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Since future microprocessors will have higher current in accordance with Moore's law, there are still challenges for voltage regulators (VRs). Firstly, high efficiency is required not only for easy thermal management, but also for saving on electricity costs for data centers, or battery life extension for laptop computers. At the same time, high power density is required due to the increased power of the microprocessors. This is especially true for data centers, since more microprocessors are required within a given space (per rack). High power density is also required for laptop computers to reduce the size and the weight. To improve power density, a high frequency is required to shrink the size of the output inductors and output capacitors of the multi-phase buck VR. It has been demonstrated that the output bulk capacitors can be eliminated by raising the VR control bandwidth to around 350kHz. Assuming the bandwidth is one-third of the switching frequency, a VR should run at 1MHz to ensure a small size. However, the efficiency of a 12V VR is very poor at 1MHz due to high switching losses. As a result, a 12V VR can only run at 300kHz to 600kHz, and the power density is very low. To attain high efficiency and high power density at the same time, two-stage power architecture was proposed. The concept is "Divide and Conquer". A single-stage VR is split into two stages to get better performance. The second stage has about 5V-6V input voltage; thus the duty cycle can be extended and the switching losses are greatly reduced compared with a single-stage VR. Moreover, a sub-20V MOSFET can be used to further improve the efficiency at high frequencies. The first stage of the proposed two-stage architecture is converting 12V to 5-6V. High efficiency is required for the first stage since it is in series with the second stage. Previous first stage which is a buck converter has good efficiency but bulky size due to low frequency operation. Another problem with using a buck converter is that light-load efficiency of the first stage is poor. To solve these problems, switched-capacitor voltage dividers are proposed. Since the first stage does not require voltage regulation, the sweet point for the voltage divider can be determined and high efficiency can be achieved. At the same time, since there are no magnetic components for the switched-capacitor voltage divider, high power density can be achieved. By very careful design, a power density of more than 2000W/in3 with more than 97% efficiency can be achieved for the proposed voltage divider. The light-load efficiency of the voltage divider can be as high as 99% by reducing the switching frequency at light load. As for the second stage, different low-voltage devices are evaluated, and the best device combinations are found for high-frequency operation. It has been demonstrated that 91% efficiency can be achieved with 600kHz frequency, and 89% efficiency can be achieved with a 1MHz frequency for the second stage. Moreover, adaptive on-time control method and a non-linear inductor structure are proposed to improve CCM and DCM efficiency for the second stage respectively. Previously the two-stage VR was only used as a CPU VR. The two-stage concept can also be applied to other systems. In this dissertation, the two-stage power architecture is applied to two different applications: laptop computers and high-end server microprocessors. The common characteristics of the two applications are their thermal design power (TDP) requirement. Thus the first stage can be designed with much lower power than the maximum system power. It has been demonstrated that the two-stage power architecture can achieve either higher efficiency or higher power density and a lower cost when compared with the single-stage VR. To get higher efficiency, a parallel two-stage power architecture, named sigma architecture, is proposed for VR applications. The proposed sigma VR takes advantage of the high-efficiency, fast-transient unregulated converter (DCX) and relies on this converter to deliver most of the output power, while using a low-power buck converter to achieve voltage regulation. Both the DCX converter and the buck converter can achieve around 90% efficiency when used in the sigma VR, which ensures 90% efficiency for the sigma VR. The small-signal model of the sigma VR is studied to achieve adaptive voltage positioning (AVP). The sigma power architecture can also be applied to low-power point of load (POL) applications to reduce the magnetic component size and improve the efficiency. Finally, the two-stage VR and the sigma VR are briefly compared. / Ph. D.

The yield response of soybean [Glycine max (L.) Merr.] to reproductive stage applications of either nitrogen (N) or boron (B) has been inconsistent. This study evaluated soybean seed yield response to foliar applications of B and soil applications of N at two stages of plant development, at two row spacings, at four irrigation levels, and on three cultivars over three years. Planting dates were either mid-May or mid-June, except the year two of the irrigated soil moisture experiment which had a second planting date of early July. In an experiment to evaluate B rate and timing, B was applied at four rates from 0 to 0.56 kg ha-1 at the R3 or R5 development stage. In an experiment to evaluate N rate and timing, N was applied at seven rates from 0 to 168 kg ha-1 at either the R3 or R5 development stage. A third experiment to evaluate row spacing and cultivar effects on N and B had four treatments: 0 N and 0 B; 56 kg ha -1 N, and 0 B; 0 N and 0.28 kg ha-1 B; and finally 56 kg ha-1 N and 0.28 kg ha-1 B. Treatments were applied to three soybean cultivars planted in either 23 or 46 cm row spacings. The above experiments were irrigated to evaluate treatments at high yield levels. To further evaluate the effect of soil moisture, the same four N and B combinations were applied to soybeans irrigated via a sub-surface micro-drip irrigation system delivering four irrigation regimes: 0%, 33%, 66%, or 100% of plant required water. Gradients were established in year one of this experiment, but late season rains eliminated gradients, and high rainfall in the second year disallowed gradient establishment. Applications of N or B had no effect on seed yields in any experiment, or at any moisture level. In the row spacing and cultivar experiment, there were significant effects of varieties, and a significant interaction between row spacing and variety in two of the three years. / Master of Science

