Analysis of self-healing properties of film capacitors

Film capacitors can be widely used and used by major manufacturers because of their self-healing capabilities. There are two different mechanisms for self-healing of film capacitors: one is self-healing by discharge; the other is electrochemical self-healing. Discharge self-healing occurs at a higher voltage, so it is also called high-pressure self-healing; electrochemical self-healing also occurs at a very low voltage, so it is called low-pressure self-healing.

Self-healing of discharge

Suppose there is a defect in the film between the two metallized electrodes and the resistance is R. Depending on the nature of the defect, it may be a metallic defect or a semiconductor or inferior insulating defect. When the defect is a metallic defect, the capacitor has self-healing at a low voltage. High-voltage discharge self-healing occurs only when semiconductors or inferior insulating defects occur.

The process of self-healing of discharge is: after a voltage V is applied to the film capacitor, an ohmic current I=V/R passes through the defect. The current density flowing through the metallized electrode is J = V / Rπr2. That is, in the metallized electrode, the closer the region is to the defect, the higher the current density. Due to the Joule heat caused by the defect power consumption W = (V2 / R) r, the resistance R of the semiconductor or insulating defect is exponentially decreased. Therefore, the current I and the power consumption W increase rapidly. As a result, in the region where the metallization electrode is close to the defect, the current density J1 = J = V / πr12 rises sharply to the melting of the metallization layer of the region by its Joule heat energy. , causing the arc to fly between the electrodes, the arc quickly evaporates and throws away the molten metal at the place, forming an insulating isolation zone without a metal layer, and the arc is extinguished to achieve self-healing.

2. Electrochemical self-healing

Generally, at low voltage, the film capacitor will have such self-healing. The mechanism of this self-healing is as follows: Assume that there is a defect in the dielectric film of the metallized organic film capacitor. After the voltage is applied to the capacitor, there will be a large leakage current through the defect, which means that the insulation resistance of the capacitor is much lower than that. The specified value in the technical conditions.

Because film capacitors have a certain water absorption rate, and capacitors may be wetted during the manufacture, storage, and use of capacitors, they may contain ion currents in the leakage current and may also contain electron currents. Therefore, a considerable part of the ion current is the O2-ion and H-ion currents generated by the electrolysis of water. After the O2-ion reaches the AL metallization anode, it combines with AL to form AL2O3. As time goes on, the AL2O3 insulating layer is gradually formed to cover and isolate the defects, so that the capacitor insulation resistance is greatly improved and self-healing is achieved.

It should be noted that the film capacitor needs to be self-healing and requires a certain amount of energy. There are two sources of energy: one is from the power source, and the other is the oxidation and nitridation exothermic reaction from the metal of the defect.