New energy-efficient high-voltage DC-DC power conversion technology

Abstract

This thesis presents the findings of research on new high-voltage medium-power dc-dc conversion technologies. A current-fed full-bridge step-up high output voltage converter and two multi-level step-down high input voltage converters have been investigated. A switched-capacitor snubber is proposed for zero-current-switching (ZCS) of the four IGBTs in the full-bridge converter. The ZCS can be achieved with minimum circulating current under different loading condition due to the self-adaptable resonant energy in the snubber. Two kinds of energy-efficient solutions are presented to perform the conversion from high input voltage to low output voltage. One is by using a multi-level multi-phase topology with zero-voltage-switching (ZVS) to reduce the voltage stress on the primary-side switches and improve the output current capacity. The other is by employing a three-level converter featured with different voltage stresses on the two series-connected switch pairs, allowing optimal selection of switching devices and wide soft-switching load range. In practice, the low output voltages of the high-voltage converters are usually used to supply various low-voltage converters which are exposure to momentary loads. To improve the dynamic response, a generalized fast transient controller is proposed for different type of low-voltage converters based on the derivation of a uniform second-order switching surface. The considerable power loss and short lifetime of aluminum electrolytic capacitors in power converters impose massive challenges to push up efficiency and lifetime. Therefore, a novel concept to reduce the dc-link capacitance by introducing a voltage compensator connecting in series with the dc bus line is proposed and studied. It explores the possibilities to replace the electrolytic dc-link capacitors in high-voltage power converters by long lifetime low power loss film capacitors without sacrificing power density and cost effectiveness. The contents of this thesis are as follows: In Chapter 1, the motivation of the research on high-voltage medium-power dc-dc converters will be discussed. Prior-art approaches to improve the efficiency of power converters will be reviewed from component level, circuit level to system level. The principle and associated application limitation of switching surface control for dc-dc converters will be illustrated. Available concepts and solutions to reduce dc-link capacitors in power electronic systems will be presented. In Chapter 2, the concept of adaptive snubber energy for ZCS will be presented. The operating principles of the proposed current-fed full-bridge converter will be described. The trade-off design and small-signal model will be given. Implementations and evaluations of a 530 V / 15 kV 5 kW experimental prototype will be discussed. In Chapter 3, a solution for high-to-low voltage conversion based on a generalized multi-level multi-phase topology will be discussed. The switching mechanism and operation of a typical switch pair will be analyzed. A dc analysis will be carried out to determine the dc conversion ratio and the ZVS conditions in an analytical form. The selfbalance property of the voltages across the input capacitors will be described. A 1500 V / 48 V, 2 kW prototype with four switch pairs in the primary-side is designed, implemented, and evaluated. In Chapter 4, another solution to convert 1500 V dc to 48 V dc will be presented based on a novel concept, by which the voltage stresses on the series-connected two switch pairs are asymmetric. The advantages of a three-level converter adopted the proposed concept will be illustrated in terms of utilization of switching devices and load range for soft-switching. A new concept of hybrid ZVS-ZCS scheme will be proposed. The evaluations on a 2 kW prototype will be given to verify the theoretical predictions. In Chapter 5, a uniform second-order switching surface for a fast transient controller of dc-dc converters will be derived. Stability analysis and controller implementation will be discussed. Simulation results on four kinds of dc-dc converters (i.e., boost converter, buck-boost converter, Ćuk converter and SEPIC) will be presented to verify the universal applicability of the proposed control method. Experimental results on a 48 V/48 V buckboost converter prototype will be also analyzed to exhibit the performance of the controller. In Chapter 6, a patent-pending technology for reducing dc-link capacitance in capacitor-supported power electronic systems will be presented. The operation principle and implementation of the proposed voltage compensator will be discussed. Simulation and experimental results will be provided to verify the theoretical analysis. In Chapter 7, conclusions and suggestions for future research on this topic will be given.

Date of Award	16 Jul 2012
Original language	English
Awarding Institution	City University of Hong Kong
Supervisor	Shu Hung Henry CHUNG (Supervisor) & S.Y. Hui (Co-supervisor)