Thesis (M. Phil.)--Hong Kong University of Science and Technology, 2004. / Includes bibliographical references. Also available in electronic version. Access restricted to campus users.

Thesis (M.S.)--University of Wisconsin--Madison, 1969. / eContent provider-neutral record in process. Description based on print version record. Includes bibliographical references.

Thesis (Ph. D.)--University of Hong Kong, 1992.

Thesis (Ph. D.)--University of Washington, 1996. / Vita. Includes bibliographical references (leaves [102]-109).

Thesis (Ph.D.)--Aberdeen University, 2009. / Title from web page (viewed on June 26, 2009). Includes bibliographical references.

This thesis presents the operation, analysis, simulation and experimental results of a single-phase single-stage soft-switched high frequency (HF) transformer isolated ac-to-dc bridge converter with low total harmonic distortion (THD) and its extension to ac-to-dc multiphase converter. A single-phase single-stage soft-switched ac-to-dc bridge converter cell has been proposed based on a new gating scheme. Due to the discontinuous current mode (DCM) operation of the boost inductor, natural power factor correction and low THD are ensured. The single-stage ac-to-dc multiphase converter is realized based on this bridge converter cell to reduce HF harmonic components in the line current. The steady-state operation of the single-stage bridge converter is explained for all operating modes. Intervals of operation in these modes are identified and analyzed. The steady-state solutions are presented. Based on these solutions, design curves are obtained. Design example of a 1.7 kW converter is presented to illustrate the design procedure. Operational characteristics are obtained for different line and load conditions. PSPICE simulation results for the designed converter are presented. An experimental prototype is built to verify the operation and performance of the converter. All four switches in the fixed frequency bridge converter undergo soft switching (common switch requires an auxiliary circuit) for a wide line and load range. A single-stage HF transformer isolated ac-to-dc multiphase converter is proposed. The analyses of the single-stage bridge converter cell are extended to the multiphase converter. It is shown that N = 3 is near the optimum number of cells to reduce the input current HF harmonic components. A design example of a 166 to 260 V (rms) input, 420 V output, 5 kW converter switching at 50 kHz is presented. PSPICE simulation results are obtained for the designed converter to study its performance for varying load and line conditions. A 3-cell 1.5 kW experimental prototype is built and experimental results are obtained. All the results show that HF harmonics in the line current are reduced and output voltage ripple frequency is increased. Each cell handles equal power and all bridge-switches are soft switched. As a result, uniform thermal distribution is obtained. Small-signal analysis of the single-stage ac-to-dc bridge converter cell is presented for all operating modes using state-space averaging technique. Based on this analysis, small signal transfer functions are obtained. Frequency response of the transfer functions are plotted using MATLAB program and verified by PSPICE simulation results. A closed loop control system is designed and frequency response of the overall loop gain is presented. Large-signal transient behavior of the converter cell is studied with open loop using PSPICE simulation program for step change in line and load conditions. The simulation results show that the closed loop system is required to improve the converter performance during step increase in line voltage. / Graduate

D.Ing. / Changes in the preferred technologies used in modem switched mode power supplies are driven by the need for a decrease in cost of manufacturing while maintaining the highest possible power density. Modem materials allow smaller components to be manufactured without affecting their electromagnetic properties but thermal problems still impair efforts to further reduce their size. Increased switching frequencies cause increased conductor and core losses in magnetic components such as inductors and transformers. Amongst other advantages the increased surface area of low-profile planar structures allow better removal of the heat generated in the component and this work reevaluates conductor optimization for specific planar inductor windings. Conductor optimization has been a topic thoroughly investigated over the last few decades and these techniques have been adopted for planar winding design. It will be shown that the process involved in the manufacturing of helical planar windings allows further exploitation of the basic optimization process. A per-layer optimization technique, earlier proposed by other researchers and deemed impractical at the time, is revised and it is shown here that it can be successfully implemented in the chosen winding type. The per-layer optimization is also extended to accommodate non-sinusoidal current waveforms in this work. Other facets of loss reduction are also addressed. Windings are shaped in the region of air gaps in magnetic circuits to reduce the losses caused by the fringing flux intersecting the conductors but since the shaping influences both the ac-resistance and dc-resistance of the winding an optimization process is required to find the optimal conductor arrangement. Contributions are made with regard to speeding up the process of finding the optimal layout of conductors around the air gap by devising a simple model for the flux distribution of the fringing flux. The effect of combining the per-layer optimization technique with these shaping techniques is exploited to its full potential in this work and contributes largely to loss reduction in helical inductor windings carrying currents containing ac- and dc components. The optimization time is reduced by allowing winding shaping while maintaining the dc-resistance of each layer. The optimal winding shape then becomes a function of the window constraints for a chosen core rather than finding it through conventional methods. Case studies, complete with FEM-simulations and experimental measurements, are offered in support of the proposed solutions. Manufacturing issues are dealt with and loss measurement techniques developed as part of this work.

