Development of sliding mode controllers for DC-DC converters

Abstract

Despite being a popular research subject, sliding mode (SM) control is still rarely applied in practical DC-DC converters. This is mainly due to the apparent complexity of implementation, the lack of strong motivation for its development, and the inherent requirement of high and variable switching frequency. This thesis is written to address collectively the various issues concerning the development of sliding mode controllers for practical DC-DC converters. An analog approach of implementing conventional sliding mode controllers for DC-DC converter is illustrated. Switching frequency variation issues in these controllers are elaborated. Practical solutions to these issues and their implementation methods are described. An alternative type of analog fixed-frequency sliding mode controller based on pulse-width-modulation (PWM) is proposed. Systematic and unified critical design rules and conditions for these controllers as applied to the buck, boost, and buck-boost converters are proposed. The discussions take into consideration the practical aspects of the converter, and it is shown that the fixed-frequency pulse-width-modulation based sliding mode control technique can he easily realized with simple analog circuitries. Experiments are conducted to compare the difference between the sliding mode controlled converters and the conventional linearly controlled converters. The results show that the former complies to the design with a similar response for all operating conditions, while the response of the latter will only comply to the design at a specific operating condition. The main contributions of this thesis can he summarized as follows: 1. A simple approach that can he applied in the development of hysteresis-modulation (HM) based SM voltage controlled DC-DC converters, to bridge the gap between the control principle and circuit implementation, has been introduced. 2. A thorough investigation into the problem of switching frequency variation due to the deviation of operating conditions in the HM based SM controlled DC-DC converters has been conducted. The effectiveness of an adaptive control scheme in alleviating the problem has been studied. Methods of implementing the proposed adaptive control strategies have been provided. 3. A general approach of deriving the PWM based SM voltage controller for DC-DC converter has been introduced. Simple and ready-to-use control equations for the immediate implementation of the PWM based SM voltage, controller has been given. 4. The design and implementation of the PWM based SM voltage controllers have been presented from a circuit design perspective, along with experimental verification. This thesis contains nine chapters. The first three chapters give a detailed exposition on the related theories, operating principles, and information that are relevant to the development of SM controllers for DC-DC converters. The purpose is to serve as an introduction to the more advanced topics discussed in the subsequent chapters, which report the core results of the various areas of investigation in this research subject. The final chapter concludes the thesis and discusses some possible future work.