Research On Topology And Control Strategy Of Bidirectional LLC Converter For On-Board Charger

Due to the gradual depletion of global energy,countries around the world have shifted the focus to the development of new energy vehicles.With the increasing number of electric vehicles,people pay more and more attention to the research on related technologies of on-board chargers.The traditional on-board charger can only realize the one-way transmission of energy,while,with the increase of people’s life demand,people need on-board charger to reverserelease the energy in the car battery on many occasions,henceon-board bidirectional charger has a great application space.First,the background and significance of the concept of new energy vehicles are explained.Then the development of on-board chargers is studied,meanwhile,various bidirectional DC/DC circuit topologies are analyzed,non-isolated and isolated topologies are introduced as well.Next it focuses on a variety of bidirectional DC/DC converters evolved from LLC circuits.The traditional Bidirectional LLC circuit has some problems,such as asymmetric topology,inappropriate control method,slow output feedback response,and high output voltage overshoot.Through the comparative study of different LLC circuit structures,the symmetrical CLLLC topology is finally selected as the DC/DC conversion topology in the bidirectionalon-boardcharger.Secondly,the operating principles of the bidirectional CLLLC converter are studied under variable frequency and phase shift control.Simultaneously,the characteristics of the converter voltage gainare deduced byusing mathematical modeling analysis.Aiming at the problem that a large frequency range needs to be changed when the converter realizes a wide voltage gain under variable frequency control and the converter can only work in step-down mode under phase shift control,a control method based on phase shift and frequency conversion hybrid is adopted to achieve the requirement of wide voltage gain.Finally,aiming at the problem of high overshoot and poor robustness of traditional PI control in nonlinear systems,a fuzzy PI control CLLLC converter strategy is proposed to improve the problems of high overshoot and long dynamic response time.The simulation model is established using MATLAB software tools and the simulation was carried out.The simulation results show that the bidirectional CLLLC converter based on hybrid control studied in the thesis can automatically switch to the theoretically designed control mode during boost and buck operation,and the output voltage can reach the set value.The CLLLC circuit model of traditional PI control and fuzzy PI control are built.The simulation results show that the bidirectional CLLLC converter based on fuzzy PI control has lower output voltage overshoot and shorter response time than traditional PI control.The simulation results verify the feasibility and effectiveness of the theoretical design.