Design Considerations of Efficiency Enhanced LLC PEV Charger Using Reconfigurable Transformer

LLC topology demonstrates optimal efficiency near its resonant frequency, where its normalized voltage gain is close to unity. However, in plug-in electric vehicle (PEV) charging applications, the voltage gain of the LLC converter needs to vary in a wide range. This makes the switching frequency deviate away from the resonant frequency. In this paper, an improved LLC-based PEV charger is proposed to cope with this challenge. The proposed converter adopts a reconfigurable two-level turns-ratio transformer. With the increase of output voltage, the transformer secondary side reconfigures from low turns to high turns. Thus, the switching frequency range can be effectively squeezed around the resonant frequency in constant current charging stage. In constant voltage charging stage, the switching frequency of the LLC converter is exactly tuned to the resonant frequency. Therefore, the overall efficiency can be enhanced over the entire charging process. The design considerations of the proposed PEV charger are addressed in detail. A 1.35-kW prototype to charge a 250-450 V battery pack is designed and tested both in steady state and dynamically. The prototype demonstrates 97.47% peak efficiency and enhanced efficiency performance over the wide output range.