U.S. patent application number 10/358877 was filed with the patent office on 2003-09-18 for metallized film capacitor for use in implantable defibrillator.
This patent application is currently assigned to Intermedics Inc.. Invention is credited to Munshi, Mohammed Zafar Amin.
Application Number | 20030176893 10/358877 |
Document ID | / |
Family ID | 22060547 |
Filed Date | 2003-09-18 |
United States Patent
Application |
20030176893 |
Kind Code |
A1 |
Munshi, Mohammed Zafar
Amin |
September 18, 2003 |
Metallized film capacitor for use in implantable defibrillator
Abstract
A thin film capacitor for use in an implantable defibrillator. A
first dielectric polymer filmlayer has a metallized film on one
side thereof. A second dielectric polymer film layer has a
metallized film on one side thereof. The first and second layers
are overlain on each other and wound spirally with the metallized
film of one layer adjacent the dielectric polymer of the other
layer. The beginnings and ends of the first and second metallized
films are offset from the respective beginnings and ends of the
first and second polymer film layers. The dielectric layers can be
tapered in increasing thickness toward the respective beginnings
and ends of the layers. The dielectric layers can themselves
comprise at least two layers of differing polymer materials, the
preferred materials being polyvinylidene fluoride and polyester for
improved energy density and self-healing properties.
Inventors: |
Munshi, Mohammed Zafar Amin;
(Missouri City, TX) |
Correspondence
Address: |
SCHWEGMAN, LUNDBERG, WOESSNER & KLUTH, P.A.
P.O. BOX 2938
MINNEAPOLIS
MN
55402
US
|
Assignee: |
Intermedics Inc.
|
Family ID: |
22060547 |
Appl. No.: |
10/358877 |
Filed: |
February 4, 2003 |
Related U.S. Patent Documents
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Application
Number |
Filing Date |
Patent Number |
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10358877 |
Feb 4, 2003 |
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09782443 |
Feb 13, 2001 |
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6514276 |
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09782443 |
Feb 13, 2001 |
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09065131 |
Apr 23, 1998 |
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6187028 |
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Current U.S.
Class: |
607/5 |
Current CPC
Class: |
H01G 4/015 20130101;
H01G 4/18 20130101; H01G 4/32 20130101 |
Class at
Publication: |
607/5 |
International
Class: |
A61N 001/39 |
Claims
I claim:
1. A thin film capacitor for use in an implantable defibrillator,
comprising: a first dielectric polymer film layer, said first layer
having a metallized film on one side thereof, said first layer
having a beginning and an end, said first metallized film having a
beginning and a end; a second dielectric polymer film layer, said
second layer having a metallized film on one side thereof, said
second layer having a beginning and an end, said second metallized
film having a beginning and a end; said first and second layers
being overlain on each other and wound spirally with the metallized
film of one layer adjacent the dielectric polymer of the other
layer; and said beginnings of said first and second metallized
films being offset from said respective beginnings of said first
and second polymer film layers.
2. The thin film capacitor of claim 1, in which said ends of said
first and second metallized films are offset from said respective
ends of said first and second polymer film layers.
3. A thin film capacitor for use in an implantable defibrillator,
comprising: a first dielectric polymer film layer, said first layer
having a metallized film on one side thereof, said first layer
having a beginning and an end, said first metallized film having a
beginning and a end; a second dielectric polymer film layer, said
second layer having a metallized film on one side thereof, said
second layer having a beginning and an end, said second metallized
film having a beginning and a end; said first and second layers
being overlain on each other and wound spirally with the metallized
film of one layer adjacent the dielectric polymer of the other
layer; and said ends of said first and second metallized films
being offset from said respective ends of said first and second
polymer film layers.
4. The thin film capacitor of claim 3, in which said beginnings of
said first and second metallized films are offset from said
respective beginnings of said first and second polymer film
layers.
5. A thin film capacitor for use in an implantable defibrillator,
comprising: a first dielectric polymer film layer, said first layer
having a metallized film on one side thereof, said first layer
having a beginning and an end; a second dielectric polymer film
layer, said second layer having a metallized film on one side
thereof, said second layer having a beginning and an end; said
first and second layers being overlain on each other and wound
spirally with the metallized film of one layer adjacent the
dielectric polymer of the other layer; and said first layer having
a middle portion of substantially uniform thickness and a first
portion that tapers in increasing thickness from said middle
portion to said beginning of said first layer.
6. The thin film capacitor of claim 5, in which said first layer
tapers in thickness over a distance in the direction of winding at
least several multiples of the inner circumference of the
capacitor.
