A Hybrid Pulse Frequency Modulation Control Strategy for L-LLC Resonant Converter

The traditional control strategy for the L-LLC resonant converter is the synchronous unequal width pulse frequency modulation (SUW PFM). However, this strategy causes a large circulating power in the forward buck and reverse boost modes, especially when the voltage gain deviates from 1. To solve the limitations of the SUW PFM, a novel constant width PFM (CW PFM) is proposed in this article, which can reduce the circulating power and improve the converter efficiency in the forward buck and reverse boost modes. Furthermore, a hybrid PFM control strategy is proposed by merging the appropriate working conditions of the CW PFM and the SUW PFM, which can flexibly switch the two modulations depending on the gain and load conditions. Therefore, the L-LLC resonant converter has good soft-switching characteristics in entire load and gain ranges. Finally, a 7.5-kW L-LLC resonant converter prototype is built to verify the validity and superiority of the proposed control strategy.