A study on maximum turn-off current of a high-power GTO

A 6 kV, 6 kA pnpn-type GTO was fabricated and its maximum turn-off current was investigated. The simulation uses a three-segment device model together with an equivalent circuit model. Results of simulations with segments having different on-state voltages showed current concentration in the lower on-state voltage segment in the fall period. By introducing a homogeneous fabrication process, the on-state voltage distribution band was decreased by a factor of 2. The circuit model simulation results showed the maximum peak current of the improved GTO with smaller on-state voltage band was reduced by a factor of 1.33 compared to that of the conventional GTO. A high-power GTO turn-off limiting model was proposed from the measured 6 kA GTO turn-off locus and the segment SOAs. This turn-off limiting model showed that the GTO would be destroyed when the segment current exceeds the SOA at the time a spike voltage occurs in the fall period. It was demonstrated that the maximum turn-off current estimated by this limiting model coincided well with the experimental results.