The Design Of LLC Converter Considering Parasitic Parameters And Its Control

With the development of the third-generation wide-bandgap semiconductor technology,miniaturized switching power supplies are increasingly widely used in aerospace,communications,data centers and other fields,high frequency and high power density switching power supplies have become a development trend.With its resonant characteristics,LLC resonant converter can achieve soft switching characteristics within the full input voltage and load range,its superior soft switching performance can effectively reduce high frequency switching losses and improve system efficiency.In order to further increase the switching frequency and meet the needs of high frequency,new wide-bandgap semiconductor GaN devices are widely used.GaN devices have great advantages in switching speed,parasitic parameters,on-resistance,etc.,to achieve high frequency and high power density,which is the best switching device selection for switching power supply.However,at high switching frequency,the existence of circuit parasitic parameters will bring a series of problems to the circuit and affect the normal operating performance of the LLC converter.The control strategy at high frequency will also directly determine the overall operating efficiency and performance of the LLC converter.In order to solve the above problems,this paper conducts related research from the angle of circuit parameter design optimization and control strategy optimization.The specific research contents include:(1)Research on the influence of high frequency parasitic parameters.In this paper,an improved equivalent model of resonant network is established,and the related research on circuit parasitic parameters is carried out based on this model.Aiming at the influence of parasitic parameters on the DC voltage gain,a phasor model describing the magnitude and phase relationship of each resonant parameter is established,and its influence mechanism is described;Aiming at the influence of parasitic parameters on the switching process,an equivalent time domain model is established to quantitatively describe the influence of parasitic parameters on the realization of soft switching on the primary side and the turn-off process of the secondary side rectifiers.(2)Optimize the circuit parameter design considering parasitic parameters.Aiming at the influence of parasitic parameters on DC voltage gain,this paper considers parasitic parameters and proposes a new LLC parameter design method based on phasor model,which solves the problem of mismatch between parameter design and actual model.In view of the influence of parasitic parameters on the switching process,the time domain expression of the resonant current is solved to obtain the dead time boundary conditions to ensure the effective realization of soft switching;the time domain solution of the rectifier turn-off voltage peak value is used to guide reasonable selection of the rectifier.(3)Optimize the control strategy of LLC converter.Aiming at the problems of wide voltage gain range under rated load and insufficient frequency resolution of the digital controller,the control strategy under rated load is optimized,and a variable frequency and phase shift hybrid control strategy is proposed.Aiming at the problems of wide voltage gain range and low control efficiency under light load,the intermittent mode control strategy is adopted to effectively reduce the light load loss.Aiming at the problem of large inrush current in the process of LLC high-voltage startup,the optimal soft-start scheme is studied,and the optimal soft-start scheme for frequency reduction and phase shift hybrid startup is determined to achieve the optimization goal of minimum startup inrush current.Finally,in order to verify the above theoretical analysis,an experimental prototype with a rated power of 2kW and a resonant frequency of 500 kHz was built to verify the above-mentioned parameter optimization design method and control performance optimization strategy.The experimental results are basically consistent with the theoretical analysis results.