every
component in an implantable medical device be kept very small, so
that the size and weight of the overall medical device is
comfortable for the patient and also convenient for surgical
implant. However, in some devices there is considerable height that
is available. Accordingly, it is preferred that the inductor slab
46 have as much height as possible for a given design.
[0147] FIGS. 21 and 23 illustrate a novel feature of the present
invention in that ferrite slabs with a very small center hole can
be manufactured and then layered to provide the overall height to
optimize both the inductive and resistive properties. In FIG. 21
one sees that there are two ferrite slabs 46 and 46' which have
been bonded together with a non-conductive insulating washer 50"
(see FIG. 16). This allows one to increase the overall height of
the ferrite slab without running into the fixturing problems if one
tried to manufacture this as a single element. As previously
mentioned, for a single inductor slab, the height and inside
diameter ratio could be quite problematic in the manufacturing
operation.
[0148] It will be obvious to one skilled in the art that two, three
or a number of ferrite slabs 46 can be co-bonded together to
achieve any desired height and total inductance that is
required.
[0149] The schematic diagram shown in FIG. 22 illustrates the
effect of having these two inductors 46 and 46' acting in series
with their two resistive properties acting in series. These
elements simply add up which increases the overall inductance and
the overall resistance of the ferrite slab. However, this does not
change the basic L circuit EMI filter configuration. In other
words, the addition of a second ferrite slab 46' means that the EMI
filter of FIG. 21 still acts as a two element L section filter. It
is only when you separate the ferrite slabs by a capacitor element
that you increase the number of poles or elements of the EMI
filter, as described further herein.
[0150] Referring now back to FIG. 21, one can see that a plurality
of ferrite slabs 46 and 46' can be co-bonded together. These slabs
can be of various initial permeabilities and properties. For
example, the first slab 46 could be of manganese zinc material and
slab 46' could be of cobalt zinc material. These two materials have
markedly different electrical properties. One material has higher
inductance at low frequency whereas the other material has higher
inductance at higher frequencies. By co-bonding beads or slabs 46
and 46' of various materials together, one can optimize inductance
throughout wider frequency ranges. The same is true of the
resistive property R.sub.L1 and R.sub.L2 of the two ferrite slabs
46 and 46'. Each type of ferrite material has different resistance
versus frequency properties. By combining various materials one can
also optimize the amount of resistance versus frequency.
[0151] Another novel method of building an L circuit filter is the
embedded approach, illustrated in FIG. 24. In this case, the
ceramic capacitor 40 has been placed completely inside a
surrounding ferrule 12. The inductor slab 46 is then co-bonded to
the capacitor 40, preferably oriented toward AIMD circuitry as
illustrated. This electrical connection from the capacitor outside
diameter metallization 24 and gold braze 44' of ferrule 12 is
performed using connection material 42' in accordance with U.S.
patent application Ser. No. 10/377,086, the contents of which are
incorporated herein, utilizing oxide resistant biostable conductive
pads. An optional epoxy cap 52 can be placed over the top of the
ferrite inductor 46, primarily for cosmetic purposes. The resulting
L circuit is illustrated in the schematic diagram of FIG. 25, which
as shown in FIG. 10 gives rise to an attenuation slope of 40
dB/decade.
[0152] The present invention is not limited whatsoever in the
number of terminal pins or the EMI feedthrough terminal assembly
design. For example, FIG. 26 illustrates an exploded view of an
internally grounded pentapolar feedthrough capacitor hermetic
terminal 54 with mounted feedthrough capacitor 40, inductor 46, and
alumina insulator 20 with five gold brazed leadwires 18 which are
typically platinum, or platinum-iridium or the like. A ground plate
56 is typically attached to the ferrule 12 by laser welding or the
like. An insulating washer 50 is then placed on top of the ground
plate 56. An internally grounded feedthrough capacitor 40 in
accordance with U.S. Pat. No. 5,905,627 is then placed and attached
to the lead wires 18. Insulating washer 50' is then placed on top
of capacitor 40 to which inductor 46 is assembled in accordance
with the present invention. Accordingly this makes a very efficient
pentapolar L section filter.
[0153] Adding even more inductor elements to the EMI filter
additionally increases its attenuation rate per decade. FIG. 27
illustrates a three element "T" section low pass filter assembly 58
wherein there are ferrite slab inductor elements 46 and 46' which
appear electrically on both sides of the feedthrough capacitor 40.
Such a three-element filter will have an attenuation rate of 60 dB
per decade, as shown in FIG. 10, which is even more highly
desirable. This is also shown as the T circuit schematic of FIG.
29. Another feature of the invention as shown in FIG. 27 is that
there is a biocompatible conformal coating over the ferrite bead
inductor element 46'. This provides an additional level of
protection from intrusion of body fluid in the inductor element
46'. In addition, the inductor element 46' is encapsulated
underneath the pacemaker or implantable defibrillator header block
(not shown). This provides additional protection from the intrusion
of body fluid.
[0154] In summary, placing the inductor 46' on the body fluid side
is accomplished in three main ways. That is, the ferrite material
is a hard fired material that in and of itself is not prone to
leaching out and therefore has its own degree of biocompatibility.
The adjunct conformal coating of silicone, Paralyne or other
biocompatible coating assists in its biocompatibility as well as
the placement of adjunct sealants in the header block.
[0155] With continuing reference to FIG. 27, a ferrite slab
inductor 46 is co-bonded directly to the ceramic capacitor 40, such
as by an adhesive washer 50'. In this regard, it is identical to
the structure shown in FIG. 17. However, on the body fluid side, a
second inductor slab 46' is bonded directly to the hermetic
terminal 12, such as by an adhesive washer 50'". In this case, the
inductor ferrite bead or the inductor slab 46' is directly exposed
to body fluid or on the body fluid side of the device 14. In a
typical implantable medical device, a header or connector assembly
(not shown) is placed over this area with some sort of a sealant
such as silicone and the like. However, it is still possible for
body fluids and electrolytes to penetrate down to the layer of the
ferrite. A unique aspect of the invention is the use of ferrite
material which during sintering is highly bound to various elements
including iron. This makes the composite structure biocompatible.
Examples of such ferrite material include Manganese Zinc, Nickel
Zinc or Cobalt Nickel.
[0156] FIG. 30 illustrates a novel L section filter 60 of the
present invention with the inductor slab 46 bonded to the inside of
the hermetic terminal 12, such as by a washer 50'". The ceramic
feedthrough capacitor 40 is shown on the outside or body fluid side
of the device. In this case there is no conformal coating over the
top of the capacitor 40. The materials of the ceramic feedthrough
capacitor 40 must all be biocompatible. That is the internal
electrodes 26 and 28, the metallization 22 and 24 and the
connections 42' from the ferrule 12 to the outside diameter
metallization 24 and from the lead 18 to the inside diameter
metallization 22 must be of suitable biocompatible materials, as
described in U.S. patent application Ser. No. 10/778,954, filed
Feb. 12, 2004.
