Categorized | Power Devices, Thyristors


Posted on 12 April 2019

Reliable Thyristor Triggering








To ensure that a thyristor is triggered reliably and safely when the main current increases steeply, a trigger pulse current sufficiently larger in amplitude than the gate trigger current IGT (≥ 5⋅ IGT) and with a sufficient rate of rise (≥ 1 A/μs) is needed.

Even if the current in the commutation circuit rises relatively slowly, the RC element connected in parallel for overvoltage protection will often mean that a fast rising discharge current is driven through the thyristor on every trigger. Using sufficiently strong and steep drive pulses is therfore recommended. This is particularly important for thyristors connected in parallel or series, since strong and steep drive pulses will make the trigger far more synchronous. An exponential current rise is determined by the stray inductance of the pulse transformer LS.

To determine the peak current of the driver unit, a straight line is drawn between the points no-load voltage VO~VB and the short-circuit current IK~VB/R in the linearly scaled trigger diagram shown above (dashed line).

The actual input characteristics for the individual thyristors lie between the limiting characteristics in the diagram (dash-dotted line). The possible points of intersection with the driver output characteristic lie between points A and B, accordingly. The point of intersection S of a typical thyristor with the driver output characteristic results in the trigger current (here e.g. 2.3 A; 10.7 V) for this component and lies in the middle of the safe firing region. The minimum duration for the drive pulses is 10 μs.

In most cases, the latching current given in the datasheets applies to this pulse duration, too. The minimum trigger current and the latching current decrease for longer drive pulses.


For further information, please read the following articles:

Safe Firing of Thyristors

Driver Units for Thyristors

Criteria for Successful Selection of Diodes and Thyristors


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