FRI-E-1

Positioning-Free Magnetically Resonant WirelessPower Transmission Board with Staggered Repeater
Coil Array (SRCA)
Hyunkeun Lim, Koichi Ishida, Makoto Takamiya, and Takayasu Sakurai
University of Tokyo, 4-6-1 Komaba, Meguro-ku, Tokyo 153-8505, Japan
Abstract- A magnetically resonant wireless power transmission
board (MR-WPTB) on a printed-circuit board is developed. MRWPTB wirelessly supplies the power to electronic devices such as
smartphones anywhere on the board with high efficiency. Inorder to increase a power transmission efficiency degraded by amisalignment of a receiver coil, a transmitter coil array and aproposed staggered repeater coil array (SRCA) are stacked in
MR-WPTB. 1 transmitter coil closest to the receiver coil and thesurrounding 4 repeater coils are selectively activated based on
the result of a position detection of the receiver coil. By adding
SRCA to the transmitter coil array, the measured powertransmission efficiency at the worst case misalignment increases
from 4.8% to 64% at a distance of 15mm between the transmitterand receiver coils with a diameter of 40mm.

Magnetically resonant wireless powertransmission board (MR-WPTB)
1~2m

I. INTRODUCTION
Fig. 1. Usage scene of proposed magnetically resonant wireless powertransmission board (MR-WPTB).

Ambient or ubiquitous charging of electronic devices suchas smartphones is increasingly needed, because thesmartphone requires a frequent charging due to the increased
power consumption. A wireless power transmission (WPT)
sheet [1-4] with a transmitter (TX) coil array and a selectively
activation of a TX coil based on the position detection of areceiver (RX) coil is one of a possible solution for theubiquitous charging. A WPT distance of the sheet, however, isshort (e.g. 4-mm distance at 10% efficiency [1]), because anelectromagnetic induction is used in the sheet. In order toincrease the distance, a magnetic resonance [5] instead of theelectromagnetic induction is used in this paper. In the newly
developed magnetically resonant wireless power transmission
board (MR-WPTB) on a printed-circuit board (PCB), however,
WPT efficiency degradation due to a misalignment of RX coilis still large even if the magnetic resonance is used. In order tokeep a high efficiency regardless of the misalignment, astaggered repeater coil array (SRCA) is proposed in this paper.
By using MR-WPTB with SRCA, a positioning of RX coil is
not required, thereby enabling the ubiquitous charging.

WPTB, a transmitter coil is selectively activated based on theposition detection [1-4] of RX coil. The purpose of this work
is to propose the best configuration of the coil array which
achieves the highest WPT efficiency at the worst
misalignment condition of RX coil. In order to find the best
configuration, the WPT efficiency of 4 different coil arrays arecompared.
Fig. 2 shows 4 types of coils used in the coil arrays. Marksfor the 4 types of coils are defined. The transmitter coil (TXC)
in Fig. 2(a) is a power feeding coil. The repeater coil (RepC)
in Fig. 2(b) is a short coil to extend the WPT distance. The
open coil in Fig. 2(c) is a non-activated and unused coil. The 9transmitter coils (9 TXC’s) in Fig. 2(d) is 9 coils sharing asingle power source.

(a) Transmitter coil (TXC)

II. MAGNETICALLY RESONANT WIRELESS POWER
TRANSMISSION BOARD (MR-WPTB) WITH SRCA

RFig. 1 shows a usage scene of the proposed MR-WPTB.
MR-WPTB is put on a table and wirelessly supplies the power
to electronic devices such as a smartphone, a keyboard, and amouse anywhere on MR-WPTB with high efficiency. MRWPTB has transmitter coil arrays. In MR-WPTB, the target
size is 1~2m square and the target thickness is 1mm. In MR-

978-1-4673-1780-1/12/$31.00 ©2012 IEEE(b) Repeater coil (RepC)(c) Open coil(d) 9 Transmitter coils (9 TXC’s)

Fig. 2. 4 types of coils used in coil arrays in Fig. 3.

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yyy

R R R
RR
R R Rxx(a) 1 TXC scheme(b) 9 TXC scheme

Bottom TX layer(c) 1 TXC + 8 RepC schemeyTop repeater layer (SRCA)

R R
R Rx

StackedR R
R Rx(d) Proposed stacked 1 TXC + 4 RepC scheme
Fig. 3. 4 different coil arrays. (a) Conventional 1 TXC scheme [1, 3-4]. (b) 9 TXC scheme [2].(c) 1 TXC + 8 RepC scheme [6]. (d) Proposed stacked 1 TXC + 4RepC scheme.

