U.S. patent application number 10/861898 was filed with the patent office on 2005-01-06 for electrolytic capacitor for use in an implantable medical device.
Invention is credited to Bomstad, Tim T., Casby, Kurt J., Haas, David P., Hossick-Schott, Joachim, Nielsen, Christian S., Norton, John D., Rorvick, Anthony W., Taylor, William John, Viste, Mark Edward.
Application Number | 20050002147 10/861898 |
Document ID | / |
Family ID | 33131655 |
Filed Date | 2005-01-06 |
United States Patent
Application |
20050002147 |
Kind Code |
A1 |
Nielsen, Christian S. ; et
al. |
January 6, 2005 |
Electrolytic capacitor for use in an implantable medical device
Abstract
A capacitor structure comprises a shallow drawn encasement
having first and second major sides and a peripheral wall coupled
to the first and second sides. A cathode is disposed within the
encasement proximate the first and second major sides, the cathode
having a cathode lead. A central anode a having an anode lead is
disposed within the encasement, and a bipolar, insulative
feedthrough extends through the encasement through which electrical
coupling may be made to the anode lead and the cathode lead.
Inventors: |
Nielsen, Christian S.;
(River Falls, WI) ; Viste, Mark Edward; (Brooklyn
Center, MN) ; Rorvick, Anthony W.; (Champlin, MN)
; Haas, David P.; (Brooklin Park, MN) ;
Hossick-Schott, Joachim; (Minneapolis, MN) ; Norton,
John D.; (New Brighton, MN) ; Bomstad, Tim T.;
(Inver Grove Heights, MN) ; Casby, Kurt J.;
(Grant, MN) ; Taylor, William John; (Anoka,
MN) |
Correspondence
Address: |
INGRASSIA FISHER & LORENZ, P.C.
7150 E. CAMELBACK, STE. 325
SCOTTSDALE
AZ
85251
US
|
Family ID: |
33131655 |
Appl. No.: |
10/861898 |
Filed: |
June 3, 2004 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
|
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10861898 |
Jun 3, 2004 |
|
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|
10452424 |
May 30, 2003 |
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6807048 |
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Current U.S.
Class: |
361/302 |
Current CPC
Class: |
H01G 9/10 20130101 |
Class at
Publication: |
361/302 |
International
Class: |
H01G 004/35; H01G
009/08; H01G 002/10 |
Claims
1. A capacitor structure, comprising: an encasement having first
and second major sides and a peripheral wall coupled to said first
and second major sides; a cathode disposed within said encasement
proximate said first and second major sides, said cathode having a
cathode lead; a centrally disposed anode within said encasement,
said anode having an anode lead; and a bipolar, insulative
feedthrough in said encasement through which electrical coupling
may be made to said anode lead and said cathode lead.
2. A capacitor structure according to claim 1 wherein said
feedthrough comprises first and second separate apertures
therethrough through which electrical coupling may be made to said
anode and said cathode respectively.
3. A capacitor structure according to claim 2 wherein said
feedthrough comprises: a ferrule coupled through said encasement;
and a wire guide having said first and second apertures
therethrough positioned within said ferrule.
4. A capacitor structure according to claim 3 wherein said
feedthrough is elliptical in shape.
5. A capacitor structure according to claim 3 wherein said
feedthrough is made of a polymeric material.
6. A capacitor structure according to claim 2 further comprising an
electrolyte within said encasement and in contact with said cathode
and said anode.
7. A capacitor structure according to claim 3 further comprising a
first insulative separator between said anode and said cathode.
8. A capacitor structure according to claim 7 further comprising a
second insulative separator between said cathode and said first and
second major sides.
9. A capacitor structure according to claim 8 wherein said cathode
comprises at least one substrate having cathode material deposited
thereon.
10. A capacitor structure according to claim 9 wherein said cathode
comprises carbon on titanium carbide.
11. A capacitor structure according to claim 9 wherein said cathode
comprises carbon and silver vanadium oxide on titanium carbide.
