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Johann W. Kolar

Other affiliations: Alstom, Infineon Technologies, Bosch  ...read more
Bio: Johann W. Kolar is an academic researcher from ETH Zurich. The author has contributed to research in topics: Rectifier & Three-phase. The author has an hindex of 97, co-authored 965 publications receiving 36902 citations. Previous affiliations of Johann W. Kolar include Alstom & Infineon Technologies.


Papers
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Journal ArticleDOI
TL;DR: In this paper, a step by step design procedure of a pulse transformer for rectangular pulse shape with fast rise time is presented, where different transformer topologies are compared with respect of the parasitic elements, which are then calculated analytically depending on the mechanical dimensions of the transformer.
Abstract: Microseconds range pulse modulators based on solid state technology often utilize a pulse transformer, since it could offer an inherent current balancing for parallel connected switches and with the turns ratio the modulator design could be adapted to the available semiconductor switch technology. In many applications as e.g. radar systems, linear accelerators or klystron/magnetron modulators a rectangular pulse shape with a fast rise time and a as small as possible overshoot is required. In reality, however, parasitic elements of the pulse transformer as leakage inductance and capacitances limit the achievable rise time and result in overshoot. Thus, the design of the pulse transformer is crucial for the modulator performance. In this paper, a step by step design procedure of a pulse transformer for rectangular pulse shape with fast rise time is presented. Different transformer topologies are compared with respect of the parasitic elements, which are then calculated analytically depending on the mechanical dimensions of the transformer. Additionally, the influence of the core material, the limited switching speed of semiconductors and the nonlinear impedance characteristic of a klystron are analyzed.

62 citations

Proceedings ArticleDOI
02 Apr 2007
TL;DR: In this paper, an indirect air cooling system for magnetic devices which combines the transformer with a heat sink and a heat transfer component is presented, and an analytic approach for calculating the temperature distribution is derived and validated by measurements.
Abstract: In the area of power electronics there is a general trend to higher power densities. In order to increase the power density the systems must be designed optimally concerning topology, semiconductor selection, etc. and the volume of the components must be decreased. The decreasing volume comes along with a reduced surface for cooling. Consequently, new cooling methods are required. In the paper an indirect air cooling system for magnetic devices which combines the transformer with a heat sink and a heat transfer component is presented. Moreover, an analytic approach for calculating the temperature distribution is derived and validated by measurements. Based on these equations a transformer with an indirect air cooling system is designed for a 10 kW telecom power supply.

62 citations

Journal ArticleDOI
TL;DR: In this paper, the authors proposed a new balancing circuit, which results in faster switching transients and higher possible operating pulse currents, is presented and validated by measurement results, and a super cascode switch with a blocking voltage N times higher than the blocking voltage of a single JFET is presented.
Abstract: In many pulsed-power applications, there is a trend to modulators based on semiconductor technology For these modulators, high-voltage and high-current semiconductor switches are required in order to achieve a high pulsed power Therefore, often, high-power IGBT modules or IGCT devices are used Since these devices are based on bipolar technology, the switching speed is limited, and the switching losses are higher In contrast to bipolar devices, unipolar ones (eg, SiC JFETs) basically offer a better switching performance Moreover, these devices enable high blocking voltages in the case where wide-band-gap materials, for example, SiC, are used At the moment, SiC JFET devices with a blocking voltage of 12 kV per JFET are available Alternatively, the operating voltage could be increased by connecting N JFETs and a low-voltage MOSFET in series, resulting in a super cascode switch with a blocking voltage N times higher than the blocking voltage of a single JFET For the super cascode, auxiliary elements are required for achieving a statically and dynamically balanced voltage distribution in the cascode In this paper, a new balancing circuit, which results in faster switching transients and higher possible operating pulse currents, is presented and validated by measurement results

61 citations

Journal ArticleDOI
TL;DR: In this article, a homopolar magnetic bearing biased by permanent magnets is combined with a high-pole-number segment motor for contactless levitation and rotation of a wide annular rotor through the walls of a sealed process chamber.
Abstract: This paper describes a novel and compact topology for contactless levitation and rotation of a wide annular rotor through the walls of a sealed process chamber. In the proposed setup, a homopolar magnetic bearing biased by permanent magnets is combined with a high-pole-number segment motor. The paper discusses the functional principle of the motor, and gives design and optimization guidelines for the bearing and the drive unit, respectively. An experimental system is presented along with a set of measurement results verifying the theoretical considerations.

