Analysis of the causes of thyristor breakdown

1. Overvoltage breakdown:

Overvoltage breakdown is one of the main causes of thyristor breakdown, and thyristors have almost no time to withstand overvoltage. Even in a short period of milliseconds, overvoltage can be broken down. Therefore, in practical application circuits, RC absorption circuits must be connected at both ends of thyristors to avoid instantaneous overvoltage caused by various irregular interference pulses. If thyristor breakdown frequently occurs, please check whether the components of the absorption circuit are burnt out or malfunctioning.

2. Overcurrent and overheating breakdown:

Actually, overcurrent breakdown and overheating breakdown are the same thing. Overcurrent breakdown refers to the thermal effect generated inside a thyristor chip when current passes through it, causing the chip temperature to rise. When the chip temperature reaches 175 ℃, the chip will fail and cannot be restored. Under normal operating conditions, as long as the working current does not exceed the rated current of the thyristor, this type of thermal breakdown will not occur. This is because the principle of overcurrent breakdown is caused by temperature rise, and the process of temperature rise requires a certain amount of time. Therefore, overcurrent in a short period of time (several hundred milliseconds to a few seconds) generally will not break down.

3. Overheating breakdown:

The overheating breakdown referred to here refers to the thermal breakdown that occurs when the operating current does not exceed the rated current of the thyristor. The main reason for this breakdown is that the auxiliary heat dissipation device of the thyristor is not working properly, causing the temperature of the thyristor chip to be too high, resulting in breakdown. For those working with water cooling, the main check is whether the inlet water temperature is too high (generally requiring the water temperature to be below 25 ℃, but not exceeding 35 ℃ at most), and whether the flow rate is sufficient; For those working in air-cooled mode, it is necessary to check whether the fan speed is normal and the ambient temperature should not be too high. However, whether it is air-cooled or water-cooled, if you only replace the chip when replacing the thyristor, it is important to ensure good contact between the chip and the heat sink during installation. The contact surface should be flat, without scratches or bumps, and there should be no dust trapped. It is also necessary to ensure sufficient and uniform pressure, especially for water-cooled thyristors. The tension of the three bolts must be uniform, and scale should be checked and cleaned regularly. Too much scale can also affect the heat dissipation effect and lead to overheating breakdown. In addition, if the chip is replaced multiple times, it will also cause deformation of the contact surface of the heat sink and affect the heat dissipation effect. If a certain thyristor on your machine frequently breaks down and no other reasons can be found, you should consider replacing it together with the heat sink when replacing the thyristor.