SiC FETs in High-Voltage Input 15W Flyback Aux Power Supply Reference Design

January 13, 2020 by Paul Shepard

In the CRD15DD17P reference design, Cree has introduced a wide input range (300V-1200V), 15W flyback auxiliary power supply based on Cree's C2M1000170J, 1700V, 1000mΩ, (TO-263-7) silicon carbide (SiC) MOSFET. The designed flyback converter can accept 480Vac to 530Vac or 300Vdc to 1200Vdc input to provide 12Vdc at the output.

This design is targeted at high power and high voltage applications including solar inverters, energy storage systems, and traction applications. Low power subsystems that support these high power and high voltage systems (such as cooling fans, displays, and controller biasing) must have a low power auxiliary power supply that can take high voltage input (ac or dc) and can generate low dc voltage at the output.

The topology that is most commonly used in the industry for these low power subsystems is the flyback topology. In the present market, silicon based MOSFETs are widely used power switching devices for flyback converters.

However, as compared to SiC MOSFETs, most silicon based MOSFETs have 1500V (max) blocking voltage with a low design margin of voltage stress (which has significant impact on the reliability of the power supply) and a very large on-state resistance (Rdson), which leads to higher power loss, lower efficiency and high thermal stress.

Cree's CRD15DD17P, 15W flyback auxiliary power supply board is based on Cree's C2M1000170J, 1700V, 1000mΩ, TO-263-7 SiC MOSFET. Cree's C2M1000170J SiC MOSFET consists of a fast-intrinsic diode with low reverse recovery charge (Qrr) and a very low output capacitance. Cree's C2M1000170J SiC MOSFET comes in a compact surface mount package with extended leads for high voltage capability and low source inductance.

A Texas Instruments Inc. UCC28740 flyback controller has been utilized in this design. This controller contains less than 10mW no load power capability, optocoupled feedback for constant voltage (CV) mode and primary side regulation for constant current (CC) mode (which enable +/- 1% voltage regulation across line and load), a 700V start-up switch, valley switching operation for high overall efficiency, frequency dithering to ease electromagnetic interference (EMI) compliance, a clamped gate drive output for the MOSFET, and overvoltage, low line, and overcurrent protection function.