ISSCC 2007 / SESSION 20 / TD: PROXIMITY DATA AND POWER TRANSMISSION / 20.4
20.4

Design Solutions for a Multi-Object WirelessPower Transmission Sheet Based on PlasticSwitches

Makoto Takamiya1, Tsuyoshi Sekitani1, Yoshio Miyamoto1, Yoshiaki Noguchi1,Hiroshi Kawaguchi 2, Takao Someya1, Takayasu Sakurai1
1

University of Tokyo, Tokyo, Japan, 2Kobe University, Kobe, Japan

Wireless power delivery to electronic objects scattered overtables, walls and ceilings will form an infrastructure necessaryfor ubiquitous electronics, wireless sensor networks and ambient
intelligence. Described here are design innovations to overcome
the shortcomings of a previous wireless power transmission sheet[1] made with plastic MEMS switches and organic FETs (OFET)
circuits suitable for printable low-cost electronics. The sheet
delivers power to multiple objects, frees the users from position
adjustment, reduces the number of coil arrays, and takes 5V digital input by using a newly developed level shifter.
The principle of wireless power transmission is based on an arrayof coils (TX-coil) made on a plastic sheet, which is selectively driven by plastic MEMS switches and is coupled magnetically with areceiver coil (RX-coil) mounted on a power receiving object asshown in Fig. 20.4.1. The MEMS switch can transfer amperes ofcurrent but the number of life-time switching cycles is limited up
to 106 and the switching speed is as slow as a little less than a second. The selective TX-coil activation of small coils is needed forhigh power efficiency since if the RX-coil is made with one large
coil, the coupling coefficient is decreased by a factor of one thousand in some cases. Position detection of the receiving object isneeded for the selective activation of TX-coils. The position detection is carried out by OFETs as shown in Fig. 20.4.2, since scanning through many TX-coils to check if an RX-coil exists above aTX-coil by means of impedance change needs a faster speed andlonger life-time than the MEMS switches.
As the sheet size gets large, power transmission at multiple random points on the sheet is required to deliver power to multiple
objects and/or to deliver large power to a single object by using
multiple RX-coils. In order to enable the multiple selection of TXcoils, conventional row and column decoders, which can select
only one line at a time, are not sufficient. Thus, multiple lineselection using an OFET SRAM [2] as shown in Fig. 20.4.2 isadopted. At first, all SRAMs are reset to “L”. Then, only columns
and rows above which RX-coils exist are selected. Multiplecolumns and rows can be selected simultaneously. By integrating
the SRAMs and the decoders [3] on the OFET sheet, the numberof the connections with off-sheet wires is decreased from 2N to2log2N for an array of N×N coils.
Position adjustment-free power transmission is made possible by
the following design. As shown in Fig. 20.4.1, the TX-coils areplaced regularly with a pitch of 25.4mm which is called one unit.
In [1], the TX-coils and the RX-coils have the same outer diameter (= 1 unit). In this case, as shown in Fig. 20.4.3(c), the powertransmission efficiency changes from 58% to 2% with a displacement from 0 to 0.5 unit. Thus, 0.5-unit displacement is notallowed. This suggests a need for the exact positioning of thepower receiving object. On the other hand, Fig. 20.4.3(a) shows
the multiple activation technique of TX-coils, which tolerates thedisplacement. The outer diameter of the RX coils is set twice aslarge as that of the TX-coils. Figs. 20.4.3(b) and (c) show themeasurement setup for the power transmission efficiency and themeasured dependence of efficiency on the displacement of the RXcoil center to the TX-coil center with changing the number of activated TX-coils. It is seen that the efficiency becomes relatively
insensitive to the displacement by using multiple TX-coil activation. Figure 20.4.3(d) shows the dependence of the maximum,
average, and minimum power efficiency varied over the displacement amount on the number of activated TX-coils. The minimum
efficiency achieves a maximum at 3×3 coils activation, which isconsidered to be the best design choice, because the minimum

