Research On Three-level Power Conversion Technology Used In Electric Vehicle Wireless Charging System

With the increasing energy consumption and the gradual shortage,the environmental pollution problem has also increased.In order to solve these two major crises,vigorously developing electric vehicles and related industries has become a broad consensus.The advantages of wireless charging technology,such as noncontact,safety,convenient and flexible use,have become an important development direction in the field of electric vehicle charging.At present,wireless charging technology is developing in the direction of high-voltage,high-power and highefficiency,in order to shorten the charging time and alleviate users’ mileage anxiety and other problems.Based on the above practical needs in the field of wireless charging,this paper proposes a three-level wireless charging system for electric vehicles,and conducts in-depth research on its working mechanism,modulation strategy,closed-loop control scheme and online resonant frequency tracking technology.Firstly,an LC series compensation wireless charging system with a diodeclamped three-level converter as the emission source is proposed,and its working principle is analyzed.Further combining the working characteristics of the system,a phase-shifting modulation strategy that can achieve fixed output voltage phase and neutral point voltage balance is proposed to avoid phase shift while adjusting the output voltage amplitude,thereby improving the dynamic response capability of the system.A simulation model is built to simulate and verify the three-level conversion system and the proposed modulation strategy.Secondly,the steady-state characteristics of the entire wireless charging system are analyzed.Utilizing the characteristics of the resonance network characteristic that the primary and secondary currents are all sinusoidal,the step wave voltage is converted into a sinusoidal fundamental wave component based on the fundamental wave approximate analysis method,and then the steady-state equivalent circuit is used for analysis,and the coil mutual inductance,resonance frequency and The quantitative relationship between output power and efficiency and the electrical characteristics of the input and output ports prove that there is a definite linear relationship between the steady-state primary and secondary side voltage and current of the series-series compensation network,which lays a theoretical foundation for the subsequent corresponding closed-loop control research.Thirdly,the closed-loop control scheme of the secondary side current is determined,the small signal modeling of the system is carried out using the generalized state space averaging method,and the model is simplified according to the obtained model,according to the simplified Bode diagram of the transfer function between the input voltage and the secondary side current The corresponding closedloop controller parameters are designed.At the same time,in order to speed up the dynamic performance of the traditional phase-locked loop frequency tracking,a digital hardware-based frequency tracking scheme is proposed.This scheme does not require voltage sampling and has lower requirements for current phase sampling accuracy,thus reducing the response time and making it more convenient.Application,the closed-loop control system and frequency tracking technology are verified through detailed simulation results.Finally,a prototype of the experimental principle based on DSP+FPGA is built,the detailed hardware scheme and software program flow are given,and the corresponding theoretical analysis results and the proposed wireless charging system are verified in detail.