Hybrid Mode Control Strategy of a CLLC Resonant Converter with Wide Voltage Gain for EV On-Board Chargers

A large switching frequency range is typically necessary to attain a wide voltage gain with pulse frequency modulation (PFM) for CLLC resonant converters, especially under light loads. To achieve a wide voltage gain of CLLC resonant converters without increasing the switching frequency range too much, this paper proposes a hybrid modulation strategy of PFM and inner phase-shift modulation (IPSM). Both modulation schemes are analyzed in this paper, including the voltage gain and ZVS regions. IPSM is used in light-load and no-load conditions to expend the buck range and to achieve no-load control. In order to improve system efficiency, a synchronous rectification (SR) method is proposed based on the current detection. A turn-on delay is considered to avoid malfunction of the SR. The proposed control strategy is validated by simulation and an 11kW prototype with a peak system efficiency of 97.7%.