Fig. 4 shows a photograph 3 x 3 coil array fabricated on
FR4 PCB. The diameter of the octagonal coil is 40mm and thecoil pitch (p) is 44mm. “Position O”, “Position A”, and“Position B” of RX coil are defined to evaluate the WPT
efficiency at the misalignment conditions. Table I summarizes
the coil for the magnetic resonance. The number of turns of
the coil is 9 and the thickness of the metal is 35μm. Chipcapacitors are used to resonate the coil for the magneticresonance.

Fig. 3 shows the 4 different coil arrays compared in thispaper. In the conventional 1 TXC scheme [1, 3-4] in Fig. 3(a),1 TXC closest to RX coil is activated based on the result of aposition detection of RX coil and all the other coils are open.
In the 9 TXC scheme [2] in Fig. 3(b), 9 TXC’s are activated.
In the 1 TXC + 8 RepC scheme [6] in Fig. 3(c), 1 TXC and thesurrounding 8 RepC’s are activated. In the proposed stacked 1TXC + 4 RepC scheme in Fig. 3(d), a bottom TX layer and a
top repeater layer are stacked, and 1 TXC in the bottom TX
layer and the surrounding 4 RepC’s in the top repeater layer
are activated. The repeater layer is called as SRCA, because
the centers of the TX coils and the centers of the repeater coils
are staggered each other.

yIII. EXPERIMENTAL RESULTS
In order to compare the 4 different coil arrays in Fig. 3, thedependence of the WPT efficiency on the frequency, the WPTdistance, and the misalignment is measured.

xy

TABLE I SUMMARY OF COIL FOR MAGNETIC RESONANCE.

Parameters
Diameter
Coil pitch(p)Number of turns
Line widthLine spacing
Thickness of metal
Thickness of PCB
Inductance

Position B
Position O

45°

Position A x22mm
Coil pitch(p)= 44mmChip capacitor
Frequency

40mm

Value
40mm
44mm
91mm
0.1mm
35μm
1mm
3.42μH
39pF (TXC, RepC)
33~39pF (Receiver coil)
13.8~16.2MHz

Fig. 4. Photograph 3 x 3 coil array fabricated on PCB.

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Fig. 5 shows a photograph of a measurement setup. The WPT
efficiency is measured with a network analyzer. Fig. 6 shows adefinition of the WPT distance and the misalignment. The
distance is defined as d. The misalignment on x-axis and xyaxis are defined as Δx and Δxy, respectively.
Fig. 7 shows a measured frequency dependence of theWPT efficiency. Figs. 7(a) and (b) show the conventional 1
TXC scheme and the proposed stacked 1 TXC + 4 RepC
scheme, respectively. d is 15mm and Δxy is varied. The
frequency which achieves the highest efficiency at the worst
misalignment condition of Δxy = 31mm (Position B) is used in
the following measurements for each 4 different coil array.

Receiver coil

2x2 repeater coils
Networkanalyzer

10090Fig. 5. Photograph of measurement setup.

80Δx

yRX coilΔxy

RX coilddx
TX coil array

45°xy

Efficiency [%]yx

TX coil array

Position O

1 TXC +8 RepC
(14.8MHz)

Conventional 1 TXC(13.8MHz)70

Proposed Stacked
1 TXC + 4 RepC
(15.3MHz)

60
50
40302010(a)0(b)

Fig. 6. Definition of WPT distance (d) and misalignment (Δx and Δxy).

0102030d (Distance) [mm]13.8MHz

10070608016mm50403024mm

2031mm1011121370

Conventional 1 TXC(13.8MHz)60

Proposed Stacked
1 TXC + 4 RepC
(15.3MHz)

504014151617Frequency [MHz]18199 TXC
(16.2MHz)

1020

1 TXC + 8 RepC
(14.8MHz)

30
200
10Position A

908mm

Efficiency [%]Efficiency [%]100d = 15mmΔxy = 0mm

80

Target range

00(a)1015.3MHz100

1 TXC + 4 RepC
scheme90

Position B
Target range

80706050Δxy = 0mm

408mm

3016mm24mm
31mm

1001011121314 15 16 17
Frequency [MHz]40(b)90202030d (Distance) [mm]100d = 15mm

Efficiency [%]Efficiency [%]8040(a)1 TXC scheme

90

9 TXC
(16.2MHz)

Target range

7060504030

Conventional
1 TXC (13.8MHz)

20101819

Proposed Stacked
1 TXC + 4 RepC
(15.3MHz)

9 TXC(16.2MHz)

1 TXC + 8 RepC
(14.8MHz)

0200(b)102030d (Distance) [mm]40(c)Fig. 7. Measured frequency dependence of WPT efficiency. (a)Conventional 1 TXC scheme. (b) Proposed stacked 1 TXC + 4 RepC
scheme.

