Turn-off energy reduction by charge control in a cascode-switched GTO

Recently, improvements in the performance of high power gate turn-off thyristors (GTOs) have been made by operation at turn-off gains of G⩽1 (Gruning et al, 1996) and G=1 (Oetjen and Sittig, 1997). Both result in an increased SOA and therefore the turn-off snubber can be reduced or omitted. However, the very steep rise in gate current increases the device´s turn-off losses, in comparison to conventional GTO operation (G>1). This paper shows how a cascode-type turn-off process can be controlled to reduce the switching losses of a GTO. After discussing the influence of the excess charge on the turn-off losses, the method of charge control by a cascode switch is described. Finally, the experimental results for a pre-discharged cascode-type turn-off sequence of a standard 4.5 kV/3 kA GTO are presented