Film capacitors are essential electrostatic capacitors suitable for medium, higher voltage and higher current circuits. Unlike most other dielectric systems, film capacitors feature low loss factor at very low temperature.
Dielectric constant is not big, but they feature very high dielectric strength. In combination with long life and self-healing aging capabilities it makes them ideal choice for high voltage, high power systems. While we focus on the most common dielectric types there is a wide list of organic dielectric materials with different features. Overview can be seen in the article: What is a Dielectric Constant of Plastic Materials ?
The common European abbreviations for polypropylene capacitors is PP or KP for film/foil and MKP for metallized film.
Polypropylene (PP) is from a molecular point of view a non-polar dielectric with small losses and a relatively straight and moderate TC. Since the smallest film thickness is approx. 3.5 μm (0.14 mils) and εr ≈ 2.3 the capacitor can not come down to those sizes characterizing PET at low rated voltages. But remaining good characteristics in many applications have brought up PP as a replacement for polycarbonate (PC) and polystyrene (PS), not least as a precision capacitor. PP exists in both foil and metallized design and is adopted for both AC, pulse and transient suppression (X-capacitor) applications. The pulse and X-capacitor designs, however, require a specific metallizing technology.
The MKP design had from the beginning problems with the adhesion between the metallized layer and the plastic film. This problem characterizes non-polar dielectrics consisting of molecules and has caused many problems at, for example, gluing of components to circuit boards. The plastic surface film has to be raised and roughened which among other things can be done by etching, flame exposure, electron irradiation or corona. Today the MKP design is well established and the adhesion problems a passed period.
The demand for filtering / interference suppression of thyristor generated noise voltages has brought forth the same type of large capacitors as shown for PET but here the low ESR losses allow quite different r.m.s. currents, for example at high frequencies.
The recommended absolute maximum temperature is +105°C. We recommend max +85°C with the remark that developments of new films is going on to offer temperature range above +105°C as a maximum operating temperature. Current temperature limitations still make PP difficult for chip designs.
Temperature and frequency dependencies
The frequency dependence of capacitance for PP capacitors is moderate. In Figure 29. the broken part of the curve indicates an increase of capacitance. This increase is not physical but depends on influence from series capacitance measurements.
Note in Figure 30. the outstanding low loss factor for PP over the whole temperature range. The curve area in Figure 31. represents capacitance up to some hundreds of nF. The higher capacitance, the greater losses.
Designs with foil or hermetic seal have somewhat higher IR than corresponding metallized and plastic encapsulated types.
Any characteristic failure mode just for PP capacitors is not distinguishable. Here what was said about PET usually applies.