7. The thin film capacitor of claim 5, in which said second layer
has a middle portion of substantially uniform thickness and a first
portion that tapers in increasing thickness from said middle
portion to said beginning of said second layer.
8. The thin film capacitor of claim 7, in which said first layer
tapers in thickness over a distance in the direction of winding at
least several multiples of the inner circumference of the
capacitor.
9. A thin film capacitor for use in an implantable defibrillator,
comprising: a first dielectric polymer film layer, said first layer
having a metallized film on one side thereof, said first layer
having a beginning and an end; a second dielectric polymer film
layer, said second layer having a metallized film on one side
thereof, said second layer having a beginning and an end; said
first and second layers being overlain on each other and wound
spirally with the metallized film of one layer adjacent the
dielectric polymer of the other layer; and said second layer having
a middle portion of substantially uniform thickness and a first
portion that tapers in increasing thickness from said middle
portion to said beginning of said second layer.
10. The thin film capacitor of claim 9, in which said second layer
tapers in thickness over a distance in the direction of winding at
least several multiples of the inner circumference of the
capacitor.
11. The thin film capacitor of claim 9, in which said first layer
has a middle portion of substantially uniform thickness and a first
portion that tapers in increasing thickness from said middle
portion to said beginning of said first layer.
12. The thin film capacitor of claim 9, in which said second layer
has a second portion that tapers in increasing thickness from said
middle portion to said end of said second layer.
13. The thin film capacitor of claim 12, in which said first
portion of said second layer tapers in thickness over a distance in
the direction of winding at least several multiples of the inner
circumference of the capacitor.
14. The thin film capacitor of claim 12, in which said second
portion of said second layer tapers in thickness over a distance in
the direction of winding at least several multiples of the outer
circumference of the capacitor.
15. A thin film capacitor for use in an implantable defibrillator,
comprising: a first dielectric polymer film layer, said first layer
having a metallized film on one side thereof, said first layer
having a beginning and an end; a second dielectric polymer film
layer, said second layer having a metallized film on one side
thereof, said second layer having a beginning and an end; said
first and second layers being overlain on each other and wound
spirally with the metallized film of one layer adjacent the
dielectric polymer of the other layer; and said first dielectric
polymer film layer comprising at least two layers of differing
polymers.
16. The thin film capacitor of claim 15, in which said at least two
layers of differing polymers include polyvinylidene fluoride and
polyester.
17. The thin film capacitor of claim 15, in which said second
dielectric polymer film layer comprises at least two layers of
differing polymers.
18. The thin film capacitor of claim 17, in which said at least two
layers of differing polymers include polyvinylidene fluoride and
polyester.
Description
BACKGROUND OF THE INVENTION
[0001] 1. Field of the Invention
[0002] The present invention relates generally to electrical energy
storage capacitors, and more particularly to energy storage
capacitors suitable for use in an implantable cardiac
defibrillator.
[0003] 2. Background Information
[0004] Implantable defibrillators are implanted in patients who are
at risk of suffering cardiac arrhythmias, such as ventricular
fibrillation, that can cause sudden death. The defibrillator
detects the occurrence of ventricular fibrillation and
automatically delivers defibrillating therapy in the form of a
high-energy shock to the cardiac tissue. Implantable defibrillators
in their most general form include appropriate electrical leads and
electrodes for collecting electrical signals generated by the
heart, and for delivering electric shocks to the heart. Also
included are batteries and energy storage capacitors, and control
circuitry connected to the leads, batteries and capacitors. The
control circuitry senses the electrical activity of the heart and
controls the charging of the capacitors and the delivery of the
shocks through the leads to the heart.
[0005] Defibrillation therapy generally involves rapid delivery of
a relatively large amount of electrical energy to the heart at high
voltage. Typical values include 20 joules or more at 700 volts or
more. Presently available batteries suitable for use in implantable
defibrillators are not capable of delivering energy at such levels
directly. Consequently, it is customary to provide a high-voltage
energy storage capacitor that is charged by the battery via
appropriate voltage transformation and charging circuitry. To avoid
wasting battery energy, the high-voltage energy storage capacitor
is not maintained in a charged state, but rather is charged after
fibrillation has been identified by the control circuitry and
immediately prior to delivering the shock.
