Film Capacitors
Film capacitors are mostly
used in high-performance applications. Polycarbonate, polyester, and
polypropylene have been the "big three" of film capacitors. They are the
only ones that most film capacitor makers make, although polyphenylene
sulphide (PPS) is on the rise. For the lower volume capacitors, I have listed
known manufacturers. This does not mean they actually stock parts, but at
least they have the capability to make them if they want to.
Polycarbonate:
Polycarbonate has a fairly low temperature drift (lower than most films),
dissipation factor, and dielectric absorption. It can be used in timing
circuits, although C0G ceramics are a better choice for small sizes. It is
suitable for some pulse applications, and for some precision analog
applications, especially if you need its good temperature stability and
relatively high temperature rating. Moisture absorption is high compared to
most other film dielectrics, a problem for some critical applications. Good
heat resistance, to 125C, but not good enough to be found in surface-mount
packages. Some manufacturers recommend it for automotive applications.
Unfortunately,
polycarbonate is about to go the way of polystyrene. The last manufacturer of
capacitor-grade film, Bayer AG, says they will cease production at the end of
2000, but may have a substitute material. A number of manufacturers are
making recommendations for polycarbonate replacements (some of which should
not be taken seriously).
Evox-Rifa
for one, recommends polyphenylene sulfide, and this is probably the best
choice for most applications. Polycarbonate has a slight edge in
high-temperature leakage, but PPS is as good or better in most other
parameters. PPS´s only weaknesses are a higher high-temperature temperature
drift and higher DF at high temperature, >100C. Its K is about the same as
PC, so PPS caps will be about the same size. Polypropylene may also be
considered for applications where very low leakage is important, but high
temperature is not a issue, although its temperature drift is not as good as
PC or PPS. Polystyrene has had availability long after the film ceased
production, but this may not happen with polycarbonate. Unlike polystyrene,
polycarbonate has few significant advantages over replacement dielectrics.
Polyester:
Polyester is probably the most popular of the film capacitors, at least
for board-level applications. Actually, polyester is a generic term for a
class of similar polymers, the one used in polyester capacitors being
polyethylene terephthalate. Dupont's trade name is Mylar, some people call it
PET, PETE, or PETP just to be confusing (and that doesn´t even include the
various European trade names for it). It doesn't really do anything
particularly well but low cost, small size and the ability to do many things
well enough makes it a good choice for many noncritical applications. High
dissipation factor means it is best used in DC or relatively
low-frequency/low-current pulse and AC power applications. Poor temperature
drift, dielectric absorption, and leakage relegate it to non-critical analog
circuit applications. Typical applications would be where you want a
capacitor larger than a C0G but better electrical properties than an X7R.
Polyester capacitors can typically be found in values from 0.01 uF through at
least 10 uF and beyond. Polyester has a high temperature drift but can be
found layered with polypropylene to flatten the temperature curve (the two go
in opposite directions). Polyester capacitors are available to 125C. Good
heat resistance allows polyester capacitors to be made in surface-mount style.
Polypropylene:
Polypropylene (PP) capacitors have a lot going for them. They are available
in a wide range of sizes and voltages, and are used in a wide variety of
circuits. PP has a very low dissipation factor over it´s entire temperature
range and over a wide frequency range. This makes polypropylene capacitors
popular for high-frequency, high-current applications like switching power
supplies. Large film, film-oil, and paper-oil-film types are found in
power-line applications like power-factor correction. These can have
operating-voltage ratin
gs
>400 kV AC. Polypropylene motor-starter, motor-run, and SCR snubbers are
replacing older electrolytic and paper-oil types which all have much higher
dissipation factors. Sizes for these run into the 10s of uF and >500 VAC.
While PPs very low dissipation factor has made it the only viable
material for many high-power AC applications, its self-healing properties,
critical for reliable high voltage operation, are only fair. When an
arc-through of the dielectric occurs, PP tends to leave more carbon at the
site than polyester. Also, the arc is more quickly extinguished if the gas
pressure at the failure site is as high as possible. This requires that the
dielectric is as strong and heat resistant as possible. Polyester is better
in this regard as well. Oil impregnation helps make up for these deficiencies
however.
The small through-hole styles are
available in the usual range of sizes, roughly 100 pF-10 uF. Low leakage and
low dielectric absorption make small polypropylene capacitors suitable for
integrators and sample-and-hold circuits. Moisture absorption is negligible.
Only its higher temperature drift makes it inferior to polystyrene.
Polypropylene has limited heat resistance (to 105＜
C),
and is not found in surface mount.
Polystyrene:
Polystyrene (PS), (the Europeans often call it "styroflex") has long been
the material of choice for critical analog circuits. Low leakage, low
dielectric absorption and a shallow, flat temperature curve makes these
capacitors suitable for timing circuits, filters, integrators, and
sample-and-hold circuits. Moisture absorption is very low. Size, cost,
availability, and temperature range limitations make polystyrene unsuitable
for most other applications. Heat resistance is limited to about 85C, so
forget surface mount. They can be damaged by soldering and by chlorinated
board cleaning solvents. I don´t believe I have seen them in metallized film,
only in film-foil.
Because of the poor heat resistance, polystyrene has largely been replaced
by polypropylene and C0G ceramics, and the capacitor-grade film is no longer
being made. There is several year´s supply still available, and they are
still being sold, but be careful using them in new applications. Some
manufacturers have noted that polystyrene caps are "not for new designs". One
company, ITW Paktron, makes polypropylene capacitors with a guaranteed
temperature drift similar to (if not quite as good as) polystyrene. PS's
other electrical properties are mostly very similar to PP.
Whether
polystyrene capacitors will really go away any time soon is not certain. I
have been warned of polystyrene´s demise "in a few years" for at least a
decade now, and yet it is still available. There almost seems to be vast
supplys of the film stashed away in manufacturers back rooms. It may also be
that declining usage will extend available stocks for many years to come. I
imagine that many of polystyrene´s traditional applications are fading away as
newer technologies take over.
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