Wireless Energy Transfer System with Multiple
Coils via Coupled Magnetic Resonances
Sanghoon Cheon, Yong-Hae Kim, Seung-Youl Kang, Myung Lae Lee, and Taehyoung Zyung
A general equivalent circuit model is developed for a
wireless energy transfer system composed of multiple coils
via coupled magnetic resonances. To verify the developed
model, four types of wireless energy transfer systems are
fabricated, measured, and compared with simulation
results. To model a system composed of n-coils, node
equations are built in the form of an n-by-n matrix, and
the equivalent circuit model is established using an electric
design automation tool. Using the model, we can simulate
systems with multiple coils, power sources, and loads.
Moreover, coupling constants are extracted as a function
of the distance between two coils, and we can predict the
characteristics of a system having coils at an arbitrary
location. We fabricate four types of systems with relay
coils, two operating frequencies, two power sources, and
the function of characteristic impedance conversion. We
measure the characteristics of all systems and compare
them with the simulation results. The flexibility of the
developed model enables us to design and optimize a
complicated system consisting of many coils.
Keywords: Energy transfer,
resonance, model, multi-coils.
wireless,
magnetic
Manuscript received July 19, 2011; revised Feb. 29, 2012; accepted Mar. 23, 2012.
This work was partly supported by the IT R&D program of MKE/KCC/KEIT [100351812010-01, Development of RF energy Transmission under 100Watts and Harvesting
Technology] and [12ZF1170, Future Technology Researches in the Fields of Informations and
Telecommunications].
Sanghoon Cheon (phone: +82 42 860 1651, sh.cheon@etri.re.kr), Yong-Hae Kim
(yhakim@etri.re.kr), Seung-Youl Kang (kang2476@etri.re.kr), Myung Lae Lee
(mllee@etri.re.kr), and Taehyoung Zyung (thz@etri.re.kr) are with the Convergence
Components Materials Laboratory, ETRI, Daejeon, Rep. of Korea.
http://dx.doi.org/10.4218/etrij.12.0111.0461
ETRI Journal, Volume 34, Number 4, August 2012
© 2012
I. Introduction
In recent years, there has been an increasing interest in
wireless power transfer technology. In particular, significant
progress has been charted for inductively coupled systems [1][11]. Inductively coupled power transfer systems have been
developed for a wide range of applications, including vehicle
battery charging systems, and a very high end-to-end system
efficiency of up to 80% has been documented [4], [8].
However, most studies have been restricted to close range, that
is, typically shorter than 30% of the coil diameter. The
transmission distance is generally close to 1 cm [4], and 15 cm
is considered a fairly large distance [8]. Results at the midrange (that is, more than twice the coil diameter) have not been
reported.
Recently, MIT proposed a new scheme based on strongly
coupled magnetic resonances, thus presenting a potential
breakthrough for a mid-range wireless energy transfer [12],
[13]. The fundamental principle is that resonant objects
exchange energy efficiently, while non-resonant objects do not.
Figure 1 shows the basic coil system composed of four coils:
drive, transmit resonance, receive resonance, and load coils.
The transmit resonance coil is coupled to the drive coil which
is linked to a power amplifier that supplies energy to the system.
The receive resonance coil is coupled with the load coil to
provide the power to an external load. The scheme is carried
with a power transfer of 60 W and has RF-to-RF coupling
efficiency of 40% for a distance of 2 m, which is more than
three times the coil diameter. We expect that coupled magnetic
resonances will make possible the commercialization of a midrange wireless power transfer.
Magnetic resonance coupling is a new concept in wireless
energy transmission. The analyses used in early research were
Sanghoon Cheon et al.
527
Load coil
Drive coil
To verify the model, we fabricate four types of systems that
have relay coils, two operating frequencies, two power sources,
and the function of characteristic impedance conversion. We
measure the characteristics of all systems and compare them
with the simulation results.
