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.

Novel iGSE-C Loss Modeling of X7R Ceramic Capacitors

Abstract: Due to the large relative permittivity of Class II dielectrics, ceramic capacitors (CCs) from these materials promise significant volume and weight reductions in inverter and rectifier sine-wave filters, and are especially attractive in mobile applications that demand ultrahigh power density. While previous literature found large low-frequency losses in these components, no extensible loss model was proposed to accurately characterize these ferroelectric losses. In this article, we take advantage of prior art on ferro magnetic components in power electronics to propose a Steinmetz parameter-based loss modeling approach for X7R CCs, named the Improved Generalized Steinmetz Equation for CCs, or iGSE-C. This model is verified using the Sawyer–Tower circuit to measure losses in a commercially available X7R capacitor across excitation magnitude, dc bias, temperature, excitation frequency, and harmonic injection. Losses are shown to scale according to a power law with charge, with the resulting Steinmetz coefficients valid across dc bias and slightly varying as the temperature is increased. The iGSE-C accurately predicts losses for typical nonsinusoidal phase voltage waveforms with an error under 8%. Finally, the loss modeling technique is demonstrated for the sine-wave output filter of a bridge-leg arrangement with both low- and high-frequency excitations, with total capacitor losses predicted within 12% accuracy.

High-speed magnetically levitated reaction wheel demonstrator

Abstract: Reaction wheels (RWs) for small satellites with active magnetic bearings allowing for ultra-high-speed operation show advantages in angular momentum density over ball bearing RWs with limited speed according to scaling laws developed in this paper. A reaction wheel demonstrator design based on a novel dual hetero­ Ihomoploar, slotless, self-bearing, permanent-magnet synchronous motor concept with a rotational speed of 250000 rpm is investigated. The design includes the rotor dynamics, mechanical stress analysis, electromagnetics , power electronics and control, and the sensor concept. The experimental setup ready for experimental verification is presented.

Design, Control and Performance of Tracking Power Supply for a Linear Power Amplifier

Abstract: Power amplifiers are widely used in consumer and industrial applications and are based on either a linear power amplifier or switch-mode power amplifier design. By combining the linear power amplifier with a switch-mode tracking power supply, a hybrid power amplifier is formed that maintains a high quality output while having a high efficiency. This paper presents a hybrid amplifier that is constructed with a novel boost-type tracking power supply and provides an excellent performance. A design method for the output filter using the power supply ratio rejection (PSRR) of the linear power amplifier is proposed. This method ensures that the amplifier output voltage has minimal switching frequency components as well as being able to quickly track changes in the output voltage. A control system design method is presented that ensures good performance in the control of the constant inductor current of the switch-mode tracking stage and a high rejection of output voltage disturbances. A laboratory prototype experimentally verifies the theoretical analysis

Transient Behavior of Solid-State Modulators With Matrix Transformers

Abstract: Solid-state modulators based on pulse transformers offer the advantage that, with the turns ratio of the transformer, the primary voltage could ideally be adapted to available switch technology, and a series connection of switches could be avoided. To increase the power level, several semiconductor switches must be connected in parallel, and a balancing between the different switches must be guaranteed. Here, the matrix transformer concept, which is based on multiple primary and/or secondary windings, as well as flux adding, offers superior performance with respect to the achievable rise times. However, the influence of the parasitic elements on the voltage and current distribution is quite involved. In this paper, the influence of the parasitic elements of the matrix transformer on the current balancing and on the winding voltages is investigated based on reluctance models. Additionally, the inherent current balancing of the matrix transformer for windings mounted on different cores is explained. Furthermore, the influence of parasitic load/transformer capacitances on the turnoff transient is discussed in detail.

A 300 000-r/min Magnetically Levitated Reaction Wheel Demonstrator

Abstract: Magnetic bearings can be used in reaction wheel systems to avoid several drawbacks of ball bearings, such as limited lifetime due to mechanical friction and lubricant monitoring/sealing requirements. Therefore, this letter discusses an electrical machine topology with integrated reaction wheel and magnetic bearings. The slotless/ironless structure, together with a new electromagnetic arrangement resulting in a shorter rotor than in earlier works, enables efficient operation at very high speeds, which in turn enables the miniaturization of the system. Measurements taken on a demonstrator prototype at 300 000 r/min show a significant increase of the feasible operational speed range compared to the state-of-the-art reaction wheels for small satellites, which typically run below 10 000 r/min.

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.