Research On Charger Of High-speed Train Based On LLC Resonant Converter

With the rapid development of high-speed trains,lightweight and intelligent design has become an important development direction.DC charger is one of the core electrical equipment of high-speed train,which provides power for driving circuit,control circuit and battery of high-speed train.At present,the traditional high-speed train charger has several shortcomings,such as large volume,concentrated quality and low efficiency.Owing to its low loss and small size,LLC resonant converter has been widely researched in low power applications.The research of this paper is LLC resonant converter,which is applied in DC charger in high speed train.Moreover,it is of great importance to guarantee the safe operation of high speed train.Firstly,this paper analyzes the topology and working principle of LLC resonant converter.Then,the full cycle energy flow relationship of LLC resonant converter is deduced including intermittent mode,critical continuous mode and continuous mode.Besides,the soft switching characteristics of primary and secondary switching devices in each mode are discussed,which is the basis of control strategy and work area selection in LLC resonant converter.According to the analysis of MATLAB/ Simulink simulation,the soft switching characteristics of the primary and secondary switching devices under different working modes are verified.Besides,the working areas of discontinuous working mode and critical continuous working mode are determined.In order to calculate the exact resonant parameters of LLC resonant converter,a unified optimization model with all parameters and all states is proposed in this paper.The effects of parasitic parameters and absorption parameters on the gain of LLC resonant converter are analyzed,and a novel resonant model with all parameters is established in frequency domain.By analyzing the influence of intermittent area in intermittent mode,a novel model with all states is established in time domain.Finally,the influence relationships between DC gain and various parameters of LLC resonant converter are deduced,and the iterative algorithm based on small variable disturbance is designed,which provides reference for the accurate design of engineering parameters.Based on the resonant model,the integrated control strategy of LLC resonant converter is designed.In the normal state,the frequency conversion control strategy is used to adjust the switching frequency of LLC resonant converter according to the output voltage value realtimely.In the start-up state,the overshoot suppression strategy is used to deduce the consistency between variable duty cycle control and variable frequency control.By reducing the duty cycle,the start overshoot is reduced and the smooth start is realized.In light load state,intermittent control strategy is used to reduce the output voltage.Under the full load state,the digital synchronous rectification control strategy is adopted to control the secondary switching devices to turn on and off in a certain order.Therefore,the switching loss and heat loss of the secondary side can be reduced certainly.The effectiveness of the integrated control strategy of LLC resonant converter is verified by MATLAB/ Simulink software,and its loss analysis is preliminarily explored.Finally,a low power experimental prototype is developed to verify the topology,mathematical model and control strategy of LLC resonant converter.Based on FPGA control controller,the integrated control strategy program of LLC resonant converter is designed,and the algorithm verification under different working conditions is completed.On this basis,the loss of LLC resonant converter is analyzed and optimized.In conclusion,the proposed resonant model and integrated control strategy lay a good foundation for its engineering application.