Gate Protection in Thyristors
Gate circuit should also be protected against overvoltages and over currents. Overvoltages across the gate circuit can cause false triggering of the SCR. Overcurrent may raise junction temperature beyond specified limit leading to its damage. Protection against over-voltages is achieved by connecting a zener diode ZD across the gate circuit. A resistor R2 connected in series with the gate circuit provides protection against overcurrents.
A common problem in thyristor circuits is that they suffer from spurious, or noise, firing. Turning-on or turning-off of an SCR may induce trigger pulses in a nearby SCR. Sometimes transients in a power circuit may also cause unwanted signal to appear across the gate of a neighbouring SCR. These undesirable trigger pulses may turn on the SCR leading to false operation of the main SCR. Gate protection against such spurious firing is obtained by using shielded cables or twisted gate leads. A varying flux caused by nearby transients cannot pass through twisted gate leads or shielded cables. As such no e.m.f. is induced in these cables and spurious firing of thyristors is thus minimised. A capacitor and a resistor are also connected across gate to cathode to bypass the noise signals, Fig. 4.29. The capacitor should be less than 0.1 µF and must not deteriorate the waveshape of the gate pulse.