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

This thesis first explains the eddy current and the phenomenon of skin effect, where the resultant flux flows near the surface of the metal. A new flux direction perspective is created for steel laminations, from which derivations of the eddy current resistance and power losses in different directions are developed assuming uniform flux conditions. The developed method compares with a proposed theory through experimental data. The results from the comparison support the validity of the developed derivations. Two uniform flux generators and their billets construction are introduced. The power loss between two cubic billets with different orientations is compared. A Finite Element Analysis (FEA) program is used to show the difference between lamination alignments. To prove the validity of the developed theory, two experiments were performed using two different electroheating apparatus. The results give scale factors from which the theoretical values can be matched to the experimental ones. Due to the poorer construction of the first apparatus, the scale factor of measured to computed losses is 1.15. The scale factor for the second apparatus can be taken as unity, revealing a good match between theory and measurements. After verification of the developed equations for uniform flux experiments, the focus of the eddy current loss calculation turned to partial core transformers. The flux background of a cubical core is reviewed. Three key factors ( L', Kec and βa) are introduced into the eddy current power loss model. L' is a length which indicates the region of the flux spreading at the ends of the core. Kec as a ratio indicates how much of the main flux spreads at the ends of the core. βa is the ratio of the winding axial length and winding thickness. Using simulations from the Finite Element Analysis (FEA) program MagNet, a partial core side view with the flux distribution and flux density from two orthogonal angles is created. A flux linkage comparison between the experimental results and the returned values from MagNet verifies the high accuracy of the flux plot in MagNet. The eddy current power loss model is then built up with equations. The relationships amongst the three key factors are studied and confirmed using the experimental results. Normally, a partial core transformer uses a cylindrical partial core rather than a cubical partial core, to reduce the amount of winding material. Therefore, a further goal was to prove the developed model for cylindrical partial core transformers. The construction differences between the cubical and cylindrical core is discussed. The orthogonal flux assumptions for the cylindrical core in two directions are reviewed. The flux penetration between two adjacent blocks is considered and explained. The mathematical core loss model is created for a cylindrical core composing by ten blocks. Three tests were performed using the developed core loss model. The results visualize the power loss from the core by its temperature distribution, and consequently prove the validity of the developed core loss model. An eddy current loss comparison and the discussion are made between the previous method and the developed method. Overall, the results confirm a significant improvement using the developed core loss model, and a generic form of the partial core can be used for designing future models of partial core transformers which have a stacking factor greater than 0.96.

Superconductivity Centennial Conference 2011- EUCAS–ISEC–ICMC (18-23 Sep 2011, The Hague, The Netherlands)

Graduation date: 1997

Larval fish assemblages were investigated during a three-year multidisciplinary project conducted off the coast of south-western Australia. Larvae were sampled using replicated oblique bongo net tows along a five-station transect extending from inshore (18m depth) to offshore waters (1000m depth). A total of 148 taxa from 93 teleost families were identified. Larvae of Gobiidae, and Blenniidae were abundant inshore, while larvae of pelagic and reef-dwelling families, such as Clupeidae, Engraulidae, Carangidae and Labridae were common in continental shelf waters. Larvae of oceanic families, particularly Myctophidae, Phosichthydae and Gonostomatidae, dominated offshore assemblages. Inshore larval fish assemblages were the most seasonal, in terms of species composition and abundance, with offshore assemblages the least so. Multivariate statistical analyses revealed larval fish assemblages to have a strong temporal and spatial structure. Assemblages were closely correlated to water masses, with species distributions reflecting both cross shelf and along-shore oceanographic processes and events. The strength and position of the warm, southward flowing Leeuwin Current, and of the cool, seasonal, northward flowing Capes Current were shown to drive much of the variability in the marine environment, and thus larval fish assemblages. Many of the distinctions between larval fish assemblages on the continental shelf were attributable to patterns of abundance in clupeiform larvae. While larvae of Engraulis australis and Spratelloides robustus showed clear seasonal and spatial distribution patterns, larvae of Sardinops sagax and Etrumeus teres were found throughout the year, with high interannual variability in abundance. Abundances of larvae from all pelagic clupeiform species were negatively correlated to microzooplankton concentrations. Peaks of abundance of S. sagax and E. teres, in particular, appeared to be better aligned with favourable transport and retention conditions. A detailed comparison of the horizontal and vertical distribution of larval fishes highlighted the influence of contrasting oceanographic conditions between summer and winter on larval fish assemblages. Although most fish larvae were found above the thermocline, depth distributions differed between taxa, and were shown to influence their offshore transport. Neustonic fish larvae showed potential for significant dispersion during summer, as a result of offshore Ekman transport. Mesoscale Leeuwin Current eddies were a feature of the oceanography of the region, and their influence on larval fish assemblages was examined in both an anti-cyclonic eddy (warm-core) and a cyclonic eddy (cold-core). The warm-core eddy contained larval fish assemblages that were distinct from those in the cold-core eddy, with lower larval fish concentrations, especially in the eddy centre. Although the eddies originated near the continental shelf, larval fish assemblages within both eddies were largely oceanic, probably a result of the age of the eddies when they were sampled (about 5 months). Overall, larval fish assemblages showed strong temporal and spatial structure, and were well aligned to water masses in the region. The unique oceanography off south western Australia thus has considerable implications for both larval fish transport, and potential recruitment to regional fisheries.

