Research On Topology And Control Of Wide-Gain And High-efficiency Series-parallel LLC Resonant Converter

LLC resonant converter is widely used in electric vehicle charging piles,photovoltaic energy storage systems and other applications due to its soft switching characteristics of zero-voltage turn-on performance of the primary side switches and zero current turn-off performance of the secondary side rectifier side diodes within the full load range.Such as for the wide battery voltage level characteristics of electric vehicles,LLC resonant converters is difficult to obtain the wide voltage gain range and high-efficiency operation at the same time.Secondly,as the power conversion level continues to increase,single-module LLC resonant converters can no longer meet the demand.Multi-module series-parallel combined operation has become an important solution for expanding power capacity.When operating at multi-module combination,the tolereance of resonant tank parameters between modules will cause serious current and voltage imbalance problems,resulting in a decrease in the reliability of the converter.Therefore,studing wide voltage gain strategy and the current and voltage balacing method of multiple modules in series and parallel architecture is of great significance.In order to widen the voltage gain range of the converter,this article aims at the control strategy and proposes a hybrid control strategy of frequency modulation and phase shift modulation,analyzes the working modes of the converter under frequency modulation and phase shift modulation,explains the soft switching characteristics of different modes.The gain characteristic of the converter is analyzed based on fundamental harmonic analysis and time-domain analysis method respectively,and the similarities and differences are compared.The turn-on and turn-off currents of the power switches under different working conditions are calculated based on time domain analysis,the hybrid control strategies of frequency modulation and phase shift modulation are designed based on the two criteria of zero-voltage switching(ZVS)constraint and minimum total turn-off current.Finally,simulation and experiment results verifies the hybrid control strategy,and various indictators of the results are compared with single-frequency modulation and single-phase-shift modulation,the results show that the hybrid control strategy can guarantee the ZVS characteristic with wide load range,reduce the turn-off current and improve the efficiency of the converter.Aiming at the combined operation mode of multi-module LLC resonant converter,the current and voltage imbalance characteristics caused by the difference of resonant tank parameters under input-parellel output-parellel(IPOP)and input-parellel outputseries(IPOS)architectures of the converter are analyzed,for this problem,this paper proposes a passive balancing method that is suitable for both IPOP and IPOS architectures and analyzes the current and voltage sharing error of the method under different parameter tolerance combinations.The theoretical analysis results show that this method can improve the current and voltage balancing performance of the converter.Finally,based on simulation and experiment results,the excellent current and voltage sharing characteristics of the passive balancing method are verified.In order to realize the flexible and seamless switching between the IPOP and IPOS architectures of the converter,and widen the voltage gain range and power level of the converter,this paper proposes a impoved topology and control strategy of the seriesparallel seamless switching of LLC resonant converters,so that it can be seamlessly switched between IPOP and IPOS architectures,and maintain a small transient impact.This paper analyzes the structure,working mode and the transient switching process of the topology in detail,based on the goal of minimizing the transient impact,this paper discusses the design rules of the key parameters of the topology,and gives the control method of seamless switching,and finally the feasibility and effectiveness of the topology are proved through simulation and experiments results.