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The concept of soft-switching pulse-width-modulated (PWM) technique was proposed aimed at combining the advantages of both the conventional PWM technique and the resonant technique. This work presents four new families of soft-switching PWM converters: the zero-voltage-switched (ZVS) PWM converters, the zero-current-switched (ZCS) PWM converters, the zerovoltage- transition (ZVT) PWM converters, and the zero-current-transition (ZCT) PWM converters. The family of ZVS- and ZCS-PWM converters are developed to improve the performance of the ZVS and ZCS quasi-resonant converters, respectively. The principles of operations of these two families of converters are presented, and the merits and limitations are assessed. A number of experimental converters are breadboarded to verify the theoretical analysis. Both the ZVT-PWM and ZCT-PWM techniques use the concept of shunt resonant network to achieve soft-switching. In this way, the new converters achieve soft-switching without increasing the voltage and current stresses of the power switches and diodes. By using the boost topology as an example, a complete dc analysis of the ZVT-PWM and ZCT-PWM converters is presented, and the dc Voltage-conversion ratio characteristics are derived. Design trade-offs are examined, and design procedures are established. The theoretical analysis and novel features of the proposed converters are verified on a number of breadboarded converters. Finally, the typical small-signal characteristics of the ZVT -PWM converters are analyzed and verified experimentally by using the boost converter as an example. / Ph. D.

Two control schemes for PWM converters, average current-mode (ACM) control and charge control, are studied in this dissertation. The small-signal models are derived for continuous-conduction mode PWM converters employing these two controls. Sampled-data modeling is applied to the current loop modeling, and the obtained models are accurate up to half the switching frequency. The relationships between current loop instability and converter operating conditions for both controls are found for the first time. The derived models are verified by both time-domain simulations and experiments. The models can be used for both voltage loop and current loop analysis and designs. Comprehensive design guidelines for PWM converters with both controls are also provided. The small-signal characteristics of these two controls are compared with those of peak current-mode control. The applications of ACM control and charge control to power factor correction (PFC) circuits are studied. Charge control is applied to continuous-conduction mode flyback converter to achieve a single-stage PFC. The current loop instability in PFC circuit and its effect on the input EMI filter design are investigated. The trade-off between the current loop stability nlargin and line current distortion is also discussed. / Ph. D.

A new small-signal model is developed for pulse-width-modulated converters operating with current-mode control. Four different control schemes are analyzed, all of which use the instantaneous value of the inductor current as part of the control modulation. Continuous-conduction mode and discontinuous-conduction mode models are provided. Sampled-data modeling is applied to the current-mode cell which is common to all converters, and the important results obtained are simplified to give a powerful model for analysis and design. The new model explains all of the observed phenomena of current-mode control, including the subharmonic oscillation which can occur with constant-frequency modulation. It is shown that the model provides transfer functions with three poles for two-state converters. Predictions of the new model are accurate to half the switching frequency. / Ph. D.

A nonlinear programming technique using the penalty function method,which. is especially suitable for power. converter design optimization, is utilized for the comparison of the conventional BuckBoost and Cuk converters. Detailed comparisons are made with respect to the loss and weight breakdowns. of the optimum design of the two converters for a given set of performance specifications. The comparison shows that the Cuk converter outperforms the conventional Buck-Boost converter in both operating efficiency and weight considerations for both the step up or step down modes of operation. Also, a detailed comparison of the coupled inductor version of the Cuk converter under both the balanced and unbalanced current ripple reduction modes of operation is made. From this comparison, it is shown that the unbalanced current ripple reduction is more advantageous. The effects of using different input voltages and different core materials with different saturation flux densities on the global power converter optimization were also studied. The new Cuk converter was disclosed for the first time in 1977. Since then, there are several controversies about the claims made by Cuk of the advantages. of his. converter with respect to the conventional Buck-Boost converter. The comparisons made by Cuk of the two converters did Iiot satisfy the same performance specifications. Consequently, his conclusions of the superiority of his converter cannot be fully justified. In this work the comparisons are made of these two converters based upon the optimum designs. £or a given set of performance specifications. With the detailed comparisons of the loss/weight breakdowns of these two converters, the controversies surrounding the Cuk converter are solved. / Ph. D.

