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.