Wireless Power Transfer via Electric Coupling
Mitsuru Masuda*1, Masahiro Kusunoki*1, Daiki Obara*1, Yujiro Nakayama*1,
Hiroki Hamada*1, Shoichi Negami*1, Kei Kaizuka*2

This paper describes our research on wireless power transfer via electric coupling with planar electrodes and coils. Major researches of wireless powertransfer using electric field are restricted to an area of almost no gap power transfer by thebreakdown voltage of air. However, we have achieved long distance and high efficiency powertransfer; the power transfer distance is 200 mm and the efficiency is over 90%. Moreover, we
succeeded 300 W power transfer experiment. We discovered that the wireless power transfer
using electric field has an advantage of that misalignment characteristic of one direction is better
than using magnetic field and that it can make power transfer distance longer with repeaters.

ABSTRACT1.

INTRODUCTION

Wireless power transfer technologies are greatly attractive
and paid special attention. The reason is because midrange wireless power transfer named as magnetic resonance was presented by MIT in 20071), 2). This phenomenon also holds in the electric field3). However, the wirelesspower transfer using the electric field remained in the
study for supplying the electric power in close range
because of the limitation of the dielectric breakdown
strength of air and the concern about the influence of anelectromagnetic wave since human body is constituted ofthe dielectric materials.
The long distance power transfer can be achieved by aresonance circuit that had a large magnitude Q-value withthe main energy of electric field and the frequency 13.56
MHz and 27.12 MHz of Industrial, Scientific and Medical
Band (ISM band). However, in case that the electromagnetic radiation increases by the electrode shape, theenergies of induction and electrostatic field which carry
power by resonance phenomenon cannot be utilized
effectively. For that reason, long distance wireless powertransfer using electric field has not been much reported.
We indicate, with electromagnetic field simulation, thatthe electromagnetic radiation level can be lowered withan LC series resonance circuit and a high efficient powertransfer is possible by electric resonance coupling andhave achieved high a wireless power transfer performance.

*1 Automotive System & Device Laboratories,
Research & Development Division*2 Industrial Cable & Power Cable accessories Div.
Energy Backcast Group2.

THE STUDY OF THE WIRELESS POWER
TRANSFER USING THE ELECTRICRESONANCE COUPLING

2.1

The Wireless Power Transfer Using the Magnetic
Field and the Electric Field
When we consider the space for transferring the energy
wirelessly, there are 2 methods. One is such as the soundwave which transfers energy through material and theother is the transferring energy through fields. The field
can be divided into an electric field and a magnetic field.
When we classify the wireless power transfer to thinkphysical system, we can list up (1) Use an electric field.
(2) Use a magnetic field. (3) Use an electric field and amagnetic field. (Electromagnetic waves, light) (4) Use asound wave and etc.
In these systems, (3) the energy transfer using the electric field and the magnetic field has been studied for longtime as per Nikola Tesla’s experiment that was performedin the early 20th century shows. Recently the study suchas Solar Power Satellite (Space Solar Power Systems) isalso on-going.
Meanwhile, in recent years, the wireless power transfer
using the magnetic field is being applied to a broad range
of applications such as portable cell phone based on Qi
standard for the electromagnetic induction system and,such as Radio Frequency Identification (RFID) which is awireless power transfer system that many people use it
without being aware.
The study of the wireless power transfer using electricfield coupling is continuing in some research organizations.
2.2

About the Wireless Power Transfer Using theElectric Field Coupling Resonance
The wireless power transfer using the electric field is limitFurukawa Review, No. 44

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Wireless Power Transfer via Electric Coupling

C1

Ｌ1 - L m

C2

Ｌ2 - L m

and in the electric resonance coupling, the ratio between
ω×L 1 and 1/ω(C 1-C m) is output. In other words, wethought that if the difference between C1 and Cm is small,the power transfer can be done efficiently even in case of
the electric field.
The series resonance type power transfer system canbe configured if L1 and L 2 of external coils are connectedto the plate electrodes of dimensions 118 mm×250 mm.
For example, when L1 and L2 are equal to 5 µH, C1 and C2
are equal to 7 pF. Cm is equal to 0.8 pF. The power transfer efficiency was calculated by SPICE system circuit simulator. These numbers were calculated based on themeasured data. As shown in Figure 3 and 4, it was foundthat the transfer efficiency of more than 85% can beachieved in this configuration.

