Investigation of Adaptive Synchronous Rectifier (SR) Driving Scheme for LLC/CLLC Resonant Converter in EV On-Board Chargers

For LLC/CLLC resonant converters, synchronous rectifiers (SR) are desired to be turned off at the current zerocrossing moment to avoid the additional conduction loss and the circulation current. In this paper, an adaptive SR driving scheme is proposed for LLC/CLLC resonant converter, which guarantees the ZCS turn-off of the SRs and improves the system efficiency. The proposed method senses the drain-to-source voltage of only one switch, which makes it easy to implement. Meanwhile, a simple drain-to-source voltage sensing circuit is proposed. The proposed circuit can be directly interfaced with the digital controller without any signal conditioning circuit. Moreover, external RC circuits are employed to compensate the turnon moment discrepancy between the SR and its corresponding primary side switch. A 6.6 kW/500 kHz CLLC resonant converter prototype is built, which is used as the isolated dc-dc stage for an EV on-board charger. The proposed adaptive driving scheme is validated by experimental tests.