12. A capacitor structure according to claim 9 wherein said cathode
comprises carbon and crystalline manganese dioxide on titanium
carbide.
13. A capacitor structure according to claim 9 wherein said cathode
comprises platinum on titanium.
14. A capacitor structure according to claim 9 wherein said cathode
comprises ruthenium on titanium.
15. A capacitor structure according to claim 9 wherein said cathode
comprises silver vanadium oxide on titanium.
16. A capacitor structure according to claim 9 wherein said cathode
material comprises barium titanate on titanium.
17. A capacitor structure according to claim 9 wherein said cathode
comprises carbon and crystalline ruthenium oxide on titanium
carbide.
18. A capacitor structure according to claim 9 wherein said cathode
comprises carbon and crystalline iridium oxide on titanium
carbide.
19. A capacitor structure according to claim 3 wherein said ferrule
forms a hermetic seal with said encasement.
20. A capacitor structure according to claim 3 wherein said
encasement comprises: a shallow drawn case comprising: said first
major side and said peripheral wall; and a lid including a second
major side and sealing coupled to said case along adjacent edges of
said lid and said wall.
21. A capacitor structure according to claim 20 further comprising
a protective layer on said anode adjacent said peripheral wall to
protect said at least one of said first and second anodes when said
lid is sealing coupled to said case.
22. A capacitor structure according to claim 21 wherein said
protective layer comprises a metalized ring.
23. A capacitor structure according to claim 22 wherein said
metalized ring comprises a polymer spacer having a metalized
surface.
24. A capacitor structure according to claim 21 wherein said
protective layer comprises a metallized tape.
25. A capacitor structure for use in an implantable medical device,
said capacitor structure comprising: an encasement having first and
second major sides and a peripheral wall coupled to said first and
second major sides; a cathode disposed within said encasement
proximate said first and second major sides, said cathode having a
cathode lead; a centrally disposed anode within said encasement,
said anode having an anode lead; and a bipolar polymeric
feedthrough in said encasement through which electrical coupling
may be made to said anode lead and said cathode lead, said
feedthrough comprising: a ferrule coupled through said encasement;
and a wire guide having first and second apertures therethrough
positioned within said ferrule.
26. A capacitor structure according to claim 25 wherein said
feedthrough is eliptical in shape.
27. A capacitor structure according to claim 25 further comprising
an electrolyte within said encasement and in contact with said
cathode and said anode.
28. A capacitor structure according to claim 27 further comprising
a first insulative separator between said anode and said
cathode.
29. A capacitor structure according to claim 28 further comprising
a second insulative separator between said cathode and said first
and second major sides.
30. A capacitor structure according to claim 25 wherein said
ferrule forms a hermetic seal with said encasement.
31. A capacitor structure according to claim 29 wherein said
encasement comprises: a shallow drawn case comprising: said first
major side and said peripheral wall; and a lid including a second
major side and sealingly coupled to said case along adjacent edges
of said lid and said wall.
Description
CROSS REFERENCE TO RELATED APPLICATIONS
[0001] This application is a Continuation Application of U.S.
application Ser. No. 10/452,424, filed May 30, 2003, the entire
content being hereby incorporated by reference.
FIELD OF THE INVENTION
[0002] The present invention generally relates to capacitors, and
more particularly to a thin electrolytic capacitor suitable for use
in an implantable medical device such as an implantable cardiac
defibrillator (ICD) and wherein the capacitor encasement remains
electrically neutral.
BACKGROUND OF THE INVENTION
[0003] ICDs are devices that are typically implanted in a patient's
chest to treat very fast, and potentially lethal, cardiac
arrhythmias. These devices continuously monitor the heart's
electrical signals and sense if, for example, the heart is beating
dangerously fast. If this condition is detected, the ICD can
deliver one or more electric shocks, within about five to ten
seconds, to return the heart to a normal heart rhythm. These
defibrillation electric shocks may range from a few micro-joules to
very powerful shocks of approximately twenty-five joules to forty
joules.