Receive
resonance coil
Transmit
resonance coil
⎛ iL1 ⎞ ⎛ iC1 ⎞
0
⎞
⎜
⎟ ⎜
⎟ ⎛
⎟
⎜ iL2 ⎟ ⎜ iC 2 ⎟ ⎜
0
⎟
⎜ # ⎟ ⎜ # ⎟ ⎜
⎜
⎟
#
⎜
⎟ ⎜
⎟ ⎜
⎟
⎜
⎟ ⎜
⎟ ⎜
⎟
⎜
⎟ ⎜
⎟
⎟
0
⎜ iLi−1 ⎟ ⎜ iCi−1 ⎟ ⎜
⎜
⎟
⎜i ⎟ ⎜i ⎟
i − i / ( jωCi ) / Z ⎟
⎜ Li ⎟ ⎜ Li ⎟ ⎜ S Ci
⎟
⎜ iL ⎟ + ⎜ iL ⎟ = ⎜
0
i +1
⎜
⎟.
i +1
⎜
⎟ ⎜
⎟ ⎜
⎟
#
⎜ # ⎟ ⎜ # ⎟
⎜
⎟
⎜
⎟ ⎜
⎟ ⎜
⎟
⎜
⎟ ⎜
⎟
⎟
0
⎜ iL ⎟ ⎜ iLk −1 ⎟ ⎜
k −1
⎜
⎟
⎜
⎟ ⎜
⎟
⎜ iLk ⎟ ⎜ iLk ⎟ ⎜ −iCk / ( jωCk ) / Z 0 ⎟
⎟
⎜
⎟ ⎜
⎟ ⎜⎜
0
⎟
i
i
C
⎜ Lk +1 ⎟ ⎜ k +1 ⎟ ⎜
⎟
#
⎜ # ⎟ ⎜ # ⎟ ⎝
⎠
⎠
⎝
⎠ ⎝
Fig. 1. Schematic of wireless energy-transfer system using
coupled magnetic resonances.
based on pure physical theory and failed to provide tangible
findings for electrical engineers [12]-[18]. The reports [8]-[23]
do not clearly present practical design methods. Reports [24][26] accurately present the characteristics of a coupled
magnetic resonance system, and these reports can be
practically applied to the design. Through the circuit analysis of
an equivalent model, the system transfer function can be
obtained as a function of frequency, and several key parameters
of the system can be analyzed [24]. Also, the coupled mode
theory [12] can be extended for an analysis of other systems
[25]. The proposed methods are easily applied to a system
composed of several coils and having negligible weak coupling.
However, if the system is composed of many coils and weak
coupling affects the system characteristics, it is very difficult to
apply the methods for analysis.
RF engineering is also a candidate method for the analysis of
a coupled magnetic resonance system. In [26], the equivalent
circuit model with a compensation source is developed to take
into account both weak coupling and loss. The model has been
established in an RF simulator and the parameters for the
model are extracted through a measured S-parameter.
Simulation results show strong agreement with the
measurement results [26].
Previous studies have analyzed the system consisting of four
coils. In this paper, the node equation in the form of a matrix is
written for a system composed of n-coils by expanding the
node equation of [26]. The general model of the coupled
magnetic resonance system is established using an electric
design automation tool, and the model enables us to design a
system composed of several coils, power sources, and loads.
⎛
⎜
⎜
⎜
⎜
⎜
⎜
⎜
⎜
⎜
⎝
jω L1
jω M 12
jω M 13
jω M 21
jω M 31
jω L2
jω M 32
jω M 23
jω L3
⋅
⋅
528
jω M n 2
⋅
⋅
⋅
⋅
jω M n1
⋅
⋅
jω M n 3
Sanghoon Cheon et al.
