Also known as plastic film, polymer film or film dielectric, film capacitors are some of the most versatile and widely used components around.
This article is part of The engineer’s complete guide to capacitors. If you’re unsure of what type of capacitor is best for your circuit, read How to choose the right capacitor for any application.
What is a film capacitor?
Film capacitors are used in many applications because of their stability, low inductance and low cost. They can also tolerate overvoltage surges. There are several types of film capacitors including polyester film, metallized film, polypropylene film, polycarbonate film, polytetrafluoroethylene (PTFE, sometimes branded as Teflon) film and polystyrene film.
How film capacitors work
Like all capacitors, metallized film capacitors incorporate metal plates separated by a dielectric. Film capacitors are also known as plastic film, polymer film, or film dielectric capacitors. Film capacitors are inexpensive and come with a nearly limitless shelf life. The film capacitor uses a thin dielectric material with the other side of the capacitor metalized. Depending on the application, the film capacitor is rolled into thin films. The general voltage range of these capacitors is from 50 V to 2 kV.
Metallized film capacitors are not affected strongly by DC bias. Their volumetric efficiency is not as great as that for multilayer ceramic chip (MLCC) capacitors or electrolytic capacitors. These capacitors (as well as ceramics) are used in safety applications for EMI/RFI reduction and safe failure modes.
Capacitor self-healing
Self-healing is the ability of a metallized capacitor to clear a fault area where a momentary short occurs due to dielectric breakdown under an over-voltage event.
Metallized film capacitors have self-healing properties, while discrete foil electrode capacitors do not. Polypropylene film/foil capacitors are commonly used as snubber capacitors in low pulse applications. In comparison, polypropylene metallized film capacitors and double-sided metallized film capacitors have a self-healing property, and they are suitable for use in low pulse and medium pulse applications. These two types of capacitors are suitable for protecting various switching devices including thyristors, FETs and IGBT modules.
Applications of film capacitors
Metallized film capacitors find their way into a wide variety of applications, with different dielectrics suited to different use cases as detailed in the table below:
|
General classification |
Dielectric |
Applications |
|
General purpose |
Metallized polyester |
DC blocking, AC coupling, bypassing, energy reservoirs
|
|
Interference suppression (safety capacitors, Class X1 and Y1 for industrial applications and class X2 and Y2 for domestic appliances) |
Metallized polypropylene |
Switched mode power supplies (SMPs), EMI filters, e-ballast (capacitors used for power factor correction to counter the choke inductance), domestic appliances
|
|
AC and pulse |
Double-sided metallized polypropylene |
High frequency, high current, high pulse |
|
|
Metallized polyphenylene sulfide (PPS) |
High current, high temperature |
|
Motor run capacitors |
Metallized polypropylene |
Motor run |
Film capacitors are used in electromagnetic interference (EMI) suppression and as safety capacitors (Classes X and Y). While ceramic capacitors offer better dv/dt capabilities, film capacitors are good (with a maximum value of 2200 V/µs) making them suited for use in snubber circuits. Film capacitors also have low equivalent series resistance (ESR), low equivalent self-inductance (ESL) and can tolerate large peak currents. Snubber capacitors in series with a small resistor are placed across solid-state switching devices. Snubbers protect the devices from voltage spikes and reduce the rate of rise (dv/dt) across them. A large dv/dt across a solid-state switch can cause it to turn on and power control will be lost. Snubbers are used widely in power electronics applications.
In fluorescent light ballasts, film capacitors are used to provide power factor correction to counter the choke inductance. Lighting ballasts are employed to provide proper operating conditions (including startup) of fluorescent lights. Older ballasts used an inductor (choke) but provided a poor power factor. Current ballast designs employ switched power supplies that rely on film capacitors to improve the power factor.
Power film capacitors are used in radar, pulsed laser, defibrillator and x-ray equipment. Low-power applications of film capacitors include coupling, decoupling, bypassing and filtering. In high power applications, power film capacitors can be rated to handle thousands of volts.
Polystyrene is an important metal film capacitor. It has a low dielectric absorption (DA) characteristic which makes it a great choice for sample-and-hold and peak detector applications. Polycarbonate capacitors provide a wide temperature range of operation (-55 °C to 125 °C).
Alternatives to film capacitors
Film capacitors are interchangeable with Class 1 (NPO/COG) ceramic capacitors in some applications (see the comparison table in The engineer’s guide to ceramic capacitors). Power film capacitors are large and unique and cannot be replaced with ceramic devices.
