Research On Time Domain Analysis Based On Full-bridge LLC DC/DC Converter

In order to obtain a wide voltage gain range and satisfy the ZVS condition of switches,a small normalized inductance ratio of the magnetizing inductance to the resonant inductance is designed in the traditional LLC converter which increases the primary-side circulating currents and the high frequency ac resistant of resonant inductor,consequently,the conduction loss and the ZVS boundary of switches is not considered based on the FHA method which directly affect the overall operating efficiency of the converter;burst mode is adopted to obtain a high effiency by reducing the load-independent losses under lightload,but there exist some hard switching processes during the initial stage of burst-on time,the voltage ripple is increased and the interaction between parasitics capacitances and the resonant elements leads to high frequency oscillation and produce high EMI noises in the burst on time,which have a negative effect on the improvement of efficiency and power density.With the progress and development of PFC,the required voltage gain range is decreased,a full bridge LLC resonant converter cooperate with front-end AC/DC stage for PFC is studied,a big inductance ratio is designed to reduce the primary-side currents and conduction loss,and the ZVS switching under different loads are achieving in this paper.Commutation energy is stored in the resonant inductance and the load current is established to ensure the ZVS switching under heavy-load;based on the time domain design and control method,parasitic parameters are considered,the equivalent time domain model in different states and circuit formulas are established,steady state trajectory of LLC resonant converter under different loads are analyzed,and the steady state plane is plotted to simplify the derivation of ZVS condition;the relation curve of output power and resonant parameters under heavy-load is matched and the state switching points of different operation modes and loss are analyzed in detail.Then,aiming at the problem of low control efficiency under light load,the stage of freewheeling in the primary side is proposed and the switching frequency is reduced to obtain lower power;the continuous working mode is adopted to eliminate the problems of hard switching and oscillation existing in the traditional mode and the large output filter capacitance;the energy is stored in the magnetizing inductance to solve the problem of insufficient energy storage caused by the use of a small resonant inductor and the steadystate trajactory is combined with time domain analysis to derive the ZVS condition of switches;the time domain equation of output power and resonant parameters is derived and the working mode switching point is disscussed;in order to prevent control strategy from frequent switching when the load is step up or step down in a small range,the boundary of switching frequency is designed to make the upper limit of output power under light load higher than the lower limit of output power under heavy load.According to the selection of the main parameters of the resonant converter,the SIMETRIX simulation model is build and the ZVS switching under different loads are verified by simulation;the primary-side circulating currents of the proposed converter are compared with traditional converter under the same output conditions and the feasibility of efficiency improvement is verified.Finally,a 3000 W output experimental prototype was built,in order to verify the ZVS conduction of switches under different loads,the performance at full load and upper limit of light load are tested respectively,the primary-side RMS currents of the proposed converter are compared with traditional converter under the same output conditions and the efficiency under different loads are tested,the soft switching under different loads is proved and the efficiency is improved obviously based on the experimental results,and the peak efficiency can reach 98.4%,the experimental results are basically consistent with the theoretical analysis results,the feasibility and accuracy of the design and analytical method are verified.