Design and Testing of High Frequency Converters for Photovoltaic System Integration

This thesis presents simulation, modelling, and testing for the complete photovoltaic (PV) system, including the PV panels themselves, the converters necessary for grid connection, and the testbed environments required for their testing. Investigating such systems is crucial, as PV systems are becoming increasingly prevalent. Converters are evolving and so too are the semiconductor devices of which they are comprised. Advanced topologies such as the modular multilevel converter (MMC) are allowing for radical changes in converter design and control, raising performance and efficiency ever higher. Buoying that growth is the development of wide bandgap (WBG) semiconductors, which are enabling faster, smaller, and more efficient converters. In light of these advancements, significant room for modeling and analysis of the various phenomena that occur in PV systems is critical. In order to address this need, this thesis will present analysis, simulation, and testing of a number of key elements within the PV system, such that the whole may be better understood. It will begin with the DC-DC converter itself, modeling transient events in synchronous buck converters, as well as demonstrating the implementation of maximum power point tracking (MPPT) in boost converters. Next, the inverter portion of the system will be examined, focusing on development of a single phase, low voltage version of the MMC topology that has been previously demonstrated in high voltage direct current (HVDC) systems. Finally, a design for a test bank and workbench will be discussed, shedding light on the laboratory apparatus necessary for proper evaluation and testing of new power electronic devices and systems. In short, the complete PV system is presented, its individual components are modeled and analyzed, and the conditions and materials necessary for testing are established, such that the understanding of PV integration in modern power systems can be better understood

High Density Power Conversion Electronics Enabled by GaN-Based Modular Topologies

This dissertation explores the use of modular multilevel converter (MMC) architectures, coupled with wide-bandgap semiconductors, to achieve high power-density in power electronics converters. At the converter level, the capabilities of the modular multilevel converter are investigated for their use in low voltage, low power, DC-DC and DC-AC applications. This investigation shows that the use of modular multilevel architectures enables low voltage Gallium Nitride high electron mobility transistors (GaN HEMTs) to be used in applications for which their voltage thresholds are not typically suited. This results in lightweight, compact, conversion systems. GaN HEMTs have been shown to provide a low loss, low volume alternative to Silicon transistors for power conversion, but require several enabling technologies to make them ideally suited to high-density converters. This work therefore presents two enabling technologies for GaN-based conversion circuits. First, a technique is developed that optimizes the gate resistance for driving GaN HEMTs in order to ensure safe, rapid device turn on. Next, the development of planar magnetic transformers is discussed, with a focus on high-frequency converter operation. For each of these technologies mathematical analysis, circuit simulation, and hardware development are performed and compared to ensure proper functionality. Taking advantage of those two enabling technologies, two converter architectures based on the MMC structure are developed. First, a DC-AC MMC is presented, taking advantage of GaN HEMTs and minimal filtering requirements to achieve high power density in low voltage systems. Next, that topology is extended and a novel DC-DC converter based on two coupled DC-AC MMCs is presented. Both systems are described mathematically, simulated, and developed as hardware prototypes to prove functionality. While both converter systems are relevant for applications in DC microgrids, the DC-AC converter will be specifically investigated for its application as a variable speed drive in naval power systems. Likewise, the DC-DC MMC will be shown to provide new solutions for high voltage spacecraft power systems. Based on the work presented in this dissertation, engineers will be presented with alternatives to traditional methods of achieving high density in power conversion systems. By coupling the low filtering requirements and low losses of the modular multilevel converter with low voltage, highly efficient GaN HEMTs, the presented converter systems achieve high power density and efficiency with minimal filtering requirements. The result of this work is two novel converter systems that will enable further research into lightweight, low volume, power conversion

Soft-Switching GaN-Based Isolated Power Conversion System for Small Satellites with Wide Input Voltage Range

As we pursue the advancement of small satellites for space missions with more capabilities, there is a significant need for cutting-edge, modularly configurable, high density power converters. This article proposes a fixed switching frequency, high efficiency, compact isolated converter for sensitive loads such as radar, communication systems, or other instruments on small satellites

Genomic and proteomic profiling of responses to toxic metals in human lung cells.

Examining global effects of toxic metals on gene expression can be useful for elucidating patterns of biological response, discovering underlying mechanisms of toxicity, and identifying candidate metal-specific genetic markers of exposure and response. Using a 1,200 gene nylon array, we examined changes in gene expression following low-dose, acute exposures of cadmium, chromium, arsenic, nickel, or mitomycin C (MMC) in BEAS-2B human bronchial epithelial cells. Total RNA was isolated from cells exposed to 3 M Cd(II) (as cadmium chloride), 10 M Cr(VI) (as sodium dichromate), 3 g/cm2 Ni(II) (as nickel subsulfide), 5 M or 50 M As(III) (as sodium arsenite), or 1 M MMC for 4 hr. Expression changes were verified at the protein level for several genes. Only a small subset of genes was differentially expressed in response to each agent: Cd, Cr, Ni, As (5 M), As (50 M), and MMC each differentially altered the expression of 25, 44, 31, 110, 65, and 16 individual genes, respectively. Few genes were commonly expressed among the various treatments. Only one gene was altered in response to all four metals (hsp90), and no gene overlapped among all five treatments. We also compared low-dose (5 M, noncytotoxic) and high-dose (50 M, cytotoxic) arsenic treatments, which surprisingly, affected expression of almost completely nonoverlapping subsets of genes, suggesting a threshold switch from a survival-based biological response at low doses to a death response at high doses

Increased focal adhesion kinase- and urokinase-type plasminogen activator receptor-associated cell signaling in endothelial cells exposed to asbestos.

