Browsing by Author "Chen, Zheng"
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- Characterization and Modeling of High-Switching-Speed Behavior of SiC Active DevicesChen, Zheng (Virginia Tech, 2009-12-18)To support the study of potential utilization of the emerging silicon carbide (SiC) devices, two SiC active switches, namely 1.2 kV, 5 A SiC JFET manufactured by SiCED, and 1.2 kV, 20 A SiC MOSFET by CREE, have been investigated systematically in this thesis. The static and switching characteristics of the two switches have firstly been characterized to get the basic device information. Specific issues in the respective characterization process have been explored and discussed. Many of the characterization procedures presented are generic, so that they can be applied to the study of any future SiC unipolar active switches. Based on the characterization data, different modeling procedures have also been introduced for the two SiC devices. Considerations and measures about model improvement have been investigated and discussed, such as predicting the MOSFET transfer characteristics under high drain-source bias from switching waveforms. Both models have been verified by comparing simulation waveforms with the experimental results. imitations of each model have been explained as well. In order to capture the parasitic ringing in the very fast switching transients, a modeling methodology has also been proposed considering the circuit parasitics, with which a device-package combined simulation can be conducted to reproduce the detailed switching waveforms during the commutation process. This simulation, however, is inadequate to provide deep insights into the physics behind the ringing. Therefore a parametric study has also been conducted about the influence of parasitic impedances on the device's high-speed switching behavior. The main contributors to the parasitic oscillations have been identified to be the switching loop inductance and the device output junction capacitances. The effects of different parasitic components on the device stresses, switching energies, as well as electromagnetic interference (EMI) have all been thoroughly analyzed, whose results exhibit that the parasitic ringing fundamentally does not increase the switching loss but worsens the device stresses and EMI radiation. Based on the parametric study results, this thesis finally compares the difference of SiC JFET and MOSFET in their respective switching behavior, comes up with the concept of device switching speed limit under circuit parasitics, and establishes a general design guideline for high-speed switching circuits on device selection and layout optimization.
- Electrical Integration of SiC Power Devices for High-Power-Density ApplicationsChen, Zheng (Virginia Tech, 2013-10-24)The trend of electrification in transportation applications has led to the fast development of high-power-density power electronics converters. High-switching-frequency and high-temperature operations are the two key factors towards this target. Both requirements, however, are challenging the fundamental limit of silicon (Si) based devices. The emerging wide-bandgap, silicon carbide (SiC) power devices have become the promising solution to meet these requirements. With these advanced devices, the technology barrier has now moved to the compatible integration technology that can make the best of device capabilities in high-power-density converters. Many challenges are present, and some of the most important issues are explored in this dissertation. First of all, the high-temperature performances of the commercial SiC MOSFET are evaluated extensively up to 200 degree C. The static and switching characterizations show that the device has superior electrical performances under elevated temperatures. Meanwhile, the gate oxide stability of the device - a known issue to SiC MOSFETs in general - is also evaluated through both high-temperature gate biasing and gate switching tests. Device degradations are observed from these tests, and a design trade-off between the performance and reliability of the SiC MOSFET is concluded. To understand the interactions between devices and circuit parasitics, an experimental parametric study is performed to investigate the influences of stray inductances on the MOSFETs switching waveforms. A small-signal model is then developed to explain the parasitic ringing in the frequency domain. From this angle, the ringing mechanism can be understood more easily and deeply. With the use of this model, the effects of DC decoupling capacitors in suppressing the ringing can be further explained in a more straightforward way than the traditional time-domain analysis. A rule of thumb regarding the capacitance selection is also derived. A Power Electronics Building Block (PEBB) module is then developed with discrete SiC MOSFETs. Integrating the power stage together with the peripheral functions such as gate drive and protection, the PEBB concept allows the converter to be built quickly and reliably by simply connecting several PEBB modules. The high-speed gate drive and power stage layout designs are presented to enable fast and safe switching of the SiC MOSFET. Based on the PEBB platform, the state-of-the-art Si and SiC power MOSFETs are also compared in the device characteristics, temperature influences, and loss distributions in a high-frequency converter, so that special design considerations can be concluded for the SiC MOSFET. Towards high-temperature, high-frequency and high-power operations, integrated wire-bond phase-leg modules are also developed with SiC MOSFET bare dice. High-temperature packaging materials are carefully selected based on an extensive literature survey. The design considerations of improved substrate layout, laminated bus bars, and embedded decoupling capacitors are all discussed in detail, and are verified through a modeling and simulation approach in the design stage. The 200 degree C, 100 kHz continuous operation is demonstrated on the fabricated module. Through the comparison with a commercial SiC phase-leg module designed in the traditional way, it is also shown that the design considerations proposed in this work allow the SiC devices in the wire-bond structure to be switched twice as fast with only one-third of the parasitic ringing. To further push the performance of SiC power modules, a novel hybrid packaging technology is developed which combines the small parasitics and footprint of a planar module with the easy fabrication of a wire-bond module. The original concept is demonstrated on a high-temperature rectifier module with SiC JFET. A modified structure is then proposed to further improve design flexibility and simplify module fabrication. The SiC MOSFET phase-leg module built in this structure successfully reaches the switching speed limit of the device almost without any parasitic ringing. Finally, a new switching loop snubber circuit is proposed to damp the parasitic ringing through magnetic coupling without affecting either conduction or switching losses of the device. The concept is analyzed theoretically and verified experimentally. The initial integration of such a circuit into the power module is presented, and possible improvements are proposed.
