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.

On-state voltage measurement of fast switching power semiconductors

Abstract: The on-state resistance R ds,on is a key characteristic of unipolar power semiconductors and its value depends on the operating conditions, e.g. junction temperature, conducted current and applied gate voltage. Hence, the exact determination of the R ds,on value cannot rely on datasheet information and requires the measurement of current and on-state voltage during operation. Besides the determination of the conduction losses, the on-state voltage measurement enables dynamic R ds,on analysis, device temperature estimation, condition monitoring and consequently time-to-failure prediction. However, in contrast to a switch current measurement, several challenges arise in the design of an on-state voltage measurement circuit (OVMC), i.e. high measurement accuracy (mV-range) during on-state, high blocking voltage capability (kV-range) during off-state and fast dynamic response (ns-range) during switching transitions are demanded. Different OVMC concepts are known from IGBT applications, however, the more severe requirements introduced from the high switching frequency and low OV characterizing the operation of fast switching power semiconductors, prevent their usage. Off-the-shelf products hardly satisfy the mentioned specifications, whereas the performance of state-of-the-art OVM research prototypes require further investigations and/or improvements. With this aim, an innovative OVMC concept is designed, analyzed, calibrated and tested in this paper. Furthermore, the conduction losses of different power semiconductors are measured as function of their operating conditions to validate the performance and highlight the potential of the proposed OVMC.

Comparative evaluation of SiC and Si PV inverter systems based on power density and efficiency as indicators of initial cost and operating revenue

Abstract: In this paper, two three-level three-phase all Si PV inverter topologies are compared to a standard two-level three-phase topology employing SiC-based power transistors In a comparative evaluation based on multi-objective component modeling, the performance trade-offs between achievable efficiency and power density are systematically analyzed for all systems On the one hand, this is to investigate the potential of SiC to decrease the system complexity while achieving similar or better performance The analysis shows that a similar power density and efficiency can be obtained with the SiC two-level system while requiring only one tenth of the chip area when compared to the three-level Si inverters On the other hand, in a next step, using power density and efficiency as indicators for initial inverter cost and operational revenue, the trade-off analysis will allow to determine the economically optimal system dimensioning

Advanced Cooling Methods for High-Speed Electrical Machines

Abstract: High-speed electrical machines are gaining increasing attention, as they enable higher power densities in several applications such as micromachining spindles and turbo compressors. This brings along an important challenge in thermal management due to the higher loss densities in the machine. Therefore, a careful thermal analysis is required along with the electromagnetic and mechanical considerations during the design phase of the machines. In this paper, different forced cooling options are compared for a slotless-type high-speed permanent-magnet machine. Fast, yet sufficiently accurate thermal models are derived for analyzing these cooling concepts. This enables their coupling with electromagnetic models and incorporation into the machine optimization procedure, which would not be feasible when using computationally very intensive methods such as three-dimensional finite element method or computational fluid dynamics. The developed thermal models are first verified on mechanically simplified stator designs (in which no rotor coupling is possible), and later on fully functional high-speed electrical machine prototypes. Using an integrated cooling method instead of a standard cooling jacket, the power density can be nearly doubled while keeping the maximum winding temperature below 80 °C, without altering the rotor or the stator core geometries.

Gate driver unit for electrical switching device

Abstract: An exemplary apparatus and method for using intelligent gate driver units with distributed intelligence to control antiparallel power modules or parallel-connected electrical switching devices like IGBTs is disclosed. The intelligent gate drive units use the intelligence to balance the currents of the switching devices, even in dynamic switching events. The intelligent gate driver units can use master-slave or daisy chain control structures and instantaneous or time integral differences of the currents of parallel-connected switching devices as control parameters. Instead of balancing the currents, temperature can also be balanced with the intelligent gate driver units.

All-silicon 99.35% efficient three-phase seven-level hybrid neutral point clamped/flying capacitor inverter

Abstract: With the increasing use of photovoltaic systems, a large demand for efficient, power-dense and lightweight grid-interface inverters is arising. Accordingly, new concepts like multi-level converters, which are able to reduce the converter losses while still keeping a low construction volume, have to be investigated. The hybrid seven-level topology analyzed in this paper comprises an active neutral point clamped stage, followed by a flying capacitor stage. Compared to a pure flying capacitor converter, the combination of these two stages allows to save more than half of the capacitor volume, while still having the same requirement for the output filter stage, and hence, the same output filter volume. Moreover, the topology employs low-voltage devices and ensures low conduction and switching losses, resulting in a higher efficiency. The principle of operation of the system is briefly reviewed, and based on a detailed component modeling, an efficiency vs. power density optimization is carried out, for which switching loss measurements of state-of-the-art 200 V semiconductors are performed. From the optimization, a high-efficiency design is selected and the practical hardware realization is discussed. The simulation and optimization results are then verified by realizing an all-silicon 99.35% efficient three-phase seven-level system, featuring a volumetric power density of 3.4 kW/dm 3 (55.9 W/in 3 ), a gravimetric power density of 3.2 kW/kg, and fulfilling CISPR Class A EMI requirements. Finally, it is shown that an all-silicon realization with next generation silicon switches can achieve 99.5% efficiency with the same hardware, and 99.6% with commercial state-of-the-art GaN switches.

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.