Characterizing the Stability Limits of 1.2 kV SiC MOSFET Body Diodes Under Hard Switching

Abstract:
The stability limits of hard switched SiC MOSFET body diodes, in half-bridge configuration, is explored as a function of the turn-off current switching rate (dIDS/dt), the junction temperature and the turn-off gate voltage. It is shown that body diode turn-off at high junction temperatures with dIDS/dt > 4 kA/mus and turn-off VGS of 0 volts results in electromagnetic instability, excessive EMI and high shoot-through currents. The ringing in the drain-source voltage of the MOSFET turning ON is transmitted to the gate voltage signal (through Miller coupling and parasitic source inductance) thereby resulting in potentially destructive gate voltage oscillations beyond the maximum positive and negative gate voltage rating. The high shoot-through currents are thought to be due to dynamic avalanche during body diode reverse recovery. Miller induced parasitic turn-on is ruled out due to the temperature invariance of the turn on dI/dt and dV/dt. Measurements indicate that this phenomenon is triggered at body diode junction temperatures higher than 75deg C when switching with dIDS/dt > 4 kA/mus. It is shown that the while the recovery current dIrr/dt of the body diode decreases with increasing temperature, at a certain point during the diode turn-off, a potentially destructive ringing is induced. This phenomenon is a technology dependent as it is triggered in SiC Planar MOSFET and not in the SiC Trench MOSFET. The problem is mitigated either by using a Schottky diode, which has space and cost penalties, limiting the turn-off dIDS/dt (which has a switching penalty) or turning off with a negative VGS.