Film Capacitor: also known as Plastic Film Capacitor. It uses plastic film as a dielectric.

Basic Information

Depending on the medium, there are many kinds of capacitors, such as: electrolyte capacitors, paper capacitors, film capacitors, ceramic capacitors, mica capacitors, air capacitors, etc. However, the most frequently used in audio equipment are electrolytic capacitors and Film capacitors. Electrolytic capacitors are mostly used in the need for large capacity, such as the main power part of the filter capacitor, in addition to filtering, and as a storage of electrical energy. Thin film capacitors are widely used in analog signal cross-linking, power supply noise bypass (reverse cross-linking) and other places.

Structure classification

Film capacitor is a kind of capacitor which is composed of metal foil as electrode, and plastic film such as polyethylene, polypropylene, polystyrene or polycarbonate, which is overlapped from both ends and wound into a cylindrical structure. According to the type of plastic film, it is also called polyethyl ester capacitor (also known as Mylar capacitor), polypropylene capacitor (also known as PP capacitor), polystyrene capacitor (also known as PS capacitor) and polycarbonate capacitor.

Typical diagram of a film capacitor

Basic Features

Film capacitor is a kind of excellent capacitor because of its many excellent characteristics. Its main characteristics are as follows: non-polarity, high insulation impedance, excellent frequency characteristics (wide frequency response), and small dielectric loss. Based on the above advantages, film capacitors are widely used in analog circuits. Especially in the signal interconnect part, must use the frequency characteristic good, medium loss very low capacitor, in order to ensure that the signal in the transmission, there is not too much distortion occurs.

Its structure and paper capacitor is the same, the medium is polyester or polystyrene. Polyester film capacitor, high dielectric constant, small volume, large capacity, good stability, suitable for bypass capacitor. Polystyrene film capacitor, dielectric loss is small, insulation resistance is high, but the temperature coefficient is large, can be used in high-frequency circuits.

Among all the plastic film capacitors, the characteristics of polypropylene (PP) capacitors and polystyrene (PS) capacitors are the most significant, of course, the price of these two capacitors is also relatively high. However, in recent years, audio equipment in order to improve the quality of sound, the use of parts of the material has been more and more advanced, the price is not the most important factor to consider, so in recent years PP capacitor and PS capacitor is used in the frequency and quantity of audio equipment is also more and more high. Readers can often see a certain brand of equipment, known as the number of a certain brand of PP capacitors or PS capacitors, in order to act as an endorsement on the sound quality, the truth is here.

Summary of characteristics: the capacity range of the film capacitor is 3pF-0.1μF, the DC working voltage is 63-500V, suitable for high frequency and low frequency, and the leakage resistance is greater than 10000Ω.

Basic parameters

Rated DC voltage: is the DC voltage that is allowed to be continuously applied over the entire temperature range.

Test voltage: the voltage applied to the capacitor before the factory form test, generally in 1.5~2 times, the duration of 2 minutes or 500 hours.

Dielectric strength: The voltage that the dielectric of the capacitor can withstand, which is higher than the test voltage.

Rated AC voltage: the effective value of AC voltage that the capacitor can continuously apply under AC voltage.

capacitor

Metallized film capacitor

The usual method of making film capacitors is to make aluminum and other metal foil as electrodes and plastic film after overlapping and winding together. But there is another way of making Film capacitors, called Metallized Film, which is made by vacuum-evaporating a thin layer of metal on a plastic film to act as an electrode. In this way, the thickness of the electrode foil can be omitted and the volume of the capacitor per unit capacity can be reduced, so the film capacitor is easier to make a small, large capacity capacitor. For example, the common MKP Capacitor is a synonym for Metailized Polypropylene Film Capacitor, and MKT is a synonym for Metailized Polyester capacitor.

The films used in metallized film capacitors are polyethyl ester, polypropylene, polycarbonate, etc., in addition to winding type, there are also stacked type. This type of capacitor has a so-called Self Healing Action, that is, assuming that a small part of the electrode is short-circuited due to the fragile electrical boundary, the electrode metal around the short-circuited part will be affected by the electrostatic energy or short-circuit current carried by the capacitor at that time. And cause a larger area of melting and evaporation to restore insulation, so that the capacitor again to restore the role of the capacitor.

Characteristics of metallized film capacitors

Metallized film capacitor is to evaporate a layer of metal film on the surface of polyester film instead of metal foil as the electrode, because the thickness of the metallized film layer is far less than the thickness of the metal foil, so the volume is much smaller than the volume of the metal foil capacitor after winding. The biggest advantage of metallized film capacitors is the "self-healing" property. The so-called self-healing characteristic is that if the film medium has a defect at a certain point and a breakdown short circuit under the action of overvoltage, and the metalized layer at the breakdown point can melt and evaporate momentarily under the action of the arc to form a small metal-free area, so that the two pole pieces of the capacitor can re-insulate each other and still continue to work, so that the reliability of the capacitor is greatly improved. From the principle analysis, there should be no short-circuit failure mode of metallized film capacitors, while there will be many short-circuit failures of metal foil capacitors. Although the metallized film capacitor has the above great advantages, compared with the metal foil capacitor, it also has the following two disadvantages:

First, the capacity stability is not as good as the foil capacitor, which is due to the metallized capacitor in long-term working conditions prone to capacity loss and self-healing can lead to capacity reduction, so such as in the oscillating circuit with high capacity stability requirements, should choose the metal foil capacitor better.

Another major drawback is the poor ability to withstand high current, which is because the metallized film layer is much thinner than the metal foil, and the ability to carry high current is weak. In order to improve this shortcoming of metallized film capacitor, there are improved high-current metallized film capacitor products in the manufacturing process. The main improvement ways are as follows:

1. Use double-sided metallized film as electrode;

2. Increase the thickness of the metallized coating;

3. Improve the end metal welding process to reduce the contact resistance.

Application status

Thin film capacitors are mainly used in many industries such as electronics, home appliances, communications, electric power, electrified railway, hybrid electric vehicle, wind power generation, solar power generation, etc. The stable development of these industries has promoted the growth of the thin film capacitor market.

With the development of technology, the upgrading cycle of electronics, home appliances, communications and other industries is becoming shorter and shorter, and the film capacitor has become an indispensable electronic component to promote the upgrading of the above industries by virtue of its good electrical performance and high reliability. In the next few years, with the further development of digitalization, information technology and network construction, the country will increase investment in power grid construction, electrified railway construction, energy-saving lighting, hybrid electric vehicles and other aspects, as well as the upgrading of consumer electronic products, the market demand for thin film capacitors will further show a trend of rapid growth.