Learn more about film capacitors

Film capacitors are made of a thin dielectric film which may or may not be metalized on one side. The film is extremely thin, with the thickness being under 1 µm. After the film is drawn to the desired thickness, the film is cut into ribbons. The width of the ribbons depends on the capacity of the capacitor being produced.

Film capacitors are designed with either a separate metal foil electrodes or with metalized electrodes. Metalized electrodes offer the advantage of decreased capacitor size and weight as well as self healing properties. The advantage provided by foil electrodes is they handle high peak currents. This is a quality preferred in snubber applications. There is little size difference between metalized and film-foil capacitors in lower capacitance values such as from 0.001uF to 0.01uF. Film foil capacitors generally have a lower cost than metalized film capacitors.

Power film capacitors are used in power electronics devices, phase shifters, X-ray flashes and pulsed lasers, while the low power variants are used as decoupling capacitors, filters and in A/D convertors.

Other notable applications are safety capacitors, electromagnetic interference suppression, fluorescent light ballasts and snubber capacitors. Lighting ballasts are used for proper starting and operation of fluorescent lights. When ballast is faulty, the light will flicker or fail to start properly. Older ballasts used only an inductor, a solution which provides a poor power factor. New designs use a switched power supply which relies on film capacitors for power factor correction.

Snubber capacitors are protective devices which damp or “snub” inductive kickback voltage spikes. These circuits often use film capacitors because of their low self-inductance, high peak current and low ESR, which are all critical factors in a snubber design. Polypropylene film capacitors are most often used in this type of circuit. Snubbers are used in many areas of electronics, especially power electronics in devices such as fly back DC-DC converters and others. Film capacitors can also be used in a more conventional way as voltage smoothing capacitors, in filters, audio crossovers. They can be used to store energy and release it in a high-current pulse when needed. High-current electrical pulses are used to power pulsed lasers or generate lighting discharges.

Film capacitors can also be used in a more conventional way as voltage smoothing capacitors, in filters, audio crossovers. They can be used to store energy and release it in a high-current pulse when needed. High-current electrical pulses are used to power pulsed lasers or generate lighting discharges.

The type of film used as the dielectric makes a difference in the capabilities of the capacitor. Here are some of the more popular dielectrics used in the construction of film capacitors:

Polyester:

Characterized by its high dielectric strength and being able to operate at temperature as high as 125ºC polyester capacitors have become the most commonly used style of film capacitor. Long term reliability and dependability as well as the low cost for this style have contributed to the popularity of this dielectric with electronic engineers.

Polypropylene:

This versatile film has very low dielectric absorption and high insulation resistance. It also has a low dissipation factor that is similar to polystyrene. It has a temperature rating of 105ºC and excellent stability qualities as well as resistance to severe environmental conditions.

Polycarbonate:

This dielectric was the standard for precision film capacitors. The fact that this material stopped being made in the year 2000 has been a design problem for many electrical engineers. Its temperature rating of 125ºC along with dielectric strength allowed it to be used in production when size and temperature are critical factors in the application. Despite it’s obsolesce it is still sought for use in small scale production because it works so well.

Polyphenylene Sulfide (PPS):

This is the most popular replacement for polycarbonate as a dielectric in film capacitors. It has a low dissipation factor and has a temperature rating of 150ºC. This film dielectric is most often used in surface mount or high temperature applications.

Polystyrene:

This dielectric like polycarbonate has been discontinued. It was very popular because it offers excellent long term stability and high insulation resistance. It has a temperature of 85ºC and was most often produced in wrap and fill construction. Again like polycarbonate capacitors it is still used when available in audio applications because of its superior Q factor.