- The Role of Research in Landscape Architecture PracticeChen, Zheng (Virginia Tech, 2013-05-24)The profession of landscape architecture has not managed to sufficiently build a body of solid knowledge through research, which weakens the profession in terms of justifying its practice. In order to investigate why the profession has not built its knowledge-base sufficiently, this dissertation collected first-hand empirical data on the use and need of research in current landscape architecture practice, as well as the perceptions about research among landscape architects. Four questions were asked in this study: 1) What are the concerns of landscape architecture practice? 2) What is the significance of research in landscape architecture? 3) How do landscape architects perceive the need of research? 4) How are research findings disseminated in landscape architecture? To answer the questions, an online survey was given to randomly sampled ASLA members (adjusted response rate = 31%, n=239). The data was then analyzed through descriptive statistics, comparative statistics, and dimension analysis. Modern professions are expected not only to successfully perform professional actions, but also to justify these actions with rational explanations. To meet this expectation, the scope of landscape architecture knowledge has expanded from design knowledge into systems knowledge. While design knowledge concerns how to do design, systems knowledge concerns why certain design actions should be taken. Meanwhile, with expanding systems knowledge, research becomes more and more important to landscape architecture practice. Sixty-seven percent of landscape architects are using research findings often in making design decisions. However, results indicates that landscape architects expect research to generate rational solutions based on solid understanding of the phenomena and problems involved in design. Based on a review of literature, this expectation is unrealistic. The profession, if it expects to build a research-oriented practice, needs to change its perceptions about research, and advance its knowledge through studies and evaluations of built design work. Despite the increasing use of research, this study also found that landscape architects today still make their design decisions largely based on a body of tacit knowledge, such as professional experience and intuition. This body of tacit knowledge is often learned in an apprentice manner between practitioners in their workplace, and is rarely shared in the whole profession. While practitioners do not share much beyond their workplace, educators primarily share within academia, which limits the profession from improving its work in a fast changing world. The profession should encourage practitioners to do research by promoting the examples of practicing researchers, and offer places to share knowledge. The profession should also encourage educators to share knowledge beyond academia and to be more aware of the potential implications of their research findings.
- Scenery Management Study of Claytor LakeMiller, Patrick; Lui, Song; Gilboy, Elizabeth; Steika, Kim; Calorusso, Christine; Philen, Melissa; Abu Bakar, Shamsul; Chen, Zheng; Xu, Yining (Virginia Tech. Community Design Assistance Center, 2009-05-01)This study focuses on Claytor Lake in Pulaski County, Virginia. Claytor Lake is an important water-oriented recreation destination in Southwest Virginia, as well as being home to many local residents. The scenic experience of the lake is important to both recreationists and residents (Recreation Assessment Study: Claytor Project, Ferc No. 739). This particular scenery management study helps identify and protect the scenic integrity and desirable landscape characteristics for the Claytor Lake region. It also proposes scenery management guidelines to keep this area attractive, while also accommodating appropriate development and land uses.
- Spatial-Temporal Pattern of Agricultural Total Factor Productivity Change (Tfpch) in China and Its Implications for Agricultural Sustainable DevelopmentZhang, Haonan; Chen, Zheng; Wang, Jieyong; Wang, Haitao; Zhang, Yingwen (MDPI, 2023-03-21)With increasing tension between humans and land, and arising pressure on food security in China, the improvement of total factor productivity is important to realize agricultural modernization and promote rural revitalization strategy. In this study, we applied the DEA-Malmquist index method to measure the growth of China’s agricultural total factor productivity and its decomposition indexes at the prefecture-level city scale from 2011 to 2020. We found the average annual growth rate of agricultural total factor productivity was 4.5% during this period, with technical change being the driving factor and technical efficiency change being the suppressing factor. There is an initial decrease and then an increase in the Dagum Gini coefficient. The cold and hot spot areas of agricultural Tfpch were clearly formed. During the decade, the gravity center of agricultural Tfpch has migrated from the northeast to the southwest in general. Based on the characteristics of agricultural Tfpch, China is classified into four zones. In the future, the Chinese government should balance the government and the market mechanism, improve the agricultural science and technology innovation system and technology adoption promotion system, and implement classified policies to improve agriculture production efficiency.