3.4 Phase-Shifted Full-Bridge, Fixed Leg

Figure 3-4. Partially Soft Switching (ZVS) PSFB Leg. VIN = 800V, VOUT = 12V, IOUT = 100A, Po=1200W

Figure 3-4 shows switch node voltage of PSFB Leg-1. The switch node voltage swings to the next value before High-Side or Low-Side MOSFETs are switched ON. It is important to have enough leakage energy and enough dead time to achieve ZVS.

The switching losses for PSFB primary full bridge are inversely related to output load. In the absence of ZVS, under high input voltage (800V) and light load conditions, the switching losses of this leg increase significantly. This can drive the junction temperature of SiC MOSFETs outside of safe area of operation. Under light load operation, Burst mode should be implemented to reduce switching losses.

Figure 3-5. TURN ON of High Side SiC FET
Figure 3-6. TURN ON of Low Side SiC FET

Drain-Source Voltage swings high, right before HSS is turned on. Leakage inductance energy charges HSS Coss during the deadtime of 160 ns.

Drain-Source Voltage swings low, right before LSS is turned on. Leakage inductance energy discharges LSS Coss during the deadtime of 160 ns.