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.

Sub-Optimum Design of a Forced Air Cooled Heat Sink for Simple Manufacturing

Abstract: In order to maximize the power density of a converter, a systematic procedure for optimizing the cooling system is necessary. Solving the equations of the optimization procedure described in this paper can be performed numerically without much effort, but manufacturing the resulting optimum heat sink is often extremely difficult and expensive, and therefore impractical. Based on the optimization theory, the sensitivity of the thermal resistance of the heat sink to changes in the geometric design parameters is discussed. Diagrams show the expected increase in thermal resistance if the manufacturing process constrains the fin thickness to certain values. Based on this, one can design a sub-optimum heat sink that gives the minimum thermal resistance for a certain manufacturing procedure. Furthermore, it is shown how much additional manufacturing effort would be necessary to further improve the performance of the heat sink. The proposed procedure is verified experimentally.

Optimization of the current distribution in press-pack high power IGBT modules

Abstract: Today's IGBT modules achieve high current ratings by paralleling several semiconductor switches. For high power and high voltage applications, the press-pack IGBT design is a technology of increasing importance, since it is designed for a low inductance series connection in a module stack. This work examines the module-internal current distribution in a press-pack configuration during switching transients by means of the PEEC simulation method. Parasitic effects which result in current unbalances between the paralleled switches are determined and quantified, where special emphasis is put on the power module external interconnection wiring and its influence on the current distribution and power loss in the distinct switches. A hardware test setup is discussed in detail, and a layout design optimization is which balances the loss distribution among the switches.

η-ρ Pareto optimization of 3-phase 3-level T-type AC-DC-AC converter comprising Si and SiC hybrid power stage

Abstract: In this paper, the η-ρ (efficiency - power density) Pareto front of a 20 kVA Uninterruptible Power Supply (UPS), comprising a Si and SiC hybrid 3-level T-type rectifier and inverter stage, and input / output side filters, is determined. A multi-objective optimization procedure is detailed, which employs an electrical converter model and coupled electro-thermal component models. Each component model determines the corresponding losses, volumes, and temperature rises. A detailed description of the inductor model is given which considers high frequency winding losses as well as core losses, and calculates the temperature rises inside the inductor, e.g. the hot-spot winding temperature. Based on the determined η-ρ Pareto front, the most suitable realization of the proposed UPS system is determined, which is based on the resulting sensitivities of the design parameters (switching frequency, current ripples, magnetic materials, etc.) on the performance of power converter system (efficiency and power density). As a result, a switching frequency of 16 kHz, maximum relative input and output side current ripples of 20%, amorphous core materials for DM inductors, and nanocrystalline core materials for CM inductors are selected in order to achieve a converter efficiency of 96.6% at a power density of 2.3 kVA/dm 3 .

Optimal Design of a 3.5-kV/11-kW DC–DC Converter for Charging Capacitor Banks of Power Modulators

Abstract: For the generation of short high-power pulses in many applications, power modulators based on capacitor discharge are used, where the peak power is drawn from the input capacitor bank. In order to continuously recharge the energy buffer during operation at a lower average power, usually, power supplies connected to the mains are used. Due to the worldwide variation in mains voltages and the desired ability to adapt the capacitor voltage of the modulator, the power supply has to support a wide input and output voltage range, whereby the supply should draw a sinusoidal current from the mains due to EMI regulations. Additionally, depending on the modulator concept, a galvanic isolation also has to be provided. In order to achieve the mentioned specifications for the considered power supply, a combination of an ac-dc and a dc-dc converter is proposed, whereas the mains voltage is rectified by a three-phase buck-boost converter to 400 Vdc, and thereafter, an isolated dc-dc converter charges the input capacitor bank of the power modulator up to 3.5 kV. This paper focuses on the basic operation and the design of the 3.5-kV/11-kW isolated dc-dc converter, which includes transformer design, efficiency-volume optimization, and component selection. In this paper, compared with the well-known flyback converter, the proposed full-bridge-based topology results in a much higher efficiency and power density.

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

Abstract: An ultra high power density 10-kW/500-kHz three-phase pulse-width modulation rectifier (Vienna Rectifier) is under development at the Power Electronic Systems Laboratory, ETH Zurich. From 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 multichip power module that realizes a bridge leg of the rectifier. In order to maintain the required 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 the water flow rate) the geometry of different water channel structures below the module base plate is systematically optimized based on equations which are formulated using well-established fluid dynamics theory. The design optimization is constrained by the desire to keep the geometry of the water channels in a range which 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 for various heights of the slot channel, and an optimized channel height is determined using the condition of simple manufacturability. The shortcomings of the simple slot structure are discussed, and a novel metallic inlay structure is introduced and optimized that results in a reduction of the thermal resistance of the direct water cooling scheme as compared to the slot channel system. All theoretical considerations are experimentally verified. The general optimization scheme introduced in this paper can easily be adapted to other cooling problems.

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.