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.

Medium frequency transformers for solid-state-transformer applications — Design and experimental verification

Abstract: Solid-state-transformer technology pushes the specifications of electric transformers in the high-power medium-frequency range. This combination results in larger-sized transformers operating at higher frequencies whereby parasitic phenomenon should be carefully accounted for. This paper presents an analytical description of the limiting factors and the connection between frequency/power rating and the available core and copper conductor technology on the power density and efficiency of medium-frequency transformers. Furthermore, two designed transformers for 166kW/20kHz based on two different core materials and cooling systems are presented. Extensive copper, core and cooling system loss measurements on one of these transformers are discussed in order to analyze the transformers' behavior from a practical point of view.

Passive and active hybrid integrated EMI filters

Abstract: Two new planar integrated EMI filter structures which reduce the filter volume and which are based on standard PCB process technology are presented in this paper. First, a passive integrated EMI filter is presented, which results in a volume reduction of 25% compared to the discrete solution. However, this filter requires a planar ferrite core for the CM inductor. In order to eliminate the ferrite core and reduce the filter volume further (-40% vs. discrete filter) a passive integrated structure is combined with an active EMI filtering circuit. The transfer function, the volume and the losses of the discrete and the two integrated filters, which are designed for a 600W PFC converter, are compared.

Full-order averaging modelling of zero-voltage-switching phase-shift bidirectional DC-DC converters

Abstract: Full-order small-signal modelling and dynamic analysis of zero-voltage-switching (ZVS) phase-shift bidirectional DC-DC converters is studied. A general modelling method is proposed to develop the discrete-time average model. This full-order model takes into account the leakage inductance current and the resonant transition intervals in order to realise ZVS. Both the leakage inductance current and the resonant transition intervals are the key to accurately predict the dynamic behaviour of the converter. A control-to-output-voltage transfer function is derived for the dual active bridge DC-DC converter, which is taken as an example to illustrate the modelling procedure. Experimental results confirm that the new model correctly predicts the small-signal frequency response up to one-third of the switching frequency and is more accurate than the previously presented models.

Performance trends and limitations of power electronic systems

Abstract: In 2003 the Roadmapping Initiative of the European Center of Power Electronics (ECPE) has been started based on a future vision of society in 2020 in order to define the future role of power electronics, and to identify technological barriers and prepare new technologies well in time. In the framework of this initiative a new mathematically supported approach for the roadmapping in power electronics has been developed. As described in this paper the procedure relies on a comprehensive mathematical modeling and subsequent multi-objective optimization of a converter system. The relationship between the technological base and the performance of the system then exists as a mathematical representation, whose optimization assures the best possible exploitation of the available degrees of freedom and technologies. Thus an objective Technology Node of a system is obtained, whereby physical limits are implicitly taken into account. Furthermore, the sensitivity of the system performance with regard to the technological base can be calculated directly and the internal interdependence of Performance Indices directly studied. Accordingly, the improvement in performance achievable by improvements in the technology base can be tested and assessed in advance. Moreover, different system concepts, i.e. circuit topologies, control procedures, etc. can be evaluated and directly compared with regard to achievable efficiency, power density and costs in the form of the associated Pareto Front which defines the boundary of the Feasible Performance Space. If the target performance lies outside the Pareto Envelope of known system concepts and state-of-the-art technologies, a new technology must be employed. The necessity of a technological leap, i.e. the introduction of a Disruptive Technology can thus be recognized at an early stage. This offers an excellent basis for effective roadmapping for various main application areas in power electronics.

Active Gate Control for Current Balancing of Parallel-Connected IGBT Modules in Solid-State Modulators

Abstract: In modern pulsed power systems, often, fast solid-state switches like MOSFETs and insulated gate bipolar transistor (IGBT) modules are used to generate short high power pulses. In order to increase the pulsed power, solid-state switches have to be connected in series or in parallel. Depending on the interconnection of the switches, parameter variations in the switches and in the system can lead to an unbalanced voltage or current. Therefore, the switches are generally derated, which results in an increased number of required devices, cost, and volume. With an active gate control, derating and preselection of the switching devices can be avoided. In this paper, an active gate control of paralleled IGBT modules, which has been developed for converters with inductive load, is explained in detail and adapted to a solid-state modulator. This paper focuses on achieving a low-inductance IGBT current measurement, the control unit implementation with a field-programmable gate array and a digital signal processor, as well as the balancing of the pulse currents.

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.