In the field of power electronics there is always a push to create smaller and more efficient power conversion systems. This push is driven by the industry that uses the power systems, and can be realized by new semiconductor devices or new techniques. This examination describes a novel technique for a small and highly efficient method of converting relatively high DC voltage to a very low voltage for use in the telecommunications industry. A modification to the standard Forward Resonant converter results in improvements in component stress, system efficiency, response time, and control circuitry. This examination describes background information needed to understand the concepts in DC to DC power systems, "soft-switching" topologies, and control methods for these systems. The examination introduces several topologies that are currently being used, and several types that have been previously analyzed, as a starting point for the detailed analysis of the proposed converter topology. A detailed analytical analysis is given of the proposed topology, including secondary effects, and component stresses. This analysis is compared to the results found from both Pspice simulation, and a working DC to DC converter. Finally, the topology is examined for potential improvements, and possible refinements to the model described.

Soft-switching techniques can significantly reduce the switching loss and switching stresses of the power semiconductor devices in a power converter. This work presents several soft-switching topologies for high power PWM converters. These new topologies achieve soft-switching functions with minimum increase of device voltage/current stresses and converter circulating energy, and thus have advantages over conventional techniques in efficiency, power density, reliability, and cost of power converters. The improved zero-current transition (ZCT) converters achieve zero-current switching at both turn-on and turn-off for all main switches and auxiliary switches. These converters significantly reduce the switching loss and stress of the power semiconductor devices, while have a voltage/current stress and circulating energy similar to a PWM converter’s. The analysis, design, and experimental verification are presented. The three-phase zero-voltage transition (ZVT) boost rectifiers/voltage source inverters are developed with simple auxiliary circuits. Unlike most existing three-phase soft-switching techniques, these new topologies achieve soft-switching functions without overcharging the resonant inductors, and realize the benefits of soft-switching operation with minimum extra main switch turn-offs and fixed auxiliary circuit control timing. The operation principles of the developed techniques are experimentally verified, and their efficiency performances are evaluated with experiments and computer simulation. The three-phase ZVT buck rectifier topologies developed in this work achieves zero-voltage turn-on for all main switches with an optimum modulation schemes and simple auxiliary circuits. The auxiliary circuits, which are connected directly to each main switch, can also absorb the parasitic resonance of the bridge arms, and keep the voltage stress of the power devices at the minimum. The analysis and simulation results are presented to verify the converter operation. New ZVT dc-link schemes for three-phase ac-dc-ac converters are investigated. With coordinated control of the ac-dc converter and the dc-ac converter, a set of simple auxiliary circuit can provide soft-switching function for all switches in both the ac-dc converter and the dc-ac converter. The power loss in the auxiliary circuit is also significantly lower than existing dc-link soft-switching schemes. Simulation with experimentally obtained device switching loss data proves that significant efficiency improvement can be achieved with the new ZVT dc-link techniques. New ZVT and ZCT techniques for three-level converters are also developed. The auxiliary circuits are not in the main power path, and allow the converters to be controlled with optimum PWM schemes. Analysis and simulation results are presented to demonstrate the operation principles and advantages of soft switching in three-level converters. / Ph. D.

This thesis proposes a high efficiency driving circuit for multiple light emitting diode (LED) lamps with dimming feature. The driving circuit consists of essentially a high-frequency half-bridge series resonant inverter with multiple output transformers, on which primary windings are connected in series, while secondary sides are loaded by LED lamps rated at different powers with different turn ratios. By controlling the frequency of the inverter, the resonant current as well as the lamp current can be regulated simultaneously. On the other hand, the LED lamps can be dimmed individually by the associated dimming switches with integral cycle control. The tactful circuit ensures a high circuit efficiency owing to less conducting losses and zero-voltage switching (ZVS) operation of the active power switches of the inverter and zero current switching (ZCS) operation of the dimming switches. Two prototype circuits designed for 60 W three RGB LED lamps and 50 W five white light LED lamps have been built and tested to verify the analytical predictions. Experimental results demonstrate that the driving circuit can operate the LED lamps at a high efficiency with a wide dimming range. The lamp power can be dimmed to 10% with frequency control, while whole dimming range can be achieved with integral cycle control. The circuit efficiency with integral cycle control is relatively higher than that with frequency control. The measured efficiencies for the two designed circuit are 93% and 90%, respectively, under the rated powers.

A high-frequency half-bridge series resonant inverter with multiple output transformers is developed for driving multiple cold-cathode fluorescent lamps (CCFLs) with dimming feature. The primary sides of the transformers are connected in series with the resonant inverter to have an identical current, while the secondary sides are loaded by CCFLs with galvanic isolation to each other. To ensure a high circuit efficiency, the active power switches of the inverter are designed to be switched on at zero voltage. The resonant current of the inverter can be regulated by controlling the switching frequency of the inverter, so that all CCFLs can be dimmed simultaneously. On the other hand, the primary sides of the output transformers are associated with parallel switches to dim the CCFLs individually. These dimming switches are operated at a low frequency by integral cycle control with zero current switching (ZCS) to reduce the switching losses. The resonant circuit is tactfully designed to alleviate the variation of the resonant current caused by the switching of dimming switches. A laboratory circuit is built for driving 5 CCFLs. The intended circuit performances are confirmed by test results. The variation of the resonant current is less than 10% when the dimming switches are switching, and the measured efficiency for the circuit is 96.15% under the rated powers.

