Decoupling Modeling And Stability Analysis Of Quasi-resonant Buck Converter Considering Different Time Constants

Soft switching converter is widely used in aerospace,wireless communication,computer,electric vehicle,and other fields because of its significant advantages,such as high-power density,high integration,small size and portability,etc.Especially the quasi-resonant switching converter has become one of the top topics in the field of power electronics,due to its simpler closed-loop control network and high reliability.However,the traditional modeling method based on state space averaging is no longer suitable for resonant converters,as these converters usually have higher operating frequency and more operating modes.Therefore,a nonlinear modeling method considering different time constants is proposed for quasi-resonant switching converter.The method decouples the resonant state variables and the filter state variables,and then the transient characteristics and stability of the quasi-resonant converter are analyzed and discussed based on the established mathematical model.These works are of great significance to analyze the nonlinear operation mechanism,identify the parameter space of its stable operation,optimize the system parameter design and improve the system performance for quasi-resonant switching converter.The main work of this paper is as follows(1)Taking the zero-current-switching quasi-resonant(ZCS QR)Buck converter with pulse frequency modulation(PFM)as an example,a nonlinear modeling method is proposed to decouple the resonant state variables and the filtered state variables according to the characteristics of different time constants.And the approximate analytical solutions of all the state variables of the quasi-resonant converter system are obtained by this improved equivalent small parameter method.The modeling results of this method are very close to the simulation results of PSIM circuit model,which shows that the method can analyze PFM-ZCS QR Buck effectively and accurately.(2)Considering that the pulse width modulated(PWM)quasi-resonant converter has wider application,the above decoupling method is expanded to the modeling and analysis of PWM-ZCS Buck converter.The approximate analytical solutions of the state variables in the open-loop and closed-loop states are obtained respectively by the improved equivalent small parameter method,in addition,the influence factors of the transient process of the system are discussed.The simulation and experimental results show that the proposed decoupling modeling method can be applied to the analysis of quasi-resonant converter accurately and effectively.(3)Taking PWM-ZCS Buck converter as an example,the stability of the quasi-resonant converter is studied.According to the established mathematical model of PWM controlled quasi-resonant converter closed-loop system,the monodromy matrix based on Filippov and Floquet theory is applied to the stability analysis of the system,and the influence of key parameters of the converter on the stability is discussed,then the parameter space of its stable operation is identified.The effectiveness of the stability analysis method is verified by simulation analysis.The analysis example of PFM-ZCS QR Buck and PWM-ZCS Buck shows that the decoupling modeling method of quasi-resonant converter considering different time constants proposed in this paper can effectively analyze the transient and steady-state process of the state variables,and the obtained transient and steady-state approximate periodic solutions of the state variables have preferable precision;on the basis of the established mathematical model of the closed-loop PWM controlled quasi-resonant converter,the stability analysis can effectively and visually reflect the influence of circuit parameters on the performance of the converter.