The effect of emitter shortings on turn-off limitations and device failure in GTO thyristors under snubberless operation

The influence of the anode shorting on the turn-off failure of GTO thyristors is investigated. Two types of shorting patterns, viz., "finger"and "ring"-type anode designs, are compared with a non-shorted design. The inductively loaded GTO unit cells are measured under snubberless operation close to the safe-operating area (SOA) limit. The critical anode blocking voltage was obtained for a constant anode current density by varying the turn-off gain, G. The investigation has shown that the turn-off voltage capability of both types of anode-shorted GTO's is deteriorated for high G values. The "ring"-type anode design, however, shows better voltage blocking capability than the "finger" type. This behavior is attributed to the formation of a cylindrical high drift-current density filament at the end of the storage and beginning of the fall-time periods. The electric field in this region, which is referred to as a quasi space-charge region (QSC), is strongly G dependent. Measurements based on the time-resolved free-carrier absorption (FCA) technique are used to map the carrier-density redistribution inside the unit cell during the turn-off switching cycle. For the first time, 3-D simulations of the GTO cell are also presented in order to support the model of dynamic punch-through failure at high G values, which was proposed in earlier papers.<
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