An Improved Second-order Sliding-mode Control For Tri-level Buck Dc-dc Converters

With the advancement of science and technology,the requirements for the integration and reliability of power devices are getting higher and higher.To Buck circuits,the current-carrying capacity of the two-level Buck circuit and the withstand voltage capability of the switch tubes are far inferior to the multi-level Buck circuit.Compared with two-level Buck converters,tri-level Buck converters have many advantages.For example,the voltage stress on the switch tube is reduced by half,the size of the filter inductor and capacitor is small,the effective switching frequency is doubled,and the energy density is high.However,the number of switch tubes of a tri-level Buck dc-dc converter is two times that of a two-level Buck dc-dc converter,and its structure is complicated and working between multiple states,besides,there is a coupling relationship between the output voltage and the flying capacitor voltage of the converter,so it is difficult to control it.The paper focuses on an improved second-order sliding-mode control for tri-level Buck dc-dc converters,which achieves the decoupling control of the output voltage and the flying capacitor voltage,improves the dynamic response of the output voltage of the converter,and ameliorate the stability of the system.The research and design work of this paper is summarized as follows:(1)According to the switching states of the tri-level Buck dc-dc converter,split the converter into multiple working modes,analyze the operating mechanism of the converter under different working modes.Study the coupling relationship between the output voltage and the flying capacitor voltage of the converter,establish a nonlinear mathematical model of the converter,and compare and analyze the existing control methods.(2)In the voltage tracking process,the optimal trajectories of the output voltage and the flying capacitor voltage in the phase plane are analyzed for the various working modes of the tri-level Buck dc-dc converter,and the optimal switching conditions between one mode and another are given.(3)A second-order sliding mode control method for a finite state-machine structure is designed,and the switching conditions of the state-machine are deduced.Through the switching of the finite state-machine,the decoupling control of the output voltage and the flying capacitor voltage is realized,it not only make the output voltage of the converter reach the reference value quickly,but also make the flying capacitor voltage balanced at the half of the input voltage value quickly.Then,the designed control method was verified by simulation.(4)Design and implement a hardware experiment platform for the tri-level Buck dc-dc converter,complete the schematic and circuit design,and debug the control algorithm.The proposed control method was verified on the experimental platform,the startup process of the converter and output response test of load disturbance was completed.Finally,the effectiveness of the proposed control method is proved by simulation and experimental results,and the output voltage can be adjusted accurately and quickly.The controller not only has the robustness to the load disturbance and uncertainty of circuit parameter,but also can realize the quick adjustment and no overshoot control of the output voltage of the converter without detecting the current.