Research On The Analysis Method Of LLC Resonant Converters

The DC-DC converter is being an important interface device between the new energy generation and the post-stage converter,since it has the function of realizing the interconnection of different types of DC units.In order to meet the requirements of high efficiency and high power density for switching power supply,high frequency operation and soft switching technology emerge.The LLC resonant DC-DC converter has excellent characteristics of regulating output voltage under wide input voltage and load fluctuation.It can also reduce switching losses of converters by soft-switching technology,which improves the efficiency and power density of switching power supply.The design of the LLC resonant converter usually uses the FHA method to obtain the AC equivalent circuit and the voltage gain characteristics.Thus the resonant parameters are obtained.However,when the switching frequency deviates from the resonant frequency,the FHA method will lead to the increase of the voltage gain error,which can not accurately describe the working characteristics of the DC-DC converter,and can not improve the converter efficiency.In order to accurately solve the voltage gain of the converter,improving the efficiency and power density of the converter,this paper adopts operation mode analysis method to analyze the LLC resonant converter,verified the operation mode analysis has higher accuracy of the voltage gain and efficiency of the prototype in all frequency ranges through the comparison with FHA method by simulation and experiment.Thus we obtained the applicability of two methods.The main research contents of this paper are as follows:First of all,the circuit topology,the gain characteristics,operating waveform in ZVS region and circuit stages of the LLC resonant converter are analyzed in detail.The differences of soft switching between the above resonant region and below resonant region of ZVS region has been emphasized,including the power loss and its related effects which plays an important role in the selection of operating area for the design of LLC resonant converter.Secondly,the analysis methods of LLC resonant converter are theoretically derived and simulated.The frequency domain and time domain model of the converter are established by FHA method and operation mode analysis method.The curve of voltage gain characteristic under different parameters and the combination mode waveform with different operating ranges are drawn by programming and simulation.The parameter design of LLC resonant converter is achieved based on the above theory.The circuit simulation model is built in Psim to simulate the design parameters and to contrast the voltage gain based on the FHA and mode analysis.The simulation verified the voltage gain based on operation mode analysis method has higher accuracy in the whole frequency range,providing a reference for the design of LLC parameters resonant converter in different applications.Finally,based on the theoretical analysis and simulation of the converter,a experimental prototype using FAN7688 as the core control unit is built.The main circuit,feedback circuit,pulse control unit,drive circuit is included.And the experimental prototype is tested.It is shown that that resonant parameter design based on operation mode analysis have higher prototype efficiency in all frequency ranges,which verified the correctness of the comparative analysis in this paper.