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.

Volume minimization of the main DM/CM EMI filter stage of a bidirectional three-phase three-level PWM rectifier system

Abstract: Three-phase voltage source AC-DC converters disclose a differential mode (DM) and a considerable common mode (CM) bridge-leg output voltage at the switching frequency, which need to be filtered by an EMI input filter to comply with conducted EMI standards (e.g. CISPR 11) at the mains terminal. Additionally, the EMI filter (which in this paper includes the boost inductor) should usually limit the maximum peak-to-peak bridge-leg output current ripple, to which a DM and CM component contribute, to 20%-40% of the nominal peak current. Typically, the DM current ripple is defined by the boost inductance Ldm, 1 and the CM current ripple by the CM inductance Lcm, 1. Therefore, a degree of freedom in the filter design exists: different combinations of Ldm, 1 and Lcm, 1 lead to the same peak-to-peak bridge-leg output current ripple. Accordingly, this paper investigates the optimal ratio kL, opt = Lcm, 1/Ldm, 1, which minimizes the volume of the main LC stage of the DM/CM EMI input filter. To achieve a compact filter, it is demonstrated for a 10 kW PWM rectifier system that for kL ≥ 3 the boxed filter volume is almost constant. Additionally, kL, opt = 14 minimizes the stray field of the CM choke Lcm, 1 and leads to a minimal boxed filter volume of 0.43 ltr. for a filter cooled by natural convection.

Accurate Measurement of the Switching Losses of Ultra High Switching Speed CoolMOS Power Transistor / SiC Diode Combination Employed in Unity Power Factor PWM Rectifier Systems

Abstract: The topic of this paper is a detailed analysis of the switching behavior of a CoolMOS / SiC diode combination applied in a 3.5kW single-phase unity power PWM rectifier system. The focus is on the development of guidelines to accurate measure the switching behavior and the turn-on and turn-off power losses at high switching speeds. Further subjects are the improved design of the power circuit, and the comparison of two high bandwidth current measurement methods applied in the power circuit.

Accurate transient calorimetric measurement of soft-switching losses of 10kV SiC MOSFETs

Abstract: The characterization of soft-switching losses of modern high-voltage SiC MOSFETs is a difficult but necessary task in order to accurately model converter systems such as medium-voltage-connected Solid-State Transformers (SSTs), where soft-switching techniques are employed for an increased efficiency. Usually, switching losses in general are measured with the well-known double pulse method. However, in case of soft-switching loss measurements, this method is very sensitive to several effects such as probe skew and limited measurement accuracy, among others, and thus unsuitable for the characterization of fast switching high-voltage MOSFETs. This paper presents an accurate and reliable calorimetric method for the measurement of soft-switching losses using the example of 10 kV SiC MOSFETs. Finally, measured soft-switching loss curves of these 10 kV SiC MOSFETs are presented for different DC-link voltages, currents and gate resistors.

Towards virtual prototyping and comprehensive multi-objective optimisation in power electronics

Abstract: The constant demand for higher efficiency and power density and lower costs of power electronics systems could be met by application of new topologies and/or modulation schemes and future wide-band gap semiconductor technology. However, the performance of state-of-the-art systems also could be improved significantly by multidomain/objective optimisation, i.e. by assigning overall optimal values to the design variables in the course of the design process. In order to perform such an optimisation first a comprehensive mathematical model of the main converter circuit has to be established, including thermal component models and the measures for DM and CM EMI filtering. Based on this model, an optimisation for multiple objectives, as e.g. efficiency and power density, can be performed. The optimisation makes best use of all degrees of freedom of a design and also allows to determine the sensitivity of the system performance on base technologies like Figures of Merit of the power semiconductors or properties of the magnetic core materials. Furthermore, different topologies can be easily compared and inherent performance limits can be identified. In the paper, analytical approaches for designing the main functional elements of a power electronics converter are described and arranged to a linear design process in a first step. Moreover, the linking of the component models, i.e. of the electric, magnetic, thermal and thermo-mechanic design domains and an overall optimisation of the respective design variables based on the linked models is discussed. Finally, the coupling of the different domains and for example the utilisation of electrical equivalent circuits for implementing these couplings are investigated.

"Magnetic Ear"-based balancing of magnetic flux in high power medium frequency dual active bridge converter transformer cores

Abstract: Semiconductor switches posses non-ideal behavior which, in case of isolated DC-DC converters, can generate DC voltage components in the voltage applied to the transformer. This DC voltage component is translated into a DC flux density component in the transformer core, increasing the risk of driving the core into saturation. In this paper, a novel non-invasive flux density measurement principle, called the "Magnetic Ear", based on sharing of magnetic path between the main and an auxiliary core is proposed. The active compensation of the transformer DC magnetization level using this transducer is experimentally verified. Additionally, a classification of the previously reported magnetic flux measurement and balancing concepts is performed.

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.