Accurate Estimation of Switching Losses in SiC Power MOSFET’s

The increasing reduction of the manufacturing costs of SiC devices and in particular SiC Power MOSFETs will soon make them cost competitive with other Si devices such as Si IGBTs. For the same voltage and current ratings, SiC Power MOSFETs offer better switching characteristics. Therefore, it is a matter of time that these new devices overtake the role of Si IGBTs in several applications.

Before that moment comes, there is still a lack of uniformity among SiC Power MOSFET manufacturers, especially in terms of the information related to the switching losses that is given in the Datasheet. If we take a look at the Datasheet information for Si Power MOSFETs, most of the manufacturers provide the transition times but not the switching losses. Besides those transition times are typically referred to resistive switching conditions.

Nevertheless, Si Power MOSFETs do not have the same voltage and current ratings as their SiC counterparts. Hence, they are typically used in different applications. Therefore, it makes more sense to compare SiC Power MOSFETs with their competence. These are Si IGBTs.

After analyzing Si IGBTs and their Datasheets we can notice that more information is given for the switching losses. In fact, many manufacturers choose to provide not only the switching times but also the switching losses. Besides, the switching losses are given under inductive switching conditions. For that purpose a freewheeling diode is required to provide a current path when the switch is turned on. This implies that the diode can affect switching performance of the device under test (MOSFETs or IGBT), especially during the turn on, due to the reverse recovery. To prevent from this, Schottky barrier diodes, with zero reverse recovery, are used as freewheeling diodes.

The question then is, is it correct to give the switching losses under inductive conditions? What is the impact of the freewheeling diode on the switching losses? Would a resistive switching test provide more accurate information.