Electrical transients of snubber diodes in GTO circuits

During GTO operation the snubber diode is subjected to successive reversals of applied voltage bias. Such changes in bias products voltage transients in the diode. Under conditions of large forward dl/dt the diode exhibits a transient forward volt drop. Under conditions of commutating dl/dt in an inductive circuit the diode recovery produces a reverse voltage overshoot. Optimising the forward recovery performance of the diode can have a detrimental effect on the reverse recovery performance. It was found that all soft recovery diodes regardless of design can be made to exhibit snappy recovery. The diode has a soft recovery up to a critical dl/dt after which the device `snaps-off´ producing dangerous voltage spikes. Such spikes can destroy the diode and cause a subsequent circuit failure. This change in recovery characteristic at high dl/dt and circuit inductance must be allowed for in the design of the GTO circuit