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.

Inductive power transfer system and method for operating an inductive power transfer system

Abstract: An inductive power transfer system, in particular a battery charging system comprises: • a transmitter coil (3) and a receiver coil (4); • a transmitter-side power converter (8) comprising a mains rectifier stage (11) powering a transmitter-side dc-bus (10) and controlling a transmitter-side dc-bus voltage U 1,dc ; • a transmitter-side inverter stage (9) with switching frequency f sw supplying the transmitter (3) coil with an alternating current; • a receiver-side power converter (12) comprising a receiver-side rectifier stage (13) rectifying a voltage induced in the receiver coil (4) and powering a receiver-side dc-bus (14) and a receiver-side charging converter (15) controlling a receiver-side dc-bus voltage U 2,dc ; • power controllers (19, 18) determining, from a power transfer efficiency of the power transfer, reference values U 1,dc *, U 2,dc * for the transmitter and receiver side dc-bus voltages; • an inverter stage switching controller (20) controlling the switching frequency f sw to reduce losses in the transmitter-side inverter stage (9).

Three Levels Are Not Enough: Scaling Laws for Multilevel Converters in AC/DC Applications

Abstract: Single-phase inverters and rectifiers in 230 V $_{\text{rms}}$ applications, with a dc-side voltage of 400 V, achieve ultrahigh efficiency with a simple two-level topology. These single-phase designs typically utilize a line-frequency unfolder stage, which has very low losses and essentially doubles the peak-to-peak voltage that can be generated on the ac side for a given dc-link voltage. For certain applications, however, such as higher power grid-connected photovoltaic inverters, electric vehicle chargers, and machine drives, three-phase converters are needed. Because of the three-phase characteristic of the system, unfolders cannot be similarly used, leading to a higher minimum dc-link voltage of the three-phase line-to-line voltage amplitude, which is typically set to 800 V for 230 V $_{\text{rms}}$ phase voltage systems. Previous demonstrations indicate that significantly more levels—and the associated higher cost and complexity—are required for ultrahigh-efficiency three-phase converters relative to their single-phase counterparts. In this article, we seek to determine the fundamental reason for the performance difference between three-phase 800 V dc-link converters and single-phase 400 V converters. First, we build a 2.2 kW dc/ac hardware demonstrator to confirm the necessity of higher complexity converters, showing a simultaneous reduction in efficiency and power density between a two-level 400 V benchmark (99.2% peak efficiency at 18.0 kW/L) and a three-level 800 V inverter phase-leg (98.8%, 9.1 kW/L). With the motivation confirmed, we derive general scaling laws for bridge-leg losses across the number of levels and dc-link voltage, finding the efficiency-optimal chip area and the minimum semiconductor losses. With commercially available Si or GaN power semiconductors, the scaling laws indicate that six or more levels would be required for an 800 V three-phase ac/dc converter to meet or exceed the bridge-leg efficiency of a two-level 400 V GaN benchmark for a fixed output filter. With a complete Pareto optimization, we find that at least seven levels are necessary to recover the efficiency of the two-level 400 V benchmark, and we validate this theory with a seven-level 800 V 2.2 kW hardware prototype with a power density of 15.8 kW/L and a peak efficiency of 99.03%. Finally, two practical solutions that make use of the benefits of unfolder bridges familiar in single-phase systems are identified for three-phase systems.

Active Magnetizing Current Splitting ZVS Modulation of a 7 kV/400 V DC Transformer

Abstract: The LLC series resonant converter (SRC) is one of the most popular galvanically isolated dc–dc converters since it provides zero voltage switching (ZVS), reduces rms currents, and tightly couples the input and output voltages, when it is operated at (or below) the resonance frequency, and, therefore, acts as a dc transformer (DCX) without requiring closed-loop voltage control. Hence, this topology is of particular importance for the dc–dc converter stage of high power medium voltage to low voltage solid-state transformers (SSTs). This paper first highlights the limitations of passive and synchronous rectification (e.g., oscillations, current distortion, load-dependent voltage transfer ratio) for bridges employing semiconductors with large output capacitances. Afterward, a magnetizing current splitting ZVS (MCS-ZVS) modulation scheme, which allows an active sharing of the magnetizing current between the primary side and secondary side metal oxide semiconductor field effect transistor ( mosfet )-based bridges, is analyzed. It is shown that the ZVS mechanism is acting equivalent to a controller, allowing for a robust open-loop operation of the converter. The proposed modulation scheme features a load-independent voltage transfer ratio, load-independent ZVS for both bridges, and quasi-sinusoidal currents. Finally, the phase shift modulation scheme is experimentally verified for the SiC mosfet -based dc–dc converter of a ${\text{25}}$ kW ac–dc SST, which operates at ${\text{48}}$ kHz between a ${\text{7}}$ kV and a ${\text{400}}$ V dc bus with an efficiency of ${\text{99.0}}{\%}$ .

A High Efficiency Indirect Matrix Converter Utilizing RB-IGBTs

Abstract: This paper presents the design and construction of an Indirect Matrix Converter (IMC) utilizing newly available IGBTs with reverse voltage blocking capability (RB-IGBTs). The design process involves the characterization of the RB-IGBTs by measuring the on-state parameters and the switching behavior in the rectifier stage of an IMC topology. Based on analytical equations the semiconductor losses are calculated and are used to perform the thermal design and simulation of the cooling system. Experimental measurements demonstrate that the designed IMC is capable of generating sinusoidal input and output currents and that the RB-IGBTs ensure low rectifier stage conduction losses and/or high IMC power conversion efficiency.

Comprehensive evaluation of GaN GIT in low- and high-frequency bridge leg applications

Abstract: In power electronics applications with power ratings around several kilowatts, wide band gap semiconductors are more and more replacing state-of-the-art Si MOSFET. SiC MOSFETs with blocking voltage rating up to 1200V and low-voltage GaN devices are already commercially available on the market since a couple of years. Now also 600V GaN devices are entering the market, which are a cost-effective solution in many 400V key applications in order to increase the system performance in terms of achievable efficiencies or power density. Besides the employed semiconductor devices also the design of the appropriate gate drive circuit is important. In this paper a simple and reliable gate drive circuit for driving GaN switches is presented. In addition, the proposed gate drive is used to evaluate the switching performance of a GaN Gate Injection Transistor (GIT) under soft- and hard-switching condition, which provides a basis for further optimization of totem-pole converter systems.

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.