[0157] Another advantage of designing with an L or T circuit has to
do with the timing of the output circuitry of implantable
cardioverter defibrillators. It has been noted that the presence of
an EMI ceramic feedthrough capacitor in the high voltage output
circuits of an implantable defibrillator can interfere with its
timing or cause microprocessor re-sets. This is particularly true
when the implantable defibrillator is fired into a no load
situation. In other words, this is when the ICD lead wires would
not be connected to cardiac tissue.
[0158] It is theorized that the leading edge from the implantable
defibrillator's pulse causes excessive charging current into the
feedthrough capacitor. The energy stored can then reflect back and
disrupt implantable defibrillator timing circuitry. The presence of
the slab inductor 46 as described throughout this patent
application is an advantage in that the series inductance will slow
the rise time of this leading edge pulse before it gets to the
feedthrough capacitor. In this way, it is a novel aspect of the
present invention that higher capacitance value feedthrough
capacitors can be used in combination with an inductor without
disrupting the sensitive output circuitry of the implantable
defibrillator. This is because the series inductance decouples the
feedthrough capacitor from the ICD's output circuitry.
[0159] With reference now to FIGS. 31 and 32, a novel T filter
assembly 62 includes two inductor ferrite slabs 46 and 46', which
are co-bonded to opposing top and bottom surfaces of the ceramic
capacitor 40 within the ferrule 12 such as by insulating washers 50
and 50'". The schematic of the FIG. 31 "T" filter is shown in FIG.
32. This filter assembly 62 has an attenuation slope of 60 dB per
decade, similar to the filter illustrated in FIG. 27.
[0160] Further describing the assembly shown in FIG. 31, insulating
washer 50'" is first placed into the cavity formed by the ferrule
12. The inductor 46 is then placed on top of the adhesive layer of
50'" and cured in place. Then an insulative layer 50' and a
capacitor 40 are placed and cured thereby forming a laminate
structure. The electrical connections 42' between the capacitor
outside diameter metallization 24 and the ferrule 12 and the
capacitor lead wire 18 and the inside diameter metallization of the
capacitor 22 are then formed, such as by the insertion of a
conductive thermosetting polymer, a solder, liquid solder, solder
paste, brazing or the like (42). The thermal setting conductive
material 42 or 42 can be injected using a syringe into the annular
space between the ferrule and the feed through capacitor-inductor
stack and between the annular space surrounding lead wire 18 in the
inside diameter of the capacitor and corresponding conductor stack.
However, it is very difficult using small needle syringes to inject
the relatively viscous conductive thermal setting materials. A
preferred methodology of injecting the conductive material 42 and
42' is through centrifuge methods. This is best accomplished by
inverting the assembly shown in FIG. 31 and injecting a thermal
setting conductive adhesive in its liquid state flooding the entire
surface of inductor 46. This material would fill the entire cavity
that is formed above inductor 46 and within the inside diameter of
ferrule 12. The entire assembly is then centrifuged which injects
the thermal setting conductive material 42 and 42' down into the
annular spaces as previously described. Typically, a cleaning
operation would be followed after this step. At this point an
insulating washer 50' with adhesive backing is placed and an
inductor 46' is placed on top of the capacitor 40 and seated. There
is a final curing operation which co-bonds the entire structure
which results in a laminate beam consisting of the inductor 46, the
capacitor 40 and the inductor 46'.
[0161] As one can see in FIG. 31, insulative washers 50' and 50'"
are designed to be the same as dimensionally and to conform to the
outside diameter of the capacitor 40 and the outside diameter of
the ferrite slabs 46 and 46'. This is important because it allows
the conductive thermal setting polyimide or solder 42 and 42' to
directly contact the gold braze 44 and 44'. Directly contacting the
gold braze as opposed to contacting the titanium is very important
to avoid the formation of titanium oxides which can preclude the
proper operation of the EMI filter capacitor. This is fully
described in pending U.S. patent application Ser. No. 10/377,086.
By having the conductive thermal setting polyimide or solder 42
contact the lead wire gold braze 44, this eliminates the necessity
for a direct contact between the capacitor metallization 22 and the
lead wire 18. This is also described in pending U.S. patent
application Ser. No. 10/377,272. The lead wire 18 can then be of
any biocompatible material including the group of niobium, tantalum
and the like.
[0162] One can see that the conductive material 42 and 42' is also
in direct contact with ferrite slab 46. It would be undesirable to
have material 42 or 42' short out ferrite slab 46. Accordingly,
ferrite slab 46 has been conformably coated with a suitable
insulating material. This is done prior to assembling ferrite slabs
46 and 46' into the assembly shown in FIG. 31. Suitable conformal
coating materials exist in the art and would consist of the group
of thermal setting polymers and the like. Two preferred materials
are Paralyne C or Paralyne D. These materials are vapor deposited
and have excellent dielectric breakdown strength measured in volts
per mil. Paralyne D has a higher temperature rating and is ideally
suited for use in a hermetic terminal of an implantable medical
device. This is important because these terminals are designed to
be laser welded from the conductive ferrule 12 to the overall
housing 14 of the implantable medical device. This laser weld forms
a hermetic seal between the filtered terminal assembly 62 and
housing 14 and also makes the ferrule of the hermetic assembly 12
become an overall part of the continuous electromagnetic shield 14
of the implantable medical device. During laser welding a heat
pulse is generated which can travel to the ferrite slab 46 or 46'
and the feedthrough capacitor 40. Accordingly, it is desirable for
all connection materials to be of high temperature construction.
Thus, Paralyne D would be preferred insulating material. Connective
materials 42 and 42' are also desirably of high temperature
ratings. For example, a high temperature solder such as SN10 can be
used or a thermal setting conductive polyimide which can easily
withstand temperature above 300 degrees centigrade.
[0163] Another important reason to use conformal coatings on the
ferrite slabs 46 or 46' is for applications in a high voltage
device such as an implantable cardioverter defibrillator. When high
voltage therapy is applied to the lead wire 18, a very large
electric field is generated across the surfaces of the ferrite slab
46 or 46 . Paralyne coatings are preferred because they have
voltage breakdowns in excess of 1000 volts per mil. A conformal
coating of 2-3 mils allows the ferrite slab to withstand voltages
of greater than 2000 volts.
[0164] A novel PI filter 64 is shown in FIGS. 33 and 34. A PI
circuit filter is a three-element filter which offers a three
element attenuation slope of 60 dB per decade (see FIG. 10). This
is the same as the slope for the T element filter that was
previously described. In this case, a ferrite slab inductor 46, has
been sandwiched between two feedthrough capacitors 40 and 40' as
shown. Insulative washers 50' bond and prevent conductive contact
between the inductor 46 and capacitors 40 and 40'. Conformal
coating on all surfaces of inductor 46 prevents electrical contact
between the pin 18, the pin electrical connection material 42, and
the inductor 46. Space has been allowed so that the conductive
thermal setting materials, such as a silver filled conductive
polyimide, can directly contact the gold braze in accordance with
pending U.S. patent application Ser. No. 10/377,086.
