Method of Turn-ON Operation of GTO Thyristor

Conventional thyristor (CT) can be turned OFF by expensive and bulky commutation circuitry, which limits its use up-to about 1 KHz applications. A GTO Thyristor can easily be turned off by a negative gate pulse of appropriate amplitude. Constructional details with p-n junction layer and circuit symbol of GTO Thyristor is given in Fig. 1a and Fig 1b respectively.

• Self turn-OFF capability of GTO makes it the most suitable device for inverter and chopper applications.
• A GTO Thyristor is turned ON by applying a positive gate current Ig.
• Current gains α_1, α_2 begin to rise and when α_1+α_2=1, saturation level is reached and GTO Thyristor is turned ON.
• For turning-ON a GTO, First transistor TR_1 is turned ON, this in turn switches ON TR_2 to apply positive gate current pulse to turn ON GTO.

• For turning-OFF a GTO, the turn-off circuit should be capable of outputting a high peak current. Usually, a thyristor is used for this purpose.
• When gating thyristor T_1 is turned on, a large negative gate current pulse turns off the GTO.

• t_q = t_s + t_f + t_t
• Where: t_q is total turn off time, t_s is storage period, t_f is fall period, and t_t is tail period.