| The two-stage AC-DC-DC power transformation structure is widely used in charging and discharging systems of electric vehicles such as charging piles.The large capacitance value electrolytic capacitors in the intersections bring problems such as low system life and large volume,which limit the improvement of the comprehensive performance of the system in terms of power density,working efficiency and reliability.Therefore,it is urgent to study the new AC-DC power transformation technology to improve its comprehensive performance,which has important theoretical significance and practical value.In this paper,a novel three-level single-stage high frequency isolated AC-DC converter is proposed,which can realize the basic AC-DC transformation,electrical isolation and bidirectional lifting voltage simultaneously through the first-level topological structure without the need of the intermediate electrolytic capacitance.Therefore,it has higher power density,longer service life and higher working efficiency.In this paper,the topological structure,modulation strategy and closed-loop control strategy are studied in detail.Firstly,a three-level single-stage high-frequency isolated AC-DC converter topology is proposed.The basic working principle is analyzed in depth.Based on the general three-phase-shift modulation strategy,the voltage characteristics of the ac side divider capacitance are deeply analyzed,and then a staggered PWM modulation method is proposed to realize the capacitor voltage equalization within the switching cycle.This modulation method decoupled the voltage of the divider capacitor from the resonant capacitor,and equalized the capacitor voltage only by adjusting the carrier phase,thus simplifying the equalization control scheme.Secondly,the main working waveforms under the general three-shift modulation strategy are analyzed,and the equivalent circuits under different switching states are given.Based on the fundamental wave analysis,the relationship between the mean power and resonant current RMS and the three phase shift angles in a switching period is derived.Then the optimal coordination control principle between three phase shift angles under the minimum constraint of resonant current RMS is obtained to improve the efficiency of the converter.Then,in order to realize the constant control of DC voltage and sinusoidal waveform control of grid side current,a double closed-loop control scheme with dc voltage as outer loop and grid side current as inner loop is presented.The working principle of proportional resonance and quasi-proportional resonance controllers for the current closed-loop control at the grid side is analyzed and compared.The design principle of the controller parameters is analyzed to obtain the controller parameters that meet the requirements.The open-loop and closed-loop simulation models of the proposed converter were built in MATLAB/Simulink simulation environment,and the basic AC-DC transformation function of the converter and its modulation strategy was simulated and verified by open-loop method.Furthermore,the dynamic and static closed-loop control performance of the converter is verified in detail by the simulation model of dc voltage and grid side current double closed-loop control.The simulation results show that the proposed converter and its modulation and control strategy realize the excellent control performance of the current and dc side voltage on the grid side,and then verify the correctness and effectiveness of the research results.Finally,the experimental platform of the proposed novel converter based on DSP and FPGA dual controllers is built,the overall scheme of the system is given,and some hardware circuit design and software program design of DSP and FPGA are carried out. |
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