Ultimate limits of an IGBT (MCT) for high voltage applications in conjunction with a diode

IGBTs and MCTs are power devices, which in addition to the well established thyristors and GTOs, also have a utility in high voltage applications. The static characteristics, such as blocking voltage and forward voltage drop, do not demonstrate basic physical limitations. The resulting power dissipation must be handled in such a way that certain temperature limits are not exceeded. Physically imposed limits may however become restrictive during switching, if at high current densities the time dependent dynamic blocking capability of the device can not meet the requirements imposed by the load. The interaction between the switch and freewheeling diode in hard switching modes, such as those found in chopper and converter designs, is important but not considered in publications. It is shown that in such a situation the diode may prove to be the weakest element. A safe operating area can often only be defined if the switching characteristics of the switch sufficiently reduce the requirements placed on the diode. This is relatively simple to achieve in the case of an IGBT although with a higher power dissipation in the transistor. The switching characteristics of a MCT are not readily modified and as a result a reduction in stress on the diode is only possible by means of a snubber circuit.