IEEE Power Electronics Magazine - June 2022 - 26

by a varying current source in the primary side coil, a
predominantly magnetic time-varying field is generated
between the coils. Both the coil-to-coil distance and
the coil size are generally much smaller than the operating
frequency's wavelength. For most of the current
coil designs, Litz wire is used extensively instead of the
conventional solid AWG wire for its capability of reducing
high frequency eddy current loss. Usually, magnetic
cores like ferrite plates or bars are applied to better
channel the flux around the coil.
The typical structure of an IPT charger is shown in Figure
15. Compared to the wired charger of Figure 2, the main
difference is the pair of loosely-coupled ()k 1%
coils Lp and
Ls that replace the transformer of the wired charger. Due to
the low coupling resulting from the air gap between transmitter
and receiver, impedance matching networks (IMN)
are used to cancel out the large uncoupled series reactances
on both coils and limit the circulating reactive power. Each
IMN can be as simple as a single capacitor designed to resonate
with the coil inductance, either in series or in parallel.
In either event, the IMN gives the system a bandpass
characteristic and results in a dominantly single-frequency
magnetic field between the two coils. Differences in series
or parallel compensation, resonating with coil self-inductance
or uncoupled inductance, or the use of more complex
IMNs such as LCC of LCL networks will influence the charger's
dynamics, harmonic attenuation, and loading of the
inverter and rectifier over varying output power levels and
misalignment conditions [23], [24].
Different coil pad structures have been explored in the
literature, mainly including circular, rectangular, DD, and
DDQ pads. To have a fair and comprehensive comparison
Table 2. Industrial Wireless EV
Charger Specifications.
Company
WiTricity
Momentum
HEVO
Plugless Power
WiPowerOne
Frequency
[kHz]
145
NA
85
20
85
Airgap
[mm]
180
610
305
152
200
Power
rating
[kW]
3.3-11
50-200
1-10
3.3-7.2
27
Efficiency
90-93%
NA
>85%
89%
80%
on the system performance using different coil pad structures,
a multi-objective optimization is developed in [22]
considering the tradeoffs between conflicting design parameters
to evaluate and compare the performance of different
coil pad structures. The main conclusions are:
a) Circular pads have the highest coupling coefficient and
efficiencies for the same gravimetric power density
under the perfectly aligned condition.
b) Circular pads use the most ferrite and the least copper
for the same system performance.
c) Polarized pads including DD and DDQ give better misalignment
performance in the longitudinal direction.
d) Both circular and rectangular pads have lower stray
field densities compared to polarized pads.
In CPT, energy is delivered through coupled metal plates
instead of coupled inductors. Compared with IPT, CPT features
favorable characteristics including lightweight and
cost-effective design. It does not require expensive highfrequency
Litz wires or heavy magnetic cores, which aids
the high frequency design to reduce the weight of passive
components. However, CPT suffers from two major issues:
low coupling capacitance and high fringing field. Capacitive
coupling requires a relatively large coupling area to achieve
large coupling capacitance, imposing a design challenge on
high power density. For high power EV applications with a
large air gap, the transmission efficiency is low due to the
low coupling capacitance between the paralleled plates.
To compensate, MHz operating frequencies are required
to reduce the impedance, which directly imposes a design
challenge for the high power, high frequency converters. In
addition, relatively large-valued microhenry-range inductances
are required for resonant matching networks in the
MHz frequency range, leaving a design challenge for high
value, high quality factor inductors at high frequency. The
high fringing field at the edge of coupled metal plate pairs is
another concern for CPT because of electric field exposure
limits on the human body.
Whether using CPT or IPT, maximum power and coupler
design are constrained by electromagnetic compliance
and human exposure. EMC requirements, both regulatory
and system-level, are similar in WPT systems to traditional
wired chargers. The near-field electric and/or magnetic
fields used to transfer power are minimally radiative,
though the long coils and large conductors used may make
radiated compliance difficult when higher harmonics are
iac
ip
IMN
vline
PFC
Rectifier
VHV
+
-
HF
Inverter
Lp
is
k
Ls
IMN
HF
Rectifier
Vout
+
-
dc/dc
Battery
Charger
Vbatt
+
-
FIG 15 Typical structure of an IPT charger for EVs.
26 IEEE POWER ELECTRONICS MAGAZINE z June 2022
IEEE Power Electronics Magazine - June 2022

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