An Introduction to Film Capacitors
Film capacitors, also known as film caps, are electrical capacitors that feature an insulating plastic film as a dielectric, sometimes combined with paper as a carrier of the electrodes. Depending on the desired dielectric strength, the dielectric films are drawn in a special process to an extremely thin thickness and are then provided with electrodes. Film capacitor electrodes can be metallized aluminum or zinc applied directly to the film’s surface, or a separate metallic foil. Two conductive layers are wound into a cylindrical winding, typically flattened to reduce mounting space requirements on a printed circuit board, or layered to form a capacitor body.
Film capacitors, along with ceramic and electrolytic capacitors, are the most common capacitor types used in electronic equipment.
A similar component is the power capacitor. While the materials and construction techniques are similar to those of standard film capacitors, power capacitors are often classified separately due to their high power ratings for applications within power systems and electrical installations. As modern electronic equipment has developed the ability to handle power levels that were once exclusive to electrical power components, the distinction between electronic and electrical power has become less clear. Previously, the defining factor was a reactive power of 200 volt-amperes, but average modern power electronics are able to handle much greater power levels.
Film capacitors are constructed from two pieces of plastic film covered with metallic electrodes and wound into a cylindrical winding with terminals attached and then encapsulated. Generally, film capacitors are not polarized, meaning the terminals are not interchangeable. There are two types of plastic film capacitors with different electrode configurations: film/foil capacitors and metallized film capacitors. film/foil capacitors, also known as metal foil capacitors, are made with two plastic films as the dielectric. Each film is layered with aluminum or another thin metal foil as the electrodes. The advantages of this type are the easy electrical connection to the metal foil electrodes and their ability to handle very high current surges.
Metallized film capacitors, on the other hand, are made of two metallized films with a plastic film as the dielectric. An extremely thin vacuum-deposited aluminum metallization of approximately 0.03 micrometers (one millionth of a meter) is applied to one or both sides to serve as electrodes. Capacitors of this configuration are self-healing, in the sense that the dielectric breakdowns or short circuits between the electrodes do not necessarily lead to destruction of the component. With this design, it is possible to make zero defect capacitors and other high quality products, as well as being possible to produce wound capacitors with larger capacitance values in smaller cases.
Perhaps the greatest advantage of modern film capacitors is their direct contact to the electrodes on both ends of the winding, which ensures that all current paths to the entire electrode are very short. This configuration acts similarly to a large number of separate capacitors connected in parallel, therefore reducing internal ohmic losses and parasitic inductance. The inherent structure of a film capacitor results in very low ohmic losses and a low parasitic inductance, making them ideal for applications with high surge currents, AC power applications, and applications at high frequencies.
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