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EPE’15 ECCE Europe
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: [email protected], [email protected]
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.
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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.
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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
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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.
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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.
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