The film capacitor is also called a plastic film capacitor. It uses plastic film as the dielectric. Depending on the medium, there are many types of capacitors, such as electrolyte capacitors, paper capacitors, film capacitors, ceramic capacitors, mica capacitors, and air capacitors. Film capacitors are widely used in analog signal cross-linking, power supply noise bypass (reverse cross-linking), and other places.
Basic structure
The internal structure of film capacitors is mainly as follows: metal foil (or a foil obtained by metalizing plastic) is used as the electrode plate, and plastic is used as the dielectric. Obtained by winding or stacking process. The different arrangements of foils and films lead to a variety of construction methods. The figure below is a typical schematic diagram of thin-film capacitors.
Typical scehmatic diagram of thin-film capacitors
The film capacitor is a capacitor with a metal foil as an electrode, and a plastic film such as polyethylene, polypropylene, polystyrene, or polycarbonate, which is overlapped from both ends and wound into a cylindrical structure. And according to the type of plastic film, they are also called polyethylene capacitors (also known as Mylar capacitors), polypropylene capacitors (also known as PP capacitors), polystyrene capacitors (also known as PS capacitor), and polycarbonate capacitors.
Basic characteristics
The film capacitor has many excellent characteristics. Its main equivalences are as follows: non-polarity, high insulation resistance, excellent frequency characteristics (wide frequency response), and low dielectric loss. Based on the above advantages, film capacitors are widely used in analog circuits. Especially in the part where the signal is connected, capacitors with good frequency characteristics and extremely low dielectric loss must be used to ensure that the signal is transmitted without too much distortion. Among all plastic film capacitors, polypropylene (PP) capacitors and polystyrene (PS) capacitors have the most remarkable characteristics.
Its structure is the same as a paper dielectric capacitor, and the medium is polyester or polystyrene. Polyester film capacitors have a high dielectric constant, small size, large capacity, and good stability. They are suitable as bypass capacitors. Polystyrene film capacitors have a low dielectric loss and high insulation resistance, but have large temperature coefficients and can be used in high-frequency circuits.
Among all the plastic film capacitors, polypropylene (PP) capacitors and polystyrene (PS) capacitors have the most remarkable characteristics. Of course, the prices of these two capacitors are relatively high. However, in recent years, in order to improve the sound quality of audio equipment, the parts materials used have become more and more advanced, and the price is not the most important consideration. In recent years, the frequency and number of PP capacitors and PS capacitors used in audio equipment have also increased. Readers can often see the equipment of the XYZ brand, claiming how many PP-type capacitors or PS-quality capacitors of XYZ brand are used as an endorsement in sound quality, the reason 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 10000 Ω.
Metallized film capacitors
The usual film capacitors are manufactured by overlapping and winding metal foils such as aluminum which is used as electrodes and plastic films together. But there is another manufacturing method for film capacitors, called Metallized Film. Its manufacturing method is to vacuum-deposit a thin layer of metal on the plastic film as an electrode. In this way, the thickness of the electrode foil can be omitted, and the volume per unit capacity of the capacitor can be reduced. Therefore, it is easier to make a thin film capacitor into a capacitor with a small capacity and a large capacity. For example, the common MKP capacitor is the name of Metalized Polypropylene Film Capacitor, while MKT is the name of Metalized Polyester Capacitor.
The films used for metalized film capacitors include polyethylene, polypropylene, polycarbonate, etc. In addition to the wound type, there are also laminated types. Capacitors of this type of metalized film have a so-called self-healing action, that is, if the small part of the electrode is short-circuited due to the weak electrical quality, due to the electrostatic energy or short-circuit current carried by the capacitor at that time, a larger area of melting and evaporation was induced to restore the insulation, and the capacitor was restored to the role of the capacitor again.
Characteristics of metallized film capacitors
The metallized film capacitor is to deposit a metal film on the surface of the polyester film instead of metal foil as the electrode. Because the thickness of the metallized film layer is much smaller than the thickness of the metal foil, the volume after winding is also much smaller than the volume of the metal foil capacitor. The biggest advantage of metalized film capacitors is the "self-healing". The so-called self-healing property is that if the thin film medium has defects at a certain point and a breakdown short circuit occurs under the action of overvoltage, the metallization layer of the breakdown point can melt and evaporate instantly under the action of the arc to form a small metal-free zone. The two-pole pieces of the capacitor are re-insulated from each other and can still continue to work, thus greatly improving the reliability of the capacitor. From the principle analysis, there should be no short-circuit failure mode for metallized film capacitors, and metal foil capacitors will have many short-circuit failure phenomena. Although metallized film capacitors have the above-mentioned huge advantages, compared with metal foil capacitors, they also have the following two disadvantages:
First, capacity stability is not as good as foil capacitors. This is because metallized capacitors are prone to capacity loss and self-healing in long-term working conditions, which can lead to capacity reduction. Therefore, if used in an oscillation circuit that requires high capacity stability, Metal foil capacitors should be used better.
Another major disadvantage is its poor ability to withstand large currents. This is because the metallized film is much thinner than the metal foil, and its ability to carry large currents is weak. In order to improve the metallized film capacitors, the main improvement methods of capacitor products are: Use double-sided metallized film as an electrode; Increase the thickness of the metallized coating; Improved metal welding process on the end face to reduce contact resistance.
In summary, the film capacitors are known for their low leakage current, high insulation resistance, low dissipation factor, and high voltage rating. They are also known for their ability to withstand high temperatures, making them suitable for use in harsh environments.Therefore, film capacitors are essential components in electronic circuits, and their properties make them suitable for various applications.
