Research Of Parralel Current Sharing Technology Of Electric Vehicle Charing Module

Electric vehicle charging piles usually use the way of multiple power modules connected in parallel to achieve high-power output.Due to the limitation of the technological level and the difference of device characteristics,it is difficult to ensure that each module can evenly share the load current if the charging modules are directly connected in parallel.Therefore,the technology of current sharing must be used to balance the distribution of load current in the parallel system.This article is focus on the electric vehicle charging pile with rated output power of 300 KW and research and design the current sharing technology among its internal parallel power modules.The main work of this article is divided into four parts:(1)Analyzing the working principle of the half-bridge LLC resonant circuit simply.According to the analysis of the operation of the LLC resonant converter,deriving the working waveform of LLC resonant converter and calculating the relevant parameters.Designing the sampling circuit,CAN communication interface circuit and protective circuit.(2)Comparing the commonly methods of current sharing and combining master-slave control method and average current method.Adopting the method of "digital average current technology based on master-slave control of average current" to achieve the current sharing control between charging modules.(3)Analyze the double loop control of the LLC resonant circuit,knowing that the voltage loop samples the output voltage,then compares the amplification,and then adjusts the pulse width to ensure the stability of the output voltage.The double ring is generally set as the outer ring.However,due to the existence of LC and RC integrator circuits in the voltage loop,the output voltage lags behind,which affects the transient characteristics of the output.Therefore,an internal current loop is set in the voltage loop to sample the output current,and then it is amplified and then adjusted.The pulse width ensures the stability of the output voltage,and the current loop response speed is faster than the voltage loop,compensating for the system's static error and load variation.In order to realize the digital current sharing control based on the master-slave control of the average current,a current sharing loop is added in the double loop control.Three-loop control technology is used to achieve this digital average current control.The voltage loop and the current sharing loop are adjusted at the same time,and then the output of the voltage loop and the current sharing loop is compared with the output current of the charging module as the reference of the current loop.(4)Design and analyze the software control of parallel current sharing system of charging module.Mainly for the CAN communication module,the choice of the main module,the data transmission process between the master and slave modules is designed.In addition,after the sampling value of the charging module is read,the accuracy of the current sharing is improved by increasing the sampling accuracy through software filtering.(5)The Simulink simulation model is built to verify the rationality of the parameter design of the charging module and the feasibility of the three-loop control strategy finally.When two-loop control and three-loop control are adopted,the experimental results obtained under the two conditions of the system steady state and dynamics are compared and analyzed.It is concluded that when the current sharing loop is added,when the system is in a steady state,not only is the parallel module reduced the error of the output current between them and also increases the stability of the system.When the system dynamics,in addition to the same change with the steady state,at the moment of load change suddenly,the amplitude of the oscillation is reduced,the overshoot of the current is reduced,and the stability of the system is greatly improved.