
Power Electronic Converters for Motor Drives 45
collector-emitter voltage will be falling towards zero. The
peak power reached can therefore be large, before falling to
the relatively low on-state value. Of course the total energy
released as heat each time the device switches is modest
because the whole process happens so quickly. Hence if the
swiching rate is low (say once every second) the switching
power loss will be insignificant in comparison with the on-
state power. At high switching rates, (say 100 kHz) however,
when the time taken to complete the switching becomes
comparable with the on time, the switching power loss can
easily becom ...