[0165] FIG. 35 illustrates a novel Pi section filter 68 which
incorporates surface mounting techniques. The bottom capacitor 70
is a special hybrid capacitor in that it combines both external
ground and internal ground technologies. Externally grounded
feedthrough capacitors are well known in the art. Internal
grounding is described by U.S. Pat. No. 5,905,627. The schematic
for this PI circuit device is shown in FIG. 36. As shown, it is a
three element low pass EMI filter which, as shown in FIG. 10,
offers 60 dB per decade of attenuation.
[0166] The special hybrid capacitor 70 is seated to the hermetic
terminal 12 by way of an insulating washer 50. The hybrid capacitor
70 is externally grounded to the gold braze 44' of the ferrule by
conductive material 72. The capacitor 70 active electrode plates 26
are also connected to leadwires 18 by conductive material 74. An
inductor slab 76 is then bonded to the top of capacitor 70 by way
of an insulating washer 50'. A top capacitor 78 is then placed on
top of another insulating washer 50' and cured in place to form the
laminated stack assembly 68 as shown. Capacitor 78 is a
conventional internally grounded feedthrough capacitor, as
described by U.S. Pat. No. 5,905,627, the contents of which are
incorporated herein. The arrangement of FIG. 35 allows for the
upper capacitor 78 to be grounded to the lower capacitor 70 by
conductive material 80 so that it forms an effective PI circuit
filter or three element filter capable of 60 dB per decade.
[0167] The conductive material 80 can be a variety of materials
from the group of the thermosetting conductive adhesives such as a
conductive epoxy or conductive polyimide, solder or solder paste,
and a variety of other conductive materials. It should also be
noted that there is an optional insulating surface 82 which
prevents the conductive material 80 from shorting to the ferrite
bead 76. This can be a conformal coat such as Paralyne C or
Paralyne D which surrounds all surfaces of inductor 76 or can be an
inserted insulating sleeve 82.
[0168] FIGS. 37 and 38 show two different top views of FIG. 35
illustrating that this technology can be manufactured in either
round (discoidal), rectangular, or other geometries. The number of
lead wires 18 can be varied in accordance with the intended
application.
[0169] FIG. 39 is a variation of FIG. 35 in that the fill material
80 has been replaced by a conductive pin 84. In the preferred
embodiment, this pin 84 would have a nail head configuration as
illustrated, which would increase its pull strength as it is
captured by the surrounding solder or conductive thermosetting
polymeric materials. However, it would be perfectly acceptable to
have a straight leadwire without a nail head. The pin 84 could be
comprised of a variety of materials including extruded copper,
steel, titanium or the like. The pin would be electroplated with
tin, silver or a similar solder wettable coating. Since this is on
the inside (non-body fluid side) of the device, there is no need
for any of the connection materials or the pin to be
biocompatible.
[0170] FIG. 40 is yet another modification of the principals of the
PI circuit filter shown and previously described as FIG. 35. In
this case, the center pin 86 connects the ground electrode plates
28 of the hybrid capacitor 70 with the ground electrode plates 28'
of internally grounded capacitor 78. This pin 86 has the greatest
pull strength of all the configurations in that the pin 86 is
seated into the aluminum ceramic insulator 20 and mechanically
attached to the insulator 20 along with the other pins in a
co-brazing operation. Capacitor 70's ground electrode plates 28
connect to this pin 86 which grounds it. The pin 86 in turn couples
with the capacitor 78 for grounding its internal electrode plates
28' thereby forming the PI circuit filter schematic shown in FIG.
36.
[0171] FIGS. 41-44 show the bottom capacitor 70 of FIG. 35. As can
be seen, the ground electrode plates 28 electrically connect to
both the centered inner diameter hole metallization 85 and the
outside diameter metallization 24 of the capacitor.
[0172] FIGS. 45 and 46 illustrate the solid ferrite slab inductor
76 which is sandwiched between capacitors 70 and 78 in FIG. 35.
Referring to FIG. 46, one can see that a conformal coating 77 has
been placed or vacuum deposited over all surfaces of the ferrite
inductor slab 76. In a preferred embodiment, this would be done
using Paralyne in a vapor deposition process. During original
manufacture of the inductor slab 76 it is also desirable that it be
tumbled forming radius corners on all edges 79. There are a number
of advantages to doing this. One advantage is that by eliminating
sharp corners, one reduces stress risers and thereby the potential
for breakage or fracturing of the edges of the ferrite material.
However, another important reason becomes obvious in conjunction
with the present invention. When the conformal coating material 77
is applied by vapor deposition, it forms a more reliable and
continuous surface when going around the radius 79 as shown. If the
cornet 79 was sharp, the conformal coating material 77, on curing,
would shrink back away and expose a non-insulated edge at the
corner. In an implantable cardioverter defibrillator, the
insulation provided by the conformal coating material 77 is quite
important.
[0173] When the conformal coating material 77 is of Paralyne or
equivalent material, the dielectric strength of such material is
very high. For example, a two mil or 0.002 inch coating of Paralyne
D could provide over two thousand volts of dielectric breakdown
strength. This is very important in the output of implantable
cardioverter defibrillators where high electric field potentials
exist either from the lead wire 18 to the ferrule 12, or between
lead wires 18 of opposite polarity. Accordingly, high electric
fields can occur across the surfaces of the ferrite inductor 76.
The conformal coating material 77 grades these fields and prevents
surface arcing. It should also be noted throughout all of the
preferred embodiments illustrated, the ferrite slab inductors are
preferably conformally coated. This increases the insulation
resistance of the ferrite bead and also prevents it from shorting
out either to lead wires 18 or to ferrules 12.
[0174] FIGS. 47-50 illustrate the top capacitor 78 in FIG. 35. This
is a conventional internally grounded capacitor, such as that
described by U.S. Pat. No. 5,905,627.
[0175] Previously, it was not possible to form a surface-mounted PI
circuit filter. However by electrically connecting the two inside
diameter metallizations 85 and 85 on the two stacked capacitors
together with the connection, shown as 80, 84 or 86, this grounds
the electrode plate set 28' of capacitor 78. Therefore, true PI
circuit performance is achieved.
[0176] FIGS. 51 and 52 show an internally grounded three element PI
circuit configuration of the present invention. A ground lead wire
102 is electrically connected to the ferrule 12 of the hermetic
terminal assembly. This is accomplished by gold-brazed material 44.
Accordingly, after installation into the housing of an implantable
medical device, pin 102 is at ground potential of the housing of
the medical device. Lower capacitor 40 is an internally grounded
capacitor which is well known in the art (see U.S. Pat. Nos.
