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Light Load Efficiency Improvement of Three Phase CLLC Resonant Converter for On-Board Charger Applications

Test setup
Fig.1. Test setup and sample waveforms
  The three-phase CLLC (3PCLLC) resonant converter is a good candidate for the on-board-charger application due to the low current stress it incurs on the device, its automatic current-balancing among phases, and its good interleaving. However, the efficiency at light load is sacrificed due to its higher number of switching devices, driving circuits, and magnetics. To improve the efficiency and achieve bi-directional energy transfer, synchronous rectifiers (SRs) can be adopted. The conduction time of the SR will impact the operation of circuits, gain characteristics, and efficiency. In this paper, the SR ON-time is extended to achieve negative current for better regulation capability and light-load efficiency improvement. A new phase-shedding method for the 3PCLLC converter is proposed and analyzed. By shutting down one or two bridges as the load current decreases, the light-load efficiency increases significantly. Experiments are implemented on an 11 kW 3PCLLC converter with 500 kHz operation frequency to verify the extended SR ON-time control and the phase shedding method; these results show an improvement of more than 9% under 20% load output conditions as Fig. 1 shows.

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