2 Operating Voltage of the Switching Device

The maximum input voltage must be considered when selecting the power device, since the maximum switch node voltage of the flyback converter is the summation of the input voltage, the reflected output voltage, and the voltage spike. For 800-V battery systems, Silicon Carbide (SiC) MOSFETS are becoming more popular due to their improved figure of merit (FOM) and their high voltage rating, typically 1700 V. Even though there are 1500-V or 1700-V rated silicon MOSFETs on the market, their poor FOM, non-availability of auto-grade, high unit cost, and limited suppliers makes 1.7-kV SiC MOSFETs the more preferable option. Table 2-1 compares the critical parameters between a Si MOSFET and two SiC MOSFETs and quantifies the efficiency gains achieved by the SiC MOSFETs.

Careful consideration must be given to the UVLO turn-off threshold of the SiC-based flyback controller, because it is not only the minimum operating voltage of the controller, but also the gate drive voltage for the SiC MOSFET. For example, a UVLO turn-off greater than 15 V for a 20-V_GS SiC MOSFET is recommended to provide increased reliability of the device. Gate drive voltages less than 15 V can cause the power device to operate in the negative temperature coefficient region of turn-on resistance and lose the current balance nature among the clustered cells. This usually results in a thermal runaway issue. Similarly, for 18-V_GS or 15-V_GS SiC MOSFETs it is highly recommended to maintain the gate drive voltage higher than 14 V and 12 V, respectively.