Johann W. Kolar

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.

Hardware verification of a hyper-efficient (98%) and super-compact (2.2kW/dm 3 ) isolated AC/DC telecom power supply module based on multi-cell converter approach

Abstract: Due to the increasing electricity demand of data centers driven by the emergence of cloud computing and big data, the focus on the development of telecom and data center power supplies is shifted towards high efficiencies. In this paper, a multi-cell converter approach for a telecom rectifier module breaking through the efficiency and power density barriers of traditional single-cell converter systems is shown. The com- prehensive optimization of the entire system with respect to efficiency and volume is described and the applied component loss models are explained. Furthermore, the design of a hardware demonstrator based on the optimization results is presented and several important design aspects are explained in detail.

Double-stage gate drive circuit for parallel connected IGBT modules

Abstract: Solid state modulators are increasingly being used in pulsed power applications. In these applications IGBT modules must often be connected in parallel due to their limited power capacity. In a previous paper, we introduced a control method for balancing the currents in the IGBTs. In this paper, we investigate techniques to minimize the modules' rise and fall times, which can positively impact the modulator's output pulse parameters, which in turn must meet the application's specifications. Further, a reduction in rise and fall times lowers switching losses and thus increases the modulator's efficiency. To reduce the voltage rise time of the pulse without increasing the maximal over-voltage of the parallel IGBTs we have investigated a double-stage gate driver with protection circuits to avoid over-voltages and over-currents. Additionally voltage edge detection has been implemented to improve current balancing. Our measurement results reveal the dependency of the rise-time and turnoff losses on the design parameters of the gate drive. We show that our design achieves a 62% reduction in the turn-off rise time, and a 32% reduction in the turn-off losses.

P $^{3}$ DCT—Partial-Power Pre-Regulated DC Transformer

Abstract: In this paper, a new approach to regulate the output voltage of a resonant, constant voltage transfer ratio 380 V/48 V isolated dc–dc converter is presented. Rather than applying variable frequency control to the resonant converter, which would result in reactive power processing and a more complicated electromagnetic compatibility filter design, the converter remains in its optimal operating point all time and an additional partial-power (PP) processing auxiliary converter is used to tightly regulate the output voltage. The PP converter, supplied through a tertiary winding of the resonant converter's transformer, regulates the output by adding or subtracting voltage from the dc input and has only a marginal impact on the overall efficiency of the dc–dc converter. The principal of operation is explained in detail including Sankey diagrams to illustrate the power processing of the converter and a feedback control system is proposed to tightly regulate the 48 V output voltage. A hardware demonstrator rated at 1.5 kW is implemented to cope with input voltage variations between 340 and 420 V and experimental results are provided showing that the output voltage can be kept within $\pm {1}{\%}$ of the nominal 48 V even under harsh input voltage and load transients. The realized dc–dc converter with PP pre-regulation features an overall efficiency of 97.7 $\%$ at rated power and a power density of 8.6 kWdm $^{-3}$ (141w/ $^{3}$ in).

Modeling of Pulse Transformers with Parallel- and Non-Parallel-Plate Windings for Power Modulators

Abstract: The parasitic capacitances of transformers significantly influence the resulting pulse shape of a power modulator system. In order to predict the pulse shape and adapt the geometry of the pulse transformer before constructing the transformer an equivalent circuit and analytic expressions relating the geometry to the parasitic elements are needed. Therefore, in this paper equations based on the geometry of the transformer for parametrizing an equivalent circuit consisting of 6 lumped capacitors are presented. The equations are verified by measurement results for pulse transformers with parallel- and non parallel-plate windings and a solid state modulator designed for linear accelerators.

The η-α-Pareto front of inductive power transfer coils

Abstract: Inductive power transfer (IPT) has been proposed as an alternative charging technology for electric and hybrid electric vehicles (EV/HEV). Because the low magnetic coupling of the transmission coils potentially limits the achievable transmission efficiency η, the losses in the inductors are of key importance in the design of an IPT system. The available area for the receiver coil is usually limited due to the vehicle construction and thus a high area related power density α of the IPT system is needed. In this paper, it is shown that η is maximized if the resonant compensation is chosen such that only active power is transmitted through the air gap. It is shown that a physical boundary termed the η-α-Pareto front exists, where a trade-off between high efficiency and high area related power density is encountered. A thermal model is used to include the effect of an increased winding temperature into the efficiency calculation. The comparison of the η-α-Pareto fronts of three example designs shows that the shielding of the inductor yields a lower η due to additional losses in the shielding materials and that a higher transmission frequency does not necessarily lead to an efficiency improvement.

I and i

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 …

Multilevel inverters: a survey of topologies, controls, and applications

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.

Recent Advances and Industrial Applications of Multilevel Converters

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.