There is ever-increasing demand in telecommunication system, data server and computer equipment for low voltage, high current power supply. LLC resonant converter is a good topology on primary side of the converter because it has soft switching and resonant conversion. However, the passive rectifier in the secondary side has high power dissipation. Synchronous rectifier is a popular method to reduce this rectification loss. Although there are many types of synchronous rectifier for PWM converter, most of them do not function well in LLC resonant converters. It is because the wave form of LLC resonant converter is different from PWM. The objective of this research is to reduce the power dissipation and physical size at the same time. In this thesis, a novel current driven synchronous rectifier with saturable current transformer and dynamic gate voltage control for LLC resonant Converter is presented. This novel circuit reduces the rectification loss and size of the current transformer in the synchronous rectifier. This synchronous rectifier has several outstanding characteristics compared with generic voltage driven and current driven synchronous rectifier. The saturable feature reduces the current transformer turns. Inherent dynamic gate voltage controlled by saturable current transformer reduces gate loss in the MOSFET. A novel driving circuit is proposed for accurate turn off time. It reduces loss significantly. This synchronous rectifier is completely self-contained which can replace the rectifier diode as a drop in replacement. It is insensitive to parasitic inductance. In order to explain the current transformer saturable, a model of saturable current transformer is proposed. A prototype demonstrates the advantages of the proposed current driven synchronous rectifier. Furthermore, a novel integrated synchronous rectifier is presented which provides a more compact system. The synchronous rectifier current transformer is integrated with the main transformer which reduces the number of circuit joints in power path. Each soldering joint generates significance loss in power converter. A pair of 0.5mΩ soldering joint in 25A current path produces 0.62W loss. The placement of the integrated current transformer is important. A criterion for the placement of the current transformer within the main transformer is to avoid interference to the current transformer from the magnetic flux of the main transformer. Thus, a placement method to integrate the current transformer into the main transformer is proposed. An integrated current transformer model is suggested to explain the operation of the integrated synchronous rectifier. A prototype demonstrates the advantages of the integrated synchronous rectifier. / published_or_final_version / Electrical and Electronic Engineering / Master / Master of Philosophy

High power conversion efficiency is desirable in power supplies. Design optimization of on-line power converter is presented in this thesis. High efficiencies over a wide load range, for example 20%, 50% and 100% load, are often required. It is a challenge for on-line pulse-width modulation (PWM) converters to maintain good efficiencies with light load as well as full load. A two-stage multi-objective optimization procedure is proposed to optimization power converter efficiencies at 20%, 50% and 100% load. Two-FET forward prototype converters are built to verify the optimization results. The LLC (abbreviation of two resonant inductor L and one resonant capacitor C ) series resonant converter can provide high power conversion efficiency because of the resonant nature and soft switching. The design of LLC resonant converter is more difficult than that of PWM converters since the LLC resonant converter has many resonant modes. Furthermore, the LLC resonant converter does not have analytical solution for its resonant operation. In this thesis, a systematic optimization procedure is proposed to optimize LLC series resonant converter efficiency. A mode solver technique is developed to solve LLC resonant converter operations. The proposed mode solver employs non-linear programming techniques to solve a set of LLC state equations and determine the resonant modes. Loss models are provided which serve as the objective-function to optimize converter efficiency. Optimization results show outstanding efficiency performance and experimental agreement with optimization. The optimization work extends to the LLC resonant converter with power factor correction (PFC) circuits where the effect of LLC converter input voltage variation cased by the PFC circuit is considered. Detail comparisons of PWM converter and LLC resonant converter loss profiles are also presented. The reasons that LLC resonant converter has higher efficiency are given and supported by quantitative data. Converter lifetime is highly related to component losses and temperature. The lifetime analysis is presented. The analysis reveals that the LLC resonant converter output capacitor is the weakest component concerning life. / published_or_final_version / Electrical and Electronic Engineering / Doctoral / Doctor of Philosophy

M. Tech. Electrical Engineering. / Aims to analyze, design and develop a transformer-less 5-level converter topology with an improvement in the output waveform and to investigate its suitability for AC/DC, DC/AC and back-to-back configuration.

Graduate

abstract / Electrical and Electronic Engineering / Doctoral / Doctor of Philosophy

published_or_final_version / Electrical and Electronic Engineering / Master / Master of Philosophy

The intention of this dissertation is to determine, on the Eskom system, if the pre-insertion closing resistors installed on the Alpha-Beta 765 kV line breakers are preventing overvoltages from being caused during auto-reclosing. Other possible solutions of reclosing protection are investigated. It has been shown on two occasions, from actual field data that overvoltages have occurred on the lines after reclosing. High overvoltages on this network could be the cause of the many reactor failures that have occurred. A mathematical model of the Alpha-Beta 765kV system was produced on Matlab/Simulink to simulate the resonance of the line during opening and then the effect on the voltage when reclosing takes place. The effects of installing pre-insertion resistors to reduce overvoltages on reclosing were analysed, as well as looking at controlled reclosing at the optimal voltage across the line breakers. It was shown from the studies, that pre-insertion resistors do limit the overvoltages to within the surge capabilities of the line (10% overvoltage) and that the cause of the previous overvoltages were actually due to insertion resistor operations failure. It was also shown that the method of controlled closing at optimal voltage across the breakers is also a successful method of preventing overvoltages. This dissertation also evaluates a design specification for a switching relay for controlled re-closing of the line. / Thesis (M.Sc.)-University of KwaZulu-Natal, Durban, 2009.

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