⋅
⋅
⋅
⋅
jω M 1n ⎞ ⎛ iL1 ⎞ ⎛ 1 /
⎟ ⎜ ⎟ ⎜
jω M 2 n ⎟ ⎜ iL 2 ⎟ ⎜
jω M 3n ⎟ ⎜ iL 3 ⎟ ⎜
⎟ ⎜ ⎟ ⎜
⋅ ⎟×⎜ ⋅ ⎟ = ⎜
⋅ ⎟ ⎜ ⋅ ⎟ ⎜
⎟ ⎜ ⎟ ⎜
⋅ ⎟ ⎜ ⋅ ⎟ ⎜
jω Ln ⎠⎟ ⎝⎜ iLn ⎠⎟ ⎜⎝
(1)
II. Modeling for System Composed of n-coils
1. Node Equation and Equivalent Circuit Model
In general, the wireless power transfer system has three
types of coils: a coil with a power source, a floating coil, and
a coil with a load. The resonance frequency of floating coils
is an operating frequency. Figure 2 shows the proposed
equivalent circuits of three such types of coils. The equivalent
circuit of a system composed of n-coils has a number of
resonant circuits, n, and each resonant circuit has a number of
compensation sources of n–1. The compensation source
represents the effect of mutual inductance related to coupling
with other coils [26]. Here, Li,j,k is a self-inductance, and Ci,j,k
is a capacitance of the coil.
From the equivalent circuit of each coil, we can obtain (1)
and (2) in the form of a matrix by writing the current in each
branch at the node whose voltage is Vi,j,k. This matrix can be
jωC1
0
0
0
0
1 / jωC2
0
0
1/ jωC3
⋅
0
⋅
⋅
⋅
⋅
0
⋅
0
⎞ ⎛ iC1 ⎞
⎟ ⎜ ⎟
⎟ ⎜ iC 2 ⎟
⎟ ⎜ iC 3 ⎟
⎟ ⎜ ⎟
⋅
⎟ × ⎜ ⋅ ⎟ . (2)
⎟ ⎜ ⋅ ⎟
⋅
⎟ ⎜ ⎟
⋅
⋅
⎟ ⎜ ⋅ ⎟
⋅ 1/ jωCn ⎟⎠ ⎜⎝ iCn ⎟⎠
0
⋅
⋅
⋅
⋅
0
0
ETRI Journal, Volume 34, Number 4, August 2012
Vk
Vj
Vi
iC
iL
iC
iL
Ci
is
+
–
+
–
iL
Lj
Li
Z0
Table 1. Extracted parameters.
iC
Cj
+
M1
–
+
M2
–
+
+
Mn–1
–
–
(a)
Lk
Ck
M1
+
–
+
M2
–
Z0
(b)
(c)
Coupling constant (A. U.)
0.5
Coil-1 vs. coil-1
Coil-1 vs. coil-A
0.3
0.2
Coil-1 vs. coil-1
2.72E+07
–3.64
138
Coil-1 vs. coil-A
1.14E+06
–3.19
97
κ = A( x + C ) B ,
(3)
where x is the distance between the coils and κ is the coupling
constant. This procedure is performed for the possible
combinations of coils. Table 1 shows the extracted coefficients.
Each coil used in Table 1 has a diameter of 15 cm, and the
diameter of the conductor section is 1 mm. Coil-1 has twelve
turns and a pitch of 3 mm and coil-A has one turn.
We can predict the characteristics of a system in which the
coils have arbitrary locations by establishing (3) in the
simulator. The completed model requires just the location of
each coil for simulation.
III. Model Validation and Experiment Results
0.1
0.0
0
C
M2
Fig. 2. Equivalent circuits of three types of coils: (a) coil with
power source, (b) floating coil, and (c) coil with load.
0.4
B
M1
+
Mn–1
–
Mn–1
A
100
200
300
400
500
600
700
Distance (mm)
Fig. 3. Coupling constant as function of distance between coils.
established in Advanced Design System (ADS), which is
design software widely used for RF and microwave
applications [27].
2. Parameter Extraction for System Composed of n-Coils
To model a system composed of n-coils, we have to extract
the following parameters. Each coil has the parameters of
capacitance, self-inductance, and RLoss. Also, to model the
compensation sources, we need coupling constants for the
possible combinations of coils. As a result, there is a total of
3n+n(n–1)/2 parameters. For example, a system composed of 4
coils has 18 parameters, and a system composed of 6 coils has
33 parameters.
