Fundamentals and Multi-Objective Design of Inductive Power Transfer Systems

Fundamentals and Multi-Objective Design of Inductive Power Transfer Systems Prof. Dr. Johann W. Kolar, Roman Bosshard ETH Zurich / Power Electronic Systems Laboratory Web: www.pes.ee.ethz.ch E-Mail: bosshard@lem.ee.ethz.ch, kolar@lem.ee.ethz.ch Abstract The main aims of the seminar are to introduce the participants to the concepts and the multiobjective design challenges of Inductive Power Transfer (IPT) systems in a comprehensive, easy-to-follow fashion, to generate an understanding of the performance limits and to finally present experimental results of optimized industry-type demonstrator systems. First, different application areas and IPT solutions existing in industry and academia are presented. The main components of an Electric Vehicle (EV) battery charging IPT system with three-phase power factor corrected mains interface are explained. Furthermore, the design challenges of an IPT system are discussed in immediate comparison to the design of a conventional isolated DC/DC converter, in order to bridge the knowledge gap between both areas and to allow practicing engineers in industry and researchers in academia to seamlessly extend and complete their understanding of the subject area based on knowledge of general power converter design. Subsequently, a multi-objective design and optimization approach for IPT transmission coils along with the required calculation models for the high-frequency power losses in the main IPT system power components are presented in detail. The method takes into account the required system performance (air gap, battery charging power), as well as the boundary conditions imposed by geometrical size limitations of the application, the electrical interface, the restrictions of the stray field as given by standards, and the thermal limitations of the components. Experimental results obtained from a 96.5% efficient 5 kw IPT system are presented to demonstrate the validity of the used calculation methods and the optimization process. In the last part of the seminar, the IPT system is discussed in the context of the complete power conversion chain from three phase mains via the IPT link to the vehicle battery. The feasibility of IPT systems for EV battery charging is discussed critically and requirements for implementation by industry are presented along with advantageous application areas. The intended audience is researchers and development engineers interested in an entry-level introduction into the concepts of IPT, and a thorough revision of the main operating principles and design challenges, as well as a critical discussion of the performance limits of this technology and the general potential in industry applications. Fundamentals and Multi-Objective Design of IPT Systems 1

Content Details Tutorial Part 1-A Introduction: Features & Limitations, Potential Applications & Industry Solutions The seminar starts with an overview of potential applications of Inductive Power Transfer (IPT) in power electronics with the main focus on high-power Electric Vehicle (EV) battery charging systems that have been presented by industry. On these examples, the characteristic features, advantages, and basic limitations of IPT technology are identified and discussed. A comparison of different existing solutions, such as stationary battery charging and in-motion power transmission systems, will be presented in order to identify their potential in future EV battery charging infrastructure. In order to establish the foundation for the concepts and optimization methods presented later in the seminar, in a next step the main elements of the power conversion chain of contactless EV battery charging systems, reaching from the three-phase mains interface to the battery charging controller, are discussed. Tutorial Part 1-B Fundamentals: Isolated DC/DC Inductive Power Transfer In order to develop an understanding of the fundamental concepts of IPT, a new didactical approach is taken in order to lower the entrance hurdle for practicing engineers that are new to the field of IPT. First, a conventional isolated DC/DC converter for EV battery charging is analyzed with a special focus on the physics of the isolation transformer and on the typically used equivalent circuit models. It is shown how an air gap in the transformer core deteriorates the converter output characteristics, due to the increased stray inductance and the reduced magnetic coupling of the two transformer windings. As a solution for an improved power transfer capability of the DC/DC converter, a series resonant compensation of the transformer stray inductance is introduced and analyzed in detail, including the different operating modes of the resulting converter. From the discussion of the isolated DC/DC converter, the main design aspects of the series-series compensated IPT system become immediately clear and an intuitive understanding of the working principles and the key design aspects of an IPT design, e.g. for ensuring maximum power transfer efficiency, is provided. In the course of the discussion, also the terminology used in recent literature ( impedance matching, magnetic resonance, pole-splitting, bifurcation, etc.) will be clarified to help newcomers to the field gain deeper insight into the existing concepts. In the last part of this section, design requirements for an efficient operation of the power electronics with minimum switching losses (Zero-Voltage Switching) and an efficiency-optimal control strategy of the overall IPT system are discussed. After the principal aspects of the operation and the main design considerations are clarified, the section concludes by outlining how the resulting IPT converter system can be simultaneously optimized with respect to multiple performance criteria, such as efficiency and power density. The detailed component models, which are crucial for the proposed system optimization are the topic of the following sections. Fundamentals and Multi-Objective Design of IPT Systems 2