100%
80%

η11,η21ed by the dielectric breakdown voltage in the air 4). On theother hand, the wireless power transfer using the magnetic field is being considered that a large energy of powertransfer can be done because there is no phenomenon
corresponding to discharge. It has been thought that the
maximum transmissible power using magnetic field isnine orders of magnitude greater than that of the electricfield. Further, it is being considered that the wirelesspower transfer using the electric field has a disadvantage
because the capacitance is reduced when the distance
between the transmitter and the receiver increases.
However, from the viewpoint of Electro-magnetic
Compatibility (EMC), there is an advantage to be difficult
to affect other apparatuses in the electric field, which
attenuated in proportion to the cube of the separation distance.
Therefore, we conducted a comparative study of theelectric field type and the magnetic field type. Figure 1
shows the block diagram of the magnetic field resonancecoupling scheme and Figure 2 shows the block diagramof the electric field resonance coupling.

60%
40%
20%
0%
18

Z0

22

24

26

28

30

32

34

36

38

Frequency[MHz]

Z0

Lm
R1

20

transfer efficiency η21(=|S21|^2)

R2

return loss η11(=|S11|^2)

Figure 3 Power transfer at distance 120 mm.

Figure 1 Magnetic resonance type block diagram.
100%

Cm

80%

Ｌ2

η11,η21

Ｌ1Z0
C1 - Cm

R1

Ｃ2 - Cm

Z0

60%
40%
20%
0%
18

R2

20

22

24

26

28

30

32

34

36

38

Frequency[MHz]transfer efficiency η21(=|S21|^2)

Figure 2 Electric coupling type block diagram.

return loss η11(=|S11|^2)

Figure 4 Power transfer at distance 170 mm.

In Figure 1, L1 is the inductance of the transmitter coil,
L2 is the inductance of the receiver coil and C1 and C2 are
the resonance capacitors of the power transmitter and thereceiver. R1 and R2 are the AC resistances between thetransmitter and the receiver. Lm is the coupling inductanceof the magnetic resonance coupling.
In Figure 2, C1 is the capacitance of the transmitter andC2 is the capacitance of the receiver. Cm is the coupling
capacitance of the electric resonance coupling. As theangular frequency is ω, in the magnetic field resonancecoupling, the ratio between ω(L1-Lm) and ω×Lm is output

2.3

The Structure Study Of The Wireless Power
Transfer Coupler
The wireless power transfer system of the meander line
type using electric field has been studied3). But in thismethod, the line length is increased at the wavelength of
27.12 MHz and 13.56 MHz and it is considered that loss
in this part may occur. In order to reduce this loss, we
examined the wireless power transfer coupler with a simplified structure shown in Figure 5 using a series resonance coil and a plate electrode. The parameters of the
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Wireless Power Transfer via Electric Coupling

coupler section were designed to be the value that wasused in the circuit of the simulator. The examined powertransfer coupler is a thin and light configuration and also
can be easily fabricated because it has a simple configuration. The dielectric electrodes are sandwiched by plastic resins, so that the coupler can be utilized even if it isfolded. The coupler is shown in Figure 6.

Ｚ

Z axisPlate electrode series resonancestructure

Port 2 (Rx)Z axisPhase:0-Ｙ

Electrodes
distance

Simulation result (moment method)
Center frequency 13.56 MHz
Electrodes distance 400 mm

Port1
Port
1 (Tx)

Ｘ

X axisPhase:π/4

Phase:π/2

Receiver side (Rx)

Transmitter side (Tx)
Y axisPlate（A）

Figure 7 The result of the computer simulation.
Plate（B）

Resonation CoilTransfer efficiency（η21）peak value

Figure 5 The electric coupling power system.

From Figure 7, it can be shown that the energy transfer
can be achieved between the transmitter and the receiver.
Then the change of efficiency of the energy transfer due
to the misalignment was compared between the magnetic resonance coupling method and the electric resonancecoupling method. The result of the simulation is shown in
Figure 8. As shown in Figure 8, the electric field type has
the advantage that misalignment characteristic of one
direction is better than using magnetic field.

Figure 6 Electric coupler.

Then we confirmed the behavior of the examined wireless power transfer coupler with the electromagnetic field
simulator, FEKO of FARAD. This simulator can do a kindof finite element analysis based on moment method. This
simulator can simulate the state that the dielectric constant of the dielectric material and the permeability of themagnetic material that are entered. The result of the simulation is shown in Figure 7.

Misalignment vs. Transfer efficiency
Electric resonance & magnetic resonance
Coupler diameter about 500 mm , Distance between couplers :300 mm
100%
90%
80%
70%
60%
50%
40%
30%
20%
10%
0%

0

-200

-400

-600

Misalignment from center [mm]Electric Resonance x-direction

Electric Resonance y-direction

Magnetic Resonance x-direction

Magnetic Resonance y-direction

Figure 8 The power transfer efficiency vs. the misalignment ofcoupler.

