Three main reasons of thyristor breakdown
1. Overvoltage breakdown:
Overvoltage is one of the main reasons of thyristor breakdown. SCR has almost no time to withstand overvoltage, and even within a few milliseconds, the overvoltage will be broken down. Therefore, in the practical application circuit, both ends of the thyristor must be connected with the RC return circuit to avoid instantaneous overvoltage caused by various irregular interference pulses. If the thyristor often fails, please check whether the components of the recovery circuit are burnt out or fail.
2. Overcurrent and overheat breakdown:
Actually, overcurrent breakdown and overheating breakdown are the same thing. Over-current breakdown means that when the current passes through the silicon controlled silicon chip, the thermal effect will be generated inside the chip, and the temperature of the chip will rise. When the chip temperature reaches 175℃, the chip will fail and cannot be recovered. Under normal operating conditions, as long as the working current does not exceed the rated current of the thyristor, this kind of thermal breakdown will not occur. Because the principle of overcurrent breakdown is caused by temperature rise, and the process of temperature rise takes a certain time, overcurrent (several hundred milliseconds to several seconds) will generally not break down in a short time.
3. Overheating fault:
Overheating breakdown here refers to the thermal breakdown when the working current does not exceed the rated current of SCR. The main reason for this breakdown is that the auxiliary radiator of SCR is not working well, and the temperature of SCR chip is too high, which leads to breakdown. During water cooling operation, check whether the inlet water temperature is too high (generally, the water temperature should be below 25℃, but the maximum should not exceed 35℃) and whether the flow rate is sufficient; For people working in air-cooled mode, 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 the silicon controlled rectifier is replaced, only the chip should be replaced. During installation, attention should be paid to the good contact between the chip and the radiator, and the contact surface should be smooth, free of scratches or bumps and dust, and sufficient uniform pressure should be ensured, especially for water-cooled silicon controlled rectifier. The tension of the three bolts must be uniform. In addition, if the chip is replaced many times, the contact surface of the radiator will be deformed, which will affect the heat dissipation effect. If the thyristor on your machine often fails and no other reason can be found, you should consider replacing the thyristor with the radiator.
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