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.

Efficiency optimization of an automotive multi-phase bi-directional DC-DC converter

Abstract: The paper proposes methods to improve the efficiency of a bi-directional, multi-phase buck+boost DC-DC converter for application in Hybrid Electrical Vehicles (HEV) or Fuel Cell Vehicles (FCV) Thereto, the modulation strategy for a highly-compact, 30kW/Liter, constant-frequency soft-switching converter is optimized based on a converter loss model that includes the losses in the power semiconductors and the buck+boost inductor An algorithm for numerical calculation of the optimum switching times is given, whereas the values for the loss-optimized operation of the converter are stored in a lookup-table that is accessed by the digital controller In addition, a novel method and control concept to ensure a Zero Voltage Switching (ZVS) of all semiconductor switches by determination of a zero voltage across the MOSFET switches with analog comparators is proposed that results in the lowest inductor RMS currents for ZVS operation at the same time Furthermore, at low output power an absolute efficiency gain of over 28% is achieved by partial operation of the six interleaved converter phases A detailed description on the control concept that determines the optimum number of activated phases for the current operating point of the converter is given and verified by experimental results The measurements prove the capability to instantaneously switch the number of active phases during operation without a overshoot or drop in the converter output voltage

Capacitor voltage balancing in modular multilevel converters

Abstract: A universal capacitor voltage control method for converters built from series connected modules is presented. It fully exploits both the circulating currents and the common-mode voltage without affecting the phase current control. The controllability of the capacitor voltages in various such converters is investigated. It is found that the nonzero branch currents and terminal voltages are necessary for capacitor voltage balancing. (5 pages)

Operating behavior and design of the half-cycle discontinuous-conduction-mode series-resonant-converter with small DC link capacitors

Abstract: The series-resonant-converter operated in half-cycle discontinuous-conduction-mode is a realization option for bidirectional isolated DC/DC converter modules, which are key components of modern solid state transformer (SST) concepts, among other applications. In SSTs the DC/DC converters are interfacing the input and output inverter stages via DC links. For reasons of costs as well as possible application-specific restrictions on weight and volume it is beneficial to use as small DC link capacitors as possible. However, when the resonant capacitance is higher than about 10% of the DC link capacitances, the deviation of the resonant current waveform from the sinusoidal shape becomes significant and the half-cycle duration is altered from values calculated using common approaches. Here, a comprehensive discussion of this case is given and design guidelines for the choice of the resonant capacitor are derived. The importance of the effect is illustrated by applying these to a complete design example.

Exact Analysis of Three-Phase Rectifiers With Constant Voltage Loads

Abstract: In this paper, an exact solution of a circuit model for a three-phase rectifier with constant-voltage load and AC-side reactance that operates in the continuous conduction mode is presented. Obtained results are compared to the results provided applying sinusoidal approximation, published previously. It is shown that the sinusoidal approximation provides acceptable results at low output voltages, with the accuracy being decreased for the output voltages approaching to the discontinuous conduction mode boundary. Computational complexity of the exact solution is about the same as for the solution obtained applying sinusoidal approximation.

Performance Evaluation of Series-Compensated IPT Systems for Transcutaneous Energy Transfer

Abstract: Today's implantable mechanical circulatory support devices, such as left ventricular assist devices, still rely on a percutaneous driveline, which is a frequent cause of severe infections and which reduces the quality of life for the patients. Inductive power transfer (IPT) is therefore a promising technology to replace the driveline and, hence, reducing the likelihood of an infection. This paper focuses on the series–series compensated IPT system and provides an in-depth comparison of two operating modes, i.e., the operation at resonance and the operation above resonance, and highlights the advantages and disadvantages with respect to the requirements set by the application at hand. In addition, the paper presents the design and the realization of a fully functional transcutaneous energy transfer (TET) implant hardware prototype, which includes the IPT front-end, the control circuit, the backup battery and its charging converter, as well as the communication electronics in a boxed volume of only 10.3 cl. The experimental verification shows that overall dc–dc efficiencies of up to 90% can be achieved for both operating modes when transmitting 25–30 W from the external battery to the implant backup battery, each having a nominal voltage of 14.8 V, using TET coils with 70 mm diameter and 10 mm coil separation distance.

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.