A SiC Bidirectional LLC On-Board Charger*

The LLC converters are widely used in the unidirectional chargers. However, the reverse LLC voltage gain is lower than unity. The DC-link voltage is then lower than the peak grid voltage so that the buck type DC-AC converters cannot be grid-tied directly, which causes the LLC reverse operation difficult in bidirectional charging applications. This paper proposed a SiC bidirectional LLC on-board charger architecture to achieve high efficiency and high power density. The first stage is an interleaved bridgeless totem pole PFC to achieve unity power factor. The second stage is a 300-kHz LLC taking advantage of wide ZVS range and magnetic integration. Thanks to extra control freedom of high DC-link voltage, the DC-DC voltage gain regulation burden is shared by the DC/AC stage taking advantage of high voltage SiC MOSFETs. An LLC reverse voltage gain compensation control by regulating DC-link voltage is proposed to enable the LLC bidirectional operation. A digital adaptive synchronous rectification driving scheme is proposed based on the LLC primary driver signals. The adaptive SR on-time is modified in every switching control cycle guaranteeing fast transient response and suitable for high frequency applications. A prototype of 6.6 kW SiC bidirectional LLC charger was built and the battery voltage from 240 V to 420 V. The power density is 44 W/in3 with 3 kW /kg and increases 22.2% over the Wolfspeed design. The charging peak efficiency is 95.3% through, and 1.3% higher than the state-of-the-art products. The peak discharging efficiency under 6.6 kW is 95.6%, and 0.6% higher than the state-of-the-art efficiency.