A thesis submitted to the Faculty of Science, University of the Witwatersrand, Johannesburg, in fulfilment of the academic requirements for the degree of Doctor of Philosophy. Johannesburg, 2014. / Unable to load abstract.

Thesis (Ph.D.)--Electrical and Computer Engineering, Georgia Institute of Technology, 2009. / Committee Chair: Janata, Jiri; Committee Co-Chair: Hunt, William; Committee Member: Allen, Mark; Committee Member: Brand, Oliver; Committee Member: Ferguson, Ian; Committee Member: Lyon, Andrew

Neste estudo é apresentado um procedimento de análise de dados magnéticos em casos de presença de magnetização remanente para, ao final do processo, a realização de uma inversão mais rápida e fiel ao comportamento do alvo em sub-superfície. O procedimento é composto por seis passos: (1) Delimitação lateral do alvo; (2) Estimativa de suas profundidades; (3) Estimativa das direções de inclinação e declinação aparentes de seu vetor de magnetização total; (4) Modelagem inicial com inferência de valores de susceptibilidades oriundos de estudos geológicos anteriores sobre o alvo e/ou sua região; (5) Inversão dos dados magnéticos utilizando o modelo inicial; e (6) atribuição das características magnéticas do passo (3) sobre o modelo de contraste de susceptibilidade magnética obtido com a inversão para a modelagem final do alvo, conhecendo-se seus vetores de magnetização induzida, total e, por subtração vetorial de ambos, remanente. Este procedimento foi aplicado a 108 casos sintéticos e a 8 casos reais pertencentes às províncias ígneas do Alto do Paranaíba e Rondoniana-San-Ignácio. Os resultados do uso deste procedimento indicaram uma recuperação das direções das componentes de magnetização com erro menor que 10%, em casos sintéticos, uma redução de mais de 20% no tempo de inversão com o uso de modelos iniciais, e qualitativamente, apresentaram modelos mais próximos dos originais (nos casos sintéticos) e geologicamente factíveis nos casos reais. / In this study, we present a procedure of analysis of magnetic data when remanence is present in order to, at the end of the process, obtain an inversion faster and more reliable inversion. The procedure is composed of six steps: (1) Estimation of the borders of the target; (2) Estimation of its depths; (3) A sweeping for the total apparent inclination and declination directions; (4) Initial modeling of a synthetic body, based on the recovered geometry and depth, on the directions of inclination and declination of the total magnetic field, and on previous analysis of the target and/or its region; (5) Inversion of magnetic data using the initial model; and (6) Attribution of the magnetic features of step (3) to the model recovered by the inversion for a final modeling of the target, estimating as well its remanent magnetization; the last through the vectorial resultant of the induction and total magnetization subtraction. This procedure was applied to 108 synthetic and to 8 real cases from the Alto do Paranaíba and Rondonian-San Ignacio Igneous Provinces. Their results pointed out that the error between the recovered directions of the magnetization components and the original values, in synthetic cases, was smaller than 10%; The inversions had their processing-time reduced in more than 20% and, qualitatively, presented models were more similar to the original (synthetic cases) and geologically feasible (real cases).

