Characteristics of turn-off thyristors_Detection of turn-off thyristors

Characteristics of turn-off thyristor

After ordinary unidirectional thyristors are triggered by control electrode signals, they can remain on even when the signal is removed. To turn them off, the power supply must be cut off or a reverse voltage must be applied to force them to turn off. This requires adding a commutation circuit, which not only increases the size and weight of the equipment, but also reduces efficiency, generates waveform distortion and noise. The turn-off thyristor overcomes the above defects.

When a forward voltage is applied between the anode and cathode of a turn-off thyristor and is lower than the forward transition voltage, if there is no forward voltage on the gate, the tube will not conduct; if a forward voltage is applied to the gate, the tube will be triggered to conduct, and the voltage drop of the tube after conduction is relatively large, generally 2 to 3V.

Since the applied voltage can be increased while the anode current decreases when the turn-off thyristor is turned off (unlike the ordinary unidirectional thyristor, which can only apply voltage after the anode current is equal to zero when it is turned off), the power consumption of the turn-off thyristor during the turn-off period is relatively large. In addition, because the turn-off thyristor has a large conduction voltage drop (2-3V) and a large gate trigger current (about 20mA), the conduction power consumption and gate power consumption of the turn-off thyristor are both greater than those of the ordinary unidirectional thyristor.

Detection of turn-off thyristors

1. Identification of turn-off thyristor electrodes

When judging the electrode, set the multimeter to R×1Ω and test the resistance between any two pins. When the black test lead is connected to the G pole and the red test lead is connected to the K pole, the resistance value is low. In other cases, the resistance value is infinite. From this, the G pole and the K pole can be judged, and the rest is the A pole.

2. Determination of the triggering and conduction capability of turn-off thyristors

(1) Method for testing the triggering conduction capability. When judging the triggering conduction capability of the turn-off thyristor, set the multimeter to the R×1Ω position, connect the black test lead to the A pole, and the red test lead to the K pole, and the measured resistance value is infinite. At the same time, touch the G pole with the black test lead (add a positive trigger signal), and the needle deflects to the right to a low resistance value, indicating that the thyristor has been turned on. When the tip of the black test lead leaves the G pole, the thyristor still remains turned on, indicating that the tube under test has the ability to trigger conduction.

(2) Inspection precautions: When inspecting a high-power turn-off thyristor, a 1.5V battery can be connected in series outside the R×1Ω block (connected in series with the battery polarity in the meter in the forward direction) to increase the test voltage and trigger the turn-off thyristor to conduct.

3. Determination of the turn-off capability of turn-off thyristors

Although the triggering and conducting principle of the turn-off thyristor is the same as that of the ordinary unidirectional thyristor, the turn-off principle and the turn-off method of the two are completely different. This is because the ordinary unidirectional thyristor is in a deep saturation state after being turned on, while the turn-off thyristor can only reach a critical saturation state after being turned on. Therefore, after a negative trigger signal is added to the gate of the turn-off thyristor, the on-state current begins to decrease, so that the tube cannot maintain the positive feedback of the internal current. After a certain period of time, the turn-off thyristor can be turned off.

Set multimeter 1 to R×1Ω, connect the red test lead to K pole, and the black test lead to A pole. Make the thyristor conduct and maintain, at this time, the pointer of meter 1 deflects to the right to a low resistance value. Then set multimeter 2 to R×10Ω, connect the black test lead to K pole, and the red test lead to G pole (add negative trigger signal). If the pointer of meter 1 swings to the left to infinity at this time, it means that the tube has the ability to shut down.