This thesis deals with the design and construction of a single-phase switching power supply, which is intended for direct resistive heating of iron components. The power supply is especially intended for resistive heating of horse-shoes. The supply is able to deliver an output current of up to 1500 A at a power of up to 2500 W. The first part of this work deals with the design of individual parts of the unit, the second part is focused on construction and testing of the supply and the last part contains technical documentation. The power supply was successfully tested and the required output parameters were met. However some problems do exist, especially with overheating of the output rectifier and with contacting the heated component to the output leads of the supply. These problems will be discussed in the work. The power supply can be used as an alternative solution to classic means of iron heating. The methods and ideas presented in this work can be applied in a design of a similar power supply with high output current, but most of the design rules are valid generally for the given topology.

Renewable energy source plays an important role in energy co-generation and distribution. A traditional solar-based inverter system has two stages cascaded, which has simpler controller but low efficiency. A new solar-based single-stage grid-connected inverter system can achieve higher efficiency by reducing the power semiconductor switching loss and output stable and synchronizing sinusoid current into the utility grid. In Chapter 1, the characteristic I-V and P-V curve of PV array has been illustrated. Based on prediction of the PV power capacity installed on the grid-connected and off-grid, the trends of grid-tied inverter for DG system have been analyzed. In Chapter 2, the topologies of single-phase grid-connect inverter system have been listed and compared. The key parameters of all these topologies are listed in a table in terms of topology, power decoupling, isolation, bi-directional/uni-directional, power rating, switching frequency, efficiency and input voltage. In Chapter 3, to reduce the capacitance of input filter, an active filter has been proposed, which will eliminate the 120/100Hz low frequency ripple from the PV array's output voltage completely. A feedforward controller is proposed to optimize the step response of PV array output voltage. A sample and hold also is used to provide the 120/100Hz low frequency decoupling between the controller of active filter and inverter stage. In Chapter 4, the single-stage inverter is proposed. Compared with conventional two-stage inverter, which has two high frequency switching stages cascaded, the single-stage inverter system increases the system efficiency by utilizing DC/DC converter to generate rectified sinusoid voltage. A transformer analysis is conducted for the single-stage inverter system, which proves the transformer has no low-frequency magnetic flux bias. To apply peak current mode control on single-stage inverter and get unified loop gain, adaptive slope compensation is also proposed for single-stage inverter. In Chapter 5, a digital controller for single-stage inverter is designed and optimized by the Matlab Control Toolbox. A Psim simulation verified the performance of the digital controller design. In Chapter 6, three bi-directional single-stage inverter topologies are proposed and compared. A conventional single-stage bi-directional inverter has certain shortcoming that cannot be overcome. A modular grid-connect micro-inverter system with dedicated reactive energy processing unit can overcome certain shortcoming and increase the system efficiency and reliability. A unique controller design is also proposed. In Chapter 7, a PFM ZCS flyback inverter system is invented. By using half-wave quasi-resonant ZCS flyback resonant converter and PFM control, this topology completely eliminates switching loss. A detailed mathematical analysis provides all the key parameters for the inverter design. As the inductance of transformer secondary side get smaller, the power stage transfer function of PFM ZCS flyback inverter system demonstrates nonlinearity. An optimized PFM ZCS flyback DC/DC converter design resolves this issue by introducing a MOSFET on the secondary side of transformer. In Chapter 8, experimental results of uni-direcitonal single-stage inverter with grid-connection, bi-directional single-stage inverter and single-stage PFM ZCS flyback inverter have been provided. Conclusions are given in Chapter 9.

The need to characterise a PEM electrolyser is motivated by a South African hydrogen company. One of two electrochemical characterisation methods, namely the current interrupt method or electrochemical impedance spectroscopy, is investigated to characterise the PEM electrolyser. Various literature sources can be found on the electrochemical characterisation methods. In this study the current interrupt method is used for the electrochemical characterisation of a PEM electrolyser. The current interrupt method is an electrical test method that will be used to obtain an equivalent electric circuit model of the PEM electrolyser. The equivalent electric circuit model relates to various electrochemical characteristics such as the activation losses, the ohmic losses and the concentration losses. Two variants of the current interrupt method, namely the natural voltage response method and the current switching method, are presented. These methods are used to obtain two different equivalent electric circuit models of the PEM electrolyser. The parameters of the first equivalent electric circuit, namely the Randles cell, will be estimated with the natural voltage response method. The parameters of the second equivalent electric circuit, namely the Randles-Warburg cell, will be estimated with the current switching method. Simulation models of the equivalent electric circuits are developed and tested. The simulation models are used to verify and validate the natural voltage response method and the current switching method. The parameters of the Randles cell simulation model is accurately calculated with the natural voltage response method. The parameters of the Randles-Warburg cell simulation model is accurately calculated with the current switching method. The natural voltage response method and the current switching method are also practically implemented. The results is used to indicate the various electrochemical characteristics of the PEM electrolyser. A Nafion 117 type membrane was tested with the current interrupt method. The membrane resistance parameters of Randles cell were estimated with the natural voltage response method. These values are validated with conductivity measurements found in literature. The results of the Randles- Warburg cell is validated with a system identification validation model. / Thesis (MIng (Computer and Electronic Engineering))--North-West University, Potchefstroom Campus, 2013