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The intermediate bus architecture employing the 48V bus converter is one of the most popular power architecture. 48V to 12V bus converter has wide applications in telecommunications, networks, aerospace, and military, etc. However, today's state of the art products has low power rating or power density and becomes difficult to satisfy the demand of increasing power of the loads. To improve the current design, a GaN (Gallium Nitride) based two-stage solution is proposed for the bus converter. The first stage Buck converter regulates the 40V to 60V variable input to a fixed 36V bus voltage. The second stage LLC converter convert the 36V to 12V by a 3:1 transformer. The whole solution achieves the fixed frequency control. The thesis focus on the detail design and optimization of LLC converter, especially its transformer. To have high density and high efficiency, the transformer design becomes critical at MHz frequency. The matrix transformer concept is applied and a merged winding structure is used for flux cancellation, which effectively reduces the AC winding losses. A new fully interleaved termination and via design is proposed. It achieves significant reduction in loss and leakage flux. In addition, to study the current sharing of parallel winding layers, a 1-D analytic model is proposed and a symmetrical winding layer scheme is used to balance the current distribution. The hardware is built and tested. The proposed two-stage converter achieves the best performance compared to the current market. / Master of Science / Intermediate bus architecture (IBA) has wide applications in telecommunication, server and computing, and military power supplies. The intermediate bus converter (IBC) is the key stage in the IBA, where the DC bus voltage from the front-end power supply is converted to a lower intermediate bus voltage. Traditional IBC suffers from bulky magnetic components including inductors and transformers. This work illustrates the design and implementation of a two-stage IBC, where the first-stage Buck converter will provide regulation and the second stage LLC converter will provide isolation. Thanks to the soft-switching capability of LLC, the magnetic volume can be significantly reduced by raising the switching frequency of the converter. Therefore, planar magnetics can be used and placed directly inside of the printing circuit board (PCB), which allows for higher power densities and easy manufacturing of the magnetics and overall converter. However, as the frequency goes higher, the AC losses of the transformer caused by the eddy current, skin effect, and proximity effect become dominant. As a result, high-frequency transformer design becomes the key for the converter design. First, matrix transformer concept is applied to distribute the high current and reduce the conduction loss. Second, a novel merged winding structure is proposed for better transformer winding interleaving. Third, a new terminal structure of the transformer is proposed. Finally, the current sharing between parallel windings are modeled and studied. All the efforts result in great loss reduction. The prototype were verified and compared to the current converters that are on the market in the 48V – 12V area of IBCs.

In the early 1990's, with the boom of the Internet and the advancements in telecommunications, the demand for high-speed communications systems has reached every corner of the world in forms such as, phone exchanges, the internet servers, routers, and all other types of telecommunication systems. These communication systems demand more data computing, storage, and retrieval capabilities at higher speeds, these demands place a great strain on the power system. To lessen this strain, the existing power architecture must be optimized. With the arrival of the age of high speed and power hungry microprocessors, the point of load converter has become a necessity. The power delivery architecture has changed from a centralized distribution box delivering an entire system's power to a distributed architecture, in which a common DC bus voltage is distributed and further converted down at the point of load. Two common distributed bus voltages are 12 V for desktop computers and 48 V for telecommunications server applications. As industry strives to design more functionality into each circuit or motherboard, the area available for the point of load converter is continually decreasing. To meet industries demands of more power in smaller sizes power supply designers must increase the converter's switching frequencies. Unfortunately, as the converter switching frequency increases the efficiency is compromised. In particular, the switching, gate drive and body diode related losses proportionally increase with the switching frequency. This thesis introduces a loss saving self-driven method to drive the secondary side synchronous rectifiers. The loss saving self-driven method introduces two additional transformers that increase the overall footprint of the converter. Also, this thesis proposes a new magnetic integration method to eliminate the need for the two additional gate driver magnetic cores by allowing three discrete power signals to pass through one single magnetic structure. The magnetic integration reduces the overall converter footprint. / Master of Science

The use of computers in the modern world has become prevalent in all aspects of life. The size of these machines has decreased dramatically while the capability has increased exponentially. A special DC-DC converter called a VRM (Voltage Regulator Module) is used to power these machines. The VRM faces the task of supplying high current and high di/dt to the microprocessor while maintaining a tight load regulation. As computers have advanced, so have the VRM's used to power them. Increasing the current and di/dt of the VRM to keep up with the increasing demands of the microprocessor does not come without a cost. To provide the increased di/dt, the VRM must use a higher number of capacitors to supply the transient energy. This is an undesirable solution because of the increased cost and real estate demands this would lead to in the future. Another solution to this problem is to increase the switching frequency and control bandwidth of the VRM. As the switching frequency increases the VRM is faced with efficiency and thermal problems. The current buck topologies suffer large drops in efficiency as the frequency increases from high switching losses. Resonant or soft switching topologies can provide a relief from the high switching loss for high frequency power conversion. One disadvantage of the resonant schemes is the increased conduction losses produced by the circulating energy required to produce soft switching. As the frequency rises, the additional conduction loss in the resonant schemes can be smaller than the switching loss encountered in the hard switched buck. The topology studied in this work is the 12V non-isolated ZVS self-driven presented in [1]. This scheme offered an increased efficiency over the state of the art industry design and also increased the switching frequency for capacitor reduction. The goal of this research was to study this topology and improve the magnetic design to decrease the cost while maintaining the superior performance. The magnetics used in resonant converters are very important to the success of the design. Often, the leakage inductance of the magnetics is used to control the ZVS or ZCS switching operation. This work presents a new improved magnetic solution for use in the 12V non-isolated ZVS self-driven scheme which increases circuit operation, flexibility, and production feasibility. The improved magnetic structure is simulated using 3D FEA verification and verified in hardware design. / Master of Science

