What are the reasons for the bad thyristor
1, voltage breakdown
Thyristor is damaged because it cannot withstand voltage. There is a bright and clean hole in the chip, which can sometimes be seen with a magnifying lens. The reason may be that the withstand voltage of the tube itself drops or the high voltage breakdown generated when the circuit is disconnected.
2, current damage
The trace characteristic of current damage is that the chip is burned into a pit, and it is rough, and its location is far away from the control electrode.
3, the current rise rate is damaged
Its trace is the same as the current damage, and its position is near or on the control pole.
4, edge damage
It happened at the chamfer of the outer circle of the chip, with small and clean holes. With a magnifying glass, you can see that there are fine metal scratches on the chamfered surface. This is caused by the manufacturer’s careless installation. It causes voltage breakdown.
How to protect the thyristor from being damaged
1, overvoltage protection
The thyristor is very sensitive to overvoltage. When the forward voltage exceeds a certain value of its off-state repetitive peak voltage UDRM, the thyristor will be turned on by mistake, causing circuit failure; when the applied reverse voltage exceeds a certain value of its reverse repetitive peak voltage URRM, the thyristor It will be damaged immediately. Therefore, it is necessary to study the cause of overvoltage and the method of suppressing overvoltage.
The main cause of overvoltage is that the supplied electric power or the energy storage of the system has undergone drastic changes, making the system too late to convert, or the electromagnetic energy originally accumulated in the system is too late to dissipate. Mainly found are two types of over-voltage caused by external shocks such as lightning and impulse voltage caused by the opening and closing of switches. The overvoltage caused by lightning strikes or the action of high-voltage circuit breakers is a voltage spike of several microseconds to several milliseconds, which is very dangerous for the thyristor.
2, overcurrent protection
Due to the small size and small heat capacity of semiconductor devices, especially high-voltage and high-current power devices such as thyristors, the junction temperature must be strictly controlled, otherwise it will be completely damaged. When a current greater than the rated value flows in the thyristor, the heat is too late to dissipate, causing the junction temperature to rise rapidly, and eventually the junction layer will be burned out.
There are many reasons for overcurrent, for example, the thyristor of the converter device itself is damaged, the trigger circuit fails, the control system fails, etc., and the AC power supply voltage is too high, too low or lack of phase, load overload or short circuit, phase The impact of neighboring equipment failures, etc.
The most commonly used method of thyristor overcurrent protection is fast fuse. Because the fusing characteristic of the ordinary fuse is too slow, the thyristor has been burned out before the fuse is blown; it cannot be used to protect the thyristor. The fast fuse is buried in the quartz sand by the silver fuse, the fusing time is very short, and it can be used to protect the thyristor.
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