Exposure of low-passage endothelial cells in culture to nonlethal amounts of asbestos, but not refractory ceramic fiber-1, increases cell motility and gene expression. These changes may be initiated by the fibers mimicking matrix proteins as ligands for receptors on the cell surface. In the present study, 1- to 3-hr exposures of endothelial cells to 5 mg/cm2 of chrysotile asbestos caused marked cell elongation and motility. However, little morphological change was seen when chrysotile was added to cells pretreated with either mannosamine to prevent assembly of glycophosphatidylinositol (GPI)-anchored receptors or with herbimycin A to inhibit tyrosine kinase activity. Affinity purification of GPI-anchored urokinase-type plasminogen activator receptor (uPAR) from chrysotile-exposed cells demonstrated that asbestos altered the profile of proteins and phosphoproteins complexed with this receptor. Tyrosine kinase activities in the complexes were also increased by asbestos. Immunoprecipitations with selective monoclonal antibodies demonstrated that both chrysotile and crocidolite asbestos increase kinase activities associated with p60 Src or p120 focal adhesion kinase (FAK). Further, chrysotile also changed the profile of proteins and phosphoproteins associated with FAK in intact cells. These data suggest that asbestos initiates endothelial cell phenotypic change through interactions with uPAR-containing complexes and that this change is mediated through tyrosine kinase cascades

Radiation-Tolerant, GaN-based Point of Load Converters for Small Spacecraft Missions

As computational loads for spacecraft continue to grow, the requirements levied on power-conversion electronics have become increasingly demanding. Designing for compute-intensive processing capabilities in the CubeSat form-factor further encourages the use of lightweight, compact, and efficient power-conversion electronics. However, the radiation-tolerant and radiation-hardened point-of-load converters available from existing vendors are large, expensive, and inefficient relative to their commercial counterparts. To alleviate this disparity, this paper presents the design, development, and testing of three radiation-tolerant, point-of-load (PoL) converters using Gallium Nitride (GaN) High-Electron Mobility Transistors (HEMT) and commercial controllers to enable the success of future small-satellite missions

SSIVP: Spacecraft Supercomputing Experiment for STP-H6

The Department of Defense Space Test Program (STP) provides spaceflight opportunities for conducting on-orbit research and technology demonstrations to advance the future of spacecraft. STP-H6, the next mission of the program to the International Space Station (ISS), will include a prototype spacecraft supercomputing experiment and framework, called Spacecraft Supercomputing for Image and Video Processing (SSIVP), developed at the National Science Foundation (NSF) Center for High-Performance Reconfigurable Computing (CHREC) at the University of Pittsburgh. SSIVP introduces scalable, high-performance computing (HPC) principles to a CubeSat form-factor to advance the state of the art in space computing. SSIVP adopts the CHREC Space Processor (CSP) concept, a multifaceted design philosophy for a hybrid system of commercial and radiation-hardened (rad-hard) components supplemented with fault-tolerant computing, and a hybrid processor combining fixed-logic CPU and reconfigurable-logic FPGA. SSIVP features five flight-qualified CSPv1 computers as compute nodes, to facilitate this supercomputing concept, and one μCSP smart module, for running a Gallium Nitride (GaN)-based power converter sub-experiment. SSIVP is a versatile, heterogenous platform capable of processing application workloads in the processor or on runtime-reconfigurable FPGA accelerators. In this paper, we present the flight hardware and software, frameworks for parallel and dependable computing, and mission objectives for SSIVP

A GaN-Based Four-Switch Buck-Boost Converter Using Ripple Correlation Control for Maximum Power Point Tracking in Dynamic Deep Space Environments

As the demand for high-performance power conversion in spacecraft continues to grow and spacecraft mass and volume budgets become increasingly tight, it is essential to design DC-DC converters with higher efficiency and power density. Although photovoltaic (PV) efficiency has increased over time, solar irradiance and temperatures can fluctuate dramatically in deep space. This causes significant variations in the maximum power point (MPP) of the PV array, which can decrease the overall system efficiency unless accounted for. Thus, it is imperative to track the MPP of the PV panels to maintain optimal efficiency. This paper presents the experimental development of a four-switch, GaN-based buck-boost converter with an implementation of the Ripple Correlation Control (RCC) MPPT algorithm for dynamic deep space environments. Due to the use of GaN HEMTs, the experimental system achieves better efficiency and power density compared to the previous state of the art implementations. A simulation of the prototype buck-boost converter was implemented in SaberRD (Synopsis), and a digital design of the RCC-based MPPT controller utilizing the StateAMS tool is presented. The simulation results show that this controller swiftly and precisely converged to the MPP of the source PV panels in a dynamic solar irradiance condition

Cell signaling pathways elicited by asbestos.

In recent years, it has become apparent that minerals can trigger alterations in gene expression by initiating signaling events upstream of gene transactivation. These cascades may be initiated at the cell surface after interaction of minerals with the plasma membrane either through receptorlike mechanisms or integrins. Alternatively, signaling pathways may be stimulated by active oxygen species generated both during phagocytosis of minerals and by redox reactions on the mineral surface. At least two signaling cascades linked to activation of transcription factors, i.e., DNA-binding proteins involved in modulating gene expression and DNA replication, are stimulated after exposure of lung cells to asbestos fibers in vitro. These include nuclear factor kappa B (NF kappa B) and the mitogen-activated protein kinase (MAPK) cascade important in regulation of the transcription factor, activator protein-1 (AP-1). Both NF kappa B and AP-1 bind to specific DNA sequences within the regulatory or promoter regions of genes that are critical to cell proliferation and inflammation. Unraveling the cell signaling cascades initiated by mineral dusts and pharmacologic inhibition of these events may be important for the control and treatment of mineral-associated occupational diseases