Views:3 Author:Site Editor Publish Time: 2020-12-30 Origin:Site
The reason why film capacitors can be widely used and used by major manufacturers is that film capacitors have self-healing capabilities. There are two different mechanisms for the self-healing of film capacitors: one is discharge self-healing; the other is electrochemical self-healing. Discharge self-healing occurs at a higher voltage, so it is also called high-voltage self-healing; electrochemical self-healing also occurs under very low voltage, so it is called low-voltage self-healing.
1. Discharge self-healing
Suppose there is a flaw in the film between two metallized electrodes, and the resistance is R. According to the nature of the flaw, it may be a metallic flaw, it may be a semiconductor or poor-quality insulation flaw. When the defect is a metallic defect, under low voltage, the capacitor has already discharged and self-healed. It is the self-healing of high voltage discharge when semiconductor or poor-quality insulation defects occur.
The self-healing process of discharge is: after applying voltage V to the film capacitor, an ohmic current I=V/R passes through the defect. The current density J=V/Rπr2 flowing through the metalized electrode. That is, in the metalized electrode, the closer the area to the defect, the greater the current density. The Joule heat caused by the power consumption of the defect W=(V2/R)r is an exponential decrease in the resistance R of the semiconductor or insulating defect. Therefore, the current I and power consumption W increase rapidly. As a result, in the area where the metalized electrode is very close to the defect, the current density J1=J=V/πr12 rises sharply to the point where the Joule heat energy melts the metalized layer , Causing an arc between the electrodes here, the arc quickly evaporates and dissipates the molten metal there, forming an insulating isolation zone without a metal layer, the arc is extinguished, and self-healing is achieved.
2. Electrochemical self-healing
Generally under low voltage, film capacitors will have this self-healing situation. The mechanism of this self-healing is as follows: Assuming that there is a defect in the dielectric film of the metallized organic film capacitor, after voltage is applied to the capacitor, there will be a larger leakage current through the defect, which shows that the insulation resistance of the capacitor is much lower than The specified value in the technical conditions.
Because film capacitors have a certain water absorption rate, and in the process of capacitor manufacturing, storage and use, the capacitor may be damp, and the leakage current may contain ionic current or electronic current. Therefore, a considerable part of the ion current is the O2-ion and H-ion current generated by the electrolysis of water. After O2- ions reach the AL metallization anode, they combine with AL to form AL2O3. As time goes by, the AL2O3 insulation layer is gradually formed to cover and isolate the defects, thereby greatly improving the insulation resistance of the capacitor and achieving self-healing.
It should be noted that film capacitors need a certain amount of energy to complete self-healing. The energy has two sources: one is from the power source, and the other is from the exothermic reaction of oxidation and nitridation of the defective part of the metal.