Research On Control Strategy Of Full Bridge LLC Resonant Converter

Full bridge LLC resonant converter is widely used in new energy vehicle charging pile,LED drive power supply and multi-function power supply scenarios requiring wide output due to its wide gain range,high efficiency,high power density and other characteristics.In order to broaden the wide output voltage range of the converter and improve the efficiency of the converter,this paper takes the wide output full bridge LLC resonant converter as the research object,and studies the control strategy of its transient process and steady state process respectively.The main work of this paper is as follows:(1)The application strategy of LLC resonant converter in soft start control process is studied and optimized in view of the problem that the direct start of LLC resonant converter will cause huge overshoot current in the start process.The phase shift soft start control strategy was optimized by changing the duty cycle curve with time.On the basis of this,the overshoot current in the starting process of the converter is further suppressed and the safety and stability of the converter is improved.(2)Aiming at the problem that the output voltage range of the converter is narrow under the single control strategy in steady state,the control strategy is optimized to achieve wide output voltage.When the output voltage is low,the regulating effect of frequency conversion control is not obvious.In phase shift control,the phase shift Angle is too large and the lag bridge switch tube is difficult to realize soft switch.Aiming at the above problems,Burst control strategy is added on the basis of phase-shift frequency conversion control.According to the voltage gain curves obtained from the analysis of the three control modes,the implementation of the hybrid control strategy is designed,and on the basis of the state space trajectory method,the phase shift and Burst control mode switching points are selected.In order to solve the problem of switching between two modes when the output voltage of the converter fluctuates near the mode switching point in actual control,the mode switching point is optimized.(3)Based on the above,the LLC resonant converter is designed as a whole,including the calculation of the parameters of the main circuit components,the design of magnetic components such as transformer inductance.MATLAB/Simulink is used as a simulation platform to verify the full bridge circuit.The results show that the optimized soft start strategy can better suppress the overshoot current during the start process.After adding Burst control,the phase shift frequency conversion hybrid control strategy can realize the soft switching of the converter,and further broaden the output voltage range of the converter.