[0006] The amount of electrical energy that must be transferred to
cardiac tissue to effect defibrillation is quite large by the
standards of other implantable cardiac stimulators, such as
pacemakers and cardioverters, which treat bradycardia and
tachycardia, respectively. Consequently, the physical volume of the
energy storage capacitors employed in implantable defibrillators is
substantial. Together with the battery, the energy storage
capacitor presents a major limitation to reducing the overall size
of the implanted device.
[0007] Conventional energy storage capacitors used in implantable
defibrillators have employed an aluminum electrolytic capacitor
technology that had been developed for photoflash capacitors.
Aluminum electrolytic capacitors have plates of aluminum foil
separated by a porous layer, often paper, impregnated with a
viscous liquid electrolyte comprising ethylene glycol plus
additives. Alternating layers of foil and paper are wound in a
spiral about a mandrel to form a cylindrical capacitor. Electrical
leads are attached to respective separate foil layers. The wound
capacitor is placed in a cylindrical aluminum can, or housing,
closed at one end and open at the other. The dielectric is formed
at the electrolyte-to-plate interface by applying a controlled
direct current between the leads of the capacitor. Periodically
throughout life of the capacitor, especially after periods of
non-use, that same process must be used to re-form the dielectric
of the aluminum electrolytic capacitor. To complete the
construction of the aluminum electrolytic capacitor, the open end
of the aluminum can is closed by an elastomeric seal, through which
the electrical leads project. The elastomeric seal prevents leakage
of electrolyte from the aluminum can, but does not provide an
hermetic seal. This permits venting of hydrogen gas that is
normally liberated in the aluminum electrolytic capacitor during
use.
[0008] While aluminum electrolytic capacitors have been used
successfully in implantable defibrillators, certain of their
characteristics are regarded as disadvantageous. For example, the
outgassing characteristic is undesirable in a capacitor that is
contained within an implantable device that itself must be
hermetically sealed against intrusion by body fluids. The device
either must be provided with internal hydrogen adsorbers or else
made permeable to hydrogen to prevent an internal buildup of
pressure. The relative thickness of the aluminum foil plates and
paper separators, as well as the head room required at the ends of
the capacitor housing, place upper limits on the energy density of
the aluminum electrolytic capacitor, resulting in a relatively
bulky device. This is undesirable in the context of pectorally
implanted defibrillators which, for reasons related to ease of
implantation, comfort and cosmetics, are desired to be as small as
possible. Typical aluminum electrolytic photoflash capacitors have
energy densities of about 1.8 Joules per cubic centimeter. Also,
aluminum electrolytic capacitors typically have a maximum working
voltage of about 380 V, whereas implantable defibrillators are
usually designed to deliver a shock at 700 V or more. Consequently,
two capacitors must be employed in series to achieve the desired
working voltage. This results in inefficient space utilization in
the implantable device. The need to periodically reform the
dielectric of the aluminum electrolytic capacitor is also an
undesirable characteristic of a capacitor enclosed in a self
contained, battery powered, implanted device. The periodic
reformation consumes energy from the battery that otherwise would
be available for therapeutic use, thereby reducing the longevity of
the implanted device.
[0009] Another capacitor technology that has been considered for
use in implantable defibrillators is the ceramic dielectric
capacitor. The ceramic capacitor has advantages over the aluminum
electrolytic capacitor in that it is free of outgassing and does
not need periodic reformation. Nevertheless, the ceramic capacitor
has been difficult to manufacture with the working voltage and
reliability characteristics needed for use in an implantable
defibrillator. For example, working voltages above about 400 V have
been difficult to achieve. A single local defect in the ceramic
dielectric can result in a short circuit between the plates,
resulting in catastrophic failure of the capacitor. Also, ceramic
capacitors are relatively heavy. Excess weight is undesirable in an
implantable device because it can complicate the task of reliably
anchoring the device to adjacent tissue and may raise issues of
patient comfort.
[0010] Yet another capacitor technology that has been considered
for use in implantable defibrillators is the thin polymer film
capacitor. Such capacitors employ a thin polymer dielectric film
between the metallic capacitor plates, as opposed to the
electrolyte dielectric material of the typically employed
photoflash aluminum electrolytic capacitor. The plates of the thin
polymer film capacitor usually take the form of very thin metal
layers that are vapor deposited directly to the dielectric
substrate to a thickness of about 150 to 350 angstroms. The result
is a so-called metallized polymer film that provides both the
dielectric and plate functions of the capacitor. Typically, two
layers of metallized polymer film are overlaid and are tightly
wound about a mandrel to form a wound cylindrical capacitor. The
metallized layers on the two polymer films are offset from opposite
respective edges of the films, allowing alternate plates of the
spiral-wound structure to be soldered together at opposite ends of
the cylindrical capacitor and connected to respective leads. A
capacitor wound from metallized polymer film can be constructed
with a relatively high energy density because of the efficient use
of space permitted by the extremely thin metal plates, and because
working voltages well in excess of 700 V can be achieved in a
single capacitor. The energy density that can be achieved is
limited primarily by the manufacturability of polymer films of
arbitrarily small thickness, and by the dielectric properties of
the particular polymer film, which dictate the minimum thickness
required for a particular design voltage. Energy densities of about
one (1) Joule per cubic centimeter are typical for polyester film
capacitors, for example. Polyester has a dielectric constant of
about 2.5 to 3.0.
