J.W. Kim et al.: Optimal Design of a Wireless Power Transfer System with Multiple Self-Resonators for an LED TV
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Optimal Design of a Wireless Power Transfer System with Multiple Self-Resonators for an LED TV JinWook Kim, Student Member, IEEE, Hyeon-Chang Son, Do-Hyeon Kim, and Young-Jin Park, Member, IEEE Abstract — This paper proposes an optimal design method in an asymmetric wireless power transfer (WPT) system for a 150 watt LED TV. The WPT system has three self-resonators: a Tx resonator, an Rx resonator, and an intermediate resonator. The Tx and Rx resonators are perpendicular and offset, respectively, to the intermediate resonator in the geometry. For optimal design, the WPT system is analyzed using an equivalent circuit. In particular, a calculation method for mutual inductance in the system is expressed. The calculation results of mutual inductance are used to determine the optimal position of each self-resonator for maximizing the power transfer efficiency. For verification, a WPT system for a 150 watt, 47 inch LED TV is fabricated at 250 kHz. The WPT system exhibits wireless power transfer efficiency of 80%.1 Index Terms — Mutual inductance, optimal design, rectangular self-resonator, wireless power transfer, wireless LED TV.
I. INTRODUCTION As more and more home appliances and mobile devices are used, the strong demand for the development of wireless power transfer (WPT) is increasing. WPT can allow electronic devices to be charged conveniently for power without a power cord and a cumbersome battery charger adapter. There are two implementation methods for WPT. One is using the electromagnetic wave [1]-[3] and the other is using the electromagnetic near-field. Recently, WPT using the magnetic near-field, especially, a magnetically coupled resonance phenomenon has been widely studied [4]-[9]. In such systems, two self-resonators of high quality factor (Qfactor) or low loss have been used and arranged coaxially in the geometry. Multiple self-resonators have been used to extend the effective range of WPT [10]-[14], since it is difficult to extend the coverage of WPT using only two selfresonators. However, most previous papers have mainly dealt with WPT systems of symmetric and identical self-resonators, although Tx and Rx self-resonators are usually asymmetric and arranged in an arbitrary way in the practical application. Furthermore, few application reports on a WPT system using magnetically coupled resonance for home appliances of more than 100 watt power consumption have been reported. 1 J. Kim, H.-C. Son, D.-H. Kim, and Y.-J. Park are with University of Science and Technology (UST) and Korea Electrotechnology Research Institute (KERI), 111, Hanggaul-ro, Sangnok-gu, Ansan city, Gyeonggi-do, Korea (e-mail :
[email protected]).
Contributed Paper Manuscript received 06/18/12 Current version published 09/25/12 Electronic version published 09/25/12.
In this paper, the practical application of a WPT system with three self-resonators to a wireless 150 watt LED TV is reported. The WPT system has Tx, Rx, and intermediate resonators. The three self-resonators are asymmetric and arranged in a perpendicular and offset way in the geometry. Equivalent circuit of the system is presented for detailed analysis. In addition, a calculation method for mutual inductance between two asymmetric rectangular self-resonators is also shown. Finally, with the equivalent circuit and calculation of mutual inductance, an optimal design method for the WPT system is proposed with a view to obtaining maximum power transfer efficiency. The paper is organized as follows. In Section II, a circuit analysis of the proposed WPT system is presented. In Section III, a method for mutual inductance calculation between two rectangular coils in the system is shown. In Section IV, the three fabricated self-resonators and the source and load coils are explained. In Section V, the optimal positions of each selfresonator and the impedance matching condition are determined using the results from the previous sections. In Section VI, an LED TV with 150 watt wireless power supply is described and some experimental results are reported. II. CIRCUIT ANALYSIS OF A ASYMMETRIC WPT SYSTEM WITH AN INTERMEDIATE RESONATOR
Fig. 1. Configuration of the proposed WPT system with an intermediate resonator for a wireless LED TV. (a) Schematic diagram and (b) detailed drawing (coils only).
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Fig. 2. Equivalent circuit of the proposed WPT system for a wireless TV system.
Fig. 1(a) gives a schematic diagram of a wireless TV using the new WPT system that is proposed in this work. Fig. 1(b) shows the detailed configuration of the proposed WPT system. The main feature is that the WPT system has three selfresonators: Tx, Rx, and intermediate resonators. In addition, the Tx and intermediate resonators are arranged perpendicularly in the geometry, whereas the Rx and intermediate resonators are placed in an offset way. The power can be transferred to the LED TV via the intermediate resonator effectively, whereas the power transfer efficiency between Tx and Rx resonators is generally very low without the intermediate resonator. The transmitter can be installed under the floor and the thin, planar intermediate resonator can be embedded on a wall. Each selfresonator is made of a few turns of a conducting loop and a lumped capacitor for tuning a target resonant frequency. The power transfer mechanism is as follows: First, AC power from a source coil that is connected with an AC source is induced at the Tx resonator; then, the induced power is transferred to the Rx resonator through the intermediate resonator [10], [14]; finally, the receiving AC power is rectified by a rectifying circuit and DC power is supplied to the LED TV. In Fig. 1(b), D1m_x and D1m_z denote the x-directed and zdirected distance between the Tx resonator and the intermediate resonator, respectively. D2m_x and D2m_z denote the respective x-directed and z-directed distance between the intermediate resonator and the Rx resonator. DS1 and D2L represent the distances between the source coil and the Tx resonator and between the intermediate resonator and the load coil, respectively. Fig. 2 shows the equivalent circuit of the proposed WPT system for a wireless TV system (shown in Fig. 1). All selfresonators are resonant at an operating frequency of 250 kHz except the load coil. The subscripts S, 1, m, 2, and L depict the source coil, Tx resonator, intermediate resonator, Rx resonator, and load coil, respectively. Parameters R, L, and C denote the resistance, inductance, and capacitance, respectively, in each self-resonator. The AC source consists of an ideal voltage source of VS and its characteristic impedance of Z0. ZTV represents the impedance of the LED TV. Zin and ZL’ denote the input impedance looking into the WPT system from the source and the rectifier from the load coil, respectively. In the circuit analