Consider details such as RMS current, pulse rating, ripple
current, package, mounting, connectors.
Plastic Film Capacitors employ a thin film-
polystyrene, polyester, polycarbonate, polypropylene as the
dielectric. Both film and foil and metallised film
constructions are used. In a film and foil capacitor, plastic
film is interleaved with aluminium foil. Metallised film
versions are made by vacuum depositing a thin metal film on
the plastic film. The thin metal results in both high
volumetric efficiency and self healing. In metallised plastic
units an arc can rapidly vaporize the thin metal in the
immediate vicinity of the breakdown point and clear the short.
The arc then extinguishes, and the capacitor becomes open
circuited again or self healed. Plastic film capacitors are
non-polar and excellent for AC application. They have high
insulation resistance, low DF and have replaced paper
capacitors in most applications.They have a higher volumetric
efficiency than paper, mica or ceramic units.
Film/foil capacitors are generally bigger in size
compared to metallised versions, but have a higher current and
pulse ratings. Their failure mode is short circuit and not to
be used in across the line applications.
Metallised film capacitors are smaller in size
compared to film/foil versions, but have lower pulse rating.
The failure mode is open circuit. There are various special
constructions combining the higher pulse ratings of foil
types and self healing properties of metallised capacitors.
These are double metallised, series, and series metallised
/ foil constructions.
Polyester (mylar) capacitors are very popular,
because they are inexpensive. Even though most of their
characteristics are very good, capacitance varies drastically
with temperature about as badly as aluminium electrolytes.
Polycarbonate capacitors have a higher temperature
range up to 125 degree C, low tan delta and a very low change
of capacitance with temperature.
Polyphenylene Sulphide (PPS) capacitors have a higher
temperature range upto 150 degree C, low tan delta and a very
low change of capacitance with temperature and presently being
offered as replacement for polycarbonate capacitors
Polypropylene capacitors also have a very low tan
delta and a good stability but lower operating temperature
range upto 100 degree C and are ideal for AC applications,
high current and high dv/dt applications.
Aluminium Electrolytics are widely used because of
their low cost and high Capacitance X voltage (CV) product for
a given physical volume. Electrolytics are highly sensitive to
temperature, and have a limited operations and shelf life.
With time, the Dissipation Factor can rise as much as 50% and
the capacitance can drop substantially-to 10% rated.
Tantalum Electrolytics have a higher CV
product per unit volume than the aluminum electrolytics, are
more temperature-stable, and usually have hermetic seals to
eliminate humidity effects. Both their shelf and operating
lives are superior to aluminium electrolytics. But tantalum
capacitors are several times more expensive than aluminium
electrolytics. Tantalum capacitors can be obtained as either
polar or non-polar types.
Mica Capacitors use the natural mineral Mica as a
dielectric. They are very reliable and stable and particularly
good for high frequency work. Mica, a very stable material
provides high Q, but capacitance values generally do not go
above 0.1 mfd because the units become bulky and expensive.
Ceramic capacitors can be used at high frequencies
to 1000 MHz but do not have Micas stability or Q. They are
used in bypass circuits, decoupling and EMI/RFI filters.
