48 V bidirectional charger for hybrids

EPC has developed a reference design called the EPC9165 for a 2 kW bidirectional charger in 48 V hybrid vehicles using GaN transistors (writes Nick Flaherty).

Mild hybrids running on 48 V require a 48 to 12 V bidirectional converter, with power ranging from 1.5 to 6 kW. The EPC9165’s design uses a synchronous buck/boost converter with other supporting circuitry including current sensors and temperature sensor. The design also uses a controller board for the digital control and housekeeping power supply based around the dsPIC33CK256MP503 digital controller from Microchip.

The main conversion is based around the EPC2302 GaN transistor. This has a four times better figure of merit (die area ·Ron) compared to a more traditional MOSFET design. For the same gate voltage of 5 V, GaN FETs have at least five times lower gate charge than MOSFETs, according to Tiziano Morganti, senior field application engineer at EPC.

The transistors are also smaller, with lower Coss, faster voltage transition and zero reverse recovery.

 At full load, EPC eGaN FETs can operate with 96% efficiency at 500 kHz switching frequency, enabling 1 kW/phase compared to silicon-based solutions, which are limited to 600 W/phase owing to the limitation on the inductor current at 100 kHz maximum switching frequency. The design is scalable, so two converters can be paralleled for 4 kW, three converters for 6 kW or only one phase can be used for 1 kW.

The design measures 108 x 70 x 40 mm and uses a standard commercially available eighth brick heat sink for thermal management.