1,720,956 research outputs found
High Efficiency Resonant DC/DC Converter Utilizing a Resistance Compression Network
Get PDFThis paper presents a new topology for a high-efficiency dc/dc resonant power converter that utilizes a resistance compression network (RCN) to provide simultaneous zero-voltage switching and near-zero-current switching across a wide range of input voltage, output voltage, and power levels. The RCN maintains desired current waveforms over a wide range of voltage operating conditions. The use of ON/OFF control in conjunction with narrowband frequency control enables high efficiency to be maintained across a wide range of power levels. The converter implementation provides galvanic isolation and enables large (greater than 1:10) voltage conversion ratios, making the system suitable for large step-up conversion in applications such as distributed photovoltaic converters. Experimental results from a 200-W prototype operating at 500 kHz show that over 95% efficiency is maintained across an input voltage range of 25-40 V with an output voltage of 400 V. It is also shown that the converter operates very efficiently over a wide output voltage range of 250-400 V, and a wide output power range of 20-200 W. These experimental results demonstrate the effectiveness of the proposed design
Variable Frequency Multiplier Technique for High-Efficiency Conversion Over a Wide Operating Range
No full textThis paper presents a variable frequency multiplier (VFX) technique that enables the design of converters for wide input and/or output voltage ranges while preserving high efficiency. The technique is applied to an LLC converter to demonstrate its effectiveness for converters having a wide input voltage variation such as universal input power supplies. This technique compresses the effective operating range required of a resonant converter by switching the inverter and/or the rectifier operation between processing energy at a fundamental frequency and one or more harmonic frequencies. The implemented converter operates over an input voltage range of 85-340 V, but the resonant tank and conversion ratio have only been designed for half this range; a VFX mode of the inverter is used to enhance this to the full range. The experimental results from a 50-W converter show an efficiency of 94.9%-96.6% across the entire input voltage range, demonstrating the advantage of using this technique in such applications
High efficiency resonant dc/dc converter for solar power applications
Get PDFThesis (S.M.)--Massachusetts Institute of Technology, Dept. of Electrical Engineering and Computer Science, 2013.This electronic version was submitted by the student author. The certified thesis is available in the Institute Archives and Special Collections.Cataloged from student-submitted PDF version of thesis.Includes bibliographical references (p. 109).This thesis presents a new topology for a high efficiency dc/dc resonant power converter that utilizes a resistance compression network to provide simultaneous zero voltage switching and near zero current switching across a wide range of input voltage, output voltage and power level. The resistance compression network maintains desired current waveforms over a wide range of voltage operating conditions. The use of on/off control in conjunction with narrowband frequency control enables high efficiency to be maintained across a wide range of power levels. The converter implementation provides galvanic isolation and enables large (greater than 1:10) voltage conversion ratios, making the system suitable for large step-up conversion in applications such as distributed photovoltaic converters. Three 200 W prototypes were designed, built and tested. The first prototype was made as a proof of concept and operated at a switching frequency of 100 kHz. It had an efficiency of 93.5% (at 25 V input and 400 V output). The second prototype was operated at a switching frequency of 500 kHz and had an efficiency of 93% (at 25 V input and 400 V output). The high frequency losses caused by the ringing in voltage and current due to the resonating parasitics of the transformer were removed with the help of a matching network in the third prototype. This final prototype operated at a switching frequency of 500 kHz and showed that over 95% efficiency is maintained across an input voltage range of 25 V - 40 V (at 400 V output) and over 93.7 % efficiency across a wide output voltage range of 250 V - 400 V (at 25 V input). These experimental results demonstrated the effectiveness of the proposed design.by Wardah Inam.S.M
Adaptable power conversion for grid and microgrid applications
No full textThesis: Ph. D., Massachusetts Institute of Technology, Department of Electrical Engineering and Computer Science, 2016.This electronic version was submitted by the student author. The certified thesis is available in the Institute Archives and Special Collections.Cataloged from student-submitted PDF version of thesis.Includes bibliographical references (pages 215-223).Power electronics convert and control power and are an essential part of many electronic devices and systems. Increasingly, power converters need to operate over widely varying operating conditions (voltage, current and power level), which can degrade their performance. In many applications, power converters are also being interconnected to form power systems. This is challenging as the converters may have to operate in various configurations. In order to ensure best performance, power converters and systems need to adapt to the operating conditions by adjusting topology, component values or control characteristics. In this thesis, two applications are considered: universal input power supplies and off-grid electrification. More than 3 billion power supplies were sold worldwide in 2014 [1], with efficiencies ranging from 50-90% [2], which results in significant energy loss. A major portion of these are universal input power supplies (designed to operate at AC line voltages found globally). In this thesis, a control technique, Variable Frequency Multiplier (VFX), is developed which compresses the effective operating range required of a resonant converter by switching the inverter and/or rectifier operation between processing energy at a fundamental frequency and one or more harmonic frequencies. This technique was applied to the inverter stage of a stacked-bridge LLC converter for a universal input power supply. An efficiency