The internal structure of film capacitors

Calculated according to the capacitance formula, the size of the capacitance is not only related to the specifications of the capacitor, but also related to the thermal conductivity (Permittivity) of the dielectric. The performance of the dielectric affects the performance of the capacitor, and different media require different manufacturing processes.

For performance comparison of common media, you can refer to a technical document of AVX.

AVX Dielectric Comparison Chart (tips: Use Bing search, top one)

The manufacturing process of capacitors can be mainly divided into three categories:

Film capacitor

Electrolytic capacitor-ceramic capacitor

Film capacitor

Film Capacitor is generally translated into Chinese as film capacitor in China, but its processing technology is different from Thin Film. For the sake of distinction, I think it is better to translate it directly into Chinese as film capacitor. Film capacitors are made by winding two plastic films containing metal electrodes into a cylindrical shape and finally packaged. Because the medium is usually plastic, it is also called a plastic film capacitor; its internal structure is roughly as shown in the figure below:

Original image from Wikipedia

Film capacitors can be divided into two major categories based on the processing technology of their electrodes:

Metal material foil film capacitor (Film/Foil)

Metal foil film capacitors directly add a layer of thin metal foil, usually aluminum and platinum, on the plastic film as an electrode; this process is relatively simple, the electrode is easy to draw out, and can be used in high current situations.

Metallized Film Capacitor (Metallized Film)

Metallized film capacitors use the vacuum deposition (Vacuum Deposited) process to directly form a thin metal layer on the surface of the plastic film as an electrode;Because the electrode is too thin, it can be wound into a capacitor with a larger space; but because the electrode is too thin, it is only suitable for small current situations.

Metallized film capacitors have the function of self-repair, that is, if there is a penetration damage point inside the capacitor, an avalanche effect will be caused at the damage point. The vaporized metal material will form a vaporization interface at the damage point, the short circuit fault will disappear, and the damage point will be repair;Therefore, metallized film capacitors have high reliability and will not be ineffective due to short circuit faults;There are two winding methods for film capacitors: the post-winding method is before the winding machine, and the wires are already connected to the electrodes;In the non-sensing winding method, electroplating and other processing techniques are used after the coil inductance to connect the internal electrodes of the two inner holes into one surface. In this way, a smaller ESL can be obtained, and the high-frequency performance should be higher;In addition, there is also a laminated type of non-sensing capacitor, which has a similar structure to MLCC and has good performance, which is conducive to SMD packaging.

The original dielectric material of film capacitors was paper soaked in oil or paraffin, invented by American D'Fitzvaldes in 1876; the working frequency was very high.Nowadays, plastic materials are used, that is, polymers. According to the different media materials, there are mainly the following categories:

The most commonly used film capacitor is the polyester film capacitor, which is more cost-effective because its thermal conductivity is high and the specifications can be smaller; the second is the polypropylene film capacitor.Other materials include polytetrafluoroethylene, polyethylene, polycarbonate, etc.