[0011] An advantageous characteristic of the metallized, thin
polymer film capacitor is its ability to self-heal, or clear, minor
defects in the dielectric when subjected to an initial clearing
voltage greater than its designed working voltage. This
characteristic provides a capacitor of high reliability. During
clearing, imbedded foreign particles or micro-flaws in the
capacitor dielectric lead to localized breakdowns of the film
dielectric. The breakdown event results in an arc between the two
metallized layers that develops a localized high temperature and
pressure. A puncture develops in the polymer film, and the thin
metallized plate in the vicinity of the failure site is rapidly
vaporized and blown away from the puncture. The evaporation of the
electrode around the arc causes it to extinguish, which
electrically isolates the two plates on either side of the
dielectric film in the vicinity of the puncture. This prevents
large-scale damage and catastrophic failure of the capacitor. The
clearing process removes an electrode area that is a very small
percentage of the entire area of the capacitor plate electrodes,
resulting in no significant loss of capacitance. As a general rule,
the more flexible and elastic the film material is, and the lower
the pressure between the winding layers, the greater the
probability that a puncture will self-heal. When inter-layer
(radial) pressures are high, the gas pressure associated with the
arc increases rapidly, damaging adjacent layers and extinguishing
the arc prematurely. This incomplete burning leaves behind a carbon
residue that continues to conduct, leading to a thermal runaway
that melts many layers of metallized plastic film and generates a
catastrophic, high resistance short.
[0012] Some polymer films demonstrate better clearing
characteristics than others do. In general, polymers that burn
well, i.e., that will sustain a flame once ignited, have good
clearing properties. Such polymers usually have oxygen in their
molecular structure, e.g., polyester, but there are notable
exceptions, such as polypropylene.
[0013] One promising polymer film for constructing a high
energy-density thin film capacitors is polyvinylidene fluoride, or
PVDF. This material has a very-high dielectric constant, i.e.,
k=12, which presents the possibility of constructing a capacitor
with very thin films. This would permit more windings within a
given capacitor diameter, which increases the plate area within a
given cylindrical volume and increases the energy density. Energy
densities of about 4 Joules per cubic centimeter are possible with
a PVDF dielectric. Also, PVDF exhibits lower leakage than aluminum
electrolytic capacitors, with leakage currents on the order of tens
of micro-amps rather than hundreds or thousands of micro-amps.
Compared with polyester, however, PVDF has relatively poor
self-healing, or clearing, characteristics.
[0014] Evaluations of capacitors constructed using metallized thin
films of PVDF have shown electrical degradation at voltages lower
than expected, considering the inherent voltage breakdown
characteristics of PVDF. For example, two metallized layers of PVDF
were cylindrically (spirally) wound on a mandrel having a diameter
of about 2 to 3 mm. The layers were wound until the capacitor had a
diameter of about 14.5 to 15 mm, with a height of about 50 mm. The
PVDF film had a thickness of about 6 microns, and the metallized
layers were offset about 2.5 mm from respective opposite ends of
the cylindrical construct. In theory, such a capacitor should have
withstood at least 2000 V without breakdown, but in fact exhibited
voltage breakdown at about 800 V to about 1050 V. Subsequent
examination of the failed capacitors revealed many successful
clearings of minor defects, as well as some catastrophic failures
involving localized voltage breakdown through several layers of
dielectric film. The catastrophic failures had not taken place at
locations distributed uniformly over the film, but rather had been
concentrated at the beginning (near the mandrel) and at the end (on
the surface of the capacitor) of the film. It was noted that the
failures at the end of the windings were due to shorting between
the edges of the two films. The polymer film from which the
capacitor had been wound had not been de-metallized at the last few
turns. It was also noted that the film windings at the center of
the capacitor, i.e., at the beginning of the winding near the
mandrel, were very wrinkled. The wrinkling is believed to have been
caused by the winding process in which the first few turns resist
bending smoothly at the small radius involved. The wrinkling may
have resulted in localized areas of high inter-layer pressure in
which breakdown events that ordinarily would have terminated in a
self-healing, nevertheless cascaded through several layers into
catastrophic failure.