Fig. 8. Measured distance dependence of WPT efficiency in 4 different
coil arrays. (a) Position O. (b) Position A. (c) Position B.

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IMWS-IWPT2012 Proceedings

Best case100

Conventional
1 TXC (13.8MHz)

90

Efficiency
Efficiency[%][%]

TABLE II SUMMARY OF FREQUENCY AND WPT EFFICIENCY.
Proposed Stacked Worstcase
1 TXC + 4 RepC
(15.3MHz)

80
600
7060500
9 TXC(16.2MHz)

50400
4030300
200100 00.1

Scheme
1 TXC(Conventional)

13.889%

37%

4.8%

9 TXC

16.263%22%

4.2%

1 TXC + 8 RepC

14.880%36%

6.3%

Stacked
1 TXC + 4 RrepC(Proposed)

15.366%

71%

64%

1 TXC + 8 RepC
(14.8MHz)d = 15mm
p = 44mm

10200

0.2
0.3
0.4
Relative misalignment (Δx/p)

0.5

Efficiency
Position of RX coilOA (Δx=p)
B (Δxy= 2p)(Best case) (Worst case) (Worst case)Frequency(MHz)00246810 12 14 16 18
Misalignment on x-axis (Δx) [mm]2022

Table II summarizes the frequency and the WPT efficiency
at Position O, Position A, and Position B in the 4 different coilarrays. Compared with the conventional 1 TXC scheme, theWPT efficiency of the proposed stacked 1 TXC + 4 RepC
scheme increases from 37% to 71% at Position A, and from
4.8% to 64% at Position B. The increase of the WPT
efficiency at the worst case misalignment by adding SRCA isvery important, because the end-product performance of MRWPTB is determined by the worst (=minimum) efficiency.

(a)Best case

100
Proposed Stacked
1 TXC + 4 RepC
(15.3MHz)

Efficiency
Efficiency[%][%]

9080600
7060500

Conventional
1 TXC (13.8MHz)

9 TXC(16.2MHz)

504004030300
20Worstcase

IV. CONCLUSION
d = 15mmp = 44mm

102000100 01 TXC + 8 RepC
(14.8MHz)

0.1
0.2
0.3
0.4
Relative misalignment (Δxy/ 2p)

SRCA is proposed to enable the positioning-free MRWPTB for the ubiquitous charging of the electronic devices.
By stacking SRCA and the transmitter coil array, themeasured WPT efficiency at the worst case misalignment
condition increases from 4.8% to 64% at 15-mm distance. The
improvement of the worst (=minimum) efficiency is very
important, because the end-product performance of MRWPTB is determined by the worst efficiency.

0.5004812162024
Misalignment on xy-axis (Δxy) [mm]2831(b)Fig. 9. Measured misalignment dependence of WPT efficiency in 4
different coil arrays. (a) Δx dependence. (b) Δxy dependence.

REFERENCES

Fig. 8 shows a measured distance dependence of the WPT
efficiency in the 4 different coil arrays. Figs. 8(a) (b) and (c)show the efficiency at Position O, Position A, and Position B,
respectively. The target d in this work is 10-20mm. At
Position O, the conventional 1 TXC scheme achieves thehighest efficiency at d = 15-20mm. In contrast, at Position Aand Position B, the proposed stacked 1 TXC + 4 RepC scheme
achieves the highest efficiency at d > 5mm. The 15-mm
distance which achieves the highest efficiency in the proposed
stacked 1 TXC + 4 RepC scheme at Position B is used in thefollowing measurements.
Fig. 9 shows a measured misalignment dependence of theWPT efficiency in the 4 different coil arrays. Figs. 9(a) and (b)show Δx and Δxy dependence, respectively. The horizontal
axis is normalized by p. d is 15mm. As Δx or Δxy increases,
the WPT efficiency decreases except for the proposed stacked
1 TXC + 4 RepC scheme. Therefore, Position O (Δx = 0 in Fig.
9(a) and Δxy = 0 in Fig. 9(b)) is the best-case misalignment,
while Position A (Δx/p = 0.5 in Fig. 9(a)) and Position B(Δxy/p = 0.5 in Fig. 9(b)) are the worst-case misalignment.
The WPT efficiency of the proposed stacked 1 TXC + 4 RepC
scheme is more than 64% at any misalignment.

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