Tutorial Part 2 System Components and Design Considerations Based on the derived models and converter structures, the main components of the IPT system, namely the transmission coils, the resonant capacitors, and the power electronics, are discussed in more detail. On the example of two prototype systems of different power level (5 kw and 50 kw), aspects of the magnetic modeling of the transmission coils are shown. The estimation of the power loss in the typically employed copper litz wire and ferrite core elements due to the high-frequency transmission coil currents is one of the key challenges in the design and will, therefore, be discussed in depth. Furthermore, computational methods for the calculation of the inductor stray field in the vicinity of the coils are presented including experimental results obtained from a specifically designed low-cost, high-bandwidth field probe. Finally, shielding methods of other electronic components by means of either magnetic or conductive shielding materials will be discussed. For the estimation of the high-frequency power losses in the resonant capacitors, classical calculation models are refreshed and guidelines for the component selection are given. Additionally, options for the physical arrangement of the transmission coil and the resonant capacitor are discussed including the insulation requirements, current carrying capability of the connecting power lines, and emitted EMI noise. For the power electronics, different concepts and topology options for the complete power conversion chain from the mains to the vehicle battery are discussed. A short overview of possible single-phase and three-phase mains interfaces as well as of high efficiency, high power density dc-dcconverter concepts is given to clarify the main features of their control and the performance that can be achieved when looking at the complete conversion chain from the mains to the EV battery. For the resonant converter that is used in the IPT link, possible converter design options are discussed and the requirements for low semiconductor losses due to zero voltage switching conditions are shown. Prototype systems that employ the latest Silicon-Carbide (SiC) technology are presented along with insight into experimentally verified modulation schemes and control concepts for the IPT system. Tutorial Part 3 Multi-Objective Optimization of IPT Systems By taking into account the required air gap distance of the IPT system and the electrical interface (dclink, battery voltage levels, and required charging power), as well as limit values for the stray field of the IPT coils as given by the relevant standards and the thermal limitations of the employed components as boundary conditions, a multi-objective optimization of the IPT system can be conducted with the derived calculation models. An example system with a rated power of 5 kw is used to demonstrate the approach. Using a Finite Element Method (FEM), the efficiency/power density Pareto front of the circular spiral coil design is derived by means of a grid search. From the results, the trade-off between coil size and achievable transmission efficiency is immediately clear. It is shown that even with coils that are small compared to the air gap distance, a high efficiency can be achieved if the transmission frequency can be increased. Furthermore, it is explained how this effect can be exploited to increase the tolerance of the system with respect to a misalignment of the IPT coils. Limiting factors to this scaling law, such as Fundamentals and Multi-Objective Design of IPT Systems 3

component limitations and parasitic effects, are discussed in detail. Moreover, the impact of the transmission frequency as well as the coil size on the stray field is studied and design recommendations for high-power high-frequency IPT systems are given. Next, experimental results from a prototype system which was designed based on the presented optimization approach and confirmed all employed models will be presented. This includes a verification of the calculated electrical equivalent model of the transmission coil, measurements of the magnetic flux density and comparison to FEM results, measurements of the system efficiency, system performance with misaligned coils, as well as measurements of component temperatures and effectiveness of forcedair cooling. The results demonstrate a dc-to-dc efficiency of 96.5% for the designed 5 kw / 52 mm air gap / 210 mm coil diameter demonstrator and a good accuracy of all calculation models as well as the correctness of the used multi-objective optimization approach. Tutorial Part 4 Discussion of the Results and General Conclusions In the last part of the seminar, the key aspects of the presented material are highlighted in a comprehensive summary. The obtained results are compared to the state-of-the art in industry and academia and further research topics and development challenges in the field are identified. Finally, the IPT system is put again into the context of the complete EV battery charging system including mains and battery interfaces. The feasibility of IPT systems on an industrial scale is discussed critically and the challenges specific to the IPT systems and requirements for implementation by industry are presented along with advantageous application areas. Fundamentals and Multi-Objective Design of IPT Systems 4

