Stability Analysis And Improvement Of LLC Resonant Converter Based On Floquet Theory

With the development of science and technology and the improvement of the performance requirements of modern industry on electronic equipment,the development of power electronics frontier technology gradually tends to be high-frequency,high efficiency and high power density,which is especially evident in distributed power system.LLC resonant converter is widely used in power system due to its advantages of soft switching and magnetic integration.In power system,converter performance is closely related to stability analysis.Therefore,it is of great significance to analyze and improve the stability of LLC resonant converter.In engineering practice,when closedloop debugging of LLC resonant converter is carried out,instability is often encountered.It is necessary to analyze its closed-loop stability and determine the stability range of control parameters.Therefore,this paper takes the single-voltage loop system of halfbridge LLC resonant converter as the object,and researches on its closed-loop stability analysis and improvement.This paper first introduces the basic working principle of the half-bridge LLC resonant converter,analyzes its main waveforms and circuit modes one by one at different frequencies,and discusses the realization process of the soft switch.With First Harmonic Approximation(FHA),the linear sinusoidal equivalent circuit model of the converter is established,its steady-state Voltage gain characteristics are analyzed,and its necessary and sufficient conditions for Zero Voltage Switching(ZVS)are derived.On this basis,the general steps of designing the main parameters of the converter are derived,and a set of resonant parameters are designed according to the steps,and the design scheme is verified by the simulation circuit and the experimental platform.Secondly,based on the review of the stability analysis method based on Floquet theory,the closed-loop time-domain state transfer matrix based on Floquet theory is derived by taking the single-voltage loop system of the half-bridge LLC resonant converter designed in this paper as an example.According to the eigenvalue of the established state transfer matrix,the stability analysis is carried out and the corresponding simulation verification platform is built.Moreover,compared with the closed-loop stability analysis results based on the Nyquist criterion in the traditional frequency domain,the correctness and accuracy of the stability analysis method based on Floquet theory in LLC resonant converter are verified.Finally,based on eigenvalue sensitivity analysis and Floquet criterion,a stability improvement method based on additional resonant peak current integral control and another method based on additional resonant peak current proportional control are proposed to optimize the single-voltage loop system of the half-bridge LLC resonant converter in this paper.PSIM simulation verifies the correctness and effectiveness of the two additional control methods to improve the stability of single voltage loop LLC system.In this paper,the stability analysis method and eigenvalue sensitivity analysis based on Floquet theory are first applied to resonant converter with pulse frequency modulation,which provides new guidance and reference for closed-loop stability analysis,control parameter design and control optimization of resonant converter.