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.

A 150 000-r/min Bearingless Slice Motor

Abstract: In this paper, the design and operating principle of a slotless bearingless slice motor for high rotational speeds are presented. The performance of the proposed concept is evaluated using an experimental prototype. Measurement results demonstrate a stable dynamic behavior during acceleration and an achievable rotational speed of 150 000 r/min, which is, to the knowledge of the authors, the highest rotational speed attained by a bearingless slice motor to date. The system performance is outlined at its maximum speed, and power loss measurements are carried out over the entire speed range.

Pump characteristic based optimization of a direct water cooling system for a 10 kW/500 kHz Vienna rectifier

Abstract: A high power density 10 kW/500 kHz three-phase PWM rectifier (Vienna Rectifier) is under development. Due to preliminary measurements and numerical simulations the total efficiency is assumed to be 95% at full load, resulting in power losses of up to 150 W in each multi-chip power module realizing a bridge leg of the rectifier. In order to keep the power density of the system high direct water cooling is employed where water is in direct contact with the module base plate. Based on the measured characteristic of the water pump (pressure drop dependent on water flow) the geometry of different water channel structures below the module base plate is systematically optimized based on analytical expressions which are formulated based on the well-established theory of fluid dynamics. The design optimization is constrained by the desire to keep the geometry of the water channels in a range that allows simple and low-cost manufacturing. The aim is to find a channel structure resulting in a minimum thermal resistance of the power module for a given pump characteristic. In this paper a very simple slot channel is investigated. The dependency of the thermal resistance on the cooling system is calculated in dependency on the height of the slot channel, and an optimized channel height is found under the side condition of simple manufacturability. Discussing the shortcomings of the simple slot structure, a novel metallic inlay structure is introduced and optimized resulting in a reduction of the thermal resistance of the direct water cooling scheme as compared to the slot channel system. All theoretical considerations are verified via experimental measurements. The general optimization scheme introduced in this paper can easily be adapted to other cooling problems.

Design of a multi-cell, DCM PFC rectifier for a 1mm thick, 200W off-line power supply — The power sheet

Abstract: For applications like LED lightning, flat screens, mobile electronics or smart surfaces converter systems with extreme aspect ratios are increasingly under investigation. Many of these systems include an AC-DC rectifier as mains interface with an input voltage of 230Vac and an output voltage of 400Vdc. In this study the design of an ultra-flat 200W PFC rectifier with a thickness of 1mm and a footprint similar to an A4 paper is presented. Due to the low profile components have to be integrated in the PCB which results in several limitations for the design of the devices. Designs for integrated inductors and transformers are discussed and the state of the art of integrated capacitors is presented. The integration of active switches is particularly important since the heat transfer has to be performed through the PCB. The paper shows that a distributed topology with soft switching is advantageous to meet the limitations of the components. This leads to distributed losses as well as to smaller inductances since the transmitted power is divided.

Electromagnetic Modeling of EMI Input Filters

Abstract: The paper summarizes a new modeling procedure for 3D virtual design of EMI filter circuits. The proposed method, based on the coupling of the Partial Element Equivalent Circuit (PEEC) and Boundary Integral Method (BIM) methods, extends the standard PEEC approach for modeling in the presence of magnetic materials, hence allowing the PEEC-based modeling of toroidal EMI filter inductors. The PEEC-BIM coupled method is implemented into a 3D CAD PEEC-based virtual prototyping platform (GeckoEMC). The developed EM simulation tool can give a comprehensive understanding of EM behaviour of EMI filter inductors and capacitors, taking their geometrical and material properties into account. Furthermore, it enables the modeling of optimal EMI filter structures including both parasitics and the effects originating from the mutual coupling and the interconnection of filter elements. The approach is verified by impedance and transfer function measurements of different single phase single-/two-stage EMI filter structures. Good agreement between the measurement and the PEEC-based simulation results was achieved for the frequency range from DC up to 30 MHz.

Design and control of an active reset circuit for pulse transformers

Abstract: In pulse modulators applying pulse transformers reset circuits are used to achieve optimal utilization of the core material, which results in lower costs, downsized pulse transformer/system volume and therefore in an improved pulse behavior due to the smaller parasitics. Because of its simplicity the most common method to reset the core is a DC reset circuit, where a DC current is used to premagnetize the core. However, the DC reset circuit - even with an optimal design-leads to significant losses in the freewheeling path. By applying an active reset method the losses due to the passive reset circuit can be reduced significantly. So far, only the theoretical behavior of the active reset circuit has been examined. Therefore, the detailed design and measurement results are presented in this paper. Furthermore, a new control method for achieving symmetrical flux swings in the core is presented.

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.