It’s a film capacitor structure geometry that results in low ohmic losses and very low parasitic inductance. These features specifically make them desirable for applications with very high surge currents and high frequencies.
Simply, film capacitors are created using two pieces of plastic film covered with metallic electrodes and wound into a cylindrical shape. Terminals are attached and enclosed. There are two different types of plastic film capacitors. The first, a metal foil capacitor, features two plastic films as the dielectric. Each of the two films is layered with a thin metal foil as the electrodes. This type of film capacitor is able to handle high current surges.
The second variety of film capacitors are metalized film capacitors, which uses two metalized films and the plastic film is the dielectric. This variety has self-healing properties and they are used in high-quality products including zero defect capacitors, especially where larger capacitance values of up to 100 µF and larger are necessary. They also are more efficient, but they have a more limited current surge capability.
Figure 1: Cross-section of a plastic film capacitor. (Source: Wikipedia)
The self-healing properties of plastic film capacitors are showcased in the Vishay F1778433K2FBB0 interference suppression plastic film capacitor. This device features a 7.5 mm to 27.5 mm lead pitch, and can be used for temperatures up to 110°C. The capacitance range is 0.001 μF to 4.7 μF and capacitance tolerance is ± 20 percent; ± 10 percent; (with ± 5 percent available).
Figure 2: Plastic film capacitors potted in rectangular casings or dipped in epoxy [shown in red]. (Source: Wikipedia)
In comparison, the KEMET R73U10220DQ03J film-foil polypropylene capacitor is designed for such high-current applications as deflection circuits in TV-sets (fly-back tuning), switching spikes suppression in SMPS, SNUBBER and SCR commutating circuits, switching circuits in electronic ballasts and applications with high voltage and very high currents. The box material is solvent-resistant and flame-retardant according to the UL94 V0 flammability standard. They have an operating temperature range of -55 °C to +105 °C.
There are many types of plastics used for plastic film capacitors, including:
- Polypropylene (PP)
- Polyethylene Terephthalate Polyester (PET)
- Polyphenylene Sulfide (PPS)
- Polyethylene Naphthalate (PEN)
- Polycarbonate (PP)
- Polystyrene (PS)
- Polytetrafluoroethylene (PTFE)
When compared with ceramic and electrolytic capacitors, film capacitors are used in many general purpose and industrial applications in electronic equipment. They offer very low ESR and ESL values. And although they are large, they have a high surge and load capacity. In addition, since they are not polarized, they can be used in AC voltage applications.
Yet another example of a plastic film capacitors are the PHE426HF7470KR045CL2 series by KEMET. The single metalized film pulse capacitor is used in pulse operation in SMPS, TV, monitor, electrical ballast and high-frequency uses that need stable operation. The polypropylene capacitor features vacuum-evaporated aluminum electrodes. Radial leads of tinned wire are electrically welded to the contact metal layer on the ends of the capacitor winding. They are encapsulated in self-extinguishing material.
The long life expectancy for film capacitors is typically specified in terms of applied voltage, current load, and temperature. The parts are common and the selection of type is based on application.