IEEE Power Electronics Magazine - September 2016 - 28

For the passive materials of the IPT
coils, a large technological step is currently not in sight. Litz wire, ferrite,
99.5
110
99
and film capacitors are components
100
with a long-established quality and
98.5
90
little room for improvement. Some
80
98
advantages could result from novel
97.5
70
synthetic materials for the coil hous97
60
ing. Enclosure of the IPT coil in therThermal Limit
ICNIRP 2010
96.5
50
0.2 W/cm2
mally conductive polymer or epoxy
ICNIRP 1998
96
40
0.5 1 1.5 2 2.5 3 3.5 4 4.5
resin could simplify the thermal man0 20 40 60 80 100
2
agement. However, a disruptive techArea-Related Power Density α (kW/dm )
Stray Field (µT)
nology improvement seems unlikely at
(a)
(b)
the present time.
50 kHz
100 kHz
150 kHz
200 kHz
The design parameter with the
50 kHz
100 kHz
350 kHz
highest potential for crossing the barriers is the transmission frequency.
FIG 6 (a) The h-a-Pareto fronts and (b) the tradeoff between coil losses and the stray
Figure 6 shows the h-a-Pareto fronts
field of a scaled 5-kW prototype IPT system, calculated for optimum loading conditions
and
the design tradeoff for the stray
of the resonant circuit and different transmission frequencies between 50 and 350 kHz.
field
for a scaled 5-kW laboratory pro(Figure reproduced from [35].) ICNIRP: International Commission on Non-Ionizing
Radiation Protection.
totype designed in [35]. At an increased
transmission frequency, the efficiency
of
the
power
transfer
is higher for the same power density,
components. For a contactless charger, this is possible only
and reduced magnetic stray fields are possible. Additional
to a limited extent. A reduction in coil size is always tied
research is still needed to quantify the attainable increase
to the tradeoff in terms of transmission efficiency. The
in efficiency and power density if the power converters
increasing power losses due to the low magnetic coupling
are also included. However, decreasing advantages are
and the difficulty of cooling the IPT coils without eddyexpected at a sufficiently high frequency as a result of the
current-prone metal heat sinks restrict the use of small
frequency-dependent losses in the power electronics and
coils at a high power level.
the core materials. In addition, the power electronics design
becomes more and more challenging due to the impact of
Future Perspectives for IPT
the parasitic elements of the IPT coils and the converter. In
The previously discussed design tradeoffs represent techaddition, the upcoming interoperability standard SAE J2954
nology barriers that cannot be crossed using the considered
sets the transmission frequency as 85 kHz, which effectively
power electronics topology, IPT coil geometry, and materials.
blocks development in this direction.
They can be extended only by technological progress (digital
Further technological limitations result from the eleccontrol, wide-bandgap power semiconductors, etc.) or by
tromagnetic fields that are inherently required by the
novel concepts with inherently superior performance [5].
employed physical principle. When increasing the power
level of a contactless EV charger, the stray fields become a
limiting factor, and extensive shielding is necessary. Espeη (%)
Geometric
cially for the high transmission frequencies achieved with
Pareto
Properties
novel fast-switching wide-bandgap power semiconducFront
Material Cost
tors, electromagnetic compliance filtering for IPT systems
is also needed specifically to limit the electric stray field.
Frequency
Future developments in this field, therefore, could help
Positioning
Stray Field
overcome technological barriers.
Tolerance
Shielding
Limit
Considering these barriers, the outlook for significant
Cooling
Construction
improvements
is limited for IPT systems, as the size, weight,
Volume
and material cost for the receiver coil are restricted by
Thermal
design tradeoffs that foreseeable technological advances
Limit
Feasible IPT
α
are unlikely to change.
2
EV Chargers
120

Efficiency η (%)

Power Losses (W)

100

Energy Cost

(kW/dm )

FIG 7 A schematic representation of the feasible performance
space of contactless EV chargers based on IPT, including the
effects of the technological constraints and potential ways for
future improvement.

IEEE PowEr ElEctronIcs MagazInE

z September 2016

Conclusions
The analysis presented here of the design tradeoffs of
IPT systems compared to existing state-of-the-art EV
chargers highlights that realization of a compact

Table of Contents for the Digital Edition of IEEE Power Electronics Magazine - September 2016