Thyristor Gate Control or Firing Circuit Design

 

Consideration must be given to the following points when designing gate control circuits.

1. The gate signal should be removed after the thyristor has been turned on. A continuous gate signal will increase the power loss in the gate junction.
2. No gate signal should be applied when the thyristor is reversed biased. If a gate signal is applied under these conditions, the thyristor may fail due to an increased leakage current.
3. The width of the gate pulse must be greater than the time required for the anode current to rise to the holding current. In practice, the gate pulse width is made wider than the turn-on time of the thyristor.

In thyristor converters high ac voltages exists between anode and cathode of the thyristor, while low voltage level pulses are placed between gate and cathode. Isolation is necessary between the gate-cathode circuit and the anode-cathode Circuit so that unwanted short-circuits between devices are avoided..

Gate Drive Circuits

Two basic types of drive circuits are 1) dc-coupled drive circuits and 2) Electrically isolated (optocoupler or pulse transformer drive circuits.

In the case of optocoupler isolation, the low voltage gate drive circuit is optically isolated from the high voltage anode-cathode circuit as shown in figure below.

Gate Drive Circuits (Typical Example for Thyristor)

Summary

1. A thyristor is a latching device and it can be turned on with a small gate pulse, typically 100 μs . They are of various types
2. Thyristors are generally off by line commutation due to the natural behavior of the input line supply.
3. During the turn-off process, thyristors must be subjected to a reverse voltage for a certain minimum time known the turn-off.
4. Due to the junction capacitances and turn-on limit, thyristors must be protected from high di/dt and dv/dt failures.
5. Appropriate gate drive circuit together with appropriate means of isolation between power circuit and gate circuits is also necessary for firing the thyristyors.

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