Analytical expressions describing the input current spectrum of the flyback Pulse-Width-Modulated, Zero-Current-Switched Quasi-Resonant-Converter, and Zero-Voltage-Switched Quasi-Resonant-Converter are derived in terms of normalized circuit parameters. These analytical results are tested against experimental results. The same numerical example is presented for these three converters in which the input current spectrum is computed for various line/load conditions and the results are mutually compared. Detailed experimental results are presented for the same three converters as well as for the flyback Zero-Voltage-Switched Multi-Resonant-Converter. The effects of any parasitic oscillations in the experimental circuit are also considered. / Master of Science

1. The function z = w + k√(w² - 1) will represent in the z plane the lines of flow of an electric current in a plate conductor around a circle of unit radius if the constant k is unity and around an ellipse of the form x²/1 + y²/k² = 1 which is oblate if the constant k is greater than unity and prolate if the constant k is less than unity. 2. The lines of flow of an electric current approach straight lines in the z plane as y increases and the smaller the vertical axis of the deleted section the smaller will be the distortion of the lines of flow. 3. The finding of the lines of flow of an electric current in a plate conductor around a square section does not admit of a mathematical solution. 4. The magnetic lines in a field which has been partially blocked out do not conform to "bending" as do the lines of flow of an electric current in a plate conductor in which sections of certain families of geometric figures have been deleted. 5. Lines of flow in a plate conductor with certain deleted geometric sections do conform with the theoretical lines mathematically calculated and plotted when the conductor is placed in the magnetic field. / Master of Science

Quasi-Resonant Converters (QRCs) have been proposed recently in the effort of increasing the switching frequency and reducing the switching losses. There are several merits and demerits of this class of converters. The most important aspects of this research are to assess the advantages and limitations of QRCs in terms of their device current and voltage stresses and a determination of the factors affecting them. The losses in the various elements are also determined to provide a reasonable estimate of the converter efficiency. The work in both tasks is normalized permitting a direct comparison of one topology with another without performing any detailed design or knowledge of the internal parameters of each converter. As a result of the analysis of switching losses, an accurate estimation of MOSFET switching times is presented providing more insight into the switching behavior of these converters. Experimental results confirm the accuracy of the theoretical predictions of device current and voltage stresses along with the converter losses and efficiency. Factors affecting the device stresses derived theoretically are demonstrated experimentally. Computer simulation is used to confirm the theoretical analysis of transistor switching times used in determining the switching losses in the devices. / Master of Science

Two waveform-shaping techniques to reduce or eliminate the switching stresses and switching losses in switching-mode power conversion circuits are developed: the zero-current switching technique and the zero-voltage switching technique. Based on these two techniques two new families of quasi-resonant converters are derived. Since the stresses on semiconductor switching devices are significantly alleviated, these quasi-resonant (QRC) converters are suitable for high-frequency operations with much improved performances and equipment power density. Employing the duality principle, the duality relationship between these two families of quasi-resonant converters are derived. The establishment of the duality relationship provides a framework allowing the knowledge obtained from one converter family to be readily transferred to the other. Further topological refinements are derived through the utilization of parasitic elements in the devices and the circuit. In particular, the two most significant parasitic elements, the leakage inductance of the transformer and the junction capacitances of the semiconductor switch, are incorporated as part of the resonant-tank circuit required by these quasi-resonant converters. Consequently, the detrimental effects due to these parasitic elements are eliminated, and the converters can be operated at very high frequencies. / Ph. D.

Thesis (PhD)--University of Stellenbosch, 2004. / ENGLISH ABSTRACT: Multilevel converters were developed as a result of a growing need for higher power converters. This dissertation addresses a specific multilevel topology called the multicell topology. A problem associated with this topology is cell capacitor voltage unbalance. This dissertation addresses the issue of natural balancing of multicell converters. The topology is mathematically analysed and a theory is developed to explain the natural balancing mechanism. The study of the natural balancing property includes a detailed harmonic-, steady-state- and time constant analysis. The theory is verified by a comparison between the theoretical-, simulated- and experimental results. / AFRIKAANSE OPSOMMING: Veelvlakkige omsetters het ontstaan as gevolg van ’n behoefte aan ho¨er drywing omsetters. Hierdie proefskrif handel spesifiek oor die veelsellige omsetter topologie. ’n Probleem wat met hierdie topologie geassosieer word is selkapasitor onbalans. Hierdie proefskrif ondersoek die natuurlike balansering van veelsellige omsetters. Die topologie word wiskundig geanaliseer en ’n teorie word geformuleer om die natuurlike balanseringsmeganisme te verduidelik. Die ondersoek van die natuurlike balanseringseienskap bevat ’n volledige harmoniese-, bestendige toestand- en tydkonstante analise. Die teorie is gekontroleer deur teoretiese-, simulasie- en eksperimentele resultate te vergelyk.