The need to characterise a PEM electrolyser is motivated by a South African hydrogen company. One of two electrochemical characterisation methods, namely the current interrupt method or electrochemical impedance spectroscopy, is investigated to characterise the PEM electrolyser. Various literature sources can be found on the electrochemical characterisation methods. In this study the current interrupt method is used for the electrochemical characterisation of a PEM electrolyser. The current interrupt method is an electrical test method that will be used to obtain an equivalent electric circuit model of the PEM electrolyser. The equivalent electric circuit model relates to various electrochemical characteristics such as the activation losses, the ohmic losses and the concentration losses. Two variants of the current interrupt method, namely the natural voltage response method and the current switching method, are presented. These methods are used to obtain two different equivalent electric circuit models of the PEM electrolyser. The parameters of the first equivalent electric circuit, namely the Randles cell, will be estimated with the natural voltage response method. The parameters of the second equivalent electric circuit, namely the Randles-Warburg cell, will be estimated with the current switching method. Simulation models of the equivalent electric circuits are developed and tested. The simulation models are used to verify and validate the natural voltage response method and the current switching method. The parameters of the Randles cell simulation model is accurately calculated with the natural voltage response method. The parameters of the Randles-Warburg cell simulation model is accurately calculated with the current switching method. The natural voltage response method and the current switching method are also practically implemented. The results is used to indicate the various electrochemical characteristics of the PEM electrolyser. A Nafion 117 type membrane was tested with the current interrupt method. The membrane resistance parameters of Randles cell were estimated with the natural voltage response method. These values are validated with conductivity measurements found in literature. The results of the Randles- Warburg cell is validated with a system identification validation model. / Thesis (MIng (Computer and Electronic Engineering))--North-West University, Potchefstroom Campus, 2013

This thesis investigates a novel DC/AC resonant inverter of Induction Heating (IH) system presenting a Multilevel Neutral Point Clamped (MNPCI) topology, as a new part of power supply design. The main function of the prototype is to provide a maximum and steady state power transfer from converter to the resonant load tank, by achieving zero current switching (ZCS) with selecting the best design of load tank topology, and utilizing the advantage aspects of both the Voltage Fed Inverter (VFI) and Current Fed Inverter (CFI) kinds, therefore it can considered as a hybrid-inverter (HVCFI) category . The new design benefits from series resonant inverter design through using two bulk voltage source capacitors to feed a constant voltage delivery to the MNPCI inverter with half the DC rail voltage to decrease the switching losses and mitigate the over voltage surge occurred in inverter switches during operation which may cause damage when dealing with high power systems. Besides, the design profits from the resonant load topology of parallel resonant inverter, through using the LLC resonant load tank. The design gives the advantage of having an output current gain value of about Quality Factor (Q) times the inverter current and absorbs the parasitic components. On the contrary, decreasing inverter current means decreasing the switching frequency and thus, decreasing the switching losses of the system. This aspect increases the output power, which increases the heating efficiency. In order for the proposed system to be more reliable and matches the characteristics of IH process , the prototype is modelled with a variable LLC topology instead of fixed load parameters with achieving soft switching mode of ZCS and zero voltage switching (ZVS) at all load conditions and a tiny phase shift angle between output current and voltage, which might be neglected. To achieve the goal of reducing harmonic distortion, a new harmonic control modulation is introduced, by controlling the ON switching time to obtain minimum Total Harmonic Distortion (THD) content accompanied with optimum power for heating energy.

Conselho Nacional de Desenvolvimento Científico e Tecnológico / A complete study of a topology resulting from a combination of two Forward structures, attached to the same magnetic core of a transformer and operating as a Full-Bridge converter is presented. In order to reduce the switching losses and the electromagnetic interference, a soft commutation cell that provides ZCS commutation of all the switches is implemented. This converter limits the current on the main switches at the load current because diverts the sinusoidal half cycle to a auxiliary switch. This way, a new Double Forward On-Off ZCS was developed. / Um estudo completo de uma topologia, resultante de uma combinação entre duas estruturas Forward, acopladas ao mesmo núcleo magnético de um transformador, e operando como um conversor Full-Bridge, é apresentado. Com o objetivo de reduzir as perdas por chaveamento e a interferência eletromagnética, uma célula de comutação não dissipativa, que fornece uma comutação ZCS para todas as chaves do conversor é implementada. Este conversor limita a corrente nas chaves principais ao valor da corrente nominal, pois desvia o semiciclo senoidal da corrente ressonante para uma chave auxiliar. Deste modo, um novo conversor Duplo Forward On-Off ZCS é obtido. / Mestre em Ciências