5,905,627 and 6,529,103).
[0177] A thermal setting conductive polymer or solder 42 and 42'
makes electrical connection between the lead wires 18 and 102 and
the inside diameter metallization 24 of feedthrough capacitors 40
and 40'. In the case where the pins 102 and 18 are of an oxidized
material such as niobium or tantalum an oxide layer builds up and
electrically insulates said pins. This oxide layer prevents a
reliable electrical connection between the capacitor inside
diameter metallization 24 and the pins 102 or 18. However, as shown
in FIG. 52, such connection is not required because the conductive
material 42 and 42' makes direct contact with gold brazed material
44. This direct connection to gold is described in U.S. patent
application Ser. No. 10/377,018.
[0178] The grounded pin 102 couples electrically to the internally
grounded electrode plates 28 of capacitor 40. Pins 18 are
conductively coupled to the active electrode plates 26 of capacitor
40. Both pins 18 and 102 pass through the center holes of the
inductor slab 46 in accordance with the present invention. The
inductor slab 46 has been previously conformally coated with
Paralyne C or D. A non-conductive thermal setting polymer 50' is
used to bond the ferrite slab 46 to the lower capacitor 40. A top
internally grounded capacitor 40' is then bonded with a second
insulating washer 50'. The capacitors 40 and 40' are both
internally grounded. Grounded lead wire 102 in turn connects to the
ground electrode plates 28' of top capacitor 40'. Lead wires 18
also connect to the active electrode plate set 26' of top capacitor
40'. It should be noted that it is not necessary that the
capacitance value of capacitor 40 and 40' be of the same value. For
example, capacitor 40 could be a 4000 picofarad capacitor and
capacitor 40' could be a 900 picofarad capacitor. By staggering the
capacitance values, one can make adjustments to the resonance of
the PI circuit below its 3 dB cut-off point. This can be important
so that gain does not occur at low frequencies in the low pass
filter function.
[0179] With reference now to FIG. 53, more accurate (in comparison
to the generic curves shown in FIG. 10) EMI filter performance
(attenuation) curves versus frequency graphs illustrate the
advantages of adding filter elements. As can be seen, there is
substantial difference between the single element (feedthrough
capacitor or C), the L circuit and the PI circuit configurations.
One will notice that the curves become non-linear at lower
frequency. Accordingly, if the PI circuit filter is properly
designed (so that it does not resonate) it can offer substantially
higher attenuation at lower frequencies. As previously mentioned,
the slope of the PI circuit is 60 dB per decade. The slope of the L
circuit is 40 dB per decade, and the slope of the C circuit is 20
dB per decade.
[0180] In FIG. 53, one can see a resonant dip f.sub.r in the
performance curve of the single element C-section filter. This
self-resonance phenomenon is typical of all feedthrough capacitors.
Feedthrough capacitor devices resonate far differently than
standard monolithic ceramic chip capacitors (MLCCs). In an MLCC,
the resonance is caused by parasitic inductance, which in the
equivalent circuit, is in series with the capacitor. For an MLCC at
resonance, the attenuation actually increases dramatically.
However, above resonance the attenuation rapidly falls off as the
MLCC capacitor becomes increasingly inductive. The opposite tends
to happen in a feedthrough capacitor as illustrated in FIG. 53.
This is a more complicated type of parallel transmission line
resonance. The feedthrough capacitor continues to function above
its self-resonant frequency and is still an effective EMI filter.
However, as one can see from the single element C-filter graph of
FIG. 53, there is a drop in attenuation at the actual resonant
frequency f.sub.r. This is undesirable, particularly if the drop in
attenuation occurs at the frequency of an EMI emitter such as a
cellular telephone. This means that at that particular frequency
f.sub.r, the implantable medical device, like a cardiac pacemaker,
is more susceptible to outside interference. The addition of the
inductor slab element as described herein not only increases the
attenuation slope as shown in FIG. 53, but also minimizes or
eliminates the resonant dip phenomenon f.sub.r as previously
described. The inductor slab, therefore, compensates for problems
associated with the self-resonance characteristic of the
feedthrough capacitor.
[0181] As previously described for an L section filter, it is
desirable to have the inductor element point towards the AIMD input
circuits. For similar reasons, it is desirable to have a PI section
filter as illustrated in FIG. 35, as opposed to a T section filter.
This has to do with the novel impedances that are present in an
implanted medical device. In general, lead wires that are implanted
within the human body are electrically dampened by the surrounding
body tissue. Additionally body tissue also acts to reflect and
absorb high frequency EMI signals. Because of this, the source
impedance of implanted lead wires tends to be stable and
approximate 80 ohms. Adding additional resistance or inductance in
series with this resistance does not do much to improve the
attenuation of an EMI filter. Accordingly, the PI circuit or L
section configurations as described are preferred.
[0182] Another novel aspect of the ferrite slab inductor is the
ability to combine it with helium leak detection vent holes, as
described in U.S. Pat. No. 6,566,978 (the contents of which are
incorporated by reference herein). FIG. 54 illustrates a novel
ferrite slab quad-polar inductor 46 with a small hole 88 in the
center which is designed to line up with a small centered hole 90
(not shown) in the ceramic feedthrough capacitor 40. As described
in U.S. Pat. No. 6,566,978 (the contents of which are incorporated
herein), a defective gold braze 44 or 44' or defective hermetic
seal 20 could be readily detected by the centered through-hole
passage whereby helium leak gas could freely flow. A ferrite slab
inductor 46 can be bonded using a polyimide matrix washer 50'
directly to the ceramic capacitor 40. In this case, the inductor 46
has been specially modified so that it has a center hole 88 which
lines up with the helium vent hole 90 of the capacitor 40. The
center hole 88 in the ferrite slab 46 can be manufactured during
initial pressing or by drilling before or after sintering. If the
hole in the center of the ferrite slab 46 is very small it can also
be added by laser or water cutting techniques.
[0183] FIG. 55 illustrates a cross-section of this assembly showing
an air gap 92 between the ceramic capacitor 40 and the insulative
hermetic seal 20. The air gap 92 facilitates detection of a
defective hermetic seal 20 or defective braze 44 or 44'. For
example, in a typical implantable medical device hermetic
feedthrough terminal assembly, seal 44 or 44' would be a gold braze
which is attached to a sputtered surface of the alumina insulator
20. In the case where there is a pinhole or defect in such gold
braze 44 or 44', body fluid could penetrate. In the case where body
fluid penetrates to the inside of the implantable medical device,
catastrophic failure is usually the result. The electronic
components that are inside of an implantable medical device, such
as a cardiac pacemaker and the like, typically consist of sensitive
electronic circuits including hybrid chips and other components.