of 94.9% to 96.6% was achieved for a 50 W converter operating across the entire (4:1) input voltage range. Even though centralized grid infrastructure is widespread, access to electricity is still limited in many parts of the developing world. More than 1.2 billion people globally do not have access to electricity [3]. In this thesis, an ad hoc modular microgrid architecture is developed and field-tested. Contrary to how conventional power systems are designed, these microgrids do not require pre-planning and can operate in any network configuration. Smart power management units have been designed to arbitrarily connect power sources and loads, forming an autonomous microgrid. Accurate power sharing of multiple power sources is demonstrated. A methodology to attain the lowest system cost by designing power converters that reduce lifetime energy loss has also been developed. This enables affordable and reliable electricity to be provided in off-grid areas. Improvements in these applications will have a significant impact on power utilization from the existing grid infrastructure, and will help define the future of power utilization where this infrastructure does not exist. Moreover, the techniques and designs developed, in this thesis, for adaptable and efficient operation of power converters and systems can be easily extended to other applications.by Wardah Inam.Ph. D
Going Beyond Counting First Authors in Author Co-citation Analysis
Get PDFThe present study examines one of the fundamental aspects of author co-citation analysis (ACA) - the way co-citation
counts are defined. Co-citation counting provides the data on which all subsequent statistical analyses and mappings
are based, and we compare ACA results based on two different types of co-citation counting - the traditional type that
only counts the first one among a cited work's authors on the one hand and a non-traditional type that takes into
account the first 5 authors of a cited work on the other hand. Results indicate that the picture produced through this non-traditional author co-citation counting contains more coherent author groups and is therefore considerably clearer. However, this picture represents fewer specialties in the research field being studied than that produced through the traditional first-author co-citation counting when the same number of top-ranked authors is selected and analyzed. Reasons for these effects are discussed
Stability and control of ad hoc dc microgrids
No full textAd hoc electrical networks are formed by connecting power sources and loads without pre-determining the network topology. These systems are well-suited to addressing the lack of electricity in rural areas because they can be assembled and modified by non-expert users without central oversight. There are two core aspects to ad hoc system design: (1) designing source and load units such that the microgrid formed from the arbitrary interconnection of many units is always stable and (2) developing control strategies to autonomously manage the microgrid (i.e., perform power dispatch and voltage regulation) in a decentralized manner and under large uncertainty. To address these challenges we apply a number of nonlinear control techniques-including Brayton-Moser potential theory and primal-dual dynamics-to obtain conditions under which an ad hoc dc microgrid will have a suitable and asymptotically stable equilibrium point. Further, we propose a new decentralized control scheme that coordinates many sources to achieve a specified power dispatch from each. A simulated comparison to previous research is included
Variations on the Author
Get PDF“Variations on the Author” discusses two of Eduardo Coutinho’s recent films (Um Dia na Vida, from 2010, and Últimas Conversas, posthumously released in 2015) and their contribution to the general question of documentary authorship. The director’s filmography is characterized by a consistent yet self-effacing form of authorial self-inscription: Coutinho often features as an interviewer that rather than express opinions propels discourses; an interviewer that is good at listening. This mode of self-inscription characterizes him as an author who is not expressive but who is nonetheless markedly present on the screen. In Um Dia na Vida, however, Coutinho is completely absent form the image, while Últimas Conversas, on the contrary, includes a confessional prologue that moves the director from the margins to the center of his films. This article examines the ways in which these works stand out in the filmography of a director who offers new insights into the notion of cinematic authorship
Appropriate Similarity Measures for Author Cocitation Analysis
Get PDFWe provide a number of new insights into the methodological discussion about author cocitation analysis. We first argue that the use of the Pearson correlation for measuring the similarity between authors’ cocitation profiles is not very satisfactory. We then discuss what kind of similarity measures may be used as an alternative to the Pearson correlation. We consider three similarity measures in particular. One is the well-known cosine. The other two similarity measures have not been used before in the bibliometric literature. Finally, we show by means of an example that our findings have a high practical relevance.information science;Pearson correlation;cosine;similarity measure;author cocitation analysis
Dispelling the Myths Behind First-author Citation Counts
Get PDFWe conducted a full-scale evaluative citation analysis study of scholars in the XML research field to explore just how different from each other author rankings resulting from different citation counting methods actually are, and to demonstrate the capability of emerging data and tools on the Web in supporting more realistic citation counting methods. Our results contest some common arguments for the continued
use of first-author citation counts in the evaluation of scholars, such as high correlations between author rankings by first-author citation counts and other citation
counting methods, and high costs of using more realistic citation counting methods that are not well-supported by the ISI databases. It is argued that increasingly available digital full text research papers make it possible for citation analysis studies to go beyond what the ISI databases have directly supported and to employ more
sophisticated methods
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