Why Designers are Looking to SiC for Bidirectional DC-DC Converters

With pressure to reduce reliance on fossil fuels and other polluting energy sources, electrical energy becomes the preferred choice. The most obvious change is the automotive industry’s move to partial or full electrical power. With electric vehicle (EV) chargers delivering between 3.6 kW and 22 kW of energy during charging and up to 150 kW in fast DC chargers, every percentage point of efficiency improvement implies several hundred Watts of energy that need not be dissipated as heat. This enables designers to build smaller, lighter designs or more powerful solutions into existing form factors. Adding to the challenge, automotive OEMs are planning to support bidirectional power flow, enabling EVs to jump-start other EVs, power conventional appliances while camping, or even help to balance the electrical grid to avoid brown-outs.

Traditionally, IGBTs have been the chosen power device for the high-voltage stages of DC-DC converters. Robust and well understood, they have steadily improved in their switching speed allowing progress to be made in power converter designs. However, thanks to wide-bandgap (WBG) technology developments, silicon carbide (SiC) is increasingly the preferred choice in such applications. With significantly faster turn-on and turn-off speeds, the switching losses in SiC devices, such as the 1200 V Toshiba TW070J120B N-channel MOSFET with built-in SiC Schottky barrier diode, contribute to a 66% reduction in power losses compared with a comparable IGBT in the same application.

Topologies such as Dual Active Bridge (DAB) are often found in existing EV chargers and referenced in research in this area. Its high-frequency transformer provides the required galvanic isolation. Due to the lower voltages on the DC load side charging the battery, next generation silicon MOSFETs offer an optimal price-performance point. Devices such as the TK49N65W5, a DTMOS superjunction N-channel device offering superior R_DS(ON) and fast reverse recovery time (t_rr), coupled with the TLP5214A gate driver, make for an ideal choice to complement the SiC MOSFETs for a highly efficient design.

These devices’ full capability is realized in Toshiba’s latest reference design, a 50 kHz switching, isolated bidirectional DC-DC power supply. Designed to support power converter designers working on high-power designs, the design handles up to 5 kW from a 750 V DC link. This forms an excellent starting point for EV charger developers. At a 100% step-up/-down load, the design has shown to operate with efficiencies of 97%.

The full details of this reference design, along with the advantages SiC MOSFETs offer over traditional IGBT-based designs, are shared in our latest white paper available here: