Design Considerations for High Frequency Battery Charger with GaN Device
The overall performance of a 600V GaN device, which includes a low RDS(on) and a small parasitic capacitance (CGD, CDS, CGS), make it very attractive for onboard bidirectional battery charger/discharger applications that require high power density and system efficiency to boost battery performance and improve fuel economy. Preliminary test results demonstrate >98% efficiency over a wide load range with a 500 kHz, 5kW bidirectional buck-boost converter. On the other hand, a small parasitic capacitance allows a GaN device to switch very fast and introduces a high di/dt (3~6A/ns) and dV/dt (>100V/ns), which is 3~6 times higher than that of a state-of-the-art MOSFET device. These fast transients will falsely trigger a gate drive through either a typical low-transient-immunity high side gate driver or a common source inductance, causing shoot through for the GaN based bridge configuration. Therefore, typical design guidelines and device selection would not satisfy these harsh switch environments and most of commercial parts fail in testing. This presentation will first investigate the above design challenges for a gate driver and system reliability of a GaN based battery charger, and then introduce several possible solutions.