Why does the CBB capacitor fail?

Many people think that CBB is different from ordinary electrolytic capacitors in the problem of dry electrolyte with electrolytic capacitors, that is, there will be basically no life problems under specified conditions, but in fact it is not the case. For example, in the production of a high-frequency welding machine, the inverter board part made a 'la, la, la' abnormal noise. The designer believed that it was caused by excessive temperature, thermal expansion and contraction, and there was no doubt that the CBB capacitor had a problem. After many experiments, I found out that it was the sound made by the CBB capacitor after listening at close range, and this 'la, la' sound is a typical CBB discharge self-healing sound, crisp and frequent. Immediately do the capacity and withstand voltage test of the CBB capacitor, and found that the capacity is basically within the nominal range, from 3.2~3.3uF, the nominal is 3.3uF, and for the 600V DC withstand voltage nominal test, the high voltage tester is the best It can only reach 500V, and many of them have self-healing phenomenon at 300V. As they get higher and higher, they will make a 'la, la' sound. From the experimental data, the disassembled capacitors of the same model, the lower the capacity, the lower the withstand voltage, the higher the capacity, the higher the withstand voltage. This means that the discharge self-healing damages the metalized surface area of u200bu200bthe capacitor, resulting in a decrease in capacity. In the subsequent disassembly, it was found that the outer surface of the CBB capacitor was damaged very badly. The damage caused by almost one piece of discharge, the more inside, the better. This may be related to the sealing degree of the outer package and the entry of water vapor. Through searching the data, it is found that when the CBB capacitor is in a long-term working environment, it will cause the evaporation of the metallized coating and thus reduce the capacity. If the working temperature is relatively high, it will also cause the metallized coating to fall off. From these experiments, the CBB capacitors fail, the capacity is reduced, and the withstand voltage is reduced by the following points: 1. The capacitor is not sealed enough, and the water vapor enters to cause oxidation and the withstand voltage decreases. 2. The long-term transient current is too large, the voltage is too high, and the internal discharge self-healing causes the capacity to drop. 3. For long-term high-load work, the evaporation of the metal coating causes the capacity to decrease. The solution is as follows: 1. Purchase brand-name capacitors with guaranteed quality. 2. Use as high specifications as possible. For example, the safety capacitor X2 is generally rated at 275VAC, but in fact, 1KV can be tolerated in an instant test. The quality is very good. In the disassembled capacitor, whether the safety capacitor is a high voltage test or The physical decomposition is very beautiful. 3. A reasonable varistor is connected in parallel to prevent the impulse voltage from being too high. It is said that this method is more effective and has conditions to reduce the instantaneous current. Generally, the resistance is connected in series within the allowable range. This article summarizes this article based on the actual situation, a new interpretation of the failure of CBB capacitors. The failure problem not only exists in ordinary electrolytic capacitors, but also in CBB capacitors. Here we use examples to prove the existence of this problem and give some countermeasures.
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