Research On LLC Resonant Converter Based On Input Series Output Parallel

As a low-voltage power supply equipment,the performance and reliability of new energy vehicle on-board DC/DC chargers have a great influence on the performance of automotive electrical networks.LLC resonant converter has become a research hotspot in the field because of its strong anti-interference ability,low switching loss and high switching frequency in the field of application of on-board DC/DC chargers.This paper studies the control strategy and current sharing characteristics of LLC resonant converter,which has theoretical and engineering significance.Considering that the LLC resonant converter is faced with excessive output current ripple in the application process,high design requirements for filter capacitors,and excessive voltage stress on the switch tube,this paper analyzes its AC and DC characteristics and presents a hybrid control method,and the current sharing characteristics of the series-parallel converter are analyzed and the feasibility is verified.A four-phase ISOP type LLC resonant converter is designed.This paper firstly designs an improved hybrid control method based on the characteristics analysis of single-phase LLC resonant converter.Based on FHA modeling and topology simulation respectively,the AC/DC characteristics and limitations of LLC resonant converter under PFM and PWM control strategies are analyzed.Aiming at the uncontrollable problem of PFM under high frequency conditions and the uncontrollable PWM under high duty cycle conditions,based on the real-time segmented control method,an improved hybrid control strategy combining the advantages of PFM and PWM control is proposed to ensure the monotonicity of voltage gain.Experiment verify its effectiveness.In order to solve the current sharing problem of LLC-resonant converter in series-parallel mode,this paper analyzes its current sharing characteristics and feasibility,and presents the ISOP serial-parallel method.Firstly,the equivalent circuit model of IPOP and ISOP type LLC resonant converter is established,and the current balance index is introduced to analyze the current sharing characteristics of IPOP and ISOP series parallel resonant converter.For the problem that the current distribution of each phase under the IPOP series-parallel connection method is sensitive to the resonant parameters,this paper adopts the ISOP serial-parallel method to design the converter,the excitation inductor current,series resonant inductor current and low-side output current under the full operating frequency range are analysed.Analysis was performed to meet the ripple reduction requirement and achieve soft switching characteristics.Based on the improved hybrid control strategy of single-phase LLC resonant converter and the analysis of the current sharing characteristics of the ISOP type LLC resonant converter,a four-phase ISOP type LLC resonant converter was designed.In order to reduce the power level and output current ripple of each phase resonant converter,this paper uses a four-phase and 45-degree interleaving operation method.Through the division of the capacitive and inductive region of the voltage gain curve,the optimum working area is determined and the resonant parameters of the designed 4-phase ISOP type LLC resonant converter are designed.Based on Matlab simulation software,the effectiveness of the designed converter was verified.Finally,a 3 k W vehicle DC/DC charger prototype experimental platform was built to verify the rationality of the resonant parameters of the converter and the correctness of the current sharing analysis.The monotonicity of the voltage gain regulation under the improved hybrid control strategy was verified.Test and verify response and efficiency.