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.

Comparative evaluation of three-phase high power factor AC-DC converter concepts for application in future more electric aircrafts

Abstract: A passive 12-pulse rectifier system, a two-level, and a three-level active three-phase PWM rectifier system are analyzed for supplying the DC voltage link of a 5 kW variable speed hydraulic pump drive of an electro-hydrostatic actuator to be employed in future more electric aircrafts. Weight, volume and efficiency of the concepts are compared for an input phase voltage range of 98 V to 132 V and an input frequency range of 400 Hz to 800 Hz. The 12-pulse system shows advantages concerning volume, efficiency and complexity but is characterized by a high system weight. Accordingly, the three-level PWM rectifier is identified as most advantageous solution. Finally, a novel extension of the 12-pulse rectifier system by turn-off power semiconductors is proposed which allows a control of the output voltage and therefore eliminates the dependency on the mains and load condition which constitutes a main drawback of the passive concept.

Design procedure for compact pulse transformers with rectangular pulse shape and fast rise times

Abstract: Pulse modulators based on solid state technology and for pulses in the μs-range often utilize a pulse transformer, since it could offer an inherent current balancing for parallel connected power semiconductors and the turns ratio of the pulse transformer allows to adapt the modulator design to the available switch technology. The applications like radar systems, linear accelerators or klystron/magnetron modulators usually require a nearly rectangular pulse shape with a fast rise time and a as small as possible overshoot. In reality however, parasitic elements of the pulse transformer as leakage inductance and capacitances limit the achievable rise time and define the resulting overshoot. Therefore, in modulators based on pulse transformers, the design of the pulse transformer is crucial. In this paper, a step by step design procedure of a pulse transformer for rectangular pulse shape with fast rise time is presented. Different transformer topologies are compared with respect of the parasitic elements, which are then calculated analytically depending on the mechanical dimensions of the transformer. Additionally, the influence of the limited switching speed of semiconductors and the nonlinear impedance characteristic of a klystron is analyzed.

Full-ZVS modulation for all-SiC ISOP-type isolated front end (IFE) solid-state transformer

Abstract: Thanks to their comparatively low system complexity, SSTs based on an isolated front end (IFE) approach are suitable for space and weight-constrained medium voltage (MV) AC to low voltage (LV) DC power supply applications, e.g., in future traction, naval, subsea or aerospace systems. The IFE approach connects series resonant isolation stages operating in the half-cycle discontinuous-conduction-mode (HC-DCM) directly to the MV AC grid in an input-series, output-parallel (ISOP) configuration, but the entire control, i.e., the shaping of the grid current for unity power factor and output voltage regulation, is carried out by a second, non-isolated conversion stage on the LV side. However, since the isolation stages do not operate with a DC but with an AC or |AC| input voltage, the transformer magnetizing current available for ZVS as well as the voltage to be switched vary over the grid period. Taking into account also component tolerances among the cascaded converter cells, this paper provides an in-depth analysis of the ZVS behavior under these conditions, and of the associated losses and EMI considerations, presenting a loss-optimal choice of the magnetizing inductance value and of the dead time (interlock time) of the isolation stages' bridge legs. A time-dependent variation of the latter to achieve ZVS over the entire grid period without an increase of the isolation stage losses is proposed. The considerations are verified at the example of the Swiss SST (S3T), an all-SiC 25 kW, 6.6 kV MVAC to 400 V LVDC converter system, using a detailed simulation model, including non-linear MOSFET capacitances.

Distortion analysis of low-THD/high-bandwidth GaN/SiC class-D amplifier power stages

Abstract: Power amplifiers providing an output current of high precision, high bandwidth and low distortion are required in different fields like magnetic resonance imaging or motion control systems for semiconductor production processes. This paper analyzes different power stage implementations of switched-mode (Class-D) amplifiers intended for such applications. They are preferred to linear or hybrid solutions as they feature a high output power with a good efficiency at low cost and low complexity. The advent of wide bandgap (WBG) semiconductors based on gallium nitride (GaN) or silicon carbide (SiC) enables Class-D amplifiers with unprecedented performance regarding distortion and bandwidth. Comprehensive circuit simulations incorporating detailed switch- and thermal models of all relevant components are used to compare different implementations of Class-D switching stages based on GaN and SiC semiconductors with respect to output voltage distortion. The systems' sensitivities to various parameters such as power circuit topology or output current amplitude are derived as a performance measure. It is shown that the thermal behavior of the power devices has significant influence on the output distortion and that the dual buck topology, which requires no dead time in between the turn-off and the turn-on of the bridge leg transistors, is less sensitive to parameter variations and has a better distortion performance than the conventional half bridge topology.

Novel AC coupled gate driver for ultra fast switching of normally-off SiC JFETs

Abstract: Over the last years, more and more SiC power semiconductor switches became available in low production volumes in order to prove their superior behavior with respect to fast switching speed, low on-resistance per chip area, high voltage range and high temperature operation. A very promising device among those introduced in numerous publications over the last years is the 1200 V 30 A JFET introduced by SemiSouth. It features a very low on-resistance (2.8m Ω cm2), switching operation within 20 ns, a normally-off characteristic and has already been commercialized in contrast to many other SiC switches. To fully exploit the potential of the SiC normally-off JFET, conventional gate drivers for unipolar devices must be adapted to this device due to its special requirements: During on-state the gate voltage must not exceed 3 V, while a current of around 300 mA must be fed into the gate, during switching operation the transient gate voltage should be around ±15 V and the low threshold voltage of 0.7 V requires a high noise immunity which is a severe challenge as the device has a comparably low gate-source but high gate-drain capacitance. To meet these requirements, several concepts have been published recently. They deal with the challenges mentioned, but they also note certain limitations (e. g. frequency and duty cycle limitations or need for additional cooling). In this paper, a novel gate driver consisting only of one standard gate driver IC, resistors, capacitors and diodes is designed and experimentally validated. It supplies enough gate current for minimum on-resistance, allows fast switching operation, features a high noise immunity and can be used for any duty cycle and usual switching frequencies without significant self-heating.

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.