To simulate the system characteristics as the distance
between coils, we need a function whose output is a coupling
constant and whose input is the distance between coils.
Figure 3 shows an extracted graph with such a function. We
have measured the coupling constant according to the distance
and extracted coefficients of (3) by fitting with the
measurement results.
ETRI Journal, Volume 34, Number 4, August 2012
Using the developed model, we predict the characteristics of
various systems, measure their characteristics, and compare
them with the simulation results. Each system has relay coils,
two operating frequencies, two power sources, and the function
of characteristic impedance conversion. The system
configuration is very useful when applying it to a commercial
product of wireless power transfer.
1. Relay System
Although the technology based on strongly coupled
magnetic resonances is able to transmit energy over a much
longer distance than the traditional method can, degradation in
efficiency is unavoidable in a system in which the transmitter is
away from the receiver. In this case, we can improve the
efficiency by adding coils to create a relay device. Figure 4(b)
shows such a relay system. A system without relay coils,
shown in Fig. 4(a), shows an efficiency of –18 dB (1.6%).
Coil-A and coil-1 in Table 1 are used for feeding and resonator
coils, respectively. The resonance frequency of coil-1 is
11 MHz.
To improve the efficiency of the system in Fig. 3(a), we can
use the relay coils, which are the same as coil-1. The number of
the required coils and their location can be determined
through simulation using the developed model. Because of the
fast simulation time, we can easily obtain the transfer
characteristics according to the position of the relay coils, and
Sanghoon Cheon et al.
529
the optimum position for maximum efficiency can be
determined. Figure 5(a) shows the improved transfer
characteristics achieved through such a process. After adding
two relay coils, the efficiency is improved to –4.2 dB (38%).
Figure 5(b) shows the measurement and simulation results,
simultaneously. The agreement between the model and
experiment data is fairly good. This example shows that we
can predict and optimize the characteristics of the system
through the developed model without experimentation.
The proposed relay scheme overcomes the mid-range
limitation of the present wireless electric system, allowing
more flexible power transmission without sacrificing efficiency.
50 cm
2. Two-Tone System
(a)
13.2 cm
17 cm
13.2 cm
(b)
Fig. 4. Photographs of system (a) without and (b) with relay coils.
0
Gain (dB)
–20
–40
–60
–80
–100
4M
Measurement (relay)
Measurement (no relay)
6M
8M
10M
12M
Frequency (Hz)
14M
16M
(a)
0
Model
Measurement
–10
Gain (dB)
–20
–30
Some applications of wireless energy transfer require two or
more receiving devices, and magnetic resonance coupling is
expected to support multiple receivers at the same time.
However, if the resonance frequencies of receiving devices are
the same, interference caused by a coupling of receiving coils
makes the implementation very difficult. When multiple
receivers are in close proximity to each other, there is a
coupling interaction between the receivers, which makes the
resonant peak split into separate peaks [28].
We overcome this difficulty through the use of resonant coils
with different resonance frequencies. The coupling interaction
occurs only when coils have the same resonant frequency.
Thus, if the resonance frequencies of coils are different, the
resonant peak frequency does not split and it can be maintained.
Figure 6 shows the experimental setup for a system with two
receiving devices. The two coils of the transmit device in the
middle of the system have different resonance frequencies, and
the two receiving coils on both sides have corresponding
resonance frequencies. A coil with a 3 mm pitch has a
resonance frequency of 9.8 MHz, and a coil with a 4 mm pitch
has a resonance frequency of 11.3 MHz. The number of turns
of each coil is 11. Figure 7 shows the measurement and
simulation results of power transferred to two receiving devices
at both ends. The peak frequency of gain to each receiver is
shifted slightly from the resonance frequency of the resonant
coil, from 9.8 MHz to 10.9 MHz at the left receiver and from
11.2 MHz to 13.2 MHz at the right receiver. This means that
the coupling interaction is small but still exists despite the
different resonant frequencies of 9.8 MHz and 11.2 MHz.
In the proposed system, wireless power transmission to two
–40
Res. freq.