362

efficiency determines the specification of the power transmission
sheet. The proposed technique frees users from tedious position
adjustment.
In contrast to [1], the circuit shown in Fig. 20.4.4(a) is devised byusing two different frequencies and carefully eliminating interference of the two functions: the power transmission and the position detection. The circuit makes use of the fact that the conductivity of MEMS switches is more than 103 higher than that ofOFETs. Thus, the OFET does not degrade the power transmission efficiency by shorting two frequency signal sources. C1 isadded to decrease the impedance at 13.56MHz (power transmission frequency) to improve the power efficiency while monitoring
the impedance change for 3.5MHz (position detection frequency).
CP is added to increase the position detection sensitivity by peaking the impedance by LC resonance. As shown in Fig. 20.4.4(b),the measured resonant frequency of VMON for the position detection system is around 8MHz to 10MHz, which is beyond the cutoff frequency of OFETs (3.5MHz). As seen from Fig. 20.4.4(b), inorder to increase the sensitivity for the position detection, thehigher the frequency used for position detection the better. Thus
3.5MHz is chosen as the frequency for the position detection.
Figures 20.4.5(a) and (b) show the measured sensitivity for theposition detection and power transmission efficiency, respectively. A 33% swing voltage change of VMOM is observed between thetwo cases of with and without a RX-coil, and a 50% power transmission efficiency is achieved.
The integration of the silicon LSIs, OFETs, and MEMS devices isnecessary in this power transmission system. Direct digital driveof OFETs and MEMS by silicon LSIs is, however, difficult,because there is a large discrepancy in the operation voltage
between LSIs (below 5V) and OFETs/MEMS (above 40V). OFET
level-shifters from 5V to 40V are developed as shown in Fig.
20.4.6. The basic topology of the shifter is a single amplifier
whose gain depends strongly on the input level and thus adaptive
biasing to adjust the input level with the source follower is preferable. A replica circuit is used for the adaptive biasing of thesource follower in order to obtain the high voltage swing gain of
the 5V input. The differential amplifier is a key circuit for the
feedback control. It is, however, difficult to design a high-gain differential amplifier with only pMOS FETs in organic circuitdesign, because the impedance of a conventional organic pMOS
load [4] is low. Figure 20.4.6 shows the proposed differential
amplifier with enhancement/depletion mixed threshold voltages
realized by controlling the back-gate voltage [5]. The load transistors M4 and M5 are of the depletion type and have higher impedance than the conventional topology because the gate and source
are tied (off-connection if it is an enhancement FET). Figure20.4.7(a) shows the measured characteristics for the differential
amplifier. A gain of 6.4 is achieved, which is higher than 2.3 forthe organic analog amplifier reported in [4]. Figure 20.4.7(b)shows the measured successful operation of the feedback control.
Figures 20.4.7(c) and (d) show the measured DC and AC characteristics for the cascaded source follower and the single amplifier,respectively. The DC gain is measured to be 2.6 and the measured
bandwidth is 430Hz.
Acknowledgements:
This work is partially supported by CREST, JST, and MEXT.
References:[1] T. Sekitani, et al., “A Large-Area Flexible Wireless Power Transmission
Sheet Using Printed Plastic MEMS Switches and Organic Field-EffectTransistors,” presented at IEDM 2006., Dec., 2006.
[2] M. Takamiya, et al., “An Organic FET SRAM for Braille Sheet Displaywith Back Gate to Increase Static Noise Margin,” ISSCC Dig. Tech. Papers,
pp. 276-277, Feb., 2006.
[3] T. Someya, H. Kawaguchi, and T. Sakurai, “Cut-and-Paste Organic FET
Customized ICs for Application to Artificial Skin,” ISSCC Dig. of Tech.
Papers, pp. 288-289, Feb., 2004.
[4] N. Gay, et al., “Analog Signal Processing with Organic FETs,” ISSCC
Dig. Tech. Papers, pp. 278-279, Feb., 2006.
[5] S. Iba, et al., “Control of Threshold Voltage of Organic Field-EffectTransistors with Double Gate Structure,” Applied Physics Letters, 87,023509, July 2005.

• 2007 IEEE International Solid-State Circuits Conference

1-4244-0852-0/07/$25.00 ©2007 IEEE.

