Research Of Synchronous PWM Strategy For Bidirectional Resonant DC-DC Converter Based On Gan HEMT

In recent years,with the development of power electronics and the shortage of energy,there has been an increasing number of distributed new energy sources connected to the power grid.At the same time,DC distribution network has also come concerned,in which the isolated bidirectional DC-DC converter(IBDC)is required for achieving electrical isolation,voltage conversion and energy bidirectional transmission.However,how to naturely and smoothly realize energy bidirectional transmission while ensuring the efficient operation of IBDC is still an urgent problem.On the other hand,Gallium Nitride High Electron Mobillity Transistors(GaN HEMT)have attracted attention due to excellent characteristics such as high switching speed,low switching loss,low on-resistance and no reverse recovery loss.Based on above all,a synchronous PWM CLLLC(primary resonant capacitor-primary resonant inductor-magnetizing inductor-secondary resonant inductorsecondary resonant capacitor)converter based on GaN HEMT is proposed in this paper,the operating principle,characteristics,control strategy of which have been deeply studied.First of all,the operating principle of the pulse frequenct modulated(PFM)and the synchronous PWM CLLLC converter have been analyzed respectively.The time domain analysis method is used to derive the state equation of the synchronous PWM CLLLC converter under different modes,which is the basis for the analysis of operating characteristics.The analysis shows that the synchronous PWM CLLLC converter can naturally and smoothly achieve energy bidirectional transmission and forward/reverse operation switching.Based on the operating principle analysis,the detailed analysis of the voltage gain characteristics,soft-swiching features and power transmission characteristics of the proposed converter has been carried out.By analyzing the relationship between each mode duration and the original/secondary duty cycle,combined with the time-domain analysis,the voltage gain characteristic independent of the load is obtained;combined with soft-switching conditions and time-domain analysis model,the boundary conditions of full range softswitching have been derivated;based on the fundamental harmonic approximation method,it is obtained that power bidirectional transmission can be realized through the adjustment of the original/secondary duty cycle.The analysis of the operating characteristics provides an effective guide for the design of the converter parameters.For the case that the state variables of the resonant circuit are sinusoidal that varies greatly,the extended description function is used to model the synchronous PWM CLLLC converter,and the large signal model and the small signal model are derivated in turn.In order to apply the small signal model to the controller design,the small-signal model is reduced by extracting the dominant zeros and poles to obtain a simplified model,based on which a suitable controller parameter design method is proposed to form a closed-loop system with good dynamic and steady response.The close-loop system enables the system to track the output current command quickly and accurately,and realizes natural bidirectional operating and switching of the converter.Finally,with the previous analysis,a parameter design method including the original/secondary duty,resonant parameters,filter capacitance has been proposed.Based on the method,according to the technical requirements,an experimental prototype based on GaN HEMT is built.The feasibility and validity of the proposed converter and the correctness of theoretical analysis are verified by experiments.In short,through theoretical analysis,software simulation and experimental verification,it is shown that the proposed converter can maintain the natural and smooth bidirectional power transmission while maintaining the high efficiency and high power density of the resonant converter.