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Browsing by Author "Collins, George, advisor"

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    Application of distributed DC/DC electronics in photovoltaic systems
    (Colorado State University. Libraries, 2017) Kabala, Michael, author; Collins, George, advisor; Sakurai, Hiroshi, committee member; Siegel, H. J., committee member; Young, Peter, committee member
    In a typical residential, commercial or utility grade photovoltaic (PV) system, PV modules are connected in series and in parallel to form an array that is connected to a standard DC/AC inverter, which is then connected directly to the grid. This type of standard installation; however, does very little to maximize the energy output of the solar array if certain conditions exist. These conditions could include age, temperature, irradiance and other factors that can cause mismatch between PV modules in an array that severely cripple the output power of the system. Since PV modules are typically connected in series to form a string, the output of the entire string is limited by the efficiency of the weakest module. With PV module efficiencies already relatively low, it is critical to extract the maximum power out of each module in order to make solar energy an economically viable competitor to oil and gas. Module level DC/DC electronics with maximum power point (MPP) tracking solves this issue by decoupling each module from the string in order for the module to operate independently of the geometry and complexity of the surrounding system. This allows each PV module to work at its maximum power point by transferring the maximum power the module is able to deliver directly to the load by either boosting (stepping up) the voltage or bucking (stepping down) the voltage. The goal of this thesis is to discuss the development of a per-module DC/DC converter in order to maximize the energy output of a PV module and reduce the overall cost of the system by increasing the energy harvest.
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    Buck converter for on-chip reference generation
    (Colorado State University. Libraries, 2010) Rai, Abhay K., author; Collins, George, advisor; Yalin, Azer, committee member; Reising, Steven, committee member
    Most modern day chips use an on chip voltage reference, also known as a bandgap voltage reference generator, to provide a stable reference, independent of power supply voltage (VDD) ripples and compensated for temperature variations. When power supply voltage decreases as the process feature size (gate length) decreases, it imposes challenges in terms of headroom and other factors to achieve a stable bandgap voltage reference. It also needs to be scaled down to VDD/2 for practical uses and provide a common mode voltage of VDD/2 of on-chip circuits. This thesis discusses a buck converter which uses an alternative to pulse width modulation (PWM) for stable reference generation and directly generates a VDD/2 reference using a novel inductor ripple current cancellation technique, which cancels inductor ripple current and therefore does not require a large capacitance for filtering of inductor ripple. An alternative to the pulse width modulation (PWM) technique is proposed, which uses common mode bias and transconductance (gm) tuning to keep the reference output constant for variable loads, and a temperature compensation techniques is used to minimize temperature sensitivity.
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    Differential gene expression in Escherichia coli following exposure to non-thermal atmospheric-pressure plasma
    (Colorado State University. Libraries, 2008) Sharma, Ashish, author; Collins, George, advisor; Pruden, Amy, advisor
    Plasma decontamination provides a low temperature and non-toxic means of treating objects where heating and exposure to poisonous compounds is not acceptable especially in applications relating to medical devices and food packaging. The effects of various plasma constituents (UV photons, reactive species, charged particles etc.) acting independently and/or synergistically on bacteria at the biomolecular level is not well understood. High-density oligonucleotide microarrays were used to explore the differential gene expression of the entire genome of E. coli following plasma treatment. The results indicate a significant induction of genes involved in DNA repair and recombination suggesting that plasma exposure caused substantial DNA damage in the cell. There was also evidence of oxidative stress and suppression of genes involved in housekeeping functions of energy metabolism and ion transport. Experiments were also carried out to optimize plasma operating parameters to achieve a higher rate of inactivation of microbes. Overall, the results of this study will help to further optimize non-thermal plasma applications for bacterial inactivation.
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    Gallium nitride high electron mobility transistors in chip scale packaging: evaluation of performance in radio frequency power amplifiers and thermomechanical reliability characterization
    (Colorado State University. Libraries, 2017) Shover, Michael Andrew, author; Collins, George, advisor; Chandrasekar, Venkatachalam, committee member; Chen, Thomas, committee member; Ackerson, Chris, committee member
    Wide bandgap semiconductors such as Gallium Nitride (GaN) have many advantages over their Si counterparts, such as a higher energy bandgap, critical electric field, and saturated electron drift velocity. These parameters translate into devices which operate at higher frequency, voltage, and efficiency than comparable Si devices, and have been utilized in varying degrees for power amplification purposes at >1 MHz for years. Previously, these devices required costly substrates such as sapphire (Al2O3), limiting applications to little more than aerospace and military. Furthermore, the typical breakdown voltage ratings of these parts have historically been below ~200 V, with many targeted as replacements for 50 V Si LDMOS as used in cellular infrastructure and industrial, scientific, and medical (ISM) applications between 1 MHz and 1 GHz. Fortunately within the past five years, devices have become commercially available with attractive key specifications: GaN on Si subtrates, with breakdown voltages of over 600 V, realized in cost effective chip scale packages, and with inherently low parasitic capacitances and inductances. In this work, two types of inexpensive commercially available AlGaN/GaN high electron mobility transistors (HEMTs) in chip scale packages are evaluated in a set of three interconnected experiments. The first explores the feasibility of creating a radio frequency power amplifier for use in the ISM bands of 2 MHz and 13.56 MHz, at power levels of up to 1 kW, using a Class E topology. Experiments confirm that a DC to RF efficiency of 94% is easily achievable using these devices. The second group of experiments considers both the steady state and transient thermal characterization of the HEMTs when installed in a typical industrial application. It is shown that both types of devices have acceptable steady state thermal resistance performance; approximately 5.27 °C/W and 0.93 °C/W are achievable for the source pad (bottom) cooled and top thermal pad cooled device types, respectively. Transient thermal behavior was found to exceed industry recommended maximum dT/dt by over 80x for the bottom cooled devices; a factor of 20x was noted with the top cooled devices. Extrapolations using the lumped capacitance method for transient conduction support even higher initial channel dT/dt rates. Although this rate of change decays to recommended levels within one second, it was hypothesized that the accumulated mechanical strain on the HEMTs would cause early life failures if left uncontrolled. The third set of experiments uses the thermal data to design a set of experiments with the goal of quantifying the cycles to failure under power cycling. It is confirmed that to achieve a high number of thermal cycles to failure as required in high reliability industrial systems, the devices under test require significant thermal parameter derating to levels on the order of 50%.
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    Voltage reduction and automation on the residential distribution grid
    (Colorado State University. Libraries, 2018) Meller, Ryan, author; Collins, George, advisor; Borky, John, committee member; Young, Peter, committee member; Marchese, Anthony, committee member
    This paper represents the culmination of my research on the effects of voltage reduction and automation on the residential distribution grid. Although voltage reduction has been in use for many years, the strategies identified and tested through my research increase savings for utilities by reducing demand during peak periods. In addition, by automating switching to transfer load on the system, utilities will benefit not only during outage events, but in alleviating load on substations and equipment nearing capacity during load control events. The energy grid has benefited from a number of efficiencies in the past several years; however, system peaks continue to be problematic for electric utilities from both a cost and infrastructure perspective. The following presentation sets forth automated voltage reduction techniques, as well as automated switching approaches on distribution line sections, in an effort to appropriately address these concerns.