LLC Resonant DC/DC Converter Control Strategy Research

With the rapid development of the new energy industry,electric vehicles have gradually replaced fuel vehicles as the means of transportation for people’s daily travel.Electric vehicles take electric energy as the power source and DC charging system as the post-stage power module.Its high efficiency and high reliability are directly related to the endurance,service life and safety of electric vehicles.Therefore,the research on high-performance DC charger is of great significance for the future development of automobile industry.LLC resonant converter,as one of the mainstream topologies of DC/DC converters,is widely used because of its unique advantages such as soft switching within the full load range,high transmission efficiency and high power density.Therefore,LLC resonant converter is selected as the DC power module of the rear stage of electric vehicles.In this dissertation,the rear stage DC charging equipment of electric vehicles is taken as the research background,LLC resonant converter is taken as the research object,and the control strategy of LLC resonant converter is studied to improve its dynamic response characteristics and light load efficiency.First of all,based on the topology analysis and research,combined with the power requirements of the system,LLC full-bridge resonant converter is selected as the main circuit topology,and its optimal operating frequency range is determined according to the working principle analysis.The steady state analysis of the converter is carried out by using the fundamental wave approximate analysis method,the DC voltage gain function is determined,and the influence of various system parameters on the converter performance is summarized.Then the resonant parameters of the converter are designed,and the reasonableness and accuracy are verified by open-loop simulation.Based on the extended description function method,the LLC resonant converter is modeled for small signals,the transfer function from control to output is derived,and the compensation parameters of PI controller are determined by Byrd diagram to ensure the stability of the system control.When LLC works normally,to solve the problem of slow dynamic response of voltage control,the current of the rectifier circuit is added as the inner current loop to form a double closed-loop control of voltage and current.The double closed-loop transfer function is deduced and PI parameter compensation is carried out.Aiming at the low light-load efficiency under double closed-loop control,the intermittent control method is designed to reduce system loss and improve the light-load efficiency by optimizing the intermittent duty ratio.Finally,the hardware circuit is designed,and the experimental platform is built to verify the feasibility and effectiveness of the research method.In this dissertation,the dynamic response characteristics and low light-load efficiency of LLC resonant converter are studied from the aspects of theoretical analysis,system modeling,software simulation and platform experiment,and the effectiveness of double closed-loop control and intermittent control strategy is verified.The results show that the designed control strategy solves the problems of slow LLC dynamic response and low light-load efficiency,improves the working efficiency of the system,and provides application and research value for the improvement of the working performance of the post-stage DC charging equipment.