Why Specify Film/Foil Capacitors?
Multi-layer ceramic, aluminum electrolytic and tantalum capacitors achieve high capacitance per unit volume and are an obvious choice for certain applications. However, the electrical performance trade-off associated with the size efficiency of these capacitors make Film capacitors attractive when the following issues are important:
- High Reliability
- Extreme Pulse Currents
- High AC Voltage
- Non-standard Values and/or Tight Tolerance
- Low Dielectric Absorption
- Extremely small capacitance change with applied voltage
- Capacitance Stability over Frequency
- Small capacitance change with temperature change
Differences between Film/Foil and Metallized Film Capacitors?
Film/Foil capacitors consist of two plates of aluminum foil, which act as the electrodes, separated by a film used as the dielectric. These materials are non-inductively wound to form the capacitor section. The wire leads are welded directly to the aluminum foil which extends out on both sides of the capacitor section, hence the term extended foil design (see Metallized film capacitor illustrations).
In order to provide the utmost in dependability a sufficient thickness of dielectric film is essential. This construction criteria is the primary reason why the physical dimensions of Film/Foil capacitors are larger than that of the metallized types.
In contrast to the Film/Foil capacitor, where the electrode is an independent layer of aluminum foil, the electrode for the metallized film capacitor is an extremely thin layer of metal which is vacuum-deposited directly onto the dielectric film. The wire lead is connected to the electrodes by means of a metal spray applied to each end of the capacitor section; the lead wire is welded to this metal end spray (see Metallized film capacitor illustrations).
Metallized film capacitors are manufactured using a thinner dielectric film than that which is used in the same rated Film/Foil design.
This is possible due to the self-healing characteristics of the metallized film. Self-healing, or clearing, removes a fault or short circuit in the dielectric film by vaporizing the metal electrode surrounding the defect and isolating the area.
Advantages and Benefits of Film/Foil versus Metallized
Several advantages the Film/Foil capacitor design offers over metallized are a higher insulation resistance, better capacitance stability, high current carrying capabilities for pulse applications (high dV/dt capability) and a lower dissipation factor. The excellent heat dissipation of the Film/Foil design is a result of the metal foil electrodes acting as heat sinks, which draw heat out from the interior of the unit. This superior heat dissipation allows for a higher voltage application, in comparison to the metallized, at the same frequency. The extended foil design also allows for operation at higher peak currents and faster duty cycles.
In addition, the type 715P and 716P units rated 800 Volts DC and above, the 715P/717P High Voltage Orange Drop® and the 778P/779P 400 VAC units incorporate a series-section design which utilizes a metallized polypropylene common (see Metallized film capacitor illustrations). Therefore you obtain the benefits of an extended foil design plus the self-healing characteristics of the metallized.
Film/Foil characteristics and applications