[0015] It would be desirable to provide improvements in the design
of and manufacturing steps for making thin film capacitors to
permit the full potential of very thin films of PVDF to be
exploited to increase the energy density of the capacitor. These
and other advantages are provided by the present invention.
SUMMARY OF THE INVENTION
[0016] In accordance with one aspect of the present invention, a
thin film capacitor for use in an implantable defibrillator
includes first and second dielectric polymer film layers, each of
the first and second dielectric polymer film layers having a
metallized film on one side thereof. The first and second
dielectric polymer film layers are overlain on each other and wound
spirally with the metallized film of one layer adjacent the
dielectric polymer of the other layer. The beginnings of the
metallized films are offset from the respective beginnings of the
first and second polymer film layers in the direction of
winding.
[0017] In accordance with a further aspect of the present
invention, the first and second dielectric polymer film layers are
tapered in increasing thickness from a middle portion of uniform
thickness toward the respective beginnings and ends of the
dielectric polymer film layers.
[0018] In accordance with yet another aspect of the present
invention, each of the first and second dielectric polymer film
layers comprises at least two layers of differing polymer
materials, one of which provides the primary dielectric
characteristics of the capacitor and the other of which provides
enhanced self-healing characteristics.
[0019] It is an object of the present invention to provide an
improved electrical energy storage capacitor for use in an
implantable defibrillator.
[0020] Other objects and advantages of the invention will be
apparent from the following descriptions of preferred embodiments
made with reference to the drawings.
BRIEF DESCRIPTION OF THE DRAWINGS
[0021] FIG. 1 is an end view of a prior art arrangement of
metallized polymer films prior to being wound spirally about a
mandrel to form a cylindrical capacitor.
[0022] FIG. 2 is a perspective view of the prior art arrangement of
metallized polymer films of FIG. 1.
[0023] FIG. 3 is a perspective view of a first embodiment of an
arrangement of metallized polymer films in accordance with the
present invention, prior to being wound spirally about a mandrel to
form a cylindrical capacitor.
[0024] FIG. 4 is a perspective view of a second embodiment of an
arrangement of metallized polymer films in accordance with the
present invention, prior to being wound spirally about a mandrel to
form a cylindrical capacitor.
[0025] FIG. 5 is a perspective view of a third embodiment of an
arrangement of metallized polymer films in accordance with the
present invention, prior to being wound spirally about a mandrel to
form a cylindrical capacitor.
DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS
[0026] Referring to FIGS. 1 and 2, a prior art arrangement of two
layers of metallized polymer dielectric film is shown, prior to
being wound spirally on a mandrel about axis A to form a
cylindrical capacitor. The thickness of the layers as illustrated,
is greatly exaggerated. Polymer film layer 10 has deposited on the
upper side thereof a thin metallic layer 12. Along one edge of film
layer 10, perpendicular to the winding axis A, metallic layer 12 is
spaced therefrom by a margin M. Similarly, polymer film layer 14
has deposited on the upper side thereof a thin metallic layer 16.
Along one edge of film layer 14, perpendicular to the winding axis
A and opposite to the edge referred to above with regard to polymer
film layer 10, metallic layer 16 is spaced therefrom by a margin
"M". Polymer layers 10 and 14 are offset relative to each other in
the direction of the winding axis A by an offset "O", such that the
edge having the margin "M" of each polymer layer is recessed
relative to the non-margined edge of the other polymer layer. When
layers 10 and 14, with their respective metallic layers 12 and 16,
are wound spirally about winding axis A, one edge of metallic layer
12 is exposed at one end of the resulting cylindrical capacitor,
whereas the opposite edge of metallic layer 16 is exposed at the
opposite end of the resulting cylindrical capacitor. Solder is
sprayed on one end of the capacitor in electrical contact with a
continuous edge of one, but only one, of the metallic layers.
Similarly, solder is sprayed on the opposite end of the capacitor
in electrical contact with a continuous edge of only the other
metallic layer.
[0027] Referring to FIG. 3, a first embodiment of the present
invention is illustrated in which two metallized layers of polymer
dielectric film are shown prior to being wound spirally about a
mandrel to form a cylindrical capacitor. Components that correspond
to similar components described above with respect to the prior art
construction illustrated in FIGS. 1 and 2 are designated by similar
reference numerals in the one hundred series. Film layer 110 has a
metallized layer 112 that begins at a location 18 offset in the
direction of winding from the starting end 20 of layer 110.
