Second-Order Sliding-Mode Control For Three Level Buck Dc-dc Converters

The high-voltage high-power applications are more and more widely used, however it does not see dramatic breakthroughs in the rated voltage and current of the power switching devices. Traditional two level Buck dc-dc converters can't meet the requirements in high-voltage high-power applications but three level Buck dc-dc converters make it. Three level Buck dc-dc converters have advantages over the standard Buck dc-dc converters: half switch voltage stress, doubling effective switching frequency, the smaller filter inductor size, the smaller filter capacitor size and the higher power density.Note that the three level Buck dc-dc converter which has 4 MOSFETs and works under 4 modes is typical nonlinear system. What's more, the output voltage and the flying capacitor voltage are coupled strongly. So it is difficult to control. The paper does research on the second-order sliding-mode control of three level dc-dc converters in order to achieve the fast dynamic responses and robustness against load disturbances and parameter uncertainties. The paper makes the research and design as follow:(1)Do research on the operation principle of three level Buck dc-dc converters and difficulties in control. Analyze the advantages over standard Buck dc-dc converters in theory. After analyzing and comparing the existing works for the three level Buck converter, this paper proposes the second-order sliding-mode(SOSM) control for it. The optimal trajectory of output voltage is analyzed and the corresponding optimal switching parameters are derived.(2)Design a novel state-machine controller structure of SOSM control which both regulates the output voltage fast and balances the flying capacitor voltage for the three level Buck dc-dc converter. Propose the controller based on proportional control using the flying capacitor voltage error information to adjust the charging and discharging time of the flying capacitor, then to balance its voltage. The proposed controller is verified by simulation and the controller's performance are analyzed theoretically.(3)Against the shortcomings of the above controller, the improved state-machine controller is designed to limit the flying capacitor voltage in a small boundary by substituting the “redundant modes” so that the more excellent output ripples are achieved. Simulation verifies the improved controller based on redundant modes and indicates the flying capacitor voltage is limited and the shortcomings are overcome.(4)Design the schematic diagrams of the three level Buck dc-dc converter control system and build the hardware platform. The above two state-machine controllers are implemented in Cyclone IV FPGA. Experimental results verify the two state-machine controllers and results of the two controllers are compared with each other.The simulation and experimental results indicate the two state-machine controllers succeed in controlling the three level Buck dc-dc converter. For the output voltage, the proposed SOSM control achieves fast start-up without overshoots and robustness against load disturbances and parameter uncertainties without current sensing. The two proposed controllers' novel structures both decouple the output voltage and flying capacitor voltage, and balance the flying capacitor voltage without knowledge of the flying capacitor current.