Research On Wide Output LLC Converter Based On PWM Controll

Today,coal,oil,natural gas and other non-renewable energy is still the main source of energy in China,which not only aggravates the problem of energy shortage,but also brings great pressure to environmental governance.Energy transition is happening quietly,solar,wind,tidal and other renewable energy boarded the stage of the era,and rapid development.LLC resonant converter is developed from series and parallel resonant converter,because its primary side switch tube can achieve zero voltage on(ZVS),secondary side diode can achieve zero current off(ZCS),small volume,low noise,high power density advantages are widely used in the field of new energy.However,the wide range of switching frequency and the difficulty of resonant parameter design limit the further development of traditional LLC resonant converter.In this paper,LLC resonant converter with full-wave secondary rectifier is studied deeply.At present,the improvement schemes for the LLC resonant converter can be divided into four categories: changing the resonant element parameters,improving the control strategy,reconstructing the converter structure and adopting interactive structure.Based on the improved control strategy and the reconstruction of the converter structure,this paper retains the transformer structure of the traditional LLC resonant converter,and obtains six kinds of converter topologies based on PWM control with the center tapped transformer as the core.The output voltage can be adjusted by controlling the duty ratio of the secondary MOSFET,and all devices work at the resonant frequency when the output voltage changes.Through the analysis and comparison of the performance of the six converters,including soft switching and the number of devices.Finally,the best performance converter is selected as the focus of this paper.For the selected converter,PWM control,PFM control and PWM+PFM hybrid control are proposed.Several control methods are analyzed and compared from two aspects of gain range and parameter design.The PWM control is analyzed,and the modal diagram and time domain expression of each working state are given.The gain characteristics of the converter are analyzed by frequency domain method and time domain method respectively.The key parameters of the converter are designed according to the conditions of the soft switch and the current stress.A simulation platform is built according to the designed parameters.The loss is analyzed and estimated through the simulation results.Finally,the hardware circuit of the converter is designed.According to the designed hardware circuit and magnetic element parameters,two 420 W experimental platforms were built.The experimental results verify the correctness of the theoretical analysis.