Even a slight amount of moisture can cause the insulation
resistance to degrade on such components which can either lead to
immediate catastrophic failure or premature battery failure of the
device. Accordingly, it is very important that hermetic seal
testing, including helium leak testing detect any such defect. In
this regard, it has been shown that installation of the feedthrough
capacitor 40 along with its surrounding electrical connection and
bonding materials 42 and 42' can mask, for a period of time, a
defective gold braze 44 or 44'. Helium leak testing as presently
done in the industry is effected very rapidly. There is typically
not sufficient time for penetration of helium through bulk
polymers. Thus, it is highly desirable to provide such passage
holes through the center of the feedthrough capacitor that are
described in U.S. Pat. No. 6,566,978.
[0184] A novel feature of the L-circuit filter of the present
invention as shown in FIGS. 54 and 55 is that the co-bonded ferrite
slab 46 also has a corresponding leak detection vent hole 88 which
lines up with the vent hole 90 in the ceramic capacitor 40.
Accordingly, the laminating washer 50' also has a corresponding
hole 94 which aligns with the previously mentioned vent holes 88
and 90. This provides a convenient space for helium to escape from
the entrapped air spaces 92 underneath the capacitor 40 and be
readily detected during a helium leak test through vent holes 90,
94 and 88.
[0185] There is a small round insulative washer 96 placed around
each leadwire 18 before the capacitor 40 is inserted into and
seated against the bottom of the ferrule 12. These insulating
washers 96 prevent the conductive material 42 that is used to
connect the capacitor leadwire 18 to the capacitor 40 inside
diameter termination, from penetrating into the air gap 92. It
would be undesirable to have conductive materials floating around
in this air gap 92 as this could lead to short circuits or a
decrease in the insulation resistance of the device.
[0186] With reference to FIGS. 56 and 57, an internally grounded
tri-polar capacitor assembly 100 embodying the present invention is
illustrated. In this case, an internal ground lead 102 is not
required on the inside of the implantable medical device.
Accordingly, the internal ground lead 102 needs to penetrate into
the inside of the implantable medical device for a distance no
greater than the thickness of the ceramic capacitor 104. As shown
in FIG. 57, the lead wire 102 grounds the ground electrode plate
set 106. The internally grounded tri-polar feedthrough capacitor
104 is seated onto the hermetic ferrule 12 and onto an insulating
washer 108. In this case the inductor slab 114 only has three
holes. This provides inductance on the three active leadwires 18
that go to the interior of the implantable medical device. It
should be noted that there is no point in doing additional
filtering on a pin 102 that is already grounded. The grounded pin
102 is by definition shorted to the ferrule 12 which provides
infinite attenuation. The grounded pin 102 is typically
conductively coupled to the ferrule 12 by gold brazing 116 or the
like. Further methods of attaching the ground pin 102 are
resistance welding, laser welding, and the like.
[0187] It is also possible to use discrete ferrite beads as opposed
to a single ferrite slab inductor. FIGS. 58-60 illustrate an inline
multi-polar hermetic terminal assembly 118 suitable for human
implant such as in a cochlear hearing device. This unit is ideally
designed for discrete uni-polar ceramic capacitors 40. FIG. 59 is a
cross-section of this device with multiple uni-polar capacitors 40
to which multiple uni-polar ferrite inductors 46 have been
co-bonded in accordance with the present invention, such as by
washer 50'. FIG. 60 is the schematic drawing of the device shown in
FIGS. 58 and 59, illustrating two parallel L section filters. The
schematic of FIG. 60 is shown conveniently as a bipolar or two
section filter. In fact, in modern implantable pacemakers, a new
therapy known as biventricular pacing has become very popular. In
addition, cochlear implants typically incorporate fourteen to
sixteen lead wires. Accordingly, additional lead wires 18 are
required. It is now common to see hermetic terminal assemblies with
anywhere from four to sixteen lead wires. In this particular
embodiment there are two discreet uni-polar feedthrough capacitors
40 bonded between the pin 18 and the titanium ferrule 12. Also
shown is a ferrite bead 46 of sandwich construction as previously
described, added to each one of the active leads.
[0188] It will be obvious to those skilled in the art that the
inline bipolar feedthrough capacitor as shown in FIGS. 58 and 59
can be elongated to add additional filtered lead wires 18. Dual
in-line configurations are also convenient.
[0189] FIGS. 61 and 62 illustrate the same device shown in FIGS. 58
and 59, except that instead of discrete ferrite bead elements, a
ferrite slab 120 has been bonded to the uni-polar capacitors 40. In
this case, instead of using individual ferrite beads 46, a ferrite
slab 120 is employed which slips over and bonds to all of the
capacitors 40 at one time. Referring to FIG. 62, one can see that
there is an epoxy preform 121 shown and disposed between a lead
wire 18 and the inductor slab 120. This optional epoxy preform can
be placed around each lead wire to improve the cosmetic appearance
and also mechanically strengthen the assembly. Another reason to
have an optional epoxy or polyimide preform 121 is to improve the
high voltage characteristics of a device, such as an implantable
cardioverter defibrillator.
[0190] As previously mentioned, the amount of inductance that one
achieves is very important to achieve overall attenuation. This is
different than the attenuation slope measured in dB per decade. As
one increases the capacitance and the inductance, the starting
point (3 db point) goes down in frequency and the overall
attenuation increases dramatically. As an example, if one had a
very low value of capacitance and a very low value of inductance,
one might only have 5 dB at 100 MHz. Even though one had a
two-element filter, which increases at 40 dB per decade, one would
in this case be limited to only 45 dB at 1000 MHz (a decade higher
than 100 MHz). However, if one was able to increase the capacitance
value and increase the inductance value, one might have a starting
attenuation at 100 MHz of 20 dB. This would mean that at 1000 MHz,
one would have 60 dB of attenuation, which is very substantial
indeed. Accordingly, there is a need for as much inductance as
possible in the ferrite or ferrite slab element. As previously
mentioned, it is not possible to wind multiple turns around a
conventional ferrite slab or ferrite bead once it has been
co-bonded or mounted to a ceramic capacitor and the hermetic
terminal of a human implantable medical device.
[0191] FIGS. 63-66 illustrate a preferred embodiment of the present
invention wherein a novel pressed indentation or notch 122 has been
formed during the powder pressing or subsequent machining of the
ferrite bead and then sintered into a solid, monolithic inductor
structure 124. Ferrite beads are generally made of proprietary
powders, which are put into multi-stage toggle presses. This
pelletizing process (with binders) forms the ferrite element which
is then sintered at very high temperatures making it into a hard
monolithic structure. It is a simple matter of mold tooling to form
the notch 122 illustrated in FIGS. 63 and 64. As can be seen in
FIG. 67, this makes it possible to bond the ferrite slab 124
directly to the ceramic capacitor 40 placing it over a single lead
wire 18. It is then relatively easy to pass the lead wire 18 back
around through and up through the center hole 126 of the ferrite
slab 124 thereby adding another turn. In this case, we have
described a two-turn inductor which increases the inductance by a
factor of four (2.sup.2).
