48W Aux. Power Supply Evaluation Board for SiC FETs

The CRD-060DD17P-2 is a demonstration board for a single-end flyback converter design with built with Wolfspeed's commercially-available 1700V SiC MOSFET. The 48W reference design demonstrates how the 1700V SiC MOSFET can reduce total cost and simplify the design of auxiliary power supply.

It replaces the more common, but more complicated, silicon-based two-switch flyback topology, while successfully supporting a wide input voltage range (300V to 1000V). An active start-up circuit is also introduced to achieve less start-up losses with faster start-up time.

Three-phase applications, such as motor drive, UPS and PV inverter, have a front end ac-dc or dc-dc converter to boost the dc link voltage up to 600Vdc to 800Vdc. Factoring in a design margin, the maximum dc link voltage is up to 1000V. To support such systems in practice, an auxiliary power supply is used to generate power for cooling fans, displays, control logic and system protection functions with the dc link voltage as its input. For such low power applications, flyback topology is the most common type in the industry; however, the conventional single-end flyback topology has difficulty in meeting high input voltage.

The first difficulty is caused by the high input voltage (1000Vdc); the single-end flyback topology would require high blocking voltage switching devices. Currently, the Silicon MOSFET only has 1500V blocking voltage, which has low voltage stress design margin and thus affects the reliability of the power supply. The second challenge is that most of the 1500V Si MOSFETs have very large on-state resistance, and this will lead to higher losses, higher thermal and lower efficiency, especially when the whole three-phase system is operating at light output load and auxiliary power losses occupy most of total system losses. Lastly, to support a wide input voltage range, a pure resistance start up circuit is normally used. However, the start-up resistance will lead to losses at high input voltage. Larger start-up resistance will have less losses but lead to long start-up time at low input voltage.

This design proposes a single-end flyback converter to replace complicated two-switch Flyback converter by using 1700V SiC MOSFET. An active start-up circuit is also introduced to achieve less start-up losses with faster start up time. The design utilizes Wolfspeed’s C2M1000170J, which is available in a 7LD2PAK surface mount package that combines a small footprint with a wide creepage distance of 7mm between drain and source. By moving to the surface mount package, design engineers can achieve economical thermal designs without the need for heat sink on the MOSFET. The addition of a heat sink enables the same design to reach 58W of output power.

The C2M1000170J features high blocking voltage with low RDS(on), low parasitic inductance, ultra-low drain gate capacitance, and a separate driver source pin. It is easy to parallel and simple to drive, and it is well-suited for auxiliary power supplies, switch mode power supplies, and other applications involving high-voltage capacitive loads. This demonstration board is not designed to be a product and is to be only used as a tool to evaluate the performance of Cree switching devices.