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| Three-Phase CLLC Resonant Converter with Synchronous Rectification Control Based on Extended Harmonics |
| Cheng He, Xu Kai, Dong Bin, Miao Hexun, Li Pengsheng |
| School of Electrical Engineering China University of Mining and Technology Xuzhou 221116 China |
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Abstract The CLLC resonant converter has been widely used in electric vehicle (EV) chargers due to its excellent performance, including wide gain, high efficiency, and soft switching under full load. Adopting an interleaved parallel structure is a better choice for medium- to high-power chargers. Compared to half-bridge and full-bridge structures, the three-phase CLLC resonant converter meets the demands for high power density, high efficiency, and high reliability while reducing the size of filter components due to its small input and output current ripple. The EV battery charging process is divided into pre-charging, constant current charging, and constant voltage charging. The three-phase CLLC resonant converter covers all three charging stages using a phase-shedding strategy, operating in three-phase, full-bridge, and half-bridge modes, ensuring high efficiency under full load.
To further improve the converter's efficiency, synchronous rectification control is applied. By utilizing the extremely low conduction resistance of SiC MOSFETs and replacing the diodes on the secondary side with controllable SiC MOSFETs, bidirectional energy flow is enabled, and the converter’s losses are reduced. The fundamental harmonic approximation (FHA) method has traditionally been used for modeling resonant converters with high accuracy near the resonant frequency. However, in the case of the CLLC resonant converter, multiple resonant frequencies emerge due to parameter mismatches, leading to a decrease in the accuracy of the FHA model. A synchronous rectification (SR) algorithm is proposed for the three-phase CLLC resonant converter. An extended harmonic impedance model is established for different operating modes under phase-shifting control, considering the effect of dead time. The conduction time of the secondary side MOSFETs for each mode is accurately calculated, improving the converter’s efficiency across the whole load range.
The control strategy is implemented as follows. The system samples the output voltage and current to determine the output load Ro. It selects three operating modes: three-phase, full-bridge, or half-bridge modes. The appropriate control strategy is then determined according to system requirements. Once the mode and strategy are set, a fitting function calculates the conduction time of the secondary-side MOSFETs based on the converter’s operating frequency fs and output load Ro. The system updates this value through the EPWM register, optimizing the drive waveforms of the primary and secondary-side MOSFETs. After updating, the system enters standby mode and waits for the next interrupt to continue dynamic adjustments.
A 2.5 kW experimental prototype using SiC devices was built. Experimental results show that half-bridge and full-bridge modes had higher efficiency under light-load conditions than the three-phase mode, effectively improving light-load efficiency. Under full-load conditions, applying the proposed synchronous rectification (SR) control improved efficiency in all three modes. Temperature tests indicate that SR control enhances system efficiency and reduces SR MOSFET heating.
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Received: 19 December 2024
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2026, Vol. 41 
