Why Specify Film/Foil Capacitors?
Capacitors using Polymer Film technology provide the following advantages when the following system issues become important:
- High Reliability
- High Pulse Currents
- Non-standard Values and/or Tight Tolerance
- Low Dielectric Absorption
- Extremely small capacitance change with applied voltage
- Capacitance Stability over Frequency
- Small variation in capacitance with temperature change
Film/Foil capacitors consist of two aluminum foils acting as the electrodes. These foil electrodes are separated by a polymer film dielectric. These materials are non-inductively wound to form the capacitor element. The wire leads are soldered directly to the aluminum foil electrode which extends out on both sides of the capacitor element.
A sufficient thickness of dielectric film is essential to withstand the applied electric field. Because the Foil electrodes are typically about 5um thick the dielectric is usually thicker than metallised film capacitors. These are the reasons why the physical dimensions of Film/Foil capacitors are larger than that of the metallized types.
The electrodes in metallized film capacitors are 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 element, the lead wire is soldered to this metal end spray. Metallized film capacitors are manufactured using a much thinner dielectric film than that typically used in the same rated Film/Foil design.
This is because of the self-healing characteristics of the metallized film. Self-healing removes a fault or short circuit in the dielectric film by the vaporization of the metal electrode surrounding the defect and isolating the area.
Film/Foil capacitor designs offers 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 a heat conductors, which transfers the 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.