=9.8 MHz
–50
–60
6M
8M
10M
12M
Frequency (Hz)
14M
Res. freq.
=11.2 MHz
16M
(b)
Fig. 5. (a) Measured results without and with relay coils and (b)
simulation and measurement results of system in
Fig. 4(b).
530
Sanghoon Cheon et al.
Fig. 6. Photograph of experimental setup for two-tone system.
ETRI Journal, Volume 34, Number 4, August 2012
0
Model
Measurement
Gain (dB)
–10
30 cm
–20
–30
(a)
–40
30 cm
–50
8M
10M
12M
14M
30 cm
16M
Frequency (Hz)
(a)
0
Gain (dB)
–10
(b)
Model
Measurement
Fig. 8. Photographs of system for charging-zone experiment: (a)
normal transfer system. Device on the right is receiver and
has load coil inside resonance coil. (b) System composed
of two transmitting devices and one receiving device.
–20
–30
0
–40
10M
12M
14M
–6.392 dB
16M
Frequency (Hz)
Fig. 7. Comparison of experiment data to simulated results:
transmitted to (a) left and (b) right devices.
–10
Gain (dB)
–50
8M
After adding sending coil
Two transmitting devices
One transmitting device
–9.018 dB
–20
receivers can be done at the same time by generating a twotone signal at the transmitter. However, the transmitter of the
system needs as many resonant coils as receivers and a multitone signal should be generated, which makes it difficult to
implement.
Ultimately, it is possible to simulate such a system using the
developed model. As shown in Fig. 7, the predictions of the
model are in agreement with the measurement results.
10.5M
11.0M
11.5M
Frequency (Hz)
(a)
0
Model
Measurement
–20
Gain (dB)
3. System with Two Transmitting Devices
–30
10.0M
–40
As an alternative to including relay coils in the system to
improve transmission efficiency, we can add more transmit
–60
coils to the system to improve transmission efficiency. Figure 8
shows a photograph of such a system. To improve the
–80
efficiency of the normal system in Fig. 8(a), we can place a
8M
10M
12M
14M
16M
Frequency (Hz)
transmit coil at the other side, as shown in Fig. 8(b). After
(b)
adding the coil, we achieve an efficiency improvement of 2.6 dB.
Figure 9(a) shows the measurement results, and Fig. 9(b)
Fig. 9. (a) Measurement results with one transmitting device and
shows a comparison with the simulation. It also shows that we
two transmitting devices and (b) comparison with
can predict the system characteristics through simulation using
simulation results of system in Fig. 7(b).
ETRI Journal, Volume 34, Number 4, August 2012
Sanghoon Cheon et al.
531
1 turn
2 turn
X
30 cm
30 cm
Fig. 10. Photograph of system for charging-zone experiment.
Receiving device is placed in an arbitrary location.
10
–5
9
Peak frequency (MHz)
11
Gain (dB)
Fig. 12. Photograph of system for impedance conversion
experiment.
12
0
m1
S(2, 2)
–10
–27
–18
–9
0
9
18
8
27
m1
Freq.=10.93 MHz
S(2, 2)=0.447/5.784
Impedance=Z0*(2.578+j0.290)
X (cm)
Fig. 11. Efficiency vs. location of receiving coil. Red line shows
corresponding frequency.
4. Conversion of Characteristic Impedance
Depending on the structure of the power source or value of
532
Sanghoon Cheon et al.
Freq. 6.100 MHz to 16.10 MHz
(a)
0
–10
Gain (dB)
the developed model. We can find out the expression for the
coupling coefficient in terms of angle between two coils. If we
add the expression to the developed model, the characteristics
of a more complicated system can be predicted by simulation.
For mobile devices, it is very practical that the charging
position is not fixed and has a degree of freedom. Figure 8(b)
shows the possibility of a one-dimensional charging zone
consisting of two transmit coils. In a real situation, the
receiving device would be placed in any position between two
transmit coils, as shown in Fig. 10. Figure 11 shows the
measured efficiency versus location, and we obtain the
available charging area between two transmit coils. The red
line in Fig. 11 is a peak frequency of gain corresponding to the
efficiency. The change of peak frequency is due to the coupling
interaction between the resonant coils in the transmitter and
receiver. We must change the operating frequency to achieve
optimum efficiency in Fig. 11, depending on the receiver
position.
