Special Issue

Technologies supporting the achievement of Furukawa Electric Group
Medium-term Management Plan 2022 - 2025
Contribution to the achievement of SDGs
by Furukawa Electric GroupThe Development of MHz Band Wireless Power Transfer Aiming
for the Dynamic Wireless Power Transfer
Naoki Tsurutani *, Hiroyuki Yamazaki *, Hirokazu Takewaki *The wireless power transfer technology is now attracting attention as the technology which contributes to improvement of a cruising range of electric vehicles
aiming at the carbon-neutral society. We have been developing the capacitive power transfer
method, which was one of two categories of methods, classified in general, of the wirelesspower transfer technology required to realize the dynamic wireless power transfer. This time, wewill report on the coupler which is a necessary component for the wireless power transfer technology that we have realized a high-power transfer of 10 kW class by developing measures tomitigate the heat generation and other problems.

ABSTRACTINTRODUCTION

It has been required to reduce the greenhouse gas emissions to prevent the global warming. In the automotive
industry as well, the reduction of emission is promoted byspreading zero-emission vehicles, mainly electric vehicles(hereinafter shortened to EVs). Aiming at promoting thespread of EVs, we have many subjects to improve suchas the reduction of the required battery capacity, theshortening of charging time and the reduction of time andeffort for charging.
The dynamic wireless power transfer is expected tosolve these problems. The dynamic wireless power transfer is the technology that transfers electric power to an
EV wirelessly when either driving or parking and either
charge a power storage such as a battery and a capacitor
or drive a motor directly as well. Applying this technology,
electric power can be supplied without connecting any
cables and furthermore the additional reduction ofrequired battery capacity and the further improvement ofa cruising range can be realized 1).
Research and development of the dynamic wirelesspower transfer has been driven based on applying kHz or
MHz frequency band. In comparison with the kHz band,
the MHz band can contribute to especially the downsizing and weight-reduction of the system. On the other
hand, it is more difficult for the MHz band to realize thehigh-power transfer exceeding the kW range, because it
is difficult to improve the efficiency of the power transfer
because of losses in conductors generated by factors
such as the skin effect and the proximity effect as examples of main losses.
Advanced Automotive Products Dept.		 Automotive Products & Electronics Laboratories, R&D Div.
*In this paper, we will report that we have reviewed andimproved the structure of the coupler in our development
looking ahead the dynamic wireless power transfer basedon the capacitive power transfer method in the MHz band
to improve the power efficiency and realized the highpower transfer, and that we have carried out a high-power transfer as high as 10 kW class.

2.

THE TECHNOLOGY OF THE DYNAMIC
WIRELESS POWER TRANSFER

2.1 The Dynamic Wireless Power Transfer System
An example of the system expected for the dynamic wireless power transfer is shown in Figure 1 below;

Charging circuit
Matching andrectiﬁcation circuit
Receiving coupler

Power storage

1.

Transmitting coupler
High frequency
power source

Figure 1

Matchingcircuit

Dynamic wireless power transfer system.

The impedance matching is carried out through amatching circuit located before a transmitting coupler tosupply the power to a power storage at a high efficiency,
which is input from a high frequency power source. Then,the wireless power transfer is carried out through a coupler. Here, a coupler means a part for transmitting andreceiving power in the wireless power transfer system.
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The Development of MHz Band Wireless Power Transfer Aiming for the Dynamic Wireless Power Transfer

The high-frequency power is converted into DC power
through a matching and rectification circuit located after apower receiving coupler, and the power is charged to apower storage through a charging circuit.
2.2 Required Power for the Dynamic WirelessPower Transfer System
A larger power is required in the wireless power transfer
system when charging the power to a moving vehicle
which is consuming energy as well in comparison to a
current system in which the power is charged to a parked
vehicle from a charging station. Here, the power required
for the wireless power transfer is shown in Figure 2.

▼Normalcharge
3 kW
Figure 2▼Normal charge
double-speed
6.6 kW▼Dynamic wirelesspower transfer(Power)20 kW～

Power required for the dynamic wireless powertransfer.

As shown in Figure 2, we have to transfer the power
wirelessly more than approximately 22 kW in the dynamic
wireless power transfer system 2). Accordingly, the coupler
is required essentially to transfer a high power, as well.

3.

THE TECHNOLOGY OF THE WIRELESSPOWER TRANSFER

3.1

The Inductive Power Transfer Method and theCapacitive Power Transfer Method
We have two categories of wireless power transfer system, classified in general, the inductive power transfer
Table 1

