6 Summary

When designing a backup power supply for a traction inverter to achieve high converter efficiency, fast startup time, and target thermal de-rating careful consideration must be given to selecting a high voltage power device and topology. The new, ultra-small, low-cost active high voltage depletion mode startup circuit introduced in this document helps achieve fast startup, reduces both PCB area, and component cost. The total startup time with the new startup circuit is very consistent over the entire VIN operating range. The new UVLO options and 30-V rating to reliably drive SiC MOSFET with UC28C5x family enable higher thermal de-rating power and optimal switching performance of the backup power supply in the increasingly popular 800-V battery system of EV traction inverters.

For high voltage power device selection of a flyback converter, silicon MOSFETs are typically acceptable for 400-V battery systems. However, Silicon Carbide MOSFETs must be used in 800-V battery systems due to their higher voltage rating (typically 1700 V) and improved FOM. Also, primary-side regulation (PSR) does not require an optocoupler which increases system reliability, eliminates a component crossing the isolation barrier, and further reduces cost.