3.

THE PROTOTYPE OF THE POWER
TRANSFER SYSTEM

3.1 The Power Transfer Efficiency Between Couplers
Based on the results obtained from the electromagnetic
field simulation, we have developed a wireless powertransfer coupler.
The electrode size of the coupler is 110 mm×480 mm
and these 2 electrodes are placed on the FR4 substrateat the intervals of 35 mm. Further, the inductance of thecoil is adjusted to resonate at 27.12 MHz and 13.56 MHz
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Wireless Power Transfer via Electric Coupling

and the two coils are connected in series. The structuresof the transmitter side and the receiver side are same.
The Input impedance of the coupler is set to match with50 Ω.
The power transfer efficiency was measured by varying
the distance between the transmitter and the receiver ofthis coupler. A network analyzer is used for the measurement and the power transfer efficiency was calculated
from S21. The measurement results are shown in Figure
9. The power transfer efficiency of 90% was achieved at
the distance of 200 mm.

Transfer efficiency η21() peak value

Plane type coupler Measured value of power transfer efficiency
100%
95%
90%
85%
80%
75%
100

120

140

160

180

200

220

240

260

280

X'tal-Oscillater

Phase shift varriable power
supplyLPF

High- side
Gate Driver
Low- side
Gate driver

HF BandPowerMOSFETE Class drive
component

HF BandPowerMOSFETE Class drive
component

Impedance
matching
circuitDirectional
Coupler
OUTPUT

Figure 10 RF inverter block diagram.

GaN HFET device has a smaller input capacity in comparison to the Si MOSFET because the input stage has a
Schottky structure. So in the present circumstance, theloss for the circuit which drives the Si MOSFET can be
reduced. We expect that high power device such as a
GaN HFET will emerge in future.
3.3 Matching Equipment
A matching circuit was provided between the RF inverter
output and the power transmitter coupler and between
the receiver coupler and the rectifier circuit. This is to prevent the characteristic change by the reflection and thedamage of the circuit because of a large reflected wavethat returns to the RF inverter. Figure 11 shows a block
diagram of an L type circuit by the coil and the variablecapacitance device.

Distance between couplers d [mm]for f0=27.12 MHz

for f0=13.56 MHz

CM Coupler

Figure 9 The power transfer efficiency vs. distance between
couplers.
Input

3.2 RF Inverter Circuit
High frequency power supply for driving the coupler
operates the lateral Si MOSFET in a half-bridge configuration. As a feature of the Si MOSFET in this frequency
band, it is necessary to devise a circuit to drive the gate
because the input capacitance is very large about 1400
pF. In the prototype circuit, a primary amplification of asignal of the oscillation circuit is performed by class E
amplifier and the signal drives a lateral Si MOSFET by a
voltage-current conversion. In addition, the output impedance of this devise is 2.4-j6.8 Ω so it is converted to 50 Ω
by using a transformer.
For driving the device much know-how is pooled and it isan area where the power circuit technology and the highfrequency circuit technology are combined. And it is expensive, but when using the drive module with built-in gate
driver circuit, the configuration can be further simplified.
The circuit generates the high-frequency power withhigh efficiency by using a soft-switching technology.
However, a problem of the soft-switching technology isthat, when changing the power supply, the voltage
applied between the drain and the source of the FET
must be changed. So the power supply unit is equippedwith a variable voltage. The output impedance is 50 Ω tomatch the coupler. The RF inverter block diagram isshown in Figure 10.

OutputDetecter
unit
Figure 11 The Impedance matching network circuit.

3.4 Rectifying Section
The power that is required for the power electronics iscommunicated to the chopper circuit which is a control
circuit after converting the commercial frequency of 50/60
Hz into direct current. In the wireless power transfer, thecircuit for converting the power of a carrier wave (27.12
MHz and 13.56 MHz) HF band into the direct current isrequired. In this case because the power which has largely different frequency must be rectified, the impedance
matching becomes an important issue.
As an example, if the wireless power transfer is done by
applying a power of 300 W to the 50 Ω power transfer line,
the voltage is 122 V and the current is 2.45 A by Ohm’slaw. Because a reflection wave occurs when the rectifier
circuit is connected to the power transfer line that has different impedance, the matching circuit must be connected. Meanwhile, when the input impedance of the halfwave rectifier circuit that combines a capacitor and adiode is measured, it was Z=1.9-j14.5 Ω in the frequency
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Wireless Power Transfer via Electric Coupling4.

