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.

Analytically Closed Calculation of the Conduction and Switching Losses of Three-Phase AC-AC Sparse Matrix Converters

Abstract: For three-phase AC-AC power conversion a conventional matrix converter (CMC) or a DC side connection of a current DC link rectifier and a voltage DC link inverter comprising no energy storage components in the DC link could be employed. The combination of DC converters does show a lower number of turn-off power semiconductors and, therefore, has been denoted as Sparse Matrix Converter (SMC) or Very Sparse Matrix Converter (VSMC). A limitation of the phase displacement of the current and voltage fundamentals at the input and at the output to ±π/6 does allow a further reduction of the system complexity, the respective circuit topology has been introduced as Ultra Sparse Matrix Converter (USMC) in the literature. In this paper a novel concept for the analytical calculation of the current stresses on the power semiconductors of the Sparse Matrix Converter Topologies (SMC, VSMC, and USMC) is proposed. Furthermore, the switching losses of the output stage which shows identical structure for the SMC, VSMC and USMC are calculated analytically based on an experimentally determined dependency of the switching loss energy on the switching voltage and current. As a comparison to a digital simulation shows, the analytical results do show a very good accuracy in a wide modulation range and for widely varying load current phase angle and widely varying ratio of output and mains frequency and therefore do provide an excellent basis for the dimensioning of the SMC, VSMC or USMC and/or for the determination of the rated output power and efficiency which could be achieved by employing given power transistors and diodes.

Experimental verification of the efficiency/power-density (η-ρ) Pareto Front of single-phase double-boost and TCM PFC rectifier systems

Abstract: Over the last decades, the converter systems performance has been substantially improved but the endeavor for highest possible performance, especially with respect to power density, efficiency, and costs remains the most important driver of present and future developments and research. In latest publications, comprehensive analytical models have been applied in optimization procedures to calculate the design parameters of single-phase AC-DC converter systems resulting in the highest performance concerning multiple objectives. In this paper, an experimental validation of this analytical design approach is provided based on four prototype rectifier systems with Power Factor Correction (PFC) which result from the optimization with respect to power density or efficiency of the mature double-boost bridgeless Continuous Conduction Mode (CCM) and the recently published interleaved totem-pole-based Triangular Current Mode (TCM) rectifier topology. All design details, such as power component values and EMIfilter structure as well as volume and losses distributions, are provided and the measurement results regarding efficiency, EMI standards, and input current quality (power factor and total harmonic distortion) allow the direct performance comparison of the investigated rectifier topologies. All four prototype systems comply with the EMI standard CISPR 22 class B and exhibit a high power factor and a low current THD. The recently published TCM-topology is beneficially applied to achieve a higher efficiency compared to the efficiency- and volume-optimized double boost CCM rectifier systems with a similar power density. With the loss-optimized TCM-prototype an extreme efficiency of 99.23 % at nominal input voltage and rated output power has been measured.

Optimization and Performance Evaluation of an AC-Chopper Ballast for HPS Lamps

Abstract: Electronic ballasts for high-pressure sodium lamps based on an ac-chopper topology are proposed as a cheaper and less complex alternative to the industry-standard low-frequency square-wave ballasts. In this paper, the design process of the ac-chopper ballast is reviewed, and the design tradeoffs caused by the single-stage topology are discussed. The analytical and simulation results are verified with measurements of a 250-W prototype system, including a detailed list of the losses for the different circuit components and electromagnetic compatibility measurements. Due to the disadvantages of the single-stage topology, a comparison to a two-stage ballast using the inverter stage of the ac-chopper ballast with a separate power factor correction stage is provided.

Pareto-Optimal Design and Performance Mapping of Telecom Rectifier Concepts

Abstract: In the course of the design of single-phase PFC rectifiers, the demand for a high efficiency η and a high power density ρ must be met at the same time. Depending on the weight on these two design criteria different topologies could be advantageous. However, a comprehensive comparison of the topologies is difficult, since a large number of parameters must be determined and constraints in different physical domains, as e.g. magnetic or thermal properties, and EMI issues must be considered. Therefore, in this paper an approach based on relatively simple analytical equations is presented for calculating the limiting curves of conventional, bridgeless and TCM resonant-transition single-phase PFC rectifiers in the ρ-η-plane. These Pareto-Limits represent the Topology Performance Map of the considered topologies and allow a direct comparison of the achievable performance and the limitations with respect to power density and/or efficiency.

A miniature, 500 000 rpm, electrically driven turbocompressor

Abstract: The trend in compressors for fuel cells, heat pumps, aerospace and automotive heating, ventilation and air conditioning systems, is towards ultra-compact size and high efficiency. This can be achieved by using turbocompressors instead of scroll, lobe or screw compressors, increasing the rotational speed and employing new electrical drive system technology and materials. This paper presents a miniature, electrically driven two stage turbocompressor system running at a rated speed of 500 000 rpm. The design includes the thermodynamic analysis, the electric motor, the inverter, the control and the system integration with rotor dynamics and thermal considerations. Experimental measurements such as the compressor map are presented for air under laboratory conditions. The two stage turbocompressor has been tested up to a speed of 600 000 rpm, where a maximal pressure ratio of 2.3 at a mass flow of 0.5 g/s has been reached. To the authors knowledge this is the highest rotational speed achieved with an electrically driven turbocompressor.

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.