IEEE Power Electronics Magazine Compendium - March 2018 - 69

contactless charger poses a major design challenge. On
the one hand, a drastic drop in efficiency, the challenging
thermal management of the IPT coils, and an inferior
coil positioning tolerance restrict an increase of the power
density (Figure 7). On the other hand, small coils are
required to limit the necessary construction volume and
material cost of the vehicle-side components, as well as
to minimize the magnetic stray field of IPT systems. Elevating the transmission frequency can increase the efficiency of high-power-density coil designs, while at the
same time reducing the magnetic stray field. However,
the approach is limited by increasing losses in the power
electronics, and it has recently been rendered unattractive by the specification of 85 kHz as a common frequency in SAE J2954.
For a conductive EV charger, the converter switching frequency can be freely selected for design optimizations, e.g.,
for the miniaturization of the passive components. In addition, the converter topology and the high-frequency isolation
transformer can be optimized irrespective of the geometric
arrangement in the application itself. Hence, this technology
can fully benefit from technological progress, e.g., from the
steady improvement of wide-bandgap power semiconductors. This means that future performance improvements and
material cost reductions may be anticipated. In stark contrast, for IPT systems the attainable performance is governed
mainly by the geometric properties of the application and by
the physics that describe the transmission coil system. Therefore, the perspective for future improvements is limited for
contactless EV charging.
Nevertheless, contactless EV charging remains an attractive solution if the lower efficiency and the increased
on-board component volume are acceptable for the application at hand. IPT offers the unique advantage of a galvanically isolated power transfer to the vehicle without
the need for moving mechanical parts that are subject
to corrosion, wear, and fatigue. This is highly attractive
for the opportunity charging of public transport systems,
where the reliability and the maintenance requirements
of the charger are a considerable factor in the operating
costs. For passenger EVs, a contactless charger with a
lower power level, e.g., 3-7 kW, and therefore a smaller size
could be considered as a premium feature in addition to a
high-power conductive charger rated for 22 kW or more.
To reduce the cost and limit the necessary construction
volume on the vehicle, the on-board power electronics of
both charging systems could be optimally integrated into
a single unit. In this way, the contactless charger would
improve the convenience and safety for the routine use
case, while faster battery recharging would remain possible with the conductive charger.

About the Authors
Roman Bosshard (bosshard@lem.ee.ethz.ch) is with ABB
Switzerland, Turgi. Formerly, he was with the Power Electronic Systems Laboratory, ETH Zurich, Switzerland. He is a
Student Member of the IEEE.
Johann W. Kolar (kolar@lem.ee.ethz.ch) is a full professor and the head of the Power Electronic Systems Laboratory at ETH Zurich, Switzerland. He is a Fellow of the IEEE.

References
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Acknowledgment

Speed Drives, London, U.K., Oct. 1994, pp. 694-699.

We would like to thank ABB Switzerland Ltd. for their funding and for their support regarding many aspects of this
research project.

[14] M. Budhia, J. Boys, G. Covic, and C. Y. Huang, "Development of a singlesided flux magnetic coupler for electric vehicle IPT charging systems," IEEE
Trans. Ind. Electron., vol. 60, no. 1, pp. 318-328, Jan. 2013.

September 2016

z IEEE PowEr ElEctronIcs MagazInE

http://www.iea.org/media/images/Issue2_Evs.pdf http://www.iea.org/media/images/Issue2_Evs.pdf https://www.iea.org/publications/freepublications/publication/GlobalEV-Outlook_2013.pdf http://www.waveipt.com http://www.waveipt.com http://www.witricity.com/assets/WiT-3300_R2.3_DS.pdf

Table of Contents for the Digital Edition of IEEE Power Electronics Magazine Compendium - March 2018