Research Of Constant Frequency Modulation Strategy For Bidirectional CLLC Resonant Converter

As an important energy interface,isolated bi-directional DC-DC converter has applied in many applications,like DC distribution network,electric vehicle,energy storage system and so on.Among them,bidirectional CLLC resonant converter is favored by researchers because of its symmetrical structure,simple parameter design,easy high frequency and magnetic integration.In the current research,the bi-directional CLLC resonant converter works at a constant switching frequency which can reduce the control complexity when the switching frequency is improved with reduced volume of the converter.However,how to ensure the bidirectional CLLC resonant converter with constant frequency can work efficiently and reliably while relizing energy bidirectional transmission naturely is still a problem to be solved.Therefore,this paper takes the bidirectional CLLC resonant converter as the research object,proposes different constant frequency modulation strategies to improve the efficiency,reliability and power density of the converter,and the principle and characteristics of the corresponding modulation strategies are studied.First of all,this paper analyzes the modes of the bidirectional CLLC resonant converter at different working frequencies,proposes an improved fundamental equivalent circuit to obtain the gain characteristics of the converter in the under resonant region,and determines the optimal working frequency of the converter as the resonant frequency through comparative analysis.Then,the shortcomings of the current design of the excitation inductor are analyzed.The expression of the resonance current is obtained by establishing the equivalent circuit in the dead time of the converter.According to the criterion of realizing soft switch of the converter under different conditions,a new design flow of the boundary excitation inductor is proposed to improve the efficiency of the converter.In this paper,it is found that there is a problem of uneven switching loss distribution in the application of Synchronous PWM modulation strategy to the bidirectional CLLC resonant converter based on gallium nitride high electron mobility transistors(Ga N HEMTs).In order to solve this problem and improve the operation reliability of the converter,this paper proposes an improved two carriers phase-shifting modulation strategy.By analyzing the different modes of the converter under the modulation strategy,the time-domain equations of the converter are established,and the voltage gain characteristics and soft switching characteristics under the modulation strategy are obtained.The analysis results show that the converter still has the advantages of wide voltage regulation range and naturely bidirectional energy flow while solving the uneven distribution of switching losses.For the bidirectional CLLC resonant converter,the volume of the resonant cavity magnetic element is large,and the switching frequency of existing switching devices such as siliconbased MOS transistor,IGBT(insulated gate bipolar transformer)can not reach the limit working frequency of the magnetic material.Based on the existing research,this paper proposes a frequency doubled PWM modulation strategy with energy flowing freely in both directions,so as to further improve the power density of the converter.By analyzing the working principles and modes of different modes of the converter under the strategy,the basic wave equivalent circuit model of the converter is established,and the voltage gain characteristics under the modulation strategy are obtained.The voltage and current stresses of the resonant cavity devices and the switches of the converter are calculated by using the basic wave equivalent circuit,so as to provide the theory for the parameter design of the converter Guidance.According to the different modulation principles of the three modulation strategies,the corresponding simulation and experimental platform is built.Firstly,the calculation program is compiled to verify the accuracy of the proposed calculation method of boundary excitation inductance.According to the calculation results,an experimental platform is built.The full load efficiency of the converter is 97.3%,and the peak efficiency is 97.6%.The experimental results show that the parameters obtained by the proposed design method can not only ensure the full load range soft switch of the converter,but also achieve efficient operation.Then,the principle and characteristics of the improved phase-shifting modulation strategy are verified by simulation and numerical calculation,which proves that the strategy has the advantages of wide voltage regulation range and improved loss distribution.Finally,the simulation platform of frequency doubling PWM modulation strategy is built to verify the characteristics of frequency doubling and natural bidirectional power flow.In a word,this paper takes the bidirectional CLLC resonant converter as the research object.The corresponding parameter design method and constant frequency modulation strategy are porposed to solve some problems existing in the practical application of the bidirectional CLLC resonant converter.So that the advantages of high frequency and high efficiency of the bidirectional CLLC resonant converter can be brought into play.