Trigger circuit for gate-off thyristor - derives trigger power from controlled load circuit

摘要

Galvanic isolation is provided between control signals and the load circuit without the need to supply trigger power from a separate source. The circuit comprises a series combination of diode and Zener diode (D1,D2) between the gate of the gate turn off thyristor (T1) and its cathode. In parallel with these diodes are the turn-off circuit (T2,R1,C1) and the turn-on circuit (T3,L,C1). A commutating thyristor (T4) is in parallel with the main thyristor (T1), and in series with the capacitor (C1). The triggering energy is provided by the capacitor (C1), whose polarity reverses after each switching oeration. It also acts, in conjunction with the fast acting commutating thyristor, to suppress voltages across the main thyristor when the latter is turning off. The gate signal for the auxiliary thyristors (T2,T3,T4) is typically provided through an optical transmission to give the required isolation.