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In this paper, a high-order nonlinear disturbance observer-based fractional-order sliding mode control (FOSMC) strategy is proposed for DC/DC buck converters in the presence of mismatched disturbances, ensuring the properties of both stability and dynamic performance. Traditional sliding mode control can deal with matched disturbances while achieving desired closed-loop performance, unfortunately it is particularly sensitive to mismatched disturbances. In the proposed control structure, two nonlinear disturbance observers are constructed to estimate both matched and mismatched disturbances in finite time. Then, the estimated variables are used for the fractional-order sliding mode surface design, where a super twisting sliding mode controller is designed to drive the states of the system to track their desired values. Comparing with the traditional SMC, the proposed method not only reduces the sensitivity of the system to mismatched disturbance, but also improves the transient performance of the system. Simulation and experimental results have comprehensively illustrated the feasibility and effectiveness of the proposed strategy. [ABSTRACT FROM AUTHOR]

In this paper, four control strategies for DC-DC buck converters are proposed, compared and analyzed: a single-loop adaptive control strategy (SA), a double-loop adaptive control strategy (DA), a single-loop disturbance observer-based control strategy (SDOB) and a double-loop disturbance observer-based control strategy (DDOB). First, the nominal system without considering the parametric uncertainties of the DC-DC buck converter is built to help develop the SA and DA. The SA is built by adaptive and backstepping control approaches, and the DA is set up by adaptive and sliding mode control approaches. Additionally, a model considering parametric uncertainties is introduced, giving the opportunity to develop the SDOB and DDOB. The SDOB is developed using a designed disturbance observer and backstepping control technique, and the DDOB is synthesized using a designed disturbance observer and sliding mode control method. Finally, the advantages and disadvantages of the four proposed control strategies are compared and analyzed through experiments. [ABSTRACT FROM AUTHOR]

In this paper, a novel adaptive second-order sliding mode (SOSM) control method is proposed by combining a new adaptive strategy with the backstepping-like technique. The new adaptive strategy is first constructed by means of the equivalent control for which a sliding-mode-based filter is employed rather than the widely-used low-pass filter such that the parameter restriction under the usage of low-pass filter can be relaxed. Then, by applying the proposed adaptive strategy and the idea of adding a power integrator, an adaptive SOSM method is established to finite-time stabilize the sliding variables. The feature of the proposed SOSM method lies in that the gain will vary with the size of the lumped uncertainty so as to avoid the overestimation of the gain. The stability analysis is given based on the finite-time Lyapunov theory. The theoretical results are finally applied to the voltage regulation problem of a Buck converter. [ABSTRACT FROM AUTHOR]

This paper studies the high-order sliding mode (HOSM) control for a general class of uncertain nonlinear systems. Firstly, the standard sliding mode system is extended to a new sliding mode system subject to mismatched terms. The use of the new sliding mode system can reduce the system uncertainties in the control channel and relax the well defined relative degree assumption. Secondly, the conventional constant upper bounds assumption is relaxed to time-varying positive functions, which enables us to obtain a global result. For the new sliding mode system under the new global assumption, a novel HOSM controller, that has a simple relay polynomial form, is then proposed. By introducing a new continuous component in the controller, the discontinuous component of the controller only needs to be selected to suppress the uncertainties and thus a controller with minimal discontinuous component magnitude can be implemented. Finally, strict Lyapunov analysis is provided to show the globally finite-time stability of the closed-loop sliding mode system and an application to Buck converter is presented. [ABSTRACT FROM AUTHOR]

In this letter, it is verified that the main result of the above article on converging the voltage tracking error of a buck converter controlled by a fractional-order sliding mode controller cannot be logically justified. The root cause of this inconsistency, which is an improper use of an inequality, is discussed. [ABSTRACT FROM AUTHOR]