As there is an increase in portable devices, including cellular
phones, people will need a charging area, not a charging pad
made by induction technology. Figure 10 is a good example of
needing such a requirement.
–2.45 dB@
10.93 MHz
–20
–30
–40
8M
10M
12M
14M
Frequency (Hz)
(b)
Fig. 13. Measured S-parameter of system in which impedance of
input port is 50 Ω and output is 50 Ω: (a) S11 and (b) S21.
the load impedance, we may wish to design the characteristic
impedance of the system with a value other than 50 Ω. In this
case, an inductor and capacitor can be used for the matching
function. However, there may be a drawback, such as an
increase in cost and heat loss at the lumped elements. We can
solve these problems using a feeding coil and load coil with
different inductance values. Equation (4) is an analytic result
ignoring the weak coupling and heat loss in a conventional
system composed of four coils [26]. From (4), input impedance
from the power source is changed in proportion to the
ETRI Journal, Volume 34, Number 4, August 2012
m1
Model
Measurement
S(2, 2)
m1
Freq.=10.93 MHz
S(2, 2)=0.092/142.488
Impedance=Z0*(0.859+j0.097)
Freq. 6.100 MHz to 16.10 MHz
Freq. 6.100 MHz to 16.10 MHz
(a)
(a)
0
0
–10
–1.52 dB
@10.93 MHz
Gain (dB)
Gain (dB)
–10
–20
–30
–20
–30
–40
–40
8M
10M
12M
Frequency (Hz)
–50
6M
14M
Model
Measurement
8M
12M
10M
Frequency (Hz)
14M
16M
(b)
(b)
Fig. 14. Measured S-parameter of system in which impedance of
input port is 50 Ω and output is 150 Ω: (a) S11 and (b) S21.
Fig. 15. Comparison with simulation results of system in which
impedance of input port is 50 Ω and output is 150 Ω,
from Fig. 12: (a) S11 and (b) S21.
inductance ratio of the feeding and load coils.
Yin = jω 0 C1 +
=
⎛
L4
Y0 + ⎜
L1
⎝
simulation results for the system with a port impedance of 50 Ω
to 150 Ω. We can verify the usefulness of the developed model
once again from a well-predicted value of S21.
⎛
1 ⎞
⋅ ⎜ Y0 + jω0 C4 +
⎟
jω0 L4 ⎠
⎝
L4
1 ⎞
jω 0 C1 +
⎟ + jω0 C4
jω0 L1 ⎠
L1
L4
L1
L
≈ 4 Y0 .
L1
IV. Conclusion
(4)
Figure 12 shows a photograph of the system used for the
impedance change experiment. The feeding coil has one turn
and an inductance of 0.467 μH. The load coil has two turns and
an inductance of 1.505 μH. The inductance of the load coil is
three times that of the feeding coil. Therefore, we can obtain
maximum efficiency if the load impedance is three times the
characteristic impedance of the power source.
Figures 13 and 14 are the measurement results of the system,
in which the port impedance is 50 Ω to 50 Ω and 50 Ω to
150 Ω, respectively. The port impedance of 50 Ω to 150 Ω
shows well-matched results without reflected power (S11) and
higher gain (S21). Figure 15 shows the measurement and
ETRI Journal, Volume 34, Number 4, August 2012
It is clear that a field simulation is the most obvious method
for the analysis of wireless power transfer via coupled
magnetic resonances, such as HFSS or CST. However, it takes
a long time to perform a field simulation, and it is not possible
to perform such a simulation when there is a variety of different
coil configurations and characteristics according to the distance
between the coils. For example, it takes about 10 min to solve
one case with the MIT system using HFSS. Moreover, it shows
different results depending on boundary conditions.
In this paper, a novel circuit model for a wireless power
transfer system via coupled magnetic resonances was proposed.
