Because of the self-healing ability of metal film capacitor
s, this type of capacitor is currently the most widely used and the most promising capacitor in the future market. There are two different mechanisms for the self-healing of metal film capacitors: one is discharge self-healing; the other is electrochemical self-healing. The former occurs at higher voltages, so it is also referred to as high-voltage self-healing; because the latter also occurs at very low voltages, it is often referred to as low-voltage self-healing. 1 Discharge self-healing In order to explain the mechanism of discharge self-healing, suppose there is a defect somewhere in the organic thin film between two metallized electrodes, and its resistance is R. According to the nature of the defect, it may be a metallic defect, or it may be a semiconductor or poor-quality insulation defect. Obviously, when the defect is the former, at low voltage, the capacitor has already discharged and self-healed. The so-called self-healing of high-voltage discharge occurs only in the case of the latter defect. The self-healing process of discharge is that after the voltage V is applied to the metal film capacitor, an ohmic current Iu003dV/R passes through the defect immediately. Therefore, the current density Ju003dV/Rπr2 flowing through the metalized electrode, that is, in the metalized electrode, the closer the area to the defect (that is, the smaller the r), the greater the current density. The Joule heat caused by the defect power consumption Wu003d(V2/R)r is an exponential decrease in the resistance R of a semiconducting or insulating defect. Therefore, the current I and the power consumption W increase rapidly. As a result, in the area where the metalized electrode is very close to the defect, the current density J1u003dJu003dV/πr12 rises sharply to the point where the Joule heat energy melts the metalized layer in this area. It causes 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 realized. 2 Electrochemical self-healing Metal film capacitors often appear this kind of self-healing under low pressure. The mechanism of this self-healing is as follows: If there is a defect in the dielectric film of a metal film capacitor, after applying a voltage to the capacitor (even if the voltage is very low), there will be a large leakage current through the defect, which is manifested as the insulation resistance of the capacitor. Far below the specified value in the technical conditions. Obviously, the leakage current contains ionic current and may also contain electronic current. Because all kinds of organic films have a certain water absorption rate (0.01%~0.4%), and in the process of capacitor manufacturing, storage and use, the capacitor may be damp, so a considerable part of the ion current is produced by the electrolysis of water O2-ion and H-ion current. After O2- ions reach the AL metallized 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 is obvious that a certain amount of energy is required for metal film capacitors to complete self-healing. There are two sources of energy, 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. The energy required for self-healing is often called self-healing energy. 3 Advantages and disadvantages of self-healing The biggest feature of metal film capacitors is that they have self-healing ability, so the benefits of self-healing are the main ones. The circuit brings signal interference and reduces the important performance of the circuit-the signal-to-noise ratio. Therefore, for some particularly demanding circuits, such as high-fidelity audio circuits, high-precision communication circuits, etc., organic film capacitors cannot be self-healing during operation. Another disadvantage of self-healing is that the capacity of the capacitor is gradually reduced. If the capacitance is working, there are a lot of times of self-healing, it will cause its capacitance and insulation resistance to decrease significantly, the loss angle will increase greatly, and the capacitor will quickly fail.
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