Likewise, film layer 114 has a metallized layer 116 that begins at
a location 22 offset in the direction of winding from the starting
end 24 of layer 114. The amount of offset of the beginnings 18 and
22 of metallized layers 112 and 116 from the beginnings 20 and 24
of film layers 112 and 114 should be at least several multiples of
the circumference of the mandrel on which the capacitor is to be
wound. This will assure that wrinkling at the start of the winding
process will be confined to an area of the windings devoid of
metallization, thereby avoiding injury to the dielectric layer that
lies between the metallized plates. A similar offset is provided at
the ends 26 and 28 of film layers 110 and 114, respectively,
whereby the metallized layers 112 and 116 terminate at locations 30
and 32. The amount of offset of the metallized layers at the end of
the winding should be at least several multiples of the
circumference of the capacitor at the end of winding. This will
prevent failures due to shorting at the end of the windings.
[0028] Referring to FIG. 4, a second embodiment of the present
invention is illustrated in which two metallized layers of polymer
dielectric film are shown prior to being wound spirally about a
mandrel to form a cylindrical capacitor. Components that correspond
to similar components described above with respect to the prior art
construction illustrated in FIGS. 1 and 2, and the first embodiment
illustrated in FIG. 3, are designated by similar reference numerals
in the two hundred series. Film layer 210 has a metallized layer
212. Likewise, film layer 214 has a metallized layer 216. Each of
film layers 210 and 214 is of uniform thickness in a middle
portion, but is tapered to an increased thickness at the beginning
ends 220 and 224 and at the terminating ends 226 and 228. As
preferred, the film layer is tapered to an increased thickness over
a distance in the direction of winding that is at least several
multiples of the circumference of the mandrel, at the starting end,
and at least several multiples of the capacitor circumference at
the terminating end. By increasing the thickness of the dielectric
at the ends of the winding, where failure modes are more likely to
occur, the reliability of the capacitor is increased.
[0029] Referring to FIG. 5, a third embodiment of the present
invention is illustrated, in which two metallized layers of polymer
dielectric film are shown prior to being wound spirally about a
mandrel to form a cylindrical capacitor. Components that correspond
to similar components described above with respect to the prior art
construction illustrated in FIGS. 1 and 2, and the first embodiment
illustrated in FIG. 3, are designated by similar reference numerals
in the three hundred series. Film layer 310 has a metallized layer
312. Likewise, film layer 314 has a metallized layer 316. Unlike
previously described embodiments, polymer film layer 310 is itself
comprised of two layers of different polymers, the primary layer 40
being polyvinylidene fluoride, and the secondary layer 42 being
polyester. The metallized layer 312 is deposited on the secondary
layer 42. As preferred, the PVDF layer 40 with its high dielectric
constant primarily determines the energy density characteristics of
the capacitor. The polyester layer 42, with its significantly lower
dielectric constant, is preferred to be much thinner than the PVDF
layer 40 so as not to adversely affect the volume of the capacitor.
The polyester layer 42 serves to improve the clearing, or
self-healing characteristics of the composite dielectric layer 310.
Similarly, polymer film layer 314 is itself also comprised of two
layers of different polymers, the primary layer 44 being
polyvinylidene fluoride, and the secondary layer 46 being
polyester. The metallized layer 316 is deposited on the secondary
layer 46.
[0030] Three specific embodiments have been described and
illustrated in FIGS. 3, 4 and 5. It should be understood, however,
that combining the features illustrated in the first three
embodiments can make other embodiments. For example, the offsets of
the metallic layers at the starting and terminating ends of the
polymer film layers, as shown in FIG. 3, can be combined with the
tapered dielectric as shown in FIG. 4, or with the composite
dielectric as shown in FIG. 5, or with both the tapered dielectric
of FIG. 4 and the composite dielectric of FIG. 5. Another desirable
combination is the tapered dielectric of FIG. 4 in combination with
the composite dielectric of FIG. 5. In the embodiment illustrated
in FIG. 5, the preferred materials for the dielectric layer is PVDF
and the preferred material for the self-healing enhancing layer is
polyester. Other polymers could be substituted. The self-healing
enhancing layer could be any polymer material having better
self-healing characteristics than the material of the dielectric
layer, although polymers having oxygen in their molecular structure
are preferred.
* * * * *