We built general node equations of the system, composed of ncoils, and established an equivalent circuit model in ADS [27].
The model has the data of a coupling constant according to the
Sanghoon Cheon et al.
533
distance between the coils, and we can simulate a system in
which coils are placed at arbitrary points. As a result, we can
perform a very fast simulation for the different coil
configuration composed of many coils.
To verify the model, we made four types of systems,
measured their S-parameters, and made a comparison with the
simulation results. The configuration of a system is very
significant when developing a mid-range wireless power
transfer system where the magnetic resonance is expected to be
the most advantageous.
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ETRI Journal, Volume 34, Number 4, August 2012
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Sanghoon Cheon received his BS, MS, and
PhD in electronics engineering and computer
science from the Korea Advanced Institute of
Science and Technology (KAIST), Daejeon,
Rep. of Korea, in 1993, 1995, and 2001,
respectively. In 2001, he joined Knowledge*on
Inc. in Korea, where he was involved in the 6inch InGaP/GaAs HBT foundry service business. He worked on large
signal modeling for microwave devices from 2001 to 2003 and the
development of a power amplifier for the WCDMA system from 2004
to 2005. In 2006, he moved to ETRI, Daejeon, Rep. of Korea, where
he has been involved in the development of micro-bolometer for
infrared focal plane arrays. His current research interests include
wireless power transfer systems and meta-materials for antennas.
Yong-Hae Kim received his BS and PhD in
physics from the Korea Advanced Institute of
Science and Technology (KAIST), in 1993 and
1997, respectively. From 1997 to 2000, he
worked with SK Hynix Semiconductor Inc. and
developed the 0.13 μm DRAM technology. He
joined ETRI, Daejeon, Rep. of Korea, in 2000,
and he has been involved in flexible display technology, such as low
temperature poly-Si TFT on plastic substrates and active matrix OLED.
His research interests include digital paper, application of metamaterial, and wireless power transmission.
Myung Lae Lee received his BS in physics
from Dong-A University, Busan, Rep. of Korea,
in 1989 and his MS and PhD in physics from
the Korea Advanced Institute of Science and
Technology (KAIST), Daejeon, Rep. of Korea,
in 1992 and 1998, respectively. Since 1998, he
has been working for ETRI, Daejeon, Rep. of
Korea. His research interests include the design, fabrication, and
characterization of MEMS devices with signal processing ICs for
applications in optics, RF, Bio, and ubiquitous sensor networks (USNs).
In recent years, he has branched his study into several areas, including
wireless power transfer by a magnetic resonance phenomenon, metamaterials for small antennas, and highly-sensitive gas sensors using
nanogold particles.
Taehyoung Zyung graduated from Seoul
National University, Seoul, Rep. of Korea, in
1977 and worked at the Korea Institute of
Science and Technology (KIST), Seoul, Rep. of
Korea, as a researcher for three and a half years
from 1978. He received his PhD at Texas Tech
University, Lubbock, TX, USA, in 1986. He
performed post-doctoral study as a research associate at the University
of Illinois at Urbana-Champaign, Urbana and Champaign, IL, USA,
from 1986 to 1989. He joined ETRI, Daejeon, Rep. of Korea, in 1989
and was a director and/or an executive director of the future technology
research division from 2000 to 2008. His work has been published in
more than 150 SCI journals, and he has filed about 30 patents.
Seung-Youl Kang received his BS in physics
from Seoul National University, Seoul, Rep. of
Korea, in 1987 and his MS and PhD in Physics
from the Korea Advanced Institute of Science
and Technology (KAIST), Daejeon, Rep. of
Korea, in 1990 and 1994, respectively. Since he
joined ETRI, Daejeon, Rep. of Korea, in 1994,
he has been involved in flexible display technology, such as organic
TFTs on plastic substrates and electronic paper. Currently, his research
interests include electronic paper, flexible electronics, applications of
meta-materials, and wireless power transfer systems.
ETRI Journal, Volume 34, Number 4, August 2012
Sanghoon Cheon et al.
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