Research On Two-stage Isolated On-board Charging System And Control Technique

With the development of the automobile industry,electric vehicles have been gradually promoted.Compared with fuel cars,electric cars have the advantages of low noise and zero emissions,electric vehicles can be divided into hybrid vehicles and pure electric vehicles.In this dissertation,a two-stage isolated on-board charging system is proposed for electric vehicles.The charging system can realize the two-way flow of energy,not only can charge the battery pack,but also convert the energy of the battery into alternating current and provide energy to other devices.The charging system adopts two-stage topology,the pre-stage circuit uses bidirectional totem-pole converter and the rear-stage circuit uses bidirectional CLLC resonant converter.First and foremost,the commonly used bidirectional DC-DC converters are studied,and the advantages and disadvantages of different topologies are introduced.The bidirectional DC-DC topology used in this dissertation is determined,namely double-active bridge CLLC resonant converter.Furthermore,the working mode of the two-stage isolated on-board charging system was deeply explored,and the working principle of the on-board charging system in forward and reverse modes was analyzed.The small signal model of the bidirectional totem-pole converter is established by using the state-space equation,the transfer functions of the outer voltage loop and the inner current loop are derived and the parameter design of the compensation controller is completed.The First harmonic approximation analysis method is used to obtain the AC equivalent circuit of the CLLC resonant converter,and the expressions of the forward voltage gain and the reverse voltage gain are derived.The influence of the inductance coefficient,capacitance coefficient and quality factor on the resonant converter is analyzed.Above all,according to the design requirements of the on-board charging system,the input inductance and DC-link capacitance parameters of the bidirectional totem-pole converter and the key parameters of the rear-stage CLLC resonant converter are calculated,in order to ensure that the charging system can realize the zero-voltage switching and zero-current switching of power switches.In the forward working mode of the on-board charging system,which can obtain a wide range of output voltage,and at the same me,the power factor correction function can be realized.In the reverse working mode of the charging system,the charging system can convert the energy of the battery into alternating current.Based on the analysis of the main parameters and control strategy of the system,tried to make a 3.3k W on-board charging system prototype,simulation and experiment results verify the correctness of theoretical analysis and topology design.