method and the capacitive power transfer method. In theinductive power transfer method, the power is transferred
based on the principle of electromagnetic induction of acoil. Recently, this method is ahead of the capacitive
power transfer method in the standardization, as Qi
Standard has been examined, which was already applied
to mobile phones and further, SAE in U.S.A. examined tostandardize the output class, 3.7 kW, 7.7 kW and 11 kW 3).
Furthermore, it has the advantage of transferring highpower over a long distance utilizing a resonance phenomenon. On the other hand, it has a demerit to heat up
metal bodies located in the surroundings caused bymainly its magnetic field.
In the capacitive power transfer method, the power istransferred from a transmitting electrode to an opposing
receiving electrode through the electric field generated
between the electrodes when inputting high-frequency
power to the transmitting electrode based on the same
theory as the capacitor. It has the advantage that nometal bodies located in the surroundings can be easily
heated up because it works based on mainly the electricfield. Furthermore, it has also the merit of the capacitive
power transfer method to be manufactured flexibly in itsshape according to the structure of the electrode and tolerate a position misalignment of the coupler. On the other
hand, generally speaking, it is said that it is not suited forthe high-power transfer from the viewpoint of electricalinsulation, because a high voltage is loaded on the electrode at the application of high power.
Here, features of each method is shown in Table 1 andthe overview of each method applied to the dynamic
wireless power transfer is shown in Figure 3.
As shown in Figure 3, in case of applying the inductive
power transfer method to the dynamic wireless powertransfer system, the power is transferred to a receiving

Comparison of the inductive power transfer method and the capacitive power transfer method.
Inductive power transfer method

Capacitive power transfer method

Many actual results in the kHz band

Limitations due the dielectric
breakdown

Transmission distance〜 several m〜 several tens cm

Safety against a foreign metal body

Induction heating caused bymagnetism

Induction heating hardly occurs

Quantity of high-frequency powersource units for the dynamic
wireless power transfer system

Many units are required

Possibility for small quantity of units

kW class power transfer

Receiving circuit

Power storage

Receiving circuit

Power storage

Receiving
coil
Receiving
electrode
Transmitting
coil～

Figure 3～～
High-frequency
power source～
Transmitting
High-frequency electrodepower source

Overview of the dynamic wireless power transfer system. (Left: Inductive power transfer method, Right: Capacitive powertransfer method)

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The Development of MHz Band Wireless Power Transfer Aiming for the Dynamic Wireless Power Transfer

coil equipped on an EV from many transmitting coils buried under a road. Because a high-frequency powersource is necessary for each coil, it is feared that the cost
may increase further.
On the other hand, in the capacitive power transfer
method, it may be possible to transfer the power througha transmitting electrode which is several meters long andburied under a road and a receiving electrode equippedon an EV. In comparison with the inductive power transfer
method, the quantity of the high-frequency power source
units can be smaller.
3.2 Frequencies
85 kHz band (79 kHz to 90 kHz) which was selected as astandard by SAE, 6.78 MHz and 13.56 MHz band which
was an application of ISM band are examined as potential frequency bands applied to the dynamic wirelesspower transfer system. The features of the kHz band andthe MHz band are shown in Table 2.
A down-sized small coupler is one of the important elements in the dynamic wireless power transfer systemfrom the viewpoint of the cruising range. The downsizing
can be realized with applying the MHz band. Considering
this point, we have decided to develop the capacitive
power transfer method on the 13.56 MHz band in thisresearch from the viewpoint of downsizing & weightreduction, reduction of inductive heating of metal bodies
and less installation number of high-frequency powersource units. It is necessary to demonstrate first of all thatthe wireless power transfer of 10 kW class becomes realistic in order to apply the capacitive power transfer theory
to the dynamic wireless power transfer system.
Accordingly, we have started working on the developTable 2

4.

THE DEVELOPMENT OF THE CAPACITIVE
POWER TRANSFER TYPE COUPLER FOR
HIGH POWER TRANSFER

4.1 The Structure of a Coupler
The structure of a coupler under the development isshown in Figure 4. It is composed of electrode plates andcoils for resonance, and each electrode is covered with ashield case and an insulating cover. The coupler is constructed to resonate at 13.56 MHz in accordance with thecharacteristics of the electrode and the coil.
4.2 Measures Against Heat Generation of a CoilWe already worked on the development of a higher power
coupler and achieved the power transfer of 4.7 kW in2020 5). At that time, we found that the measures against
the heat generation of coils was necessary when aiming
at the further higher power transfer, and we havereviewed and improved the structure of the coil.
Until today, we applied coils wound with round magnet
wires but now we recently applied the edgewise coilwound with rectangular magnet wires shown in Figure 5.
This edgewise coil is excellent in the efficiency of heatradiation because of its flat shape. On the other hand, its
inductance is not easy to change continuously becauseof its difficulty in processing. Therefore, it is difficult tocontrol its characteristics to adjust its resonance frequency to 13.56 MHz with high accuracy. But we worked on
the electrode structure of the coupler and designed it toresonate at 13.56 MHz in this development.

Features of each method for the kHz band and the MHz band3), 4).

The kHz band

The MHz band

Inductive powertransfer method- Many actual results in the power transferof over kW- Standardization is ahead- Ferrite cores are required, large coils- No ferrite cores are required, light weightby down-sized coils

Capacitive powertransfer method- Domestic regulation of radio wave wasissued at the 400 kHz band as adepartment ordinance- Short transmission distance, large coupler- Transmission distance is efficiently
improved by improved Q value- Downsizing of a coupler

Transmission coupler

Receiving coupler
Figure 4

ment of a higher power coupler at first among the elements composing the system.