ISSCC 2007 / February 13, 2007 / 3:15 PM
OBLSEL[2:0]

3-to-8 Decoder

OWLSEL[2:0]Dataout+c00V

b0

Onb1b7

Off

SRAM

LS

MEMS switch

SRAMb7SRAM

LSb0b1a0a1LSLSa7c0a0

3.5 13.56
MHz MHzSRAM

Level shifterLSLSLS

a13-to-8 Decoder

5-mm distance
between RXand TX-coilsMWLSEL[2:0]

38 series-connected LEDs
light up by the sheet.
(without OFETs)

LSLS

BL

3-to-8 Decoder
SRAM

TX-coil array

3-to-8 Decoder

RX-coilm
25.4 mLS

21 cm

LSRX-coil

LS

MEMS switches
for powertransmission
OFETs forRX position
detection

21 cm

MBLSEL[2:0]WL

8 x 8 TX-coil array

SRAM

a7

6.4 mmRES

SRAM
LS
LS: Level shifter

22.2 mm

V MON

Fabricated and shown in Fig. 20.4.1

Figure 20.4.2: Circuit diagram of wireless power transmission sheet.

Figure 20.4.1: Wireless power transmission sheet.
1 unit = 25.4 mm4 unit

y1x1yy4x4Power efficiency (%)

3x32x23020

4x410001x1
0.2

0.4

0.6

60.8max
ave.
min

2010

Displacement (y) (unit)

0Without RX-coil120

a b0-0.1-0.2-0.3

250

500

750

Time (ns)1000 1250

51015

Frequency (f1) (MHz)

(b)

20

Transmitted power = 200mW

40 V

Level shifter

InSF

SF

SF

Out

SF

Fabricated(Fig. 1)
2.5 V+

20V+-

SF

20 V 0 V-pMOS withback gate

SF

8060

VDD

40Bias200111213

VDD

VDD

Efficiency = 50%14Frequency (MHz)

15InInOut

4000/50

4000/50

100/50

Bias100/50
100/50Single amp(b)Figure 20.4.5: (a) VMON waveform to show that 33% amplitude change is observed withand without RX-coil. (b) Measured received power spectrum.

M1

4000/50

Source followerb-a= 33%b(a)0Figure 20.4.4: (a) Circuit diagram to share one TX-coil for power transmission andposition detection. (b) Measured frequency response of monitored voltage (VMON).

Outf1 = 3.5 MHz
0Received power (mW)

VMON (V)

0.1

WithRX-coil

101000.23(a)5V0VWith RX-coil

VMON

13.56 MHzfor powertransmission

Ref[1] 1x1 2x2 3x3 4x4
Activated number of TX-coils

Figure 20.4.3: (a) Various arrangement of TX-coils and RX-coils. (b) Measurement
setup. (c) Measured power efficiency dependence on displacement (y). (d) Power
efficiency dependence on number of activated coils.

0.3

Maximum
frequency
for OFETs

42C13001

CpBest choice

40RXTX(d)50

WithoutRX-coil

5f1 =3.5 MHz
for position
detection

200 mW 100-Pm
100µm distance60(c)Ref40RXSpectrum
analyzer

6050

TX

TX

y3x32x2(b)RXPower efficiency (%)

yRef[1]RX

VMON (V)TX

TX RX(a)8000/50

Bk1
M3

M2
InOut

16000/50

Bk2
M4

Inb(Outb)

4000/50

M5
Bk3
Differential amp
M1, 4, 5 : Depletion mode
M2, 3: Enhancement mode

Figure 20.4.6: Circuit diagram of level shifter based on organic analog circuits withadaptive biasing and feedback loop.

Continued on Page 609

DIGEST OF TECHNICAL PAPERS •363

ISSCC 2007 PAPER CONTINUATIONS

In
1020+-0040In (V)

1040

Ȁ0Ȁ

-5InSFȀ

1k-3dB@ 430Hz

Out-101000Out2SF

Out / Out@10 Hz (dB)InȀOut120ȀOut1, Out2 (V)Out13030(b)ȀGain = 2.6

Ȁ20In (V)

(a)
40

Out

SF

10Out

30

In20

Ȁ

10Outb

Ȁ

20V

20V

-30
Ȁ

Out

2000+Outb
Out (V)30

Ȁ40

Gain = 6.4

ȀOut, Outb (V)40

Out21020Ȁ

In (V)

(c)

30

40

-15
10100

10kFrequency (Hz)(d)

Figure 20.4.7: Measured results of level shifter. (a) DC characteristics of differential
amplifier. (b) Feedback control. (c)(d) DC and AC characteristics for cascaded source
follower and single amplifier respectively.

609• 2007 IEEE International Solid-State Circuits Conference

1-4244-0852-0/07/$25.00 ©2007 IEEE.