THE CHARASTERISTICS OF POWER
TRANSFER SYSTEM

4.1 Power Transfer AmountThe power transfer amount was measured using thedeveloped system shown Figure 12. The experiment wasdone in an anechoic chamber. The power transfer
amount of 300 W was achieved at the electrode distanceof 200 mm.

50Ω
Load

RF power
meter

Power
receivercoupler unit

Wireless
power
transfer
RF Power
supplyImpedance
matching
network

RF power
meter

Power
transmitter
coupler unit

Figure 12 The measurement system block diagram.

EUT(Equipment under test)
Power LineAMN(Artificial
Mains Network)

Figure 13 The measurement system block diagram of CISPR11.

CISPR 11 Main terminal interface voltage
Line1ClassA Limit

ClassB Limit

1.E+06

1.E+07

70
60
50
40
30
20
10
1.E+04

1.E+05

1.E+08Frequency（Hz）
Figure 14 The feeding conduction power noise vs. frequency.

Next, the EMC noise was measured using the measurement system shown in Figure 15. The results are shownin Figure 16.
It can be seen that the radial noise level is fluctuating
depending on the direction of the coupler.

Radio wave shieldingroom
Receiving coupler

4.2 EMC NoiseThe conductive noise from the RF power supply is evaluated using the current measurement system shown in
Figure 13. As a current pseudo-power supply networks,
KNF-244 F manufactured by Kyoritsu Electronics Industry
is used. The measured results are shown in Figure 14.
The results show that the system meets the criteria of the
CISPR115) standards for such electromagnetic interference of high frequency devices.

Wood Table

Spectrum Analyzer

Filed strength（dB μV/m）of 13.56 MHz. Assuming that there is no loss of the rectifier circuit and the matching circuit when 300 W is applied
as before, the voltage developed across the capacitors
and the current flowing through the diode become 66.2 V
and 4.53 A each. This means that it became very low
impedance by the rectifier circuit.
When the rectifying is done by the device of PN junction diode, the loss will occur by the characteristic called
the reverse recovery time. This effect will be remarkable in
the frequency of 13.56 MHz and 27.12 MHz. Because ofthis fact, Schottky Barrier Diode (SBD) without PN junction is used. However, because the SBD mainly composed of silicon has the low withstand voltage of 200 V in
maximum, the diode using SiC or GaN is effective. In particular, when considering the power transfer of kW orders,high breakdown voltage characteristic may be required
for elements as voltage level after rectification is high. So
the benefits of using GaN and SiC are great. In future,easy availability for these devices is expected.

Biconical
anttenna

Transmiting coupler

Terminater

RF power
supplysystem6)

Figure 15 The measurement system block diagram of CISPR22 .

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Wireless Power Transfer via Electric Coupling

CISPR22 EMC characteristics
27 MHz Input power 200 W Distance 200 mm 50 Ω Load
Radiation PowerCISPR22 ClassA LimitCISPR22 ClassB Limit

1.000E+08

1.000E+09

100

Filed strength（dB μV/m）

90
80
70
60
50
40
30
20
10
0
1.000E+07

Frequency（Hz）

REFERENCES
1) Andre Kurs, Aresteidis Karalis, Robert Moffatt, J.D. Jannopoulos,
Peter Fisher and Marin Soljac’ic: “Wireless Power Transfer via
Strongly Coupled Magnetic Resonance”, Science Express, Vol. 317
No. 5834 (2007), 83-86
2) Aristeidis Karalis, J.D. Jaoannopoulos and Marin Soljac ic: Efficient
wireless non-radiative mid-range energy transfer” Annals ofPhysics, Vol. 323, January Special Issue (2008), 34-38
3) Takehiro Imura “Research on the electric field coupling and themagnetic coupling of the non-contact power transmission as seenfrom the equivalent circuit” Institute of Electrical Engineering
Journal D, Vol. 130 No. 1 (201), 84-92 (in Japanese)
4) Hidetoshi Matsumoto, Toshihide Takahashi “The easily understandable book for wireless power transfer” Ohmsha, Ltd. (201), 24(in Japanese)5) CISPPR11 International Special Committee on Radio Interference
Specification
6) CISPPR22 ditto

Figure 16 The feeding EMC power vs. frequency.

5.

CONCLUSION

It is demonstrated that by using the electric field resonance coupling, the thin and light wireless power transfer
system can be designed.
Furthermore, by measuring its efficiency by prototype
system, it was shown that the efficiency of the powertransfer at the distance of 200 mm between the transmitter and the receiver is more than 90%. Also, 300 W powertransfer experiment was successful. In the future, we areplanning to perform a study of the wireless power transfer
system that can transfer larger power between the transmitter and receiver.

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