[0004] Early generations of ICDs utilized high-voltage, cylindrical
capacitors to generate and deliver defibrillation shocks. For
example, standard wet slug tantalum capacitors generally have a
cylindrically shaped conductive casing serving as the terminal for
the cathode and a tantalum anode connected to a terminal lead
electrically insulated from the casing. The opposite end of the
casing is also typically provided with an insulator structure.
[0005] One such capacitor is shown and described in U.S. Pat. No.
5,369,547 issued on Nov. 29, 1994 and entitled "Capacitor". This
patent disclosed an electrolytic capacitor that includes a metal
container that functions as a cathode. A porous coating, including
an oxide of a metal selected from the group consisting of
ruthenium, iridium, nickel, rhodium, platinum, palladium, and
osmium, is disposed proximate an inside surface of the container
and is in electrical communication therewith. A central anode
selected from the group consisting of tantalum, aluminum, niobium,
zirconium, and titanium is spaced from the porous coating, and an
electrolyte within the container contacts the porous coating and
the anode.
[0006] U.S. Pat. No. 5,737,181 issued on Apr. 7, 1998 and entitled
"Capacitor" describes a capacitor that includes a cathode material
of the type described in the above cited patent disposed on each of
two opposed conducting plates. A metal anode (also of the type
described in the above cited patent) is disposed between the
cathode material coating and the conducting plates.
[0007] U.S. Pat. No. 5,982,609 issued Nov. 9, 1999 and entitled
"Capacitor" describes a capacitor that includes a cathode having a
porous coating including an amorphous metal oxide of at least one
metal selected from the group consisting of ruthenium, iridiumn,
nickel, rhodium, rhenium, cobalt, tungsten, manganese, tantalum,
molybdenum, lead, titanium, platinum, palladium, and osmium. An
anode includes a metal selected from the group consisting of
tantalum, aluminum, niobium, zirconium, and titanium.
[0008] While the performance of these capacitors was acceptable for
defibrillator applications, efforts to optimize the mechanical
characteristics of the device have been limited by the constraints
imposed by the cylindrical design. In an effort to overcome this,
flat electrolytic capacitors were developed. U.S. Pat. No.
5,926,362 issued on Jul. 20, 1999 and entitled "Hermetically Sealed
Capacitor" describes a deep-drawn sealed capacitor having a
generally flat, planar geometry. The capacitor includes at least
one electrode provided by a metallic substrate in contact with a
capacitive material. The coated substrate may be deposited on a
casing side-wall or connected to a side-wall. The capacitor has a
flat planar shape and utilizes a deep-drawn casing comprised of
spaced apart side-walls joined at their periphery by a surrounding
intermediate wall. Cathode material is typically deposited on an
interior side-wall of the conductive encasement which serves as one
of the capacitor terminals; e.g. the cathode. The other capacitor
terminal (the anode) is isolated from the encasement by an
insulator/feedthrough structure comprised of, for example, a
glass-to-metal seal. It is also known to deposit cathode material
on a separate substrate that is placed in electrical communication
with the case. In another embodiment, the cathode substrate is
insulated from the case using insulators and a separate cathode
feedthrough.
[0009] A valve metal anode made from metal powder is pressed and
sintered to form a porous structure, and a wire (e.g. tantalum) is
imbedded into the anode during pressing to provide a terminal for
joining to the feedthrough. A separator (e.g. polyolefin, a
fluoropolymer, a laminated film, non-woven glass, glass fiber,
porous ceramic, etc.) is provided between the anode and the cathode
to prevent short circuits between the electrodes. Separator sheets
are sealed either to a polymer ring that extends around the
perimeter of the anode or to themselves.
[0010] A separate weld ring and polymer insulator may be utilized
for thermal beam protection as well as anode immobilization. Prior
to encasement welding, a separator encased anode is joined to the
feedthrough wire by, for example, laser welding. This joint is
internal to the capacitor. The outer metal encasement structure is
comprised essentially of two symmetrical half shells that overlap
and are welded at their perimeter seam to form a hermetic seal.
