Research On Control Strategy Of Full-bridge LLC Resonant Converter For Charging Equipment

In order to solve the increasingly serious environmental degradation and energy shortage,electric vehicles as the representative of green new energy industry are gradually popularized and applied.In order to ensure the safe and reliable energy supply of electric vehicles,it is necessary to design and study their charging equipment.Full-bridge LLC resonant converter is widely used in the rear DC-DC power module of charging equipment because of its soft switching characteristics and magnetic integration.However,different brands of electric vehicle power battery pack specifications and different charging needs of the majority of electric vehicle users cause the charging equipment load situation is variable.In this case,the charging equipment is required to meet the different load requirements,so the charging equipment rear DC-DC power module is required to achieve a wide output voltage range.In this study,a full-bridge LLC resonant converter is used to achieve a wide output voltage range.Firstly,the equivalent model of the full-bridge LLC resonant converter is studied to derive the DC output gain function of the converter,and based on this function,the effects of inductor normalization,quality factor,impedance characteristics and resonance parameters on the DC gain are studied;the necessary conditions for the converter to achieve zero-voltage conduction are derived;finally,the converter losses are studied to derive the relative loss rate function.The above process and results lay the foundation for the design of resonant parameters of full-bridge LLC resonant converter.Secondly,Li-ion power battery pack is selected as the load of charging equipment to study the load characteristics,and the worst operating point in the charging process is derived and translated into the relationship between gain and resonance parameters;and the resonance parameters of full-bridge LLC resonant converter have a decisive role in its conversion efficiency and output voltage range,so the relative loss rate is minimized as the objective function,and the DC gain,ZVS condition,operating frequency and load characteristics are used as constraints to introduce quantum particle swarm algorithm for resonant network parameter design;finally,the resonance parameters output effect is verified by Matlab-simulink model.Again,the state transfer matrix of the closed-loop circuit of the full-bridge LLC resonant converter is derived based on Floquet theory;this matrix is used as the Floquet stability criterion to analyze the closed-loop stability of the system.In the analysis process,the control parameters are influenced on the system stability due to the introduction of the control loop,and then the stability range of the control parameters is deduced;finally,the stability range of the control parameters is verified by Matlab-simulink model simulation to confirm the feasibility of the method.Finally,since PWM control under light load cannot achieve ZVS due to the small excitation current,and Burst control causes large losses due to the large resonant current,the resonant current energy during Burst-on time is used to supplement the lack of excitation current energy for PWM control ZVS to achieve light load The PFM+PWM-Burst control strategy is designed for the full load range: PFM control is used when the load is relatively heavy or the output voltage is large,and PWM control or PWM-Burst control is used when the load is relatively light or the output voltage is very low.The effectiveness and feasibility of the control strategy to achieve a wide output voltage range is demonstrated by Matlab-simulink model simulation and prototype experiments.