[0192] FIG. 65 illustrates an improved embodiment of the novel
ferrite slab inductor 124 shown in FIG. 64 incorporating a ramp 128
upward thereby making it easier to feed the lead wire 18 back
around and up through the center hole 126 of the ferrite inductor
124. It is very important that a notch 122 not be cut all the way
through which would form an air gap in the circular toroid 124. It
is very important for a toroidal inductor that it contains and form
a very low reluctance path for magnetic fields. Field inductance in
this case will still occur throughout the toroid wherein the
magnetic field is constrained within the toroidal inductor 124. By
eliminating the air gap, we can provide a very high amount of
inductance in a very efficient manner.
[0193] A unique aspect of all implantable medical device hermetic
terminals is that the lead 18 is pre-manufactured to form a
hermetic seal. In certain hermetic terminals, the lead 18 is
attached to the alumina insulator 20 by gold brazing 44. In turn,
the alumina insulator 20 is gold brazed 44 to a titanium ferrule
12. In applications other than implantable medical device hermetic
terminals, it is easy to manufacture multi-turn inductors because a
loose lead wire is available for one or more turns around a
toroidal inductor. However, in the case of an implantable medical
device, a major problem arises in how to bond the ferrite directly
to the capacitor and then to make a multiple turn. The novel molded
notch feature, illustrated in FIG. 63, demonstrates a methodology
in which the capacitor 40 can be placed down over the lead wire 18
which is straight and then the lead wire 18 can be looped back
through and around the notch 122 and brought out through the top
yielding a two turn toroidal inductor as shown in FIG. 67. As
previously mentioned, the inductance is directly related to the
square of the number of turns. The inductor 124 shown in FIG. 67 is
known in the art as a two-turn inductor. By squaring the number
two, this means that this would have four times the amount of
inductance as simply passing a lead wire 18 directly through the
center 126.
[0194] It should be pointed out that the lead wires that are
typically used in implantable medical devices must be of suitable
biocompatible materials. Typical lead wires are platinum,
platinum-iridium, tantalum, niobium and the like. As these lead
wires 18 form multiple turns through the center of a ferrite 124,
as illustrated in FIG. 67, it is very important that the turns do
not touch one another. If for example, in FIG. 67 where the lead
wire 18 loops around and crosses past itself in area 129 physically
touched together, then this shorted turn would once again become a
single turn inductor. This would not affect the inherent operation
of the pacemaker, however, it would result in reduced EMI filter
attenuation.
[0195] Accordingly, there is a need to insulate the turns where
they pass each other through the center 129 of the ferrite inductor
124. The present invention describes a number of ways of doing
this. One way would be to slip on an insulating sleeve 130 as shown
in FIG. 67 and shown expanded in FIG. 69. Suitable insulating
sleeves 130 can be made of Teflon, Kapton, or the like and are very
thin. They also have excellent dielectric strength characteristics
and can be easily slipped over the wire 18. Other methods would
include conformal coating of the wire 18 with a thin insulating
material. It should be noted that there is very little voltage
difference between the adjacent turns of the wire 18 passing
through a ferrite or iron core inductor 124. Therefore, not very
much insulation or dielectric withstanding voltage requirement is
necessary. Accordingly, a very thin coating of Paralyne, polyimide,
epoxy or other insulating material is all that is really required.
Another methodology would be to carefully place the turns through
the center of ferrite inductor 124 and then subsequently add an
encapsulant or sealant such that the un-insulated wire turns cannot
move into electrical contact with one another and therefore become
shorted.
[0196] With reference now to FIGS. 70-72, yet another inductor 124
is illustrated having a notch 122 formed therein which is different
in configuration than that illustrated and described above. As
illustrated in FIG. 72, the ferrite slab inductor 124 is co-bonded
to the capacitor 40, such as by washer 50 similar to that
illustrated in FIG. 67, but the lead wire 18 is brought through the
center 126 of the inductor 124 and then wrapped back around through
the convenience notch 122 and back through the center hole 126 of
the inductor 124, therefore, forming a two-turn inductor.
[0197] As previously noted a two-turn inductor has four times the
amount of inductance as a single turn inductor. The difference
between this particular ferrite slab 124 and the one shown in FIG.
67, is the notch 122 is only on one side of the ferrite inductor
124. This has the effect of putting the leadwire 18 across the top
of the inductor 124. In some applications, where there is
sufficient room inside the pacemaker, this would be desirable.
However, in the preferred embodiment shown in FIG. 67, one would
not have this leadwire 18 coming across the top of the inductor
124. The choice is whether to use the configuration in FIG. 63,
with a slot on top and bottom, as compared to the single slot 122
shown in FIGS. 70 and 71. There is little performance difference in
terms of attenuation in these two approaches.
[0198] FIG. 73 illustrates an alternative method of manufacturing
the two-turn L section EMI filter previously illustrated in FIG.
72. In FIG. 73, a long lead wire 18 is elongated through the
feedthrough capacitor 40. An insulative tubing 130 is placed over
the lead wire 18. It is desirable that insulative tubing 130 has a
very low coefficient of friction. Such materials would be Teflon,
Kapton or the like. A turn would be looped through the center and
back around through the ferrite slab 124, as shown. It is desirable
that ferrite slab 124 have rounded corners to facilitate slipping
the ferrite slab down along the tubing to seat it on top of the
ceramic capacitor by way of insulating bonding material 50'. Once
the loose loop is formed, one can simply grasp the end of the lead
wire 18 and push downward on the ferrite slab 124, so that it slips
along until it seats against the top of the capacitor 40 and its
bonding washer 50'. The lead wire 18 can then be snugged up so that
it fits within the notch space 122. It is desirable that the
insulating tubing 130 be captured and cured into the inside
diameter hole of the polyimide insulating washer 50'. A
non-conductive polymer is preferred.
[0199] It is also possible to add additional turns. FIG. 74
illustrates a novel uni-polar ferrite slab inductor 136 with four
novel slots 138. Accordingly, in this design, one could place four
additional turns for a total of five turns through the inductor
slab 136. If we square the number of five this means that we would
have twenty five times the inductance of a straight lead wire
ferrite. FIG. 75 illustrates the novel five-turn inductor 136 of
FIG. 74 mounted to the hermetic terminal 12 of an implantable
medical device.
[0200] FIG. 76 illustrates a rectangular quad-polar ferrite slab
140 incorporating the features of the present invention. This
allows each of the four individual EMI filters to have a two turn
toroid, which will greatly increase the inductance by a factor of
four (2 turns squared). FIG. 77 illustrates the mounting of the
novel quad-polar ferrite slab inductor 140 to a quad-polar ceramic
feedthrough capacitor filter 144 employing a novel gold bond pad
146. In this case, the inline quad-polar feedthrough capacitor 144
has metallized ground stripes 148 on each side which are attached
to a gold brazed bond pad area 146, as described in pending U.S.
patent application Ser. No. 10/377,086, filed Feb. 27, 2003.
