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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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Proceedings ArticleDOI
TL;DR: In this article , the authors systematically investigated the weight limits of several converter topology concepts: starting from a standard two-level four-switch non-inverting buck-boost converter benchmark system, fully soft-switched operation, as well as flying-capacitor multilevel and partial-power processing variants are compared by means of a comprehensive Pareto optimization (mission profile efficiency vs. gravimetric power density).
Abstract: Hybrid battery/fuel cell power supplies for all-electric Vertical Takeoff and Landing (eVTOL) aircraft enable a high peak power capability as well as long-range operation. However, the wide and overlapping voltage ranges of the battery and the fuel cell require an interconnecting bidirectional DC-DC converter with buck-boost capability. For such a converter system, the gravimetric power density is a key metric. Therefore, this paper systematically investigates the weight limits of several converter topology concepts: Starting from a standard two-level four-switch non-inverting buck-boost converter benchmark system, fully soft-switched operation, as well as flying-capacitor multilevel and partial-power-processing variants are compared by means of a comprehensive Pareto optimization (mission-profile efficiency vs. gravimetric power density). Aiming at a mission profile efficiency of $\boldsymbol{\overline{\eta} > 98.5}$ %, the results indicate the feasibility of converter realizations with gravimetric power densities of up to $\gamma > 62\text{kW}/$ kg with a three-level flying capacitor multilevel converter. Further, a virtual prototype of a two-level four-switch non-inverting buck-boost benchmark system is presented with $\gamma=46\text{kW}/$ kg, hence illustrating the trade-off between converter performance and complexity.

2 citations

Journal ArticleDOI
TL;DR: In this article , a simplified approach to estimate a lower bound for the hard switching losses of 3LTT bridge-legs is presented, which is applicable to other three-level topolgies as well.
Abstract: Hard-switching losses in three-level T-type (3LTT) bridge-legs cannot be directly estimated from datasheet energy loss curves, which are given for symmetric two-level half-bridge configurations only. The commutations in a 3LTT bridge-leg occur between semiconductors with different blocking voltages and/or current ratings, and involve a third semiconductor device in the switching transition, which contributes additional capacitive losses. This paper, therefore, describes a simplifed approach to estimate a lower bound for the hard-switching losses of 3LTT bridge-legs (note that the approach is applicable to other three-level topolgies as well). In view of the very fast switching speeds of wide-bandgap semiconductors, the model neglects voltage/current overlap losses and considers only the dominating charge-related loss contributions (semiconductor output capacitances, body diode reverse-recovery charge), thus requiring minimal information from datasheets. A direct experimental verification with an 800 V DC-link 3LTT bridge-leg (1200 V and 650 V SiC MOSFETs) operating with output currents up to 25 A confirms the good accuracy of the simplified switching-loss model.

2 citations

Proceedings Article
01 Jan 2009
TL;DR: In this paper, a two-winding inductor bouncer circuit was proposed to achieve a flat output voltage droop of less than 1% for short pulse durations, in the range of several A‚Â?s.
Abstract: In many pulsed power applications the flatness of the output pulse is an important characteristic to enable proper system operation, whereas a pulse flatness within less than a few percent has to be achieved. In power modulators based on capacitor discharge this voltage droop is mainly defined by the input capacitance. In order to overcome this problem, in power modulators systems, compensation circuits are added, whereby in spite of a smaller storage capacitor a flat pulse top is achieved. Depending on the pulse duration, different approaches for voltage droop compensation exist. For short pulse durations, in the range of several A‚Â?s, only passive solutions or bouncer circuits are applicable. In this paper the design and optimization of a two-winding inductor bouncer circuit is presented in order to achieve an output voltage droop of less than 1%. Due to the realized galvanic isolation a new degree of freedom is obtained, which allows an adaption of the bouncer circuit's voltage and current rating to standard semiconductor switches. With an optimal design of the two-winding inductor bouncer circuit for the existing system, the volume of the input capacitor is reduced by a factor of 10.5 and the stored energy is decreased by a factor of 24 compared to system without bouncer circuit.

2 citations

Proceedings ArticleDOI
Mario Mauerer1, Lukas Schrittwieser1, G. Ortiz1, Dominik Bortis1, Johann W. Kolar1 
18 May 2014
TL;DR: In this paper, an additional sinusoidal AC-excitation of the core material provides higher sensitivity of the length measurement and overcomes the high-pass characteristic of the piezo sensor.
Abstract: Magnetostriction describes the geometrical change in length of a ferromagnetic material in dependence of its internal magnetic flux density value. By detecting the vibrations caused by these dimensional changes with a piezo-electric transducer, the instantaneous value of the magnetic flux inside a magnetic core can be sensed from DC up to a few kilohertz. This principle, together with a high bandwidth current transformer, was utilized in order to construct a current sensor capable of measuring currents ranging from DC to several MHz. As will be shown in this paper, an additional sinusoidal AC-excitation of the core material provides higher sensitivity of the length measurement and overcomes the high-pass characteristic of the piezo sensor. In order to prove the principle and to demonstrate the capabilities of this new sensor, a series of experimental measurements and implementation results are presented.

2 citations

Journal ArticleDOI
TL;DR: In this paper, an extension of a state-of-the-art three-phase buck-boost power-factor correction (PFC) rectifier topology is proposed, which is especially suitable for low-power applications.
Abstract: Three-phase buck-boost power-factor correction (PFC) rectifiers are characterised by a unity power-factor mains behaviour and/or sinusoidal input currents and are providing a wide-output voltage range. In this study, an extension of a state-of-the-art three-phase buck-boost PFC rectifier topology is proposed. The DC output of the new topology does not suffer from a high-frequency common-mode voltage with respect to the (grounded) mains star point, which alleviates electromagnetic interference concerns. Also, the blocking voltage requirements of the AC-side switches are reduced significantly (almost by a factor of two for a 400 V line-to-line mains and a 400 V DC output), which facilitates a broad selection of cost-effective power semiconductors for the system's realisation. The rectifier can be controlled with a simple feedback system and the concept is especially suitable for low-power applications. A 1 kW hardware demonstrator is employed to verify the results of theoretical considerations. The system seamlessly operates in the buck and boost regime and achieves conversion efficiencies of 95.3% and mains current total harmonic distortion figures in the range of 1-5%.

2 citations


Cited by
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Journal ArticleDOI

[...]

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