This paper presents a single-stage inverter with variable frequency modulation (VFM) based on LLC resonant converter. And LLC converter is a common topology of dc/dc conversion. LLC resonant converter can achieve high efficiency and soft-switching performance. Since the dc gain curve of the single-resonant LLC converter is flat when the switching frequency is larger than the resonant frequency, namely fs>fr, an additional L-C series resonant branch is paralleled to the original resonant tank to introduce higher-order-harmonic resonant current and a zero-gain point to the gain curve. Higher-order-harmonics help to deliver power and the zero-gain point enlarges the gain range which improves output THD and reduces the switching frequency range. A 1.2 kW prototype is built to demonstrate the performance of the proposed inverter. Zero-voltage-switching (ZVS) and zero-current-switching (ZCS) are achieved on the primary side and secondary side, respectively. And 97.3% efficiency and 2.17% voltage THD are achieved at full load condition, while 97.2% efficiency and 3.2% voltage THD are achieved at half load condition. / M.S. / The inverter is widely used to connect renewable energy into the grid by converting dc to ac waveform, like photovoltaic (PV) technology. Basically, the two-stage topology is usually used. The inverter would consist of two stages working in high frequency, the first stage is dc/dc converter which can regulate the input voltage to the desired bus voltage for the second stage, and the second stage is dc/ac converter. The first stage works at a specific switching frequency, so it can be designed to achieve higher efficiency in dc/dc conversion. The second stage also works at high switching frequency and converts dc to ac commonly by using SPWM which changes the duty cycle ratio in a sinusoidal pattern. The single-stage inverter only has one stage working in high frequency while the second stage works at twice line frequency. The first stage converts dc to rectified ac waveform and the second stage unfolds it to ac. The topology of LLC resonant converter being applied for the first stage of the single-stage inverter has been proposed. This topology uses variable-frequency-modulation (VFM) which varying switching frequency on the primary side to output different voltage levels. And it achieves zero-voltage-switching (ZVS). However, LLC converter can hardly output very low voltage due to the flat voltage gain curve at high frequency. Also, LLC converter only transfers the fundamental harmonic component to the load. If the higher-order harmonic components help transfer power when the switching frequency equals the resonant frequency, the current shape will be more like a square wave and the peak of resonant current can be reduced. This thesis proposes a topology that has two L-C resonant branches in parallel for the resonant tank in the converter. And the paralleled resonant branches produce a zero-gain frequency point into the gain curve so that the gain range is enlarged within the reduced switching frequency range and 3rd harmonic component of the resonant current helps to transfer power so that the rms value of resonant current can also be reduced.

This research's objective is to investigate the reactions of hydrogen enrichment of methane combustions, and to study the flame stabilization location and the production of emissions. The main parameter the investigation focuses on is the flame stabilization found through chemiluminescence. The testing will be conducted at different ratios of hydrogen to methane, and then also testing the different mixtures at various levels of air to fuel. The hydrogen/methane mixture entering the flow can be set to three modes of fully premixed, non-mixed, or partially mixed depending on the orientation of the gas injector. This will show the correlation of the addition of hydrogen to the changes of ignition location, flame stabilization, adiabatic flame temperature, and emissions; all characteristics that will be analyzed by the experiment.

Although research has examined the determinants of physical activity, this research has focused primarily on men and few efforts have been made to explain the interrelationships between commonly used predictors of physical activity. Descriptive data and regression analyses were conducted with 193 female students, faculty, staff and community members of a southwestern Virginia university town. Variables that were entered into the regression included age, body mass index, exercise knowledge, self-esteem, depression level,self-efficacy, stage of change, exercise goals, outcome expectations and outcome values. Because of the high correlations between some of these variables, a principal components factor analysis was conducted. The factor analysis indicated significant overlap between items on the stage of change and knowledge measures with self-efficacy. Some knowledge items were dropped to create a more succinct measure and self-efficacy and stage of change were combined into one measure. Results indicated that the variables listed above significantly predicted physical activity level as measured by kilocalories expended per day on the Aerobic Center Physical Activity Questionnaire (Kohl, Blair, Paffenbarger, Macera, & Kronenfeld, 1988) for this sample of women (R2 = .346, p < .001), but that self-efficacy was the only significant single predictor. Additional regressions were conducted to examine the indirect relationships between these variables and physical activity level. The constructs of selfesteem, depression, and knowledge seemed to have indirect, rather than direct, effects on physical activity that were mediated by self-efficacy. Goals and outcome expectations also did not seem to directly predict physical activity level but were related to some of of the other variables. Implications of the interrelationships between these variables for planning physical activity interventions include enhancing self-efficacy for exercising while at the same time assessing for depressive symptomatology and using incentives and motivators that are age appropriate. / Master of Science