After welding, the capacitor is filled with electrolyte through a
port in the encasement.
[0011] The above described techniques present concerns relating to
both device size and manufacturing complexity. The use of
overlapping half-shields results in a doubling of the encasement
thickness around the perimeter of the capacitor thus reducing the
available interior space for the capacitor's anode. This results in
larger capacitors. Space for the anode material is further reduced
by the presence of the weld ring and space insulator. In addition,
manufacturing processes become more complex and therefore more
costly, especially in the case of a deep-drawn encasement.
[0012] A further disadvantage of the known design involves the
complexity of the anode terminal-to-feedthrough terminal weld
joint. As was described, a tantalum anode lead is imbedded into the
anode and is joined via laser welding to a terminal lead of the
feedthrough. This is typically accomplished by forming a "J" or "U"
shape with one or more of the leads, pressing the terminal end of
these leads together, and laser welding the interface. In order to
accomplish this, one must either perform this step prior to welding
the feedthrough ferrule into the encasement or sufficient space
must be provided in the capacitor anode structure to facilitate
clamping and welding while the anode is in the case. This results
in additional manufacturing complexity while the latter negatively
impacts device size.
[0013] As stated previously, a separator material is provided on
the anode and may be sealed to itself to form an envelope. The
anode is typically on the order of 0.1 inch thick. As a result, the
sealing operation is complex, and significant separator material
typically overhangs the anode. This overhang must be accommodated
in the design and typically either reduces the size of the anode or
increases the size of the capacitor. Furthermore, due to the
proximity of thermally sensitive separator material to the
encasement, the separator is in direct contact with the
cathode/encasement structure. Weld parameters must therefore be
carefully selected to prevent thermal damage of the separator
material. When cathode material is deposited on a separate
substrate, as described above, substrate thickness further reduces
the space available for anode material or increases the size of the
capacitor. In a case-neutral device (i.e. the capacitor encasement
forms neither the anode terminal or the cathode terminal), the
additional space necessary to incorporate separate feedthrough
ferrules and insulators to insulator the cathode and the anode from
the case further increases the size of the capacitor.
[0014] Thus, while the development of flat electrolytic capacitors
significantly reduces size and thickness, defibrillation capacitors
are still the largest components in current ICDs making further
downsizing a primary objective.
BRIEF SUMMARY OF THE INVENTION
[0015] According to an aspect of the invention, there is provided a
capacitor structure, comprising a shallow drawn encasement having
first and second major sides and a peripheral wall coupled to the
first and second major sides. A cathode is disposed within the
encasement proximate the first and second major sides, the cathode
having a cathode lead. A central anode having an anode lead is
disposed within the encasement, and a bipolar, insulative
feedthrough extends through the encasement through which electrical
coupling may be made to the anode lead and the cathode lead.
BRIEF DESCRIPTION OF THE DRAWINGS
[0016] The present invention will hereinafter be described in
conjunction with the following drawing figures, wherein like
numerals denote like elements, and
[0017] FIG. 1 is a cross-sectional view of an electrolytic
capacitor in accordance with the teachings of the prior art;
[0018] FIGS. 2, 3, and 4 are front, side, and top cross-sectional
views of a flat electrolytic capacitor in accordance with the
teachings of the prior art;
[0019] FIGS. 5, 6, and 7 are front cross-sectional, side
cross-sectional, and scaled cross-sectional views of a novel
electrolytic capacitor;
[0020] FIG. 8 is a cross-sectional view of a capacitor/anode
encasement structure in accordance with the teachings of the prior
art;
[0021] FIG. 9 is a cross-sectional view of a novel capacitor/anode
encasement assembly;
[0022] FIG. 10 is a cross-sectional view of an alternative
capacitor/anode encasement assembly; and
[0023] FIGS. 11 and 12 are isometric and exploded views of a
bipolar feedthrough assembly for use in conjunction with the
inventive electrolytic capacitor.
