Geometrical State-Plane-Based Synchronous Rectification Scheme for LLC Converter in EVs

Traditional synchronous rectification (SR) strategies in LLC converters are insufficient to meet the stringent demands imposed by the onboard chargers (OBCs) for plug-in hybrid electric vehicles (PHEVs), which require exceptional cost-efficiency, high performance, safety compliance, bidirectional operation, and the ability to handle high output voltage. In contrast, model-based SR strategies for LLC converters, driven solely by sensed dc signals, offer numerous advantages, including cost-effectiveness and robust immunity to electromagnetic interference (EMI). Nevertheless, the precision of the model significantly impacts the model-based SR strategy's performance. Furthermore, under light-load conditions, LLC converters may enter a discontinuous current mode (DCM), leading to extended SR conduction time and additional reactive current for model-based SR strategies. To solve such issues, a novel state-plane-analysis-based SR strategy for LLC converters is proposed, enhanced with an online DCM identification algorithm. Leveraging geometric principles and tailored simplifications, analytical expressions are derived for both the SR conduction time and the DCM boundary. The proposed SR strategy exhibits remarkable versatility, spanning the entire frequency range of LLC converters, and offers an exceptional dynamic response. The proposed approach is validated with a 6.6-kW prototype, achieving a rated efficiency of 97.5% and demonstrating a weighted SR timing accuracy of 97.8%.