A Bidirectional Synchronous/ Asynchronous Rectifier Control for Wide Battery Voltage Range in SiC Bidirectional LLC Chargers

As for bidirectional LLC converter, conventional pulse frequency modulation is typically adopted in the reverse mode resulting in a low voltage gain of reverse LLC. Then the bus voltage may be lower than peak grid voltage, which causes the buck-type inverter cannot output 220 Vac and poses series challenge for bidirectional operation. A bidirectional synchronous / asynchronous rectifier control is proposed for wide battery voltage applications in LLC converter. In the forward mode, the secondary-side switching devices are regulated as synchronous rectifiers (SRs) and a simplified model is built to calculate the SR on-time. In the reverse mode, with the same conduction time, the turn-on instants of primary-side and secondary-side switches are asynchronous and controlled with a delay angle. The proposed control not only achieves the forward LLC SR function with low microcontroller calculation resources, but also helps the reverse LLC obtain high voltage gain. A 6.6-kW SiC bidirectional LLC charger was built. By using the proposed control, the forward LLC efficiency improves 0.2% compared to conventional SR method. In the reverse mode, the LLC voltage gain is up to 1.33 so that the charger can output 220 Vac in the wide battery voltage range of 200 V~500 V.