DETAILED DESCRIPTION OF THE INVENTION
[0024] The following detailed description of the invention is
merely exemplary in nature and is not intended to limit the scope,
applicability, or configuration of the invention in any way.
Rather, the following description provides a convenient
illustration for implementing exemplary embodiments of the
invention. Various changes to the described embodiments may be made
in the function and arrangement of the elements described herein
without departing from the scope of the invention.
[0025] FIG. 1 is a cross-sectional view of an electrolytic
capacitor in accordance with the teaching of the prior art. It
comprises a cylindrical metal container 20 made of, for example
tantalum. Typically, container 20 comprises the cathode of the
electrolytic capacitor and includes a lead 22 that is welded to the
container. An end seal of cap 24 includes a second lead 26 that is
electrically insulated from the remainder of cap 24 by means of a
feed-through assembly 28. Cap 24 is bonded to container 20 by, for
example, welding. Feed-through 28 of lead 26 may include a
glass-to-metal seal through which lead 26 passes. An anode 30
(e.g., porous sintered tantalum) is electrically connected to lead
26 and is disposed within container 20. Direct contact between
container 20 and anode 30 is prevented by means of electrically
insulating spacers 32 and 34 within container 20 that receive
opposite ends of anode 30. A porous coating 36 is formed directly
on the inner surface of container 20. Porous coating 36 may include
an oxide of ruthenium, iridium, nickel, rhodium, platinum,
palladium, or osmium. As stated previously, anode 30 may be made of
a sintered porous tantalum. However, anode 30 may be aluminum,
niobium, zirconium, or titanium. Finally, an electrolyte 38 is
disposed between and in contact with both anode 30 and cathode
coating 36 thus providing a current path between anode 30 and
coating 36. As stated previously, while capacitors such as the one
shown in FIG. 1 were generally acceptable for defibrillator
applications, optimization of the device is limited by the
constraints imposed by the cylindrical design.
[0026] FIGS. 2, 3, and 4 are front, side, and top cross-sectional
views respectively of a flat electrolytic capacitor, also in
accordance with the teachings of the prior art, designed to
overcome some of the disadvantages associated with the electrolytic
capacitor shown in FIG. 1. The capacitor of FIGS. 2, 3, and 4
comprises an anode 40 and a cathode 44 housed inside a hermetically
sealed casing 46. The capacitor electrodes are activated and
operatively associated with each other by means of an electrolyte
contained inside casing 46. Casing 46 includes a deep drawn can 48
having a generally rectangular shape and comprised of spaced apart
side-walls 50 and 52 extending to and meeting with opposed end
walls 54 and 56 extending from a bottom wall 58. A lid 60 is
secured to side-walls 50 and 52 and to end walls 54 and 56 by a
weld 62 to complete an enclosed casing 46. Casing 46 is made of a
conductive metal and serves as a terminal or contact for making
electrical connections between the capacitor and its load.
[0027] The other electrical terminal or contact is provided by a
conductor or lead 64 extending from within the capacitor through
casing 46 and, in particular, through lid 60. Lead 64 is insulated
electrically from lid 60 by an insulator and seal structure 66. An
electrolyte fill opening 68 is provided to permit the introduction
of an electrolyte into the capacitor, after which opening 68 is
closed. Cathode electrode 44 is spaced from the anode electrode 40
and comprises an electrode active material 70 provided on a
conductive substrate. Conductive substrate 70 may be selected from
the group consisting of tantalum, nickel, molybdenum, niobium,
cobalt, stainless steel, tungsten, platinum, palladium, gold,
silver, cooper, chromium, vanadium, aluminum, zirconium, hafnium,
zinc, iron, and mixtures and alloys thereof. Anode 40 may be
selected from the group consisting of tantalum, aluminum, titanium,
niobium, zirconium, hafnium, tungsten, molybdenum, vanadium,
silicon, germanium, and mixtures thereof. A separator structure
includes spaced apart sheets 72 and 74 of insulative material (e.g.
a microporous polyolefinic film). Sheets 72 and 74 are connected to
a polymeric ring 76 and are disposed intermediate anode 40 and
coated side-walls 50 and 52 which serve as a cathode electrode.
