Research On Topology And Control Strategy Of High-efficiency Charger Suitable For Electric Vehicle On-board Power Supply

With the widespread application of electric vehicles,how to efficiently and safely charge power batteries has become an important issue.Among them,the method of charging through an on-board charger(OBC)has received widespread attention due to its convenience and high power density.The two-stage OBC topology is composed of a front-stage AC/DC converter and a back-stage DC/DC converter.In order to achieve high-efficiency charging of medium and high power,the front-stage AC/DC converter can adopt a single-phase three-level topology to achieve power factor correction and DC link voltage regulation functions;the latter-stage DC/DC converter should adopt LLC resonant topology to improve efficiency,Adjust the charging mode.The research on the optimal modulation and control strategy of the front and rear topologies is of great significance.This article takes the medium and high-power two-stage OBC as the research object,and studies the control strategies of the front-stage single-phase three-level converter and the back-stage LLC converter respectively to improve the overall charging performance.It mainly includes:single-phase three-level converter variable switching frequency modulation in critical conduction mode,single-phase three-level converter efficiency optimization control,and LLC converter transient optimization control.First,the efficiency optimization control strategy of single-phase three-level converter is studied.When the single-phase three-level converter adopts the traditional constant on-time control strategy,the system switching frequency has a wider variation range,and the switching times of the switching tube increase,which reduces the converter efficiency.According to the nine switching states of the single-phase three-level converter,an optimized switching sequence control strategy is proposed to slow down the rate of decrease of the inductor current in a switching cycle and reduce the average switching frequency and average switching times of each switching device.Therefore,the switching loss of the switching device is reduced,and the system efficiency is improved.The simulation results show that under this control strategy,the three-level converter meets the AC side current harmonic standards,while the average efficiency in the full load range is increased by 1.2%.Secondly,the system parameters of the subsequent LLC converter are designed.Based on the basic working principle of LLC converter,the equivalent circuit of over-resonance,under-resonance and full resonance is analyzed,and the DC voltage gain model of LLC converter is established by fundamental wave analysis.First determine the transformer ratio and voltage gain range;then,based on the boundary condition that the system does not enter the capacitive region,obtain the resonance inductance,magnetizing inductance,and resonant capacitor parameters,and design the magnetic components of the resonant inductance and the magnetizing inductance;finally establish a 120W simulation and The hardware platform verifies that the parameters designed in this article meet the application requirements.Finally,the soft-start optimization control strategy of the downstream LLC converter is studied.Aiming at the soft-start process of LLC converters,the soft-start process in a switching cycle is divided into four modes through a numerical model in the time domain.Analyze the critical conditions for achieving soft switching during startup,and obtain the duration of each mode through iteration,thereby determining the initial startup frequency and the initial duty cycle.Through the comparison of theoretical calculation results,simulation results and experimental results,the LLC converter soft-start strategy based on the numerical model studied in this paper can reduce the current overshoot and ensure that the converter does not lose the soft-switching characteristics during the entire soft-start process.