M.Ing. (Electrical and Electronic Engineering) / The main purpose of the study is to propose an unique unity power factor AC-DC converter topology, with isolated output. The proposed converter is obtained by merging the unity power factor boost converter and the full bridge resonant transition converter. The converter is unique as resonant switching techniques are used to obtain a zero voltage switching boost stage, resulting in an all soft switching converter. The result is lower levels of conducted EMI, as well as lower switching losses. Conventional topologies utilize more than one converter stage to obtain the required results, whereas _ the proposed converter results in a single step solution. As the purpose of the proposed converter is power factor regulation resulting in lower levels of conducted EMI, the study is commenced with a review of the definitions and equations for both power factor and EM!. Thereafter sources and propagation of conducted EMI are discussed. To gain a better understanding of the operation of the proposed converter, the operation of the unity power factor boost converter, as well as the full bridge resonant transition converter are discussed in depth. A switching cycle of the proposed converter is divided into different intervals, and analytical equations for the relevant currents and voltages for each interval are derived seperately. Possible control schemes are introduced and where applicable, conditions for zero voltage switching are given. It is concluded that a constant frequency PWM controller is the most ideal control scheme to ensure proper operation of the converter. A simple design procedure is presented by which the size of the most important circuit parameters can be designed in terms of the desired switching frequency and permissable duty cycle variation. Finally, experimental results obtained from the practical converter, are given. Deviations from the ideal are interpreted and methods of improving the performance are suggested.

M.Ing. (Electrical & Electronic Engineering) / Induction heating is a relatively old and established process. New switching devices and ideas are, however, resulting in much improved performance at reduced cost. Some of these devices are investigated in this thesis, and some new ideas presented. From a literature study a general review of the current state of the technology is given. The fundamental principles and limits of induction heating are discussed. Power electronic topologies for induction heating, as well as switching devices which can be used in these topologies, are compared and evaluated. From the knowledge gained, possible fields for improvement are identified. One such field is that of semiconductor switch drives, and a high-performance gate drive was developed. Using such fast drives, special attention must be given to the electromagnetic layout of the circuit. Another new field is that of integrated resonant elements. These elements find application in most resonant converters, and offer reduced weight and losses compared to conventional technologies. They are evaluated both experimentally and theoretically. A prototype induction heater was built, using a newly released switching device named a zero turn-off thyristor. The high-performance gate drive developed earlier was tested in the inverter and gave excellent results.

M.Ing. (Electrical and Electronic Engineering) / The purpose of this investigation is mainly the study of various topologies of force commutated distortion compensators. This is done against the background of distortion in power systems in general. At the time of the investigation confusion reigned the domein of terminology and definition of power components and power compensators. Part of the investigation was the functional categorizing of power compensators. Forced commutated distortion compensators were then highlighted as element of this suggested frame work. There is a general increase of distortion in power networks which emphasises the limits of passive filters to compensate for non-active power. The use of power converters as forced commutated distortion compensators proves an effective means whereby to counteract these limits. In this document the different topologies of forced commutated distortion compensators are labelled according to the position where the converter is coupled to the network, as well as the characteristics of the energy storage element used. The topologies are studied through modelling, mathematical analysis and simulation. An experimental system of one of the topologies is presented. Control strategies for forced commutated distortion compensators are discussed with emphasis on the control of energy loss in the energy storage elements of the compensators. The investigation is concluded with proposed ideas for futher study as well as a philosophical approach to the role of forced commutated distortion compensators in the future.