Research On Current-source Full Bridge LLC Resonant Converter

In recent years,the isolated DC/DC converter has become an indispensable part of the charging pile of new energy vehicles.With the rise of the new energy vehicle industry,various isolated DC/DC converters have become the current research hotspot.Full bridge LLC resonant converter has become one of the research hotspots because of its wide input-output range,low switching loss and high working efficiency.The common control strategies of resonant converter are voltage control strategy and current control strategy.The voltage-mode control strategy is popular because of its simple control structure,but it is difficult to meet the requirements of fast and accurate converter because of its slow reaction speed and low control accuracy.A type of full bridge LLC resonant converter controlled by PWM is studied in this paper.The main work of this paper is as follows:(1)Based on the comparative analysis of the working principle and characteristics of common resonant converters,the full-bridge LLC resonant converter is selected as the main circuit topology;The operating mode and principle of the circuit are analyzed in detail when the switching frequency is less than the resonant frequency.The basic wave component approximation(FHA)is used to simplify the circuit and obtain the equivalent circuit.The input-output gain characteristics of the system are calculated based on the equivalent circuit,and the working region of the resonant converter is determined by Mathcad.(2)In the case of determining the system gain,the main circuit parameters are designed according to the performance index of a new energy vehicle quick charging pile on the market.The main content of the design includes the design of switching network,resonant cavity,transformer and rectifier filter network.(3)Through mathematical derivation,the idea of establishing small signal model by extended function description method is detailed;Based on the FAH equivalent circuit,the small signal model of full-bridge LLC resonant converter is established by using the extended function description method.The open loop transfer function of the system is obtained by small signal model.The characteristics of voltage-type PWM and current-type PWM control strategies are analyzed.(4)The open-loop simulation model of the system was built in MATLAB/Simulink environment to verify the correctness of the design of the main circuit parameters of the full-bridge LLC resonant converter;After the open-loop transfer function of the system is obtained by fitting the open-loop simulation model,the compensation link of the resonant converter is designed and the closed-loop model of the full-bridge LLC resonant converter is established.(5)The soft switching characteristics of current-type full-bridge LLC resonant converter are verified by analyzing the switching tube operating waveform obtained by the closed-loop simulation model under the working conditions of rated input and rated load.The output waveform of the full-bridge LLC resonant converter is simulated under the condition of switching from light load to heavy load,and the transmission efficiency of the converter is calculated,which verifies the correctness of the closed-loop system design.The results show that the current full-bridge LLC resonant converter designed in this paper can keep the transmission efficiency between 0.92 and 0.96 under the full range of load conditions,and the output adjustment time is less than 2s,which has good rapidity and the whole system has good stability.