Formation and analysis of switched-capacitor converter systems with modular circuit configuration

Abstract

Switched-capacitor (SC) converter has been reported for fully integrated design because only capacitor is used for energy storage. Recently, the SC concept has been extended to medium power processing. A novel power conversion technique based on the SC technology is investigated in this thesis matching the demand of the revolution in smart mobility and renewable energy (RE). The merits and shortcomings of SC power conversion are identified by an overall review of previous development of the SC technology and the features of existing SC converters. Control techniques and circuit configurations are developed to address the limitations and weaknesses of traditional SC converters. The associated analysis and design of power converters based on systems of SC units for DC-DC and DC-AC applications are discussed. The development of SC converter systems can be divided into two sections. The former part of this thesis focuses on the design of DC-DC power conversion systems based on zero-current switching (ZCS) SC units; the latter part explores the design of step-up DC-AC multilevel inverters (MLIs) based on the series-parallel SC technique. The principle of ZCS SC DC-DC converter is reviewed; the analysis is extended to the parallel-mode operation with a varying number of active SC converter units. Based on the system modeling, a series of centralized control techniques is developed. Featuring inherent voltage conversion ratio and voltage droop characteristic by the SC topology, these centralized control methods are robust and easy to be implemented. Besides, a family of balancing systems for energy storage system (ESS) based on the multi-port SC conversion system is presented. On top of the well-known auto-balancing feature offered by the SC technology, feasibility of improving the balancing speed by an alternative ZCS SC structure with the presence of supervisory control from a centralized management system is examined. The design and analysis of the single-input multi-output and multi-input single-output, as well as, the corresponding mutations of string-to-cells, cells-to-string and cells-to-cells configurations are developed. Investigation on the topics of SC DC-DC converters provides the theoretic foundation and working principle of SC based power processing. Attractive features of the modular SC approach motivates the development of MLIs based on the SC building blocks. A hybrid SCMLI topology offering a high number of output levels based on the ideas of series-parallel SC unit and bidirectional switched MLI is developed. Compared to independent DC sources, series-connected sources have higher availability, especially for the applications which ESS is partaking. In addition, the cell balancing issue with the hybrid SCMLI is tackled by combining the switched-ladder structure with an SC unit. Furthermore, the weaknesses of common SCMLIs such as the requirement of high voltage unfolding H-bridge is mitigated by proposing a series of modular SCMLI topologies based on the two-phase, cascaded bipolar series-parallel as well as the cross-switched structures. The charge-up power loss is alleviated by the soft-charging technique with a resonant inductor. This thesis aims at exploration and provision of the SC based power processing solutions for the applications in mobility as well as small scale power distribution systems integrated with RE and ESS through the idea of modular circuit configuration.