High energy density metallized film capacitors and methods...

Details High energy density metallized film capacitors and methods... High energy density metallized film capacitors and methods...

1999-06-22

2002-07-30

Reichard, Dean A. (Department: 2831)

Electricity: electrical systems and devices

Electrostatic capacitors

Fixed capacitor

C361S323000, C029S025420

Reexamination Certificate

active

06426861

ABSTRACT:

BACKGROUND OF THE INVENTION
The present invention relates generally to metallized plastic film capacitors, and more particularly to structure and method of manufacture of such capacitors utilizing plastic film with increased dielectric constant, and dielectric strength, improved stability, and low dissipation factor compared to metallized film capacitors of the prior art.
A brief treatment of capacitors will be advantageous to an understanding of the invention. In general, a capacitor consists of two conducting metal plates separated by high quality uniform insulating media (dielectric) capable of storing electrical energy at field stress levels approaching the ultimate voltage withstand value, or breakdown voltage value, of the media material. The static capacitance, C, of the device is related to the applied voltage as follows:
C=Q/V
  (1)
where the capacitance of the capacitor measured in units of farads (F) is equal to the quantity of charge Q in coulombs which is stored on the positively charged metallic plate of the capacitor, divided by the total potential difference V in volts across the plates. Geometrically:
C=ee
o
A/t
  (2)
where A is the area of each plate, t is the thickness of the insulating media layer of dielectric constant e, and e
o
is the dielectric constant of free space. The energy, E, in joules (J) stored in the capacitor at a potential difference V across the plates is:
 
E=
½
C
(
V
)
2
  (3)
The energy stored in a charged capacitor can be continuously increased in proportion to the increase of the voltage, up to high values of V, limited only by the electrical breakdown of the dielectric. It would appear, then, that the most significant increases in the energy density of a capacitor may be made either by increasing the dielectric constant e of the insulating media, or by increasing the applied voltage (field stress) V, or both. The solution, however, is not that simple. In some cases, an increase in the dielectric constant will lead to an increase in dielectric losses, leading to thermal management problems and, worst case, to thermal failure of the capacitor. And an increase in the applied field stress can lead to low reliability and early failure from several possible failure mechanisms which include electromechanical, thermal, chemical and partial discharge mechanisms, to name a few.
A typical conventional metallized film capacitor is the wound capacitor. Dielectric material used in this and other film-type capacitor designs include Kraft paper and various polymer films such as polyester, polypropylene and polycarbonate. The capacitor is formed by sandwiching the dielectric film between metal electrodes (the capacitor plates, which may, for example, be discrete foils or vapor deposited metal film). Use of metallized film reduces capacitor size, but at the expense of peak and average power capability. Connections are made to the electrodes either by extending one entire edge of an electrode out one end of the winding and soldering, arc, flame-spraying or silver-epoxying connections at each end, or by inserting wires or flattened tabs into the winding in contact with each electrode. Examples of wound capacitors are disclosed in U.S. Pat. Nos. 4,719,539 and 4,685,026 to Lavene, U.S. Pat. No. 5,384,684 to Sugisawa, and U.S. Pat. No. 5,406,446 to Peters.
Plastic film capacitors have been the capacitor of choice for many power electronics and pulse power applications because of their inherent low losses, excellent high frequency response, low dissipation factor (DF), low equivalent series resistance (ESR) and high voltage capabilities. Film capacitors have no capacitance coefficient with applied voltage, and metallic migration or leaching does not occur as observed in ceramic capacitors. The film molecule is stable over long term use and is not prone to dielectric dissipation factor degradation or metallic shorting mechanism. Table 1 shows typical properties of some of the common film dielectrics in use today.
TABLE I
Typical properties of some common types of capacitors.
Max
Voltge
Insulation
Oper.
Enrg
Brkdn
DF
Resistance @
Temp
Dens
Capacitor Types
K
(V/ml)
(%)
25° C. (Ohms)
(° C.)
(J/cc)
Plastic Film
Polycarbonate
2.8
13,400
<1
2 × 10
11
125
0.5-1
(PC)
Polypropylene
2.2
16,250
<0.1
8 × 10
11
105
1-1.5
(PP)
Polyester (PET)
3.3
14,500
<1.5
5 × 10
10
125
1-1.5
Polyvinylidene-
12
15,000
1-5
1 × 10
9 
125
2.4
fluoride (PVDF)
Polyethylene-
3.2
14,000
<1
5 × 10
10
125
1-1.5
napthlate (PEN)
Polyphenylene-
3.0
14,000
<0.2
5 × 10
10
200
1-1.5
sulfide (PPS)
Teflon ™ (PTFE)
2.1
7,000
<1
5 × 10
10
200
0.5-1
Polyethylene terephthalate (polyester or PET) offers a reasonable dielectric constant, has a higher operating temperature of 125° C., and is available in film thickness of less than one micron (&mgr;m). However, PET has relatively higher DF with increasing temperature and frequency. For high repetition rate, PET is unsuitable for high pulse power applications.
Polypropylene (PP) has inherently low losses, excellent frequency response and very low DF and ESR with temperature and frequency. In fact, the material possesses a negative temperature coefficient of dissipation factor. The PP chain molecules do not possess polar groups, which are oriented under the effect of electric fields. It is this phenomenon which gives rise to the above beneficial properties. It has the highest breakdown voltage of any capacitor film material. Its only negative may be its maximum operating temperature of 105° C.
Devices made with polyethylene napthalate (PEN), polycarbonate (PC) and polyphenylene sulfide (PPS) dielectrics also have extremely stable characteristics over extremes of voltage, temperature and frequency. Although the intrinsic breakdown voltage for most of these film dielectrics is quite high, in full wound capacitors these dielectrics are usually derated by a factor of 6 to 8 for improved cycle life and reliability.
The polar polymer polyvinylidene fluoride (PVDF) exhibits a large dielectric constant (~12) and demonstrates excellent piezoelectric and pyroelectric properties. PVDF is a partially crystalline linear polymer with a carbon backbone in which each monomer {CH
2
—CF
2
—} unit has two dipole moments, one associated with CF
2
and the other with CH
2
. In the crystalline phase, PVDF exhibits a variety of molecular conformations and crystal structures depending on the method of preparation. The extruded or cast material usually contains 40 and 60% crystalline material in one or both of the principal crystalline phases, alpha and beta. The alpha phase predominates in material cast from the melt. This phase is converted to the beta phase by mechanical deformation of the material at temperatures less than 100° C. In commercial production, PVDF film is extruded and mechanically stretched both parallel and perpendicular to the direction of extrusion, as are most of the capacitor grade film dielectrics. This causes a preferred orientation of the polymer chains in the plane of the film and also converts a large percentage of crystallites to beta form. It is this bi-axially oriented film material which, after polarization, forms the basis of piezoelectric and pyroelectric devices. Unfortunately, the highly crystalline structure also results in some weakness in the physical strength of the film. This causes major problems during the manufacture of very thin films in gauges of less than 5&mgr;.
Metallized film capacitors offer the highest volumetric and gravimetric energy densities and reliability of all designs of film capacitors and offer higher pulse power capabilities than foil and other designs. Early film capacitors for high pulse power applications were of dielectric film/foil construction, impregnated with dielectric fluid that filled any voids between layers, and typically had energy densities of less than one J/cc. More recent improvements to these pulse power devices include use of metallized polymer films as

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