Why does the CBB capacitor also fail?

Compared to common capacitors, people's perception of CBB capacitors is relatively rare. This has led to some misunderstandings on CBB capacitors. In this paper, we will talk about the misunderstanding of CBB metallized film capacitors. Interested friends are coming to see it.

Why does the CBB capacitor fail?

Many people think that CBB will be different from ordinary electrolytic capacitors in the problem of dry electrolyte, that is, it will not have life expectancy under the specified conditions, but it is not.

In the production of a high-frequency welding machine, the inverter board part issued a "la, la, la" abnormal sound, the designer believes that the temperature is too high, thermal expansion and contraction caused, and did not suspect that CBB capacitors have problems, and then passed Many experiments, the close-up hearing found that the sound of WIMACBB capacitors, and this "la, la" sound is the typical CBB discharge self-healing sound, crisp and frequent. Immediately test the capacity and withstand voltage of the CBB capacitor, and find 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 best. It can only reach 500V, and many of them have a self-healing phenomenon at 300V. As they get higher and higher, they will make a "la, la" sound.

From the experimental data, the disassemble capacitors of the same batch model, the lower the capacity, the lower the withstand voltage, the higher the capacity, the higher the withstand voltage. This indicates that the self-healing of the discharge damages the surface area of the capacitor metallization, resulting in a decrease in capacity. In the subsequent disassembly, it was found that the outer layer of the CBB capacitor was very damaged. The damage caused by almost one discharge was more internal, and the better, this may be related to the degree of sealing of the outer packaging and the entry of water vapor.

Through the search of the data, it is found that when the CBB capacitor is in a long-term working environment, the metallization coating is evaporated to reduce the capacity. If the working temperature is relatively high, the metallization coating may fall off.

From these experiments, the CBB capacitor fails, the capacity is reduced, and the withstand voltage is reduced as follows:

1. The capacitor seal is not enough, the water vapor enters to cause oxidation, and the withstand voltage is lowered.

2. The long-term instantaneous current is too large, the voltage is too high, and the internal discharge self-healing leads to a decrease in capacity.

3. Long-term high-load work, evaporation of metal plating leads to capacity reduction