[0028] As already mentioned, the above described capacitors present
certain concerns with respect to device size and manufacturing
complexity. In contrast, FIGS. 5, 6, and 7 are front
cross-sectional, side cross-sectional, and scaled cross-sectional
of an electrolytic capacitor suitable for use in an implantable
medical device in accordance with a first embodiment of the present
invention. As can be seen, one or more layers of an insulative
polymer separator material 142 (e.g. micro-porous PTFE or
polypropylene) are heat sealed around a thin, D-shaped anode 140
(e.g. tantalum) having an anode lead wire 144 (e.g. tantalum)
embedded therein. Capacitor grade tantalum powder such as the "NH"
family of powders may be employed for this purpose. These tantalum
powders have a charge per gram rating of between approximately
17,000 to 23,000 microfarad-volts/gram and have been found to be
well suited for implantable cardiac device capacitor applications.
Tantalum powders of this type are commercially available from HC
Starck, Inc. located in Newton, Mass.
[0029] Before pressing, the tantalum powder is typically, but not
necessarily, mixed with approximately 0 to 5 percent of a binder
such as ammonium carbonate. This and other binders are used to
facilitate metal particle adhesion and die lubrication during anode
pressing. The powder and binder mixture are dispended into a die
cavity and are pressed to a density of approximately 4 grams per
cubic centimeter to approximately 8 grams per cubic centimeter.
After pressing, it is sometimes beneficial to modify anode porosity
to improve conductivity within the internal portions of the anode.
Porosity modification has been shown to significantly reduce
resistance. Macroscopic channels are incorporated into the body of
the anodes to accomplish this. Binder is then removed from the
anodes either by washing in warm deionized water or by heating at a
temperature sufficient to decompose the binder. Complete binder
removal is desirable since residuals may result in high leakage
current. Washed anodes are then vacuum sintered at between
approximately 1,350 degrees centigrade and approximately 1,600
degrees centigrade to permanently bond the metal anode
particles.
[0030] An oxide is formed on the surface of the sintered anode by
immersing the anode in an electrolyte and applying a current. The
electrolyte includes constituents such as water and phosphoric acid
and perhaps other organic solvents. The application of current
drives the formation of an oxide film that is proportional in
thickness to the targeted forming voltage. A pulsed formation
process may be used wherein current is cyclically applied and
removed to allow diffusion of heated electrolyte from the internal
pores of the anode plugs. Intermediate washing and annealing steps
may be performed to facilitate the formation of a stable, defect
free, oxide.
[0031] Layers of cathode material 146 are deposited on the inside
walls of a thin, shallow drawn, D-shaped casing 148 (e.g. titanium)
having first and second major sides and a peripheral wall, each of
which have an interior surface. The capacitive materials may be
selected from those described above or selected from the group
including graphitic or glassy carbon on titanium carbide, carbon
and silver vanadium oxide on titanium carbide, carbon and
crystalline manganese dioxide on titanium carbide, platinum on
titanium, ruthenium on titanium, barium titanate on titanium,
carbon and crystalline ruthenium oxide on titanium carbide, carbon
and crystalline iridium oxide on titanium carbide, silver vanadium
oxide on titanium and the like.
[0032] Anode 140 and cathode material 146 are insulated from each
other by means of a micro-porous polymer separator material such as
a PTFE separator of the type produced by W.L. Gore, Inc. located in
Elkton, Md. or polypropylene of the type produced by Celgard, Inc.
located in Charlotte, N.C. Separators 146 prevent physical contact
and shorting and also provide for ionionic conduction. The material
may be loosely placed between the electrodes or can be sealed
around the anode and/or cathode. Common sealing methods include
heat sealing, ultra sonic bonding, pressure bonding, etc.