Thesis (M.S.)--Missouri University of Science and Technology, 2008. / Vita. The entire thesis text is included in file. Title from title screen of thesis/dissertation PDF file (viewed May 12, 2008) Includes bibliographical references (p. 102-107).

Thesis (M.S.)--Florida State University, 2004. / Advisor: Dr. Sharon Nicholson, Florida State University, College of Arts and Sciences, Dept. of Meteorology. Title and description from dissertation home page (viewed Aug. 26, 2004). Includes bibliographical references.

Thesis (Ph.D.) - University of Queensland, 2006. / Includes graphs in back pocket. Includes bibliography.

Researchers at Uppsala University are developing a flywheel energy storage device intended to be used in electrical vehicles. Kinetic energy storage technology has potential to make purely electric powertrain both more effective and efficient. While deployment of the third prototype is approaching there has been a request for a more precise and noise-immune circuitry to power active magnetic bearings that hold and stabilise the rotor. A similar circuit designed for powering electromagnets was recently developed at the Uppsala University’s Electricity department and is used as a template in development of the new active magnetic bearing driver circuit. Current measurement integration technique is tested and implemented as a way to increase circuit’s control feedback loop performance. To further boost precision and noise-immunity 0-20 mA current loop signals are adapted as the standard for output signals. Results of this project include a thorough analysis of the electromagnet driver circuit development, implementation of a new current sensing technique including an experimental self-inductance measurement, printed circuit board layout design and a full list of components necessary to power and control two sets of active magnetic bearings consisting of 8 individual electromagnets.

This study investigates the accuracy of the wave products retrieved by a 12-MHz high-frequency (HF) phased-array radar, and establishes their potential to characterise wave-current interactions. The two stations composing the system were deployed in 2011 to overlook the Wave Hub, a test site for marine renewable energy devices located on the south-western coast of the United Kingdom. The system was conceived and configured to reduce the inaccuracies introduced by short time averaging and minimal overlap between stations, both associated with the most traditional HF radar deployments, whose primary activity is current measurement. Wave spectra were retrieved by two independent inversion algorithms, which were evaluated both independently and relative to each other. This process helped determining the errors associated to the algorithm used, and differentiated them from those inherent to the radar technology itself. The first method investigated was a semi-empirical algorithm distributed with Wellen Radars (WERA), which was calibrated using in situ measurements collected within the radar footprint. Evaluated through comparison against measurements acquired by three in situ devices, the results revealed estimates of significant wave height with biases below 9 cm, Pearson correlations higher than 0.9, and RMS errors that range from 29 to 44 cm. The relative error of wave energy period comparisons was within 10% for periods between 8 and 13 s, while both under- and overestimations were observed above and below that range, respectively. The validation demonstrated that when locally calibrated, the algorithm performs better than in its original form in all metrics considered. Observed discrepancies were mainly attributable to single-site estimations, antenna sidelobes, and the effect of the second-harmonic peaks of the Doppler spectrum. As opposed to the semi-empirical inversion, the second method evaluated in this work provides estimates of the full directional spectrum. Compared against the in situ measurements, the radar spectra were more spread over frequencies and directions, and had a lower energy content at the peak of the spectrum. In terms of parameter estimation, this was generally translated in a slight underestimation of wave periods, but accurate estimates of significant wave heights. Pearson correlations between these parameters and the in situ measurements for the bulk of the spectrum were higher than 0.9, and both types of measurements resulted in similar standard deviations. The inversion algorithm showed a high skill estimating mean wave directions, which revealed linear correlations higher than 0.8, when compared to the in situ devices. Overall, the inversion algorithm has shown to be capable of providing accurate estimates of directional spectra and the parameters derived from them, and at present the main drawback of the method is the data return, which due to the high data quality requirements of the algorithm, did not exceed 55% over the 8-month period studied here. In the second part of this work, the validated measurements were examined to determine their ability to reproduce the effects of wave-current interactions. The fine structure of the surface current was first evaluated, and revealed a circulation dominated by tides. The residual flow was seen to respond to the wind, as well as to the stratification present in the area during the spring and summer months. These data were then used to assess their contribution to wave refraction over the radar domain. The results show modulations in the wave phase parameters, which resulted from both the temporal and spatial derivatives of the surface current velocities. The evaluation of HF radar wave measurements provided in this work has shown that, properly configured, this technology can produce accurate estimates of several statistical descriptors of the wave field. Together with the highly accurate surface currents also measured by this device, the spatial wave data obtained has proved to have great potential for studying wave-current interactions; a skill that can be of support to coastal wave modelling.