Coordenação de Aperfeiçoamento de Pessoal de Nível Superior / In this work, we have measured the domain wall velocity in the low field regime and studied the domain wall dynamics in Joule-annealed amorphous glass-covered microwires with positive magnetostriction. Such microwires are known to present magnetic bi-stability when axially magnetized. In order to measure of the single domain wall dynamics under different conditions, an electrical current was applied to the wire simultaneously to the mechanical stress and driving magnetic field. We have observed that the applied stress decreases the domain wall mobility. When the dc current is applied to the sample, an increase or a decrease is observed on the axial domain wall mobility, depending on the current direction. When we have treated the orthogonal motion of the domain wall, the current influence is not detected. On the other hand, it was verified a modification on the domain wall length. It was also observed a change in the domain wall shape from conical to parabolic one. These results are explained in terms of the change in the magnetic energy promoted by the additional Oersted field which, by its time, modifies the length and shape of the conical domain wall, in such a way that the orthogonal domain wall velocity is not changed by the applied current. / Nesse trabalho, foi medida a velocidade de paredes de domínios em regime de baixos campos e estudada a dinâmica dessas paredes em microfios amorfos recobertos por vidros com magnetostricção positiva tratados via efeito Joule. Tais microfios são conhecidos por apresentar biestabilidade magnética quando axialmente magnetizados. A fim avaliar a dinâmica de uma única parede de domínio sob diferentes condições, corrente elétrica DC foi aplicada simultaneamente a tensões mecânicas e campo magnético externo. Foi verificado que quando uma tensão mecânica externa é aplicada, a mobilidade da parede de domínio diminui. Já quando a corrente foi aplicada na amostra, um aumento ou decréscimo da mobilidade axial da parede foi observado, dependendo do sentido da corrente aplicada. Quando foi tratado da velocidade ortogonal da parede de domínio, não foi observada influência da corrente. Por outro lado, foi verificado uma modificação no comprimento da parede de domínios. Além disto, foi observado uma mudança na forma da parede de domínio, passando de um formato cônico para parabólico. Os resultados são explicados em termos da mudança na energia magnética promovida pelo campo de Oersted, gerado pela corrente aplicada, que por sua vez modifica o comprimento e a forma da parede de domínio sem que a velocidade ortogonal da parede de domínio seja alterada pela corrente aplicada.

The Collisional Orogeny in the Scandinavian Caledonides (COSC) project aims to provide a deeper understanding of mountain belt dynamics through scientific deep drilling in the central parts of the mountain belt of western Sweden. The main targets include a subduction related allochthon, the basal orogenic detachment and the underlying partially subducted Precambrian basement. Research covered by this thesis, focusing primarily on reflection seismic data, was done within the framework of the COSC project. The 55 km long composite COSC Seismic Profile (CSP) images the upper crust in high resolution and established the basis for the selection of the optimum location for the two 2.5 km deep COSC boreholes. Together with potential field and magnetotelluric data, these profiles allowed the construction of a constrained regional interpretation of the major tectonic units. Non-conventional pseudo 3D processing techniques were applied to the 2D data prior to the drilling of the first borehole, COSC-1, to provide predictions about the 3D geometry of subsurface structures and potential zones of interest for the sampling programs. COSC-1 was drilled in 2014 and reached the targeted depth with nearly complete core recovery. A continuous geological section and a wealth of information from on-site and off-site scientific investigations were obtained. A major post-drilling seismic survey was conducted in and around the borehole and included a 3D reflection seismic experiment. The structurally and lithologically complex Lower Seve Nappe proved difficult to image in detail using standard processing techniques, but its basal mylonite zone and underlying structures are well resolved. The 3D data, from the surface down to the total drilled depth, show good correlation with the initial mapping of the COSC-1 core as well as with preliminary results from on-core and downhole logging.   Good correlation is also observed between the 2D and 3D reflection seismic datasets. These will provide a strong link between the two boreholes and a means to extrapolate the results from the cores and boreholes into the surrounding rock. Ultimately, they will contribute to the deeper understanding of the tectonic evolution of the region, the Scandinavian Caledonides and the formation of major orogens. / Collisional Orogeny in the Scandinavian Caledonides

