Metallized film capacitors: Failure analysis of CBB capacitors

The difference between safety capacitor and CBB capacitor

1. Ceramic capacitors are ceramic capacitors used for high-frequency and high-voltage applications. The capacitors with rated voltages of 400V and 250V are named Y1 and Y2 capacitors, namely, safety ceramic capacitors.

2. CBB capacitors are thin film capacitors. According to experience, many manufacturers use polypropylene metal film capacitors.

3. CBB non-polar capacitors are produced by a material and process (polypropylene capacitors), while safety capacitors are a characteristic and use of capacitors.

Safety capacitors are used in such occasions, that is, after the capacitor fails, it will not cause electric shock and will not endanger personal safety. It includes X capacitance and Y capacitance. X capacitors are capacitors connected between the two lines of the power line (LN), generally metallized film capacitors are used; Y capacitors are capacitors connected between the two lines of the power line and the ground (LE, NE), and generally appear in pairs . Due to the limitation of leakage current, the Y capacitor value cannot be too large. Generally, the X capacitor is uF and Y capacitor is nF. The X capacitor suppresses differential mode interference, and the Y capacitor suppresses common mode interference. Those that meet the requirements of safety capacitors can be used as safety capacitors with sufficient voltage resistance and stability.

Safety capacitors are mainly suitable for electronic instruments and electronic equipment powered by power grids. They are used in switches, contacts and other parts where spark discharge occurs. They can absorb pulse interference and reduce electromagnetic disturbance. It is widely used in electric tools, lighting, hair dryers, water heaters and other household appliances. Safety capacitors have the characteristics of small size, light weight, safety and reliability, and can withstand higher pulse voltages caused by lightning strikes on external lines or other equipment failures.

The metal film CBB capacitor has large impact current resistance, high electrical strength, excellent electrical performance, and good self-healing. CBB capacitors are widely used in various electronic equipment, differential motors, power tools, lighting fixtures, air conditioners, refrigerators, washing machines and other household appliances and power systems.

Why does the CBB capacitor fail?

Many people think that CBB will be different from ordinary electrolytic capacitors in the issue of electrolyte drying, that is, there will be basically no life-span issues under the specified conditions, but in fact it is not the case.

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 discovered that it was the sound made by the WIMACBB capacitor when I listened to it at close range. This "la, la" sound is a typical CBB discharge self-healing sound, crisp and frequent. I immediately tested the capacity and withstand voltage 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 Most of them can only reach 500V, and many of them have self-healing phenomenon at 300V. As they become 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 shows that the self-healing of the discharge damages the metalized surface area of the capacitor, resulting in a decrease in capacity. In the subsequent disassembly, it was found that the outer surface of the CBB capacitor was very damaged. 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 to reduce the capacity. If the working temperature is relatively high, it will also cause the metallized coating to fall off.

From these experiments, the following points have led to the failure of CBB capacitors, reduced capacity and reduced withstand voltage:

1. Capacitors are not sealed enough, and water vapor enters to cause oxidation and lower pressure resistance.

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. Long-term high-load work, the evaporation of the metal coating leads to a decrease in capacity.

The solution to the failure of CBB capacitor 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, it can withstand 1KV in an instant test. The quality is very good. Among the disassembled capacitors, 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 allowed range.

Based on the actual situation, this article provides a new interpretation of the failure of CBB capacitors. The failure problem not only exists in ordinary electrolytic capacitors, but also in CBB capacitors. Examples are used here to prove the existence of this problem, and some countermeasures are given.