Buck Converter Based On Fuzzy Sliding Mode Control

DC-DC converters are widely used in power electronics field, and conventionalDC-DC converters adopt analog circuits to implement control strategy. In the lastdecade, with the cost reduction of micro-controller and programmable logic device,more and more DC-DC converters use digital control. Compared with analog control,digital control is more reliable, stabile and flexible.Traditional linear control methods for DC-DC converter neglect nonlinearitieswhich may result in deteriorated output signal or unstable behavior in the presence oflarge disturbance. So some nonlinear control methods are applied to DC-DC converter,such as auto-tuning control, time-optimal control, sliding mode control and fuzzycontrol. Sliding mode control seems to be one of the most powerful robust controlstrategies for DC-DC converter, it has some advantages of wide-range stability, fastdynamic response, robustness against uncertainties and simplicity of implementation.As with other control system, the analysis and design of sliding mode control system arebased on the measurability of state variables. Unfortunately, not all state variables canbe measured directly in DC-DC converter. To solve this problem, observer is used tocombine measurable signal with other knowledge of the control system to produceobserved signals, and then use the estimated value to achieve the control law.In this paper, Buck converter is selected as the research object. First, the detailedanalysis of sliding mode control scheme and some common analytical methods forsliding mode control used in DC-DC converter were presented. Second, in order toovercome difficulty of output voltage differential measurement, a traditional slidingmode observer and a second order sliding mode observer are designed, which realizethe estimation of the state variable. Simulation results show that the second ordersliding mode observer is more accurate than the traditional sliding mode observer. Third,digital pulse width modulation (DPWM) control is combined with sliding mode controlby the concept of equivalent control theory, then a sliding mode controller based onDPWM is designed. Finally, considering the low steady-state accuracy of the slidingmode control, a fuzzy sliding mode control strategy based on switch gain adjustment isadopted. Simulation results for sliding mode controller and fuzzy sliding modecontroller are presented. Comparative study is made between sliding mode controllerand fuzzy sliding mode controller. It is observed that fuzzy sliding mode controller not only shorten the response time, but also improve the steady-state accuracy.