The Design Of Double Close-loop Control In Three-level Buck Converter

In high input/output voltage power conversion, selecting switches with properpressure are often difficult. The Multi-Level converter, which is very suitable for highinput/output voltage power conversion, splits voltage constraints amongseries-connected switches through flying capacitors, it can not only reduce the switchvoltage stress and the size of the output filter, but also improve the transient response.However, The Multi-Level converter is a strongly coupled nonlinearly system, its outputvoltage and flying capacitor voltages are strongly coupled, so the system should bedecoupled to multiple single-input and single-output sub-systems such that each outputcan be controlled independently.With the higher requirement for dynamic characteristics and stability of switchconverter, some nonlinear control methods are introduced to control switch converters.Sliding mode control, which has some advantages of fast dynamic response, robustnessagainst uncertainties and simplicity of implementation, is one of the most powerfulrobust control strategies for switch converters. Comparing to other control method,sliding mode control has a better applicability for the nonlinear characteristics of switchconverter. Though fuzzy control method is not depend on exact model of controlledobject and has adaptive ability to nonlinear plant, the control accuracy is not high. Andthe PID control method has the characteristics of simple principle, high accuracy andgood reliability, but to obtain satisfactory control effect requires adjusting the PIDcontroller’s parameters online. So, in order to achieve the desired control effect, we candevelop a fuzzy-PID controller that uses the fuzzy control rules regulating the PIDcontroller’s parameters online.In this paper, the operation principle of three level Buck converter is analyzed indetails. As there is coupling between the closed-loop of output voltage and flyingcapacitor voltage, the decoupling is proposed to design the closed-loop of outputvoltage and flying capacitor voltage independently, and the model of three level Buckconverter is built. Then, the detailed analysis of sliding mode control scheme and somecommon analytical methods for sliding mode control used in switch converter arepresented, and a fixed frequency sliding mode controller based on the equivalent controlmethod is designed to control output voltage for the characteristics that three level Buckconverter has several operation modes, and combining the advantages of PID control and fuzzy control, a fuzzy-PID controller is designed to control flying capacitor voltage.Finally, based on the theoretical research, simulations of the double closed-loop controlsystem of three level Buck converter are carried out and results of simulations are given.The simulation results show that the theoretical analysis is correct and the superiority ofthe designed controllers.