Research On Resonant Converter And Control Strategy Of Vehicle Charger

With the rapid development of electric vehicles in China,the total amount of electric vehicle battery charge in China far exceeds the total amount of chemical energy storage capacity,the storage of new energy vehicles is becoming more and more important,and the on-board charging device as an important device for the bidirectional flow of energy in electric vehicles,the design of a high-performance bidirectional on-board charging device is an outstanding research topic in the current academic community and industry.The main works of this thesis are as follows:Firstly,the design requirements and the main circuit topology of the on-board charger are studied.The advantages and disadvantages of different resonant converter topologies are studied according to the requirements analysis.The bidirectional full-bridge L-LLC resonant converter is selected as the main circuit topology according to the scenario demand,the working principle of the selected topology is studied,the voltage gain model is established,the requirements of the converter in achieving soft switching are focused,the k and Q constraints to ensure that the converter can achieve soft switching characteristics are established.Secondly,the resonant cavity voltage and current stress overload problem in the startup process of the bidirectional full-bridge L-LLC resonant converter and the soft-start strategy are studied.Based on the state trajectory plane theory,the operating modes of the resonant converter and the equivalence circuit state trajectory equations are analyzed.The designed soft-start strategy realizes the control of resonant current and resonant voltage stress in three stages,which avoids the resonant cavity energy overload during the start-up process and threatens the safe operation of the components,and the method is easy to be implemented in hardware to realize the soft-start of the circuit without output voltage overshoot,which takes into account the start-up The resonant cavity current and voltage stress are too large and the start-up time is too long.Thirdly,the problem of insufficient dynamic response of PI control during load switching and the control strategy is investigated.The state trajectory equation is used to analyse the trajectory of the state variable when the load is suddenly increased or decreased,and the mathematical expressions for increasing or decreasing the corresponding pulse width of the converter when the load is suddenly increased or decreased are derived,so that a load switching control strategy is designed to make the state trajectory quickly reach the expected state trajectory when the load is jumped,so as to be able to respond quickly,and then the voltage compensation controller is used to The steady-state control can improve the performance of the dynamic response of the converter under PI control.Fourthly,the control strategy of the converter under light load is investigated.The modal mathematical expressions of the time domain equations of the L-LLC resonant converter under light load are analysed and derived.For the problem of low efficiency of the resonant converter under light load conditions,the multi-pulse multi-step intermittent mode control strategy under light load is proposed.Different pulses are controlled according to different light load conditions,thus improving the light load efficiency of the resonant converter.Finally,the prototype bidirectional full-bridge L-LLC resonant converter is simulated and built for experimental verification.The main circuit topology of the converter as well as the soft-start,load switching,and intermittent mode control schemes under light load are simulated and verified in the prototype.The experimental results show that the experiments prove the correctness and feasibility of the control scheme proposed in this paper in terms of soft switching implementation,reduction of soft-start resonant cavity current and voltage stress,dynamic response to sudden load changes,and efficiency under light load.