Characteristics of SiC MOSFET and Si IGBT
The key to make the best utilize of the SiC MOSFET is to fully understand the device characteristics. The different characteristics of SiC MOSFET and Si IGBT have impact on their short circuit protection schemes.
Comparing with IGBT which has similar blocking voltage and current rating, SiC MOSFET has smaller chip area, which makes the parasitic capacitance smaller than IGBT and increases the intrinsic switching speed. However, the smaller chip area means the SiC MOSFET die has lower thermal dissipation capability. During short circuit conditions, the surge current generates a significant amount of joule heating and the die can be destroyed in a short period of time without enough capability to dissipate the heat. With smaller die size, the surge current capability of SiC MOSFET is lower than that of IGBT.
The output characteristics of SiC MOSFET and IGBT are different too. IGBT typically works in the saturation region during the normal ON state. When a short circuit happens, the collector current IC increases and goes through a sharp transition from the saturation region to the active region. The collector current gets self-limited and becomes independent of VCE. Consequently, the increase in IGBT current and hence power dissipation gets self-limited.
On the other hand, SiC MOSFET works in the linear region during normal ON operation. During a short circuit event, the SiC MOSFET enters the saturation region. Different than that of an IGBT, SiC MOSFET has a larger linear region. The transition from linear region to saturation region happens at significantly higher VDS. The drain current keeps increasing along with the increasing Vds. The device will be destroyed before reaching the transition point. These characteristics make the short circuit protection for SiC MOSFETs very different than IGBT.