Stability Analysis And Improvement Of Cascaded DC-DC Converters Based On Floquet Theory

In DC distributed power system,cascaded structure is one of the basic connection forms.Since the interaction among the individual converter that can operate stably alone may lead to an instability,the stability analysis and improvement of cascaded DC-DC converters is currently a hot issue.The traditional stability analysis and improvement methods are proposed in frequency domain based on the impedance model of system.However,the impedance criterion proposed according to engineering experience is conservative and inaccurate,and it is difficult to be extended to analyze the stability of multi-stage cascaded system due to complex impedance calculation.Existing stability improvement methods are mostly derived from the perspective of eliminating system impedance overlap,and impedance calculation is still required.It has certain limitations when applied to multi-stage cascaded DC-DC converters.Different from the traditional frequency domain analysis,to seek a more accurate and concise stability analysis method,this thesis studies the stability of cascaded DC-DC converters in time domain based on Floquet theory.Firstly,the stability criterion for analyzing cascaded DC-DC converters is derived based on Floquet theory.The N-cascaded buck converters is taken as an example for time domain modeling and stability analysis.Then the simulation and experimental platform of two-stage and three-stage buck converters cascaded system are built respectively,and the correctness and accuracy of the proposed stability criterion are verified.Furthermore,considering the input voltage of cascaded DC-DC converters may have periodic AC disturbance in practical application,based on Floquet theory,the time domain modeling and stability analysis of N-cascaded buck converters when there is a periodic AC disturbance on the input voltage are carried out.Then the simulation platforms of two-cascaded and three-cascaded buck converters when there is a periodic AC disturbance on the input voltage are respectively built,and the accuracy of Floquet stability criterion applied to the stability analysis of DC-DC converters cascaded system with periodic AC disturbance is verified.Finally,based on Floquet stability criterion and eigenvalue sensitivity analysis,two stability improvement methods based on additional inductor current feedback control and voltage error mutual feedback control for cascaded DC-DC converters stability improvement are proposed.Taking the two-cascaded buck converters as an example,the effectiveness and correctness of the proposed control optimization methods are verified by simulation and experiment.Subsequently,the two control optimization methods are extended to the N-stage cascaded system,and the simulation platform of three-stage buck converters cascaded system is built to verify the effectiveness of the proposed methods.