[0201] FIG. 78 is the schematic diagram of the quad-polar L section
capacitor that is illustrated in FIG. 76.
[0202] With reference now to FIG. 79, another quad-polar ferrite
slab inductor 150 is illustrated having notches 152 adapted to
permit a terminal pin or leadwire 18 to extend therethrough.
However, in this case, each notch 152 includes dividers 154 and
154' which create multiple slots within the notch 152 such that the
leadwire or terminal pin 18 can be extended through with multiple
turns through each notch 152. Thus, these novel slots 152 allow a
second turn to be brought around and through the bonded ferrite
slab inductor 150 without shorting the adjacent turns.
[0203] The structure of FIG. 80 and 82 are very similar to those
previously described in FIGS. 54 and 55. The capacitor 40
incorporates a leak detection vent hole 90 as previously
described.
[0204] Referring now to FIG. 80, the quad polar inductor slab 46 is
loosely seated on top of capacitor 40 without any bonding material
(described in previous drawings as 50'). That is, inductor 46 sits
loosely on top of capacitor 40. This is better illustrated in the
cross-section shown in FIG. 81. There is an air gap 92' which is
formed between the quad polar feedthrough capacitor 40 and inductor
slab 46. As one can see, capacitor 40 is relatively thick. This
design can be used in cases where there is plenty of room in terms
of height inside of the active implantable medical device.
Accordingly, it is not necessary that capacitor 40 be thin as
described in previous embodiments of the present invention. The
relatively thick feedthrough capacitor 40 that is shown in FIGS. 80
and 81 is not particularly volumetrically efficient as can be seen
by the blank cover sheet area at the top and bottom of capacitor 40
that is free of electrodes. This is known as the cover sheet area
as is normally built up to add the mechanical strength to the
capacitor as previously described in FIG. 1.
[0205] Referring now back to FIG. 80, it is required that the
inductor 46 be retained so that it not fall off or separate away
from the ceramic capacitor 40 during shock and vibration loading.
Accordingly, a number of different methods of holding the inductor
in place are shown. One such method would be to place epoxy
pre-forms 51' over each or a few of the four lead wires 18. A cross
section of this heat cured epoxy pre-form is also shown in FIG. 81
as material 51'. Another methodology would be to insert a metallic
push nut 53 onto one or more of the lead wires 18. Another
methodology would be to take a swaging tool and form a crimp or
swage in the lead wire 55 as shown. This swage 55 is also shown in
the cross section in FIG. 82. Another methodology would be to
insert a retaining clip 57 as shown in FIG. 80.
[0206] In a multi-polar feedthrough capacitor assembly, it is not
necessary to put a retention device on all of the pins. For
example, in a six lead or hexpolar device, it may only be necessary
to install a retaining feature on two of the leads. This depends on
calculations based on the particular shock and vibration
requirement of the implantable medical device. It is typical that
shock requirements be between 1000 and 1500 g. One would have to
calculate the mass of the ferrite slab and then calculate the
amount of force that would be applied during such shock loading
(F=ma). One can then make a decision as to the number of retaining
devices that are required.
[0207] Referring to FIG. 81, one can see that leak detection vent
hole 90 is conveniently placed through the center of the
feedthrough capacitor 40. In this particular embodiment there is no
need for a corresponding leak detection vent hole in inductor 46.
This is because helium gas would readily escape through the air gap
92'. As shown, air gap 92' appears to have significant thickness.
However in actual practice, inductor 46 would be pressed down
firmly against capacitor 40. However, without a sealing material,
helium gas can still escape readily through very small spaces. In
fact this is why helium is used in leak detection applications
since it is a very small molecule and will escape through even the
slightest pinhole. FIGS. 82 and 83 describe another embodiment of
the quad polar capacitor assembly previously described in FIGS. 80
and 81. In this case, the inductor 46 is retained by forming or
bending one or more of the lead wires 18. It is a very common
practice in medical implantable devices that the lead wires be
formed or bent in a variety of shapes and configurations so that
they line up with appropriate connection points to the internal
electronic circuitry of the AIMD. Referring to FIG. 83 one can see
that the bend 59 in lead wire 18 firmly holds inductor 46 in
place.
[0208] Referring once again to FIG. 80, the inductor slab 46 could
also be retained by the addition of a wire bond pad. Wire bond pads
are the subject of co-pending U.S. patent application Ser. No.
60/548,770.
[0209] FIG. 84 illustrates an L-shaped wire bond pad 156 shown
attached using bonding insulating material 50' to inductor slab 46.
There is an air gap 92' between inductor 46 and capacitor 40. A
perspective view of the L-shaped wire bond pad 156 is shown in FIG.
85. This gold plated wire bond pad 156 is designed for convenient
attachment of lead wire 158 either by thermal sonic or ultrasonic
wire bonding techniques 160 that are well known in the art.
[0210] FIG. 86 is an exploded view of an octapolar feedthrough
capacitor plus ground lead of the present invention. Shown is a
wire bond substrate 162 which is also used to retain the ferrite
slab 46. The capacitor 40 is shown seated by way of insulating
washer 50 to the ferrule 12 of the hermetic terminal of the
implantable medical device. Lead wire 102 is grounded to the
ferrule 12 using a weld or gold braze. The inductor slab 46 is
slipped down over lead wires 102 and 18 and loosely fitted in place
on top of capacitor 40. In this particular case there is no need
for a non-conductive polyimide washer or adhesive to bond capacitor
40 and inductor slab 46. This is because capacitor 40 is
sufficiently thick to withstand the mechanical and thermal forces
of the assembly. In turn, alumina substrate 162 incorporating
metallized areas 164 is then slipped down over the lead wires 18
and 102 and seated loosely on top of the ferrite inductor slab 46.
Gold brazed preforms 166 are then slipped over each wire and seated
on top of the metallized area 164 of the alumina substrate 162.
Nine Kovar wire bond pads 168 are then inserted over the lead
wires. These wire bond pads 168 are typically of Kovar or Alloy 42
and the like. The wire bond pads 168 are typically plated with
Nickel and then over coated with an ultra pure gold suitable for
wire bonding.
[0211] FIG. 87 is a perspective view of the completed assembly as
shown in FIG. 86. The inductor slab 46 is shown loosely sandwiched
between capacitor 40 and wire bond substrate 162. The entire
assembly is held in place by nine laser weld connections 170 which
attach each of the lead wires to the wire bond pads 168. As
described in pending U.S. patent application Ser. No. 60/548,770,
laser weld connection 170 makes a highly reliable electrical
connection in series with the input and output circuits of an
implantable device such as a cardiac pacemaker. Many alternative
embodiments of substrates are described in the above-listed pending
application. It will be apparent to one skilled in the art that any
of these wire bonds substrates could be also used to retain an
inductor slab.