61 citations

Proceedings ArticleDOI
09 Mar 2012
TL;DR: In this paper, the design procedure for a bidirectional DAB dc-dc converter, which provides a high dc voltage of 8kV to the tether of a 100 kW Airborne Wind Turbine (AWT) system, is presented.
Abstract: The design procedure for a bidirectional DAB dc-dc converter, which provides a high dc voltage of 8kV to the tether of a 100 kW Airborne Wind Turbine (AWT) system, is presented. The maximum allowed weight of the dc-dc converter is 25 kg and, thus, the main challenge is the realization of a light-weight DAB converter. The investigated dc-dc converter is split up into 16 single DAB modules with a rated power of 6.25 kW and a dc port voltage of 2kV. Thus, a weight of less than 25 kg/16 = 1.56 kg needs to be achieved for a single DAB module. The design method used to obtain the minimum weight DAB converter, based on the evaluation of a power-to-weight ratio versus efficiency Pareto Front γ-η-Pareto Front) is presented in this paper. For this purpose the transient voltages and currents of the employed SiC JFETs during switching are discussed in detail, since the respective measurement results allow for a prediction of the switching losses. Moreover, the calculated transformer is realized and experimental results are used to verify the weight, the losses, and the cooling system performance of the transformer. For a single DAB module a weight of m = 1.43 kg, an efficiency of η = 97%, and a power-to-weight ratio of γ = 4.4kW/kg results.

60 citations


Cited by
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[...]

08 Dec 2001-BMJ
TL;DR: There is, I think, something ethereal about i —the square root of minus one, which seems an odd beast at that time—an intruder hovering on the edge of reality.
Abstract: There is, I think, something ethereal about i —the square root of minus one. I remember first hearing about it at school. It seemed an odd beast at that time—an intruder hovering on the edge of reality. Usually familiarity dulls this sense of the bizarre, but in the case of i it was the reverse: over the years the sense of its surreal nature intensified. It seemed that it was impossible to write mathematics that described the real world in …

33,785 citations

Journal ArticleDOI
TL;DR: The most important topologies like diode-clamped inverter (neutral-point clamped), capacitor-Clamped (flying capacitor), and cascaded multicell with separate DC sources are presented and the circuit topology options are presented.
Abstract: Multilevel inverter technology has emerged recently as a very important alternative in the area of high-power medium-voltage energy control. This paper presents the most important topologies like diode-clamped inverter (neutral-point clamped), capacitor-clamped (flying capacitor), and cascaded multicell with separate DC sources. Emerging topologies like asymmetric hybrid cells and soft-switched multilevel inverters are also discussed. This paper also presents the most relevant control and modulation methods developed for this family of converters: multilevel sinusoidal pulsewidth modulation, multilevel selective harmonic elimination, and space-vector modulation. Special attention is dedicated to the latest and more relevant applications of these converters such as laminators, conveyor belts, and unified power-flow controllers. The need of an active front end at the input side for those inverters supplying regenerative loads is also discussed, and the circuit topology options are also presented. Finally, the peripherally developing areas such as high-voltage high-power devices and optical sensors and other opportunities for future development are addressed.

6,472 citations

Journal ArticleDOI
TL;DR: This paper first presents a brief overview of well-established multilevel converters strongly oriented to their current state in industrial applications to then center the discussion on the new converters that have made their way into the industry.
Abstract: Multilevel converters have been under research and development for more than three decades and have found successful industrial application. However, this is still a technology under development, and many new contributions and new commercial topologies have been reported in the last few years. The aim of this paper is to group and review these recent contributions, in order to establish the current state of the art and trends of the technology, to provide readers with a comprehensive and insightful review of where multilevel converter technology stands and is heading. This paper first presents a brief overview of well-established multilevel converters strongly oriented to their current state in industrial applications to then center the discussion on the new converters that have made their way into the industry. In addition, new promising topologies are discussed. Recent advances made in modulation and control of multilevel converters are also addressed. A great part of this paper is devoted to show nontraditional applications powered by multilevel converters and how multilevel converters are becoming an enabling technology in many industrial sectors. Finally, some future trends and challenges in the further development of this technology are discussed to motivate future contributions that address open problems and explore new possibilities.

3,415 citations

01 Sep 2010

2,148 citations