Static characteristics of thyristors

Static characteristics of thyristors

The volt-ampere characteristics of the thyristor are shown in Figure 1. The first quadrant is the forward characteristic. When IG=0, the forward voltage UA of the thyristor increases to before the forward transition voltage UBO, the device is in a forward blocking state, and its forward leakage current gradually increases as the UA voltage increases; when UA reaches UBO, the tube suddenly changes from blocking state to conducting, which is a hard turn-on. When the gate current IG is turned on and it is large enough, the forward turning voltage drops to a very small value, making the thyristor conduct like a rectifier diode, plus the positive anode voltage, and this conduction is called trigger conduction. The characteristics of the device after being turned on are similar to the forward volt-ampere characteristics of the rectifier diode. The voltage drop of the thyristor after turning on is about 1V. When the thyristor anode current IA that has been turned on decreases to IH (maintenance current), the thyristor returns from conduction to positive blocking, and the thyristor can only work stably in blocking and conducting states.

Static characteristics of thyristors
Figure 1 – Volt-ampere characteristics of thyristor

The third quadrant is the reverse characteristic. When the reverse anode voltage is applied to the thyristor, only a small reverse leakage current flows. When the reverse voltage rises to a certain value, the reverse leakage current increases rapidly, which will cause the reverse breakdown of the thyristor.

In summary, the following conclusions can be drawn:

1) When the thyristor bears the reverse anode voltage, the thyristor is in the reverse blocking state regardless of whether the gate has a trigger current.

2) The thyristor can only be turned on when the thyristor bears the positive anode voltage and the gate has a trigger current. This is the thyristor’s thyristor characteristic, that is, the controllable characteristic.

3) Once the thyristor is turned on, the gate will lose its control. Regardless of whether the gate trigger current exists, the thyristor will remain on. That is, after the thyristor is turned on, the gate loses its function. The gate only serves as a trigger.

4) If the thyristor that has been turned on is to be turned off, the applied voltage and external circuit can only be used to reduce the current flowing through the thyristor to below IH (holding current).

If the gate trigger signal is not applied, the thyristor may be turned on under the following conditions: ①The anode voltage is very high, reaching UBO; ②The anode voltage rise rate du/dt is too high; ③The internal junction temperature is too high. Only gate trigger is the most accurate and effective control method.

According to the external structure of the thyristor, the characteristics and the internal structure of the thyristor, we can judge from the appearance and measure it with a multimeter, and roughly measure its quality. According to the three PN junctions inside the device, the forward and reverse resistances between the anode and the cathode and between the anode and the gate should be more than hundreds of kiloohms, and the resistance between the gate and the cathode is usually several tens of ohms (less than a few ohms or more than a few hundred ohms, generally regarded as damaged). Because there is a shunt resistor between the gate electrode and the cathode in the device, the difference between the forward and reverse resistance is usually very small. (Note: When measuring the resistance between the gate and the cathode, the high-resistance gear of the multimeter cannot be used to prevent the high-voltage battery in the meter from breaking down the gate PN junction.)