[0212] FIG. 88 is a cross section of the octapolar feedthrough
capacitor as shown in FIG. 87. The inductor 46 is shown loosely
held between the capacitor 40 and the wire bond substrate 162. A
small space or air gap 92' is formed on both the top and bottom
surfaces of the inductor slab 46. The air gap 92' in this case has
been exaggerated. In actual practice a weighting fixture would be
used to press firmly down on top of the wire bond substrate 162
while laser weld connections 170 are being made. In this way, the
alumina substrate 162 would firmly push down against the ferrite
slab 46 and the capacitor 40. Accordingly the air gaps 92' would be
very small. No additional mechanical connections using thermal
setting conductive adhesives are required in the assembly. This is
because sufficient mechanical strength to withstand mechanical
shock and vibration forces is formed by the laser weld connections
170.
[0213] From the foregoing it will be appreciated that EMI filter
feedthrough terminal assemblies constructed in accordance with the
present invention generally comprise at least one conductive
terminal pin, a conductive substrate such as a ferrule through
which the terminal pin passes a non-conductive relation, a
feedthrough capacitor associated with the ferrule and through which
the at least one conductive terminal pin extends, and an inductor
adjacent to the capacitor. The feedthrough capacitor is mounted to
a hermetic seal subassembly, such as in a manner described in U.S.
Pat. Nos. 4,424,551 and 5,333,095. The ferrule is conductively
connected to a housing or casing of the human implantable device,
as is well-known in the art. The feedthrough capacitor has first
and second sets of electrode plates, also known as the active
electrode plate set and the ground electrode plate set. The
terminal pin extends through the passageway of the capacitor in
conductive relation with the active set of electrode plates. The
ground set of electrode plates of the capacitor are in conductive
relation with the ferrule, which is in turn in conductive relation
with the housing of the AIMD.
[0214] The conductive terminal pin also extends through the
inductor. It is not necessary that the terminal pin be in
conductive relation with the inductor. In fact, in an implantable
medical device, energy consumption or battery depletion is an
important consideration. Therefore, it is desirable that the
inductor be well insulated so that leakage current does not shorten
the pacemaker battery life. As such, the inductor is preferably
electrically insulated from both the capacitor and the terminal
pin. The inductor includes a conformal coating such as Paralyne.
Such high dielectric strength coatings have a low coefficient of
friction, withstand extreme environments, and act as an electrical
insulation.
[0215] A novel aspect of the present invention is that both the
ceramic feedthrough capacitor and the inductor can be made much
thinner than conventional practice because they are co-bonded into
a monolithic laminate structure, but in non-conductive relation to
one another. The inductor can be fixed to the capacitor with a
non-conductive polyimide, (as described in FIG. 16), glass, ceramic
bonding material, epoxy, or a thermal setting plastic supportive
tape adhesive or the like. The monolithic structure of the
capacitor and conductor greatly increases the mechanical strength
of the structure without greatly increasing the overall volume
(height) of the EMI filter.
[0216] The inductor typically comprises a high permeability ferrite
material and is typically either in the form of a slab or toroid.
The inductor may also comprise a molypermalloy material, a powdered
iron, a manganese zinc ferrite material, a nickel zinc ferrite
material, or a cobalt zinc ferrite material. Such materials are
biocompatible as the various materials therein are highly
bound.
[0217] A unique property of these ferrite materials is that they
not only provide inductance but they also have a variable
resistance component versus frequency. All ferrite materials
involve a trade-off like this. Certain ferrite materials have very
high inductance at low frequency. Such ferrite materials typically
have a high initial permeability. Accordingly, at high frequency,
the inductance tends to decrease. However, in those same ferrite
bead materials the resistive loss component tends to increase at
high frequency, thereby compensating for the drop in inductance.
The important parameter is the overall impedance Z of the ferrite
bead. The impedance parameter includes both the inductive reactance
and resistive properties of the slab or bead. In general, the total
impedance is equal to the square root of the inductive reactance
squared, plus the resistance squared.
[0218] In certain embodiments, the capacitor and inductor are
housed entirely or partially within the ferrule. The inductor is
conductively isolated from the ferrule.
[0219] The assembly may include a second feedthrough capacitor
associated with the inductor. Such capacitor would have the
structure as described in relation to the first capacitor and
similar to the first capacitor can be externally grounded,
internally grounded, or be both internally and externally grounded.
In such instances, the first and second feedthrough capacitors are
typically disposed on opposite surfaces of the inductor in
non-conductive relation thereto.
[0220] In other embodiments, the EMI feedthrough filter terminal
assembly includes two or more inductors. The terminal pins extend
through the additional inductor(s). A second inductor can be
disposed above an insulator of the terminal assembly, which is
disposed between the terminal pin and the ferrule. Alternatively,
the first and second inductors are fixed in non-conductive relation
to opposite surfaces of the capacitor. A particularly preferred
embodiment stacks the plurality of inductors, such as by laminating
them one to another using an adhesive washer or the like.
[0221] In a particularly preferred embodiment of the present
invention, the terminal pin passes through the inductor so as to
create multiple turns to increase inductance. Increasing the number
of turns on a ferrite cord dramatically increases the inductance.
This is because the inductance varies as the square of the number
of the turns. For example, if one increases the number of turns
from one to two, the inductance increases by a factor of four. The
one or more turns of the terminal pin are electrically isolated
from one another. The portion of the terminal pin defining the one
or more turns can be encased within a non-conductive sleeve.
Alternatively, the one or more turns of the terminal pin are
encased in a non-conductive material.
[0222] To facilitate the passing of multiple turns through the
inductor, a notch is formed in the inductor which is adapted to
permit the terminal pin to be passed therethrough and form the one
or more turns with respect to the inductor. A ramp may be
incorporated in the notch for facilitating the passing of the
terminal pin. The inductor may include multiple notches to
accommodate multiple turns of a single terminal pin or to
accommodate multiple terminal pins. In one embodiment, each notch
includes multiple slots formed therein to permit an additional turn
of the terminal pin therethrough.
[0223] The capacitor and the inductor may include aligned apertures
which co-operate with an air gap between the ceramic capacitor and
the insulative hermetic seal. This allows a leak detection gas to
quickly pass through to readily detect defective hermetic seal
connections.
[0224] Moreover, as described above, the feedthrough capacitor
assembly incorporating an inductor can be utilized in many other
different types of designs for feedthrough terminal assemblies
advantageously.
[0225] Although several embodiments of the present invention have
been described in detail for purposes of illustration, various
modifications of each may be made without departing from the spirit
and scope of the invention. Accordingly, the invention is not to be
limited, except as by the appended claims.
* * * * *