Metallized film capacitors are capacitors made of organic plastic film as the medium, metallized film as the electrode, and made by winding (except for the laminated structure). The films used in metallized film capacitors include polyethylene, polyester Acrylic, polycarbonate, etc., in addition to the winding type, there are also laminated types. Among them, polyester film medium and polypropylene film medium are the most widely used.

effect
This type of metallized film capacitor has a so-called self-healing action, that is, if a small part of the electrode is short-circuited due to the fragility of the electrical interface, the electrode metal around the short-circuited part will be damaged. Due to the electrostatic energy or short-circuit current carried by the capacitor at that time, a larger area of melting and evaporation is caused to restore the insulation, so that the capacitor returns to the role of the capacitor again.

Disadvantages and Improvements
  From a theoretical analysis, there should be no short-circuit failure mode for metallized film capacitors, but many short-circuit failures will occur for metal foil capacitors (such as 27-pbxxxx-j0x series). Although metallized film capacitors have the above-mentioned huge advantages, compared with metal foil capacitors, they also have the following two disadvantages: First, the capacity stability is not as good as that of foil capacitors, because metallized capacitors are prone to capacity loss under long-term working conditions And self-healing can lead to a decrease in capacity, so if it is used in an oscillating circuit that requires high capacity stability, it is better to use a metal foil capacitor. Another major disadvantage is the poor ability to withstand large currents. This is because the metallized film layer is much thinner than the metal foil, and the ability to carry large currents is weak. In order to improve the shortcoming of metallized film capacitors, there are currently improved high-current metallized film capacitor products in the manufacturing process. The main improvement methods are:
1) Use double-sided metallized films as electrodes;
2) Increase the thickness of the metallized coating;
3) The metal welding process of the end face is improved to reduce the contact resistance.
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Application and Requirements
Metallized film capacitors have excellent electrical characteristics, high stability and long life, which can meet various applications. Capacitor manufacturers have been continuously improving this product to provide greater capacitance in a smaller package size. Capacitor manufacturers can optimize the characteristics of metallized film capacitors by selecting the appropriate dielectric for specific applications. For example, mylar shows good characteristics in general applications, with high dielectric constant (making it the highest capacitance per volume among metallized film capacitors), high dielectric strength, self-healing characteristics and good temperature stability sex. Polyester capacitors offer the best volumetric efficiency at a moderate cost of all types of film capacitors and are the most popular choice for DC applications such as decoupling, blocking, bypassing, and noise suppression. Capacitors made of metallized polypropylene film have low dielectric loss, high insulation resistance, low dielectric absorption and high dielectric strength. It is a durable and space-saving solution, and its long-term stability is also very good. good. These characteristics make metallized polypropylene film capacitors an important choice for applications such as AC input filters, electronic ballasts, and snubber circuits. Polypropylene film capacitors are available with rated voltages of 400vac or higher to meet the requirements of industrial three-phase applications and professional equipment. They can also be used in switching power supplies, frequency discrimination and filter circuits, as well as energy storage and sample and hold applications. Additionally, ac and pulse capacitors can be optimized for applications where steep pulses exist, such as electronic ballasts, motor controllers, switched mode power supplies (smps), crt televisions and monitors or buffers. These applications typically feature dual metallized polypropylene film construction with a low loss dielectric capable of withstanding high voltage and high pulse load applications at high frequencies. The principle of selecting film capacitors for product design is usually relatively simple. For example, the emi filter in the power line power supply product adopts a common topology, and it is easy to select the capacitance value. x capacitors placed between the mains and neutral phases, there is no theoretical upper limit, but typically between 0.1 microfarads and 1.0 microfarads. Y-capacitors placed between the mains or neutral and chassis ground need to be chosen as small a capacitance as possible to minimize leakage current to ground. For most designs, 4700pf is ideal. These devices must meet the safety and performance standards applicable to components connected to the mains, including ul 94 v-0, the European enec standard and en 132400, among others. Usually, one needs to select the appropriate capacitor technology and ensure that the package size and type meet the needs of the application. Capacitance stability includes sufficient self-restoring capacitance, which is a key performance criterion. After these requirements are met, special requirements, supply and logistics issues become the main issues to be considered.