Study Of Nonlinear Analysis Of Closed-loop DC-DC Power Switching Converters

This dissertation focuses on the study of nonlinear analysis of closed-loop DC-DC power switching converter systems. In recent years, studying the operation mechanism and large-signal stability of power switching converter systems based on its nonlinear nature has become one of the most important and difficult topics in the field of power electronics. Firstly, this thesis gives a whole reviewing of the existing large-signal analysis methods, and demonstrates that the Equivalent Small Parameter (ESP) method has more superiority than others, and it is of more significance to extend ESP to the analysis of closed-loop power converter systems. Then, this thesis makes a more thorough study of the nonlinear analysis , including the following: 1. General large-signal modeling: A general unified nonlinear large-signal equivalent circuit model for PWM converters at continuous conduction mode (CCM) and discontinuous conduction mode (DCM), together with its mathematical description , is presented by using a time-variant function to represent the switches state of ON or OFF, which can be easily extended to the modeling and analysis of quasi-resonant converters. 2.Symbolic analysis of steady state of closed-loop systems: (1) The mathematical model for duty-ratio programmed PWM power converter systems is derived , then a single-loop iteration algorithm and a two-loop iteration algorithm for solving the proposed model are introduced based on the principle of ESP. Symbolic analytic expressions of DC values and ripples of capacitor voltages and inductor currents in closed-loop systems are obtained, which are obviously helpful in mastering the circuit operation mechanism, practical engineering designs and computer symbolic analysis of the systems. (2) An extension of the above mentioned algorithm to constant frequency current-mode converter systems , with a precise method and a simple method of determining the duty-ratio of current-mode controlled converters is presented. It is shown that the precise method is more practical due to its no assumptions of linear inductor current waveform and small output ripples are required, and the simple method is very useful for simplified engineering analysis. (3) A reduced-order model and a full-order circuit model for frequency modulation (FM) quasi-resonant converters (QRC) are introduced. It is shown that the full-order model has the same analyzing mode as the PWM controlled converters. Furthermore, it is illustrated that the higher-order harmonics of high frequency resonant state variables have less effects on low frequency filter state variables, and more accurate results for filter state variables can be obtained by fewer iterated times when using ESP method. The extensions of our proposed analysis method for closed-loop systems to current-mode converters and FM QRC systems show that it is a general approach. 3. Stability analysis: A stability analysis method for equilibrium point of closed systems is proposed . Moreover, a Buck regulator with typical nonlinear large-signal low frequency oscillation is investigated by means of analytical analysis and numerical simulation. It's shown that the oscillation is aroused by the regulator's periodic switching between two operation modes . All the analytic calculation are verified by Pspice simulation or numeric simulation or experiment measurement (see Appendix), and the two kinds of results are in good agreement, respectively, which show that the proposed symbolic method provides the advantages of both simplicity and accuracy. This dissertation is supported by Guangdong Natural Science Fund (No.940089; 990585) and Doctoral Education Fund of state Education Commission (No.9456102).