| In recent years,the research on the nonlinearity of DC-DC converters has made a lot of substantial progress.Scholars have conducted extensive and indepth research through theory,simulation,and experimentation,confirming that DC-DC converters can exhibit a series of nonlinear dynamic behaviors,and correspondingly proposed a variety of methods to control nonlinear phenomena.Compared with low-order DC-DC converters,the mathematical characteristics of high-order DC-DC converters are more complicated,and the research on theory and control is relatively scarce.However,high-order circuit systems have more abundant and diverse dynamic characteristics,so they also have broader research prospects.Aiming at the nonlinear dynamic characteristics of SEPIC converters under different control modes,this thesis reveals its behavior mechanism through mathematical modeling and theoretical analysis,and proposes corresponding control strategies to achieve the purpose of suppressing nonlinear phenomena.The main research work and research results obtained in this thesis are summarized as follows:1.Analyze the nonlinear dynamic behavior of the CCM mode current-mode SEPIC converter controlled by a single front inductor,and use the front inductor current as the control object to construct a discrete iterative mapping model of the converter circuit.Combining the state space averaging method and the power balance principle of the circuit,the analytical expression of the relationship between the reference current and the system stability is derived,and it is also applicable to the stability analysis after the control method is introduced.2.Aiming at the nonlinear phenomenon of the CCM mode current-mode SEPIC converter controlled by the pre-inductor,a resonance parameter perturbation method is introduced.Based on the stability analysis of the system,the influence of different perturbation amplitudes on the control effect of the nonlinear phenomenon is studied.On the basis of considering the perturbation signal phase angle factor,this thesis proposes an improved resonance parameter perturbation method and combines the actual physical meaning of the circuit to determine the optimal control phase angle of the system.It finally achieves the optimization of the nonlinear control effect and gives the corresponding simulation results.3.The nonlinear dynamic behaviors in DCM mode voltage-type and PIcontrolled voltage-type SEPIC converters are studied.The former establishes the corresponding discrete iterative model through mathematical modeling,and studies the influence of voltage feedback gain on the bifurcation and chaotic behavior of the system;By expressing the state variables of the PI part in the form of the duty cycle of the circuit,the mathematical modeling of the fifth-order circuit system is completed,and the influence of the proportional factor P and the integral factor I on the dynamic behavior of the system is analyzed.4.A passive delay feedback control method and an improved sliding mode variable structure control method are respectively proposed for the nonlinear phenomenon of voltage-type SEPIC converter in DCM mode.The former combines the actual circuit characteristics and introduces the capacitive current as a delay feedback parameter into the voltage feedback loop,which better realizes the control of the chaotic system;the latter improves the selection of state variables according to the characteristics of the DCM operating mode,and combines The existence condition of the sliding mode surface selects the sliding mode coefficient that meets the conditions,introduces the adaptive hysteresis control to stabilize the switching frequency,and finally achieves the purpose of nonlinear control.The effectiveness of the two feedback control methods is verified by Matlab/Simulink simulation.Compared with the passive delay feedback method,the improved sliding mode variable structure control method has better output ripple and dynamic response speed. |
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