[0033] The electrodes are housed in a shallow drawn, typically
D-shaped case (e.g. titanium) that may have a material thickness of
approximately 0.005 to 0.016 inches thick. A feed-through 150 is
comprised of a ferrule 154 (e.g. titanium), a terminal lead wire
152 (e.g. tantalum), and an insulator 156 (e.g. a polycrystalline
ceramic polymer, non-conducting oxides, conventional glass, etc.)
is bonded to ferrule 154 and lead wire 152. Sealed anode 140 is
inserted into a cathode coated case and a spacer ring is inserted
around the periphery of the anode to secure the position of the
anode within the case. A J-shaped feed-though lead wire 152 is
electrically coupled to anode lead wire 144 as, for example, by
resistance or laser welding. Lead wire 152 may be joined to anode
lead wire 144 without the necessity for a J-shaped bend as is
described in copending U.S. patent application Ser. No. 009.0015
filed on May 30, 2003 and entitled "Electrolytic Capacitor for use
in an Implantable Medical Device".
[0034] After assembly and welding, an electrolyte is introduced
into the casing through a fill-port 160. The electrolyte is a
conductive liquid having a high breakdown voltage that is typically
comprised of water, organic solvents, and weak acids or of water,
organic solvents and sulfuric acid. Filling is accomplished by
placing the capacitor in a vacuum chamber such that fill-port 160
extends into a reservoir of electrolyte. When the chamber is
evacuated, pressure is reduced inside the capacitor. When the
vacuum is released, pressure inside the capacitor re-equilibrates,
and electrolyte is drawn through fill-port 160 into the
capacitor.
[0035] Filled capacitors are aged to form an oxide on the anode
leads and other areas of the anode. Aging, as with formation, is
accomplished by applying a current to the capacitor. This current
drives the formation of an oxide film that is proportional in
thickness to the targeted aging voltage. Capacitors are typically
aged approximately at or above their working voltage, and are held
at this voltage until leakage current reaches a stable, low value.
Upon completion of aging, capacitors are re-filled to replenish
lost electrolyte, and the fill-port 160 is sealed as, for example,
by laser welding a closing button or cap over the encasement
opening.
[0036] As stated previously, the outer metal encasement structure
of a known planar capacitor generally comprises two symmetrical
half shells that overlap and are then welded along their perimeter
seam to form a hermetic seal. Such a device is shown in FIG. 8.
That is, the encasement comprises a case 164 and an overlapping
cover 166. A separator sealed anode 168 is placed within case 164,
and a polymer spacer ring 170 is positioned around the periphery of
anode assembly 168. Likewise, a metal weld ring 172 is positioned
around the periphery of spacer ring 170 proximate the overlapping
portion 174 of case 164 and cover 166. The overlapping portions of
case 164 and cover 166 are then welded along the perimeter seam to
form a hermetic seal.
[0037] This technique presents certain concerns relating to both
device size and manufacturing complexity. The use of overlapping
half-shields results in a doubling of the encasement thickness
around the perimeter of the capacitor thus reducing the available
interior space for the anode. Thus, for a given size anode, the
resulting capacitor is larger. Furthermore, space for anode
material is reduced due to the presence of weld ring 172 and
insulative polymer spacer ring 170. This device is more complex to
manufacture and therefore more costly.
[0038] FIG. 9 is a cross-sectional view illustrating one of the
novel aspects of the present invention. In this embodiment, the
encasement is comprised of a shallow drawn case 176 and a cover or
lid 178. This shallow drawn encasement design uses a top down
welding approach. Material thickness is not doubled in the area of
the weld seam as was the situation in connection with the device
shown in FIG. 8 thus resulting in additional space for anode
material.