Electrode plate

Insulating coverFan

The structure of developed couplers.
(Left: The transmitting & receiving coupler, Right: Shape of thetransmitting coupler)

Figure 5

Edgewise coils.

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The Development of MHz Band Wireless Power Transfer Aiming for the Dynamic Wireless Power Transfer

The result of the power transfer test up to 10 kW, whichis the maximum transmitting power at 13.56 MHz measured in the 50Ω measuring system is shown in Figure 8.
100.0
90.0

Eﬃciency / %4.3 Characteristics of the Coupler
We measured the transmission characteristic of thedeveloped coupler by a network analyzer. The transmission characteristic of the developed coupler was optimized at 13.56 MHz when setting the distance between
the transmitting coupler and the receiving coupler at
53 mm. The transmission characteristic and the reflection
characteristic at the measurement are shown in Figure 6.
In Figure 6, S21 indicates the transmission characteristic and S11 and S22 indicate the reflection characteristicof the transmitting side and the receiving side respectively. The result of S21 has shown that the transmission efficiency at micro-signals of the coupler was 92.7%.

80.0
70.0
60.0
50.0

0.0

100.0

Figure 8

6.0

8.0

10.0

12.0

S21 is 92.68%
around 13.56 MHz

40.0

S11According to the result shown in Figure 8, it is observedthat the power transfer of the maximum power transfer10.1 kW was achieved at 91.5% in efficiency.

S22S2120.0

6.

0.0051015202530

Frequency / MHzFigure 6Evaluation results of transmission characteristic andreflection characteristic.

100.0

We have carried out the power transfer test of the coupler
shown in Figure 4.
Here, the testing system is shown in Figure 7.
PowermeterCoupler

Powermeter

CONTINUES POWER TRANSFER TEST

Next, applying the same testing system and the coupler
as mentioned above, we carried out the 45-minute continuous power transfer test of the power transfer 10 kW
class. The result is shown in Figure 9.

THE POWER TRANSFER TEST OF
10 kW CLASS

High-frequency
power source

The result of the power transfer test up to 10 kW
transmitting power.

50Ωterminalload

12.0
11.0

90.0

10.0

80.0

9.0
70.0

Eﬃciency
Transmitting power

60.0
50.0

Receiving power010203040

8.0Power / kW

60.0

Eﬃciency / %Eﬃciency / %4.0

Transmitting power / kW

80.0

5.

2.0

7.0
6.0
50

Time / min
RF inverter
Coupler

Chiller

Power
meter
50Ω
dummy load

Figure 9

According to the result shown in Figure 9, it is observedthat the power transfer was achieved at a power transfer
as high as 10.1 kW. The transmission efficiency after 45
minutes duration was 90.8%.

7.

Figure 7

10 kW class power transfer system.

The result of 45-minute continuous power transfer of
10 kW class.

CONCLUSION

We have been working on developing a higher power
coupler for the 13.56 MH band capacitive power transfer
system and validated the power transfer of 10 kW class.
We have reviewed and improved the structure of the couFurukawa Electric Review, No. 54 2023

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The Development of MHz Band Wireless Power Transfer Aiming for the Dynamic Wireless Power Transfer

pler and achieved the 45-minute continuous power transfer at as high as 90.8% in efficiency even under the condition with the insulation cover.
Presently, the power transfer through the coupler is carried out in an experimental system surrounded on allsides with metal plates. As our prospect for the near
future, we will develop the mitigations against the leakage
electromagnetic field and the radiation noise and not only
examine the application on a vehicle body but also aim to
demonstrate the power transfer of kW class to a moving
body in order to apply our system to the dynamic wireless power transfer system.

ACKNOWLEDGMENTS

REFERENCES1) Sakahisa Nagai, Osamu Shimizu, Toshiyuki Fujita, HiroshiFujimoto, Daisuke Gunji: “Reduction of Installation Rate byIncreasing Power Transmission Capability for Dynamic WirelessPower Transfer”, Transaction of SAE Japan, 2021.
2) Yuto Yamada Takehiro Imura: “A Simplified Model of The Power
Requirement and Batter y Level for Dynamic Wireless Power
Transfer by Various Vehicles”.
3) Takuya Nayuki: ”Standards and Regulations of Wireless PowerTransfer”, Transaction of IEEJ, 2021.
4) Mitsuru Masuda, Masahiro Kusunoki, Hirokatsu Umegami,Fumiya Hattori, and Masayoshi Yamamoto: “Wireless Power
Transfer via Electric Field Resonance Coupling”, Transaction ofJIEP, 2015.
5) Furukawa Electric HP > News Release > World's First Successful
Use of Electric Resonance Coupling to Wirelessly Transmit
4.7 kW of Power (Referred on Feb. 13, 2023)
https://www.furukawa.co.jp/en/release/2020/kenkai_200127.html		

A part of this research was carried out by the Cabinet
Office Council for Science, Technology and Innovation
Strategic Innovation Promotion Program (SIP) “Energy
Systems for IoE Society” (Auditing firm: JST).

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