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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 article, the authors present an optimization routine and its underlying loss and thermal models, which are used to design a 166 kW/20 kHz transformer prototype achieving 99.4% efficiency at a power density of 44 kW/dm and a voltage conversion ratio of 3.5%.
Abstract: Within a solid-state transformer, the isolated dc-dc converter and in particular its medium-frequency transformer are one of the critical components, as it provides the required isolation between primary and secondary sides and the voltage conversion typically necessary for the operation of the system. A comprehensive optimization procedure is required to find a transformer design that maximizes power density and efficiency within the available degrees of freedom while complying with material limits, such as temperature, flux density, and dielectric strength as well as outer dimension limits. This paper presents an optimization routine and its underlying loss and thermal models, which are used to design a 166 kW/20 kHz transformer prototype achieving 99.4% efficiency at a power density of 44 kW/dm 3 . Extensive measurements are performed on the constructed prototype in order to measure core and winding losses and to investigate the current distribution within the litz wire and the flux sharing between the cores.

200 citations

Journal ArticleDOI
24 Oct 2005
TL;DR: A new simple procedure for modeling parasitic capacitances that is based on the known approaches is proposed and the resulting equations are verified by measurements on four different high voltage transformers.
Abstract: Parasitic capacitances of conventional transformers can be used as resonant elements in resonant DC-DC converters in order to reduce the overall system size. For predicting the values of the parasitic capacitors without building the transformer different approaches for calculating these capacitances are compared. A systematic summary of the known approaches is given and missing links between the different theories and missing equations are added. Furthermore, a new simple procedure for modelling parasitic capacitances which is based on the known approaches is proposed. The resulting equations are verified by measurements on four different high voltage transformers.

199 citations

Journal ArticleDOI
S. Waffler1, Johann W. Kolar1
TL;DR: In this article, a low-loss, constant-frequency, zero-voltage switching (ZVS) modulation strategy for bidirectional, cascaded, buck-boost DC-DC converters, used in hybrid electrical vehicles or fuel cell vehicles (FCVs), is presented and its benefits over state-of-the-art converters and soft-switching solutions are discussed in a comparative evaluation.
Abstract: A novel, low-loss, constant-frequency, zero-voltage-switching (ZVS) modulation strategy for bidirectional, cascaded, buck-boost DC-DC converters, used in hybrid electrical vehicles or fuel cell vehicles (FCVs), is presented and its benefits over state-of-the-art converters and soft-switching solutions are discussed in a comparative evaluation. To obtain ZVS with the proposed modulation strategy, the buck+boost inductance is selected and the switches are gated in a way that the inductor current has a negative offset current at the beginning and the end of each pulse period. This allows the MOSFET switches to turn on when the antiparallel body diode is conducting. As the novel modulation strategy is a software-only solution, there are no additional expenses for the active or passive components compared to conventional modulation implementations. Furthermore, an analytical and simulation investigation predicts an excellent efficiency over the complete operating range and a higher power density for a nonisolated multiphase converter equipped with the low-loss modulation. Experimental measurements performed with 12 kW, 17.4 kW/L prototypes in stand-alone and multiphase configuration verify the low-loss operation over a wide output power range and a maximum efficiency of 98.3% is achieved.

195 citations

Journal ArticleDOI
TL;DR: In this article, an isolated 25 kW, 48 kHz, 7 kV to 400 V series resonant dc/dc converter based on 10 kV SiC MOSFETs is realized and tested.
Abstract: The power supply chain of data centers from the medium voltage (MV) utility grid down to the chip-level voltage consists of many series connected power conversion stages and accordingly shows a relatively low efficiency. Solid-state transformers (SSTs) could improve the efficiency by substantially reducing the number of power conversion stages and/or directly interfacing the MV ac grid to a 400 V dc bus, from where server racks with a power consumption of several tens of kilowatts could be supplied by individual SSTs. The recent development of SiC MOSFETs with a blocking voltage of 10 kV enables the realization of a simple and, hence, highly reliable two-stage SST topology, consisting of an ac/dc power factor correction rectifier and a subsequent isolated dc/dc converter. In this context, an isolated 25 kW, 48 kHz, 7 kV to 400 V series resonant dc/dc converter based on 10 kV SiC MOSFETs is realized and tested in this paper. To achieve zero voltage switching of all MOSFETs, a special modulation scheme to actively control the amount of the switched magnetizing current on the MV- and low voltage-sides is implemented. Furthermore, the design of all main components and, especially, the electrical insulation of the employed medium-frequency transformer are discussed in detail. Calorimetric efficiency measurements show that a full-load efficiency of 99.0% is achieved, while the power density reaches 3.8 kW/L ( $63~\text {W}/\mathrm {in^{3}}$ ).

195 citations

Journal ArticleDOI
TL;DR: It is shown that a minimal volume can be found for a certain optimal number of filter stages and a volume optimization of the needed DM filter based on the previously calculated filter attenuation and volumetric component parameters is introduced.
Abstract: The design of electromagnetic interference (EMI) input filters, needed for switched power converters to fulfill the regulatory standards, is typically associated with high development effort. This paper presents a guideline for a simplified differential-mode (DM) filter design. First, a procedure to estimate the required filter attenuation based on the total input rms current using only a few equations is given. Second, a volume optimization of the needed DM filter based on the previously calculated filter attenuation and volumetric component parameters is introduced. It is shown that a minimal volume can be found for a certain optimal number of filter stages. The considerations are exemplified for two single-phase power factor correction converters operated in continuous and discontinuous conduction modes, respectively. Finally, EMI measurements done with a 300-W power converter prototype prove the proposed filter design method.

186 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