What Is a Trigger Circuit?

A trigger circuit is a circuit that has some steady state or non-steady state, at least one of which is steady state and is designed to initiate the required transition when a suitable pulse is applied.

To ensure reliable triggering, the thyristor has certain requirements for the trigger circuit:

1.Trigger

When the single-junction transistor trigger circuit is directly connected to the thyristor main circuit, it is not safe and may easily cause false triggering. The use of a pulse transformer to output the trigger pulse can electrically isolate the rectifier main circuit and the trigger circuit so that the two circuits no longer affect each other. The trigger circuit composed of a transistor and the integrated circuit trigger also often use a pulse transformer to output the trigger pulse. The trigger circuit and the trigger synchronous power supply adopt electrostatic shielding to prevent electromagnetic interference and prevent thyristors from false triggering. Diodes, resistors, and capacitors are connected in series between the thyristor gate and the cathode or the secondary output of the pulse transformer. In the case of high requirements, a reverse bias voltage can also be applied between the gate and the cathode. ^[3]

1. Use a single-junction transistor bank trigger circuit, which is currently the most widely used. It has the advantages of simple circuit, high reliability, small temperature influence, and large peak value of output pulse current. It is suitable for small and medium capacity thyristor controllable rectifier circuits. The disadvantage is that the output pulse is not wide enough and the average pulse power is not large.

2. Use a small-capacity thyristor as a trigger circuit to trigger a high-power thyristor. Its advantages are simple, reliable, high trigger power, and wide pulses. It has the disadvantage of requiring a single junction transistor to trigger a small thyristor, which uses more components.

3. Wide pulses can also be obtained with the transistor's trigger circuit. The disadvantage is that the wiring is complicated and the anti-interference ability is poor.

4. Trigger components composed of integrated circuits and digital phase-shift trigger circuits.