
66 Electric Motors and Drives
COOLING OF POWER SWITCHING DEVICES
Thermal
resistance
We have seen that by adopting a switching strategy the power
loss in the switching devices is small in comparison with the
power throughput, so the converter has a high efficiency.
Nevertheless almost all the heat which is produced in the
switching devices is released in the active region of the semi-
conductor, which is itself very small and will overheat and
breakdown unless it is adequately cooled. It is therefore
important to ensure that even under the most onerous
operating conditions, the temperature of the active junction
inside the device does not excee ...