[0039] Cover 178 is sized to fit into the open side of shallow
drawn metal case 176. This results in a gap (e.g. from 0 to
approximately 0.002 inches) in the encasement between case 176 and
cover 178 that could lead to the penetration of the weld laser beam
thus potentially damaging the capacitor's internal components. To
prevent this, a metalized polymeric weld ring is placed or
positioned around the periphery of anode 168. Weld ring 180 is
somewhat thicker than the case to cover gap 182 to maximize
protection. Metalized weld ring 180 may comprise a polymer spacer
186 having a metalized surface 184 as shown. Metalized weld ring
180 provides for both laser beam shielding and anode
immobilization. The metalized polymer spacer 180 need only be thick
enough to provide a barrier to penetration of the laser beam and is
sacrificial in nature. This non-active component substantially
reduces damage to the active structures on the capacitor.
[0040] Metalized polymer spacer 180 is placed around the perimeter
of anode 168 during assembly and may be produced my means of
injection molding, thermal forming, tube extrusion, die cutting of
extruded or cast films, etc. Spacer 180 may be provided through the
use of a pre-metalized polymer film. Alternatively, the metal may
be deposited during a separate process after insulator production.
Suitable metallization materials include aluminum, titanium, etc.
and mixtures and alloys.
[0041] FIG. 10 is a cross-sectional view illustrating an
alternative to the embodiment shown in FIG. 9. Again, the
encasement comprises a case 176 and a cover or lid 178 resulting in
gap 182. The anode assembly 168 is positioned within the encasement
as was the situation in FIG. 9. To protect the capacitor's internal
components from damage due to the weld laser beam, a metalized tape
184 is positioned around the perimeter of anode 168.
[0042] The embodiments shown in FIGS. 9 and 10 not only have space
saving aspects in the encasement design, but the components are
simple and inexpensive to produce. The top down assembly
facilitates fabrication and welding processes. The thinness of the
weld ring/spacer 180 or metalized tape 184 reduces the need for
additional space around the perimeter of the capacitor thus
improving energy density. The design lends itself to mass
production methods and reduces costs, component count, and
manufacturing complexity.
[0043] It is not uncommon for the encasement of the capacitor
itself to serve as the cathode electrode. This may be accomplished
by depositing cathode material on an inner wall of the encasement
of, if cathode material is deposited on one or more substrates, by
electrically connecting the substrates to the encasement.
Alternatively, the encasement may be made electrically neutral by
not coupling cathode 202 to the encasement. Cathode substrate 202
may simply be sealed within separators 208 as is shown at 216. In
this situation, however, it is necessary not only to provide access
to an anode electrode at the exterior of encasement 148 but
provisions must also be made to access a cathode electrode from the
exterior of the capacitor. Prior art approaches have involved the
use of separate feedthrough ferrules and insulators.
[0044] In accordance with the present invention, a bipolar
feedthrough assembly is used to insulate both the anode and the
cathode leads from the case while at the same time minimizing the
space required to facilitate electrical connections. FIGS. 11 and
12 are isometric and exploded views of a bipolar feedthrough
assembly in accordance with the present invention. As can be seen,
the inventive feedthrough ferrule is generally elliptical in shape
although other shapes may be utilized. First and second leads (e.g.
an anode lead and a cathode lead) 214 and 216 extend through
apertures 218 and 220 respectively in an insulative wire guide 222
(e.g. made of a polymeric material). Wire guide 222 is then
positioned within elliptical case 224 as is shown in FIG. 11. In
this manner, separate feedthroughs are located in close proximity
to one another by a single feedthrough assembly. Bipolar
feedthroughs of the type shown in FIGS. 11 and 12 may be utilized
to provide both glass-to-metal and polymer-to-metal seals.
[0045] Thus, there has been provided a case-neutral electrolytic
capacitor that is not only easier and less costly to manufacture,
but one which may be made smaller for a given capacitance. The
inventive capacitor is therefore suitable for use in implantable
medical devices such as defibrillators, even as such devices become
smaller and smaller.
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