Increasing the power density of power electronic converters while reducing or maintaining the same cost, offers a higher potential to meet the current trend inrelation to various power electronic applications. High power density converters can be achieved by increasing the switching frequency, due to which the bulkiest parts, such as transformer, inductors and the capacitor's size in the convertercircuit can be drastically reduced. In this regard, highly integrated planar magnetics are considered as an effective approach compared to the conventional wire wound transformers in modern switch mode power supplies (SMPS). However, as the operating frequency of the transformers increase from several hundred kHz to MHz, numerous problems arise such as skin and proximity effects due to the induced eddy currents in the windings, leakage inductance and unbalanced magnetic flux distribution. In addition to this, the core losses whichare functional dependent on frequency gets elevated as the operating frequency increases. Therefore, this thesis provides an insight towards the problems related to the high frequency magnetics and proposes a solution with regards to different aspects in relation to designing high power density, energy efficient transformers.The first part of the thesis concentrates on the investigation of high power density and highly energy efficient coreless printed circuit board (PCB) step-down transformers useful for stringent height DC-DC converter applications, where the core losses are being completely eliminated. These transformers also maintain the advantages offered by existing core based transformers such as, high coupling coefficient, sufficient input impedance, high energy efficiency and wide frequencyband width with the assistance of a resonant technique. In this regard, several coreless PCB step down transformers of different turn’s ratio for power transfer applications have been designed and evaluated. The designed multilayered coreless PCB transformers for telecom and PoE applications of 8,15 and 30W show that the volume reduction of approximately 40 - 90% is possible when compared to its existing core based counterparts while maintaining the energy efficiency of the transformers in the range of 90 - 97%. The estimation of EMI emissions from the designed transformers for the given power transfer application proves that the amount of radiated EMI from a multilayered transformer is lessthan that of the two layered transformer because of the decreased radius for thesame amount of inductance.The design guidelines for the multilayered coreless PCB step-down transformer for the given power transfer application has been proposed. The designed transformer of 10mm radius has been characterized up to the power level of 50Wand possesses a record power density of 107W/cm3 with a peak energy efficiency of 96%. In addition to this, the design guidelines of the signal transformer fordriving the high side MOSFET in double ended converter topologies have been proposed. The measured power consumption of the high side gate drive circuitvitogether with the designed signal transformer is 0.37W. Both these signal andpower transformers have been successfully implemented in a resonant converter topology in the switching frequency range of 2.4 – 2.75MHz for the maximum load power of 34.5W resulting in the peak energy efficiency of converter as 86.5%.This thesis also investigates the indirect effect of the dielectric laminate on the magnetic field intensity and current density distribution in the planar power transformers with the assistance of finite element analysis (FEA). The significanceof the high frequency dielectric laminate compared to FR-4 laminate in terms of energy efficiency of planar power transformers in MHz frequency region is also explored.The investigations were also conducted on different winding strategies such as conventional solid winding and the parallel winding strategies, which play an important role in the design and development of a high frequency transformer and suggested a better choice in the case of transformers operating in the MHz frequency region.In the second part of the thesis, a novel planar power transformer with hybrid core structure has been designed and evaluated in the MHz frequency region. The design guidelines of the energy efficient high frequency planar power transformerfor the given power transfer application have been proposed. The designed corebased planar transformer has been characterized up to the power level of 50W and possess a power density of 47W/cm3 with maximum energy efficiency of 97%. This transformer has been evaluated successfully in the resonant converter topology within the switching frequency range of 3 – 4.5MHz. The peak energy efficiency ofthe converter is reported to be 92% and the converter has been tested for the maximum power level of 45W, which is suitable for consumer applications such as laptop adapters. In addition to this, a record power density transformer has been designed with a custom made pot core and has been characterized in thefrequency range of 1 - 10MHz. The power density of this custom core transformer operating at 6.78MHz frequency is 67W/cm3 and with the peak energy efficiency of 98%.In conclusion, the research in this dissertation proposed a solution for obtaining high power density converters by designing the highly integrated, high frequency(1 - 10MHz) coreless and core based planar magnetics with energy efficiencies inthe range of 92 - 97%. This solution together with the latest semiconductor GaN/SiC switching devices provides an excellent choice to meet the requirements of the next generation ultra flat low profile switch mode power supplies (SMPS).