Analysis, design, and applications of switched-capacitor converters

Abstract

Nowadays, with the increasing demand for smaller and lighter power converters in commercial electronics, there is an increasing popularity in both the research and the development of switched-capacitor (SC) converters. With the absence of inductors and bulky transformers, the power density of SC converters is improved as compared to traditional switching mode power supplies. Moreover, the possibility of fabricating an entire SC converter into an integrated circuit (IC) chip has resulted in many companies producing SC converter ICs and many portable electronic products adopting such ICs. The purpose of the thesis is to summarize the previous research works and report the accomplished works. A literature review is included to summarize the research efforts of SC converters. Converter topologies, modeling works, control methodologies, practical applications, and the industrial market development of SC converters are discussed. Limitations of previous works are highlighted and the motivations of the proposed research works are stated. Three major contributions of the research works in this thesis are included. First, the energy efficiency of SC converters is re-examined by dissecting the entire efficiency problem into two aspects, namely the charging efficiency and the discharging efficiency. Mathematical derivations of the energy efficiency expressions on different aspects of charging and discharging conditions are included. Clarifications on misunderstandings found in previous works are stated and factors of achieving high-efficiency in both charging and discharging processes are suggested. The entire SC converter efficiency is then evaluated by combining the considerations of charging and discharging efficiencies. Experimental results are provided to verify the theoretical analysis. It is experimentally proved that resistance in the charging path is independent of the overall efficiency of SC converters, but the resistance in the discharging path affects the overall efficiency of SC converters. Additionally, the flawed concept of lossy charging process in quasi-switched-capacitor (QSC) converters is clarified. Furthermore, higher switching frequency and larger capacitance of flying capacitors can reduce the voltage ripple on the flying capacitors to improve the overall efficiency of SC converters. Second, a universal SC converter capable of achieving DC-DC, AC-DC and DC-AC power conversions is proposed. Previous SC converters are designed and are only applicable to some particular applications, which are unsuitable for application in situations requiring multi-purpose power conversions. A multi conversion purpose SC converter can resolve the problem. Detailed circuit operations on each type of power conversions are described. The control methodology of the overall system is explained. The required circuitries for programming the control sequence of power switches for achieving various types of power conversions are given. Simulations have been performed to a preliminary proof of the proposed idea. An experimental prototype has been built to demonstrate the proposed idea. It is shown that different types of power conversions can be achieved by only adjusting the control signals of power MOSFETs. Both line and load regulations capabilities, efficiency analysis, and dynamic response under the step change of the load current of the proposed converter are discussed. Third, a multi-port SC converter is introduced for multiple sources or loads applications. Due to the distributed energy resources (DER) characteristic in renewable energy systems, multiple converters are required for different power conversions in the entire system. An alternative approach is proposed to resolve the aforementioned problem, which is through the means of using a multi-port SC converter. The multi-port structure of the proposed SC converter allows the connections of different power sources or loads to a single SC converter, while different power conversions among the connected sources and loads can be performed. An example of an energy system having photovoltaic (PV) panel, battery, and AC voltage grid is considered to discuss the feasibility of the proposed converter. Three different operating modes, including the battery recharging by the PV panel or the AC voltage source, and the AC voltage generation from the PV panel, are defined. Detailed explanations on different operating modes and the overall control methodology are given. Simulation results and experimental verifications, including the line and load regulation capabilities, the efficiency analysis, and the transient response, are given. It has been shown that different power conversions among the connected sources or loads can be readily achieved by using only a single SC converter. This thesis contains seven chapters. The first three chapters give respectively the introduction of the thesis, a literature review on the previous works, and an overview of the SC converters. The accomplished research works in this study are discussed in Chapters 4-6. The objective, theoretical analysis, and experimental verifications of each work are included. The last chapter gives a conclusion to the thesis and a discussion on some possible future works.