Formulation of DC energy factor and energy-based control method for high efficiency power electronics conversion systems

Abstract

This thesis is to examine the energy handling in DC-DC converter systems. The management of the energy in terms of energy factor is the core area of the examination. The theory derived is extended to the family of DC-DC power converters from which the efficiency and its implication in power factor as that in the AC counterpart are studied. The main purpose of this thesis is the extension of the understanding of the energy handling in power converters. The conventional AC power factor is a start point. Its exploration to DC system, mainly the DC-DC power converters are thoroughly done. There are a number of new concepts that have been explored. These include the variation energy factor, non-active energy and non-active power. Besides the classical switched-mode power converters, the tapped-inductor converters are used as examples of study in their energy handing. Systematic formulations indicated energy handling have been obtained for the converters. Five main aspects of works carried out in this thesis have been clarified below. A series of novel concepts such as Energy Storage (ES), Energy Factor (FE), Buffer Energy (EB) and Buffer Energy Factor (FEB) are reviewed and presented to quantitatively describe the energy behaviour in DC-DC converters. Energy storage and energy factor are used to represent the amount and ratio of energy stored in DC-DC converters during operation. Buffer energy and buffer energy factor are used to evaluate the variation of energy storage in DC-DC converter. Explicit definitions of these energy-related concepts have been also presented. Formulations of these concepts have been deducted for inductor, capacitor and transformer in various DC-DC converters. The definitions of buffer energy and buffer energy factor are further extended on the basis of theory of non-active power. Through the extended definitions buffer energy and buffer energy factor can be applied not only to DC systems but also to AC systems. The correlations and distinctions among buffer energy and reactive energy, buffer energy factor and power factor have been clarified. On the basis of energy factors and buffer energy factors, comparison study has been carried out for basic topologies of DC-DC converters. Useful results for design of DC-DC converter have been obtained. The correlations between efficiency and buffer energy factor are also presented. In general, efficiency of DC-DC converter decreases when buffer energy factor increases. Tapped-inductor converters have been researched carefully by this thesis. A number of special topologies of tapped-inductor converters have been explored. Static performances of tapped-inductor Boost converter have been examined. Comprehensive comparisons between tapped-inductor Boost converter and conventional Boost converter have been carried out. The comparisons include stress of components, efficiency, energy factors and buffer energy factors. Experimental results reveal the superiority of tapped-inductor converter over their conventional counterpart on the occasion of large voltage conversion ratio. Tapped-inductor converter is the promising single-stage high-efficiency choice for future DC distribution systems. Experimental results reveal the main drawbacks of tapped-inductor converter including spike and power loss caused by non-ideal coupling of tapped inductor. Compared with conventional Boost converter, the design of closed loop controller for tapped-inductor converter is more challenging. This thesis has researched the buffer energy factors during transient time. The design of controller for tapped-inductor Boost converter has been studied. The application of one famous energy-based control method: passivity-based control to tapped-inductor Boost converter has been researched. The overall performances of control are analyzed. The comparison between peak-current mode control and passivity-based control has been carried out through theory and experiment. These researches can provide useful references for the controller design of tapped-inductor DC-DC converters.