With the development of fuel cell based power systems, the need for more advanced DC-DC power converters has become apparent. In such applications DC-DC converters provide an important link between low voltage fuel cell sources and inverter buses operating at significantly higher voltages. Advancements in converter efficiency, cost reduction, and size reduction are the most necessary. These challenges are formidable, even when considering the improvements made to conventional DC-DC topologies. However, it can be possible to achieve these criteria through the implementation of more advanced topologies. A recently developed efficient three-phase DC-DC topology offers benefits over standard designs. Passive component sizes and output ripple voltage were reduced as a result of an effective boost in switching frequency. Converter output voltage was reached more easily due to an increased transformer voltage boost ratio in addition to the turns ratio. For cost reduction, the converter was designed and built with discrete components instead of more expensive integrated modules. This thesis presents an overview of the three-phase converter, with a detailed focus on the design, implementation, and performance of the switching stage. The functionality of the three-phase topology is covered along with the selection of converter components. Simulation results are shown for both ideal and real converter models. Considerations for the switching device package with respect to circuit board and heat sinking configurations are discussed in support of the selection of an insulated metal substrate (IMS) circuit board. An effective circuit layout designed to minimize parasitic trace inductances as well as provide favorable component positioning is presented. Experimental converter test results are shown and the causes of undesired effects are identified. Switching stage modifications and their results are discussed along with the benefits of proposed future design enhancements. / Master of Science

The use of digital tools such as Sketchup, Rhino, AutoCAD and others is now commonplace in the architectural design process. These tools have advantages for time management and support the iterative nature of design, while also providing an efficient connection to later stages of design development and construction documentation. However, it could be argued that the use of digital tools during the ideation and concept phases of designing limits the cognitive connection between thought and the hand that is present in freehand sketching. If we accept that this cognitive connection is foundational to the design process, then new digital sketching tools are needed that have features similar to freehand sketching while processing sketches to 2D and 3D representations that are translatable to 3D printing, digital fabrication, and eventually Building Information Modeling. What this new tool might be is currently unclear. The purpose of this research is to identify the characteristics of a next generation of sketching tools that could support ideation activities in the concept stage of architectural design. The results will help to design a roadmap for the implementation of digital technologies in the architectural design process. This research will contribute to a shift in the implementation of digital tools in the architectural design process. This research relied on multiple data sources. First, a logical argument tactic was used to develop the research design model through a comprehensive review of the scholarly work that addresses the design process in the concept stage and the role of sketching and digital tools throughout these processes. Second, case study observation and interviews were used to explore how architects and architectural students at Virginia Polytechnic Institute and State University (Virginia Tech) use sketching, 3D modeling, and existing digital tools to represent, develop, iterate, and communicate their design ideas in the concept stage and how the tools they used affect their design process. Third, two immersive case studies were conducted to identify and develop the characteristics of a digital sketching tool. The last step was to determine consensus for the appropriateness of the To-Be model through Delphi tactics. / Doctor of Philosophy / The use of digital tools such as Sketchup, Rhino, AutoCAD and others is now commonplace in the architectural design process. These tools have advantages for time management and support the iterative nature of design, while also providing an efficient connection to later stages of design development and construction documentation. However, it could be argued that the use of digital tools during the ideation and concept phases of designing limits the cognitive connection between thought and the hand that is present in freehand sketching. If we accept that this cognitive connection is foundational to the design process, then new digital sketching tools are needed that have features similar to freehand sketching while processing sketches to 2D and 3D representations that are translatable to 3D printing, digital fabrication, and eventually Building Information Modeling. What this new tool might be is currently unclear. The purpose of this research is to identify the characteristics of a next generation of sketching tools that could support ideation activities in the concept stage of architectural design. The results will help to design a roadmap for the implementation of digital technologies in the architectural design process. This research will contribute to a shift in the implementation of digital tools in the architectural design process. This research relied on multiple data sources. First, a logical argument tactic was used to develop the research design model through a comprehensive review of the scholarly work that addresses the design process in the concept stage and the role of sketching and digital tools throughout these processes. Second, case study observation and interviews were used to explore how architects and architectural students at Virginia Polytechnic Institute and State University (Virginia Tech) use sketching, 3D modeling, and existing digital tools to represent, develop, iterate, and communicate their design ideas in the concept stage and how the tools they used affect their design process. Third, two immersive case studies were conducted to identify and develop the characteristics of a digital sketching tool. The last step was to determine consensus for the appropriateness of the To-Be model through Delphi tactics.