In this paper, we present a design method for reducing local losses in a Gate Turn-Off Thyristor (GTO) at turn-on and protecting GTOs, particularly those of the flat-package type used in the high-frequency switching from overheating failure. This method utilizes the measurement of cathode-current spread obtained by infrared measurement. The performance of this was verified observing the gate-current at turn-on in a GTO used in the LC resonant commutation circuit for a power supply rated 10kHz, 1.0MW.