About the Presenters Johann W. Kolar (F 10) received his M.Sc. and Ph.D. degree (summa cum laude / promotio sub auspiciis praesidentis rei publicae) from the University of Technology Vienna, Austria. Since 1984 he has been working as an independent international consultant in close collaboration with the University of Technology Vienna, in the fields of power electronics, industrial electronics and high performance drives. He has proposed numerous novel converter topologies and modulation/control concepts, e.g., the VIENNA Rectifier, the SWISS Rectifier, the Delta-Switch Rectifier, the isolated Y-Matrix AC/DC Converter and the threephase AC-AC Sparse Matrix Converter. Dr. Kolar has published over 450 scientific papers at main international conferences, over 180 papers in international journals, and 2 book chapters. Furthermore, he has filed more than 110 patents. He was appointed Assoc. Professor and Head of the Power Electronic Systems Laboratory at the Swiss Federal Institute of Technology (ETH) Zurich on Feb. 1, 2001, and was promoted to the rank of Full Prof. in 2004. Since 2001 he has supervised over 60 Ph.D. students and PostDocs. The focus of his current research is on AC-AC and AC-DC converter topologies with low effects on the mains, e.g. for data centers, More-Electric-Aircraft and distributed renewable energy systems, and on Solid-State Transformers for Smart Microgrid Systems. Further main research areas are the realization of ultra-compact and ultra-efficient converter modules employing latest power semiconductor technology (SiC and GaN), micro power electronics and/or Power Supplies on Chip, multi-domain/scale modeling/simulation and multi-objective optimization, physical model-based lifetime prediction, pulsed power, and ultra-high speed and bearingless motors. He has been appointed an IEEE Distinguished Lecturer by the IEEE Power Electronics Society in 2011. He received 10 IEEE Transactions Prize Paper Awards, 8 IEEE Conference Prize Paper Awards, the PCIM Europe Conference Prize Paper Award 2013 and the SEMIKRON Innovation Award 2014. Furthermore, he received the ETH Zurich Golden Owl Award 2011 for Excellence in Teaching and an Erskine Fellowship from the University of Canterbury, New Zealand, in 2003. He initiated and/or is the founder/co-founder of 4 spin-off companies targeting ultra-high speed drives, multidomain/level simulation, ultra-compact/efficient converter systems and pulsed power/electronic energy processing. In 2006, the European Power Supplies Manufacturers Association (EPSMA) awarded the Power Electronics Systems Laboratory of ETH Zurich as the leading academic research institution in Power Electronics in Europe. Dr. Kolar is a Fellow of the IEEE and a Member of the IEEJ and of International Steering Committees and Technical Program Committees of numerous international conferences in the field (e.g. Director of the Power Quality Branch of the International Conference on Power Conversion and Intelligent Motion). He is the founding Chairman of the IEEE PELS Austria and Switzerland Chapter and Chairman of the Education Chapter of the EPE Association. From 1997 through 2000 he has been serving as an Associate Editor of the IEEE Transactions on Industrial Electronics and from 2001 through 2013 as an Associate Editor of the IEEE Transactions on Power Electronics. Since 2002 he also is an Associate Editor of the Journal of Power Electronics of the Korean Institute of Power Electronics and a member of the Editorial Advisory Board of the IEEJ Transactions on Electrical and Electronic Engineering. Roman Bosshard (S 10) received the M.Sc. degree from the Swiss Federal Institute of Technology (ETH) Zurich, Switzerland, in 2011. During his studies, he focused on power electronics, electrical drive systems, and control of mechatronic systems. As part of his M.Sc. degree, he participated in a development project at ABB Switzerland as an intern, working on a motor controller for traction converters in urban transportation applications. In his Master Thesis, he developed a sensorless current and speed controller for a ultrahighspeed electrical drive system with CELEROTON, an ETH Spin-off founded by former Ph.D. students of the Power Electronic Systems Laboratory at ETH Zurich. In 2011, he joined the Power Electronic Systems Laboratory at the Swiss Federal Institute of Technology (ETH) Zurich, where he is currently pursuing the Ph.D. degree. His main research area is inductive power transfer systems for electric vehicle battery charging, where he published five papers at international IEEE conferences and one paper in the IEEE Journal of Emerging and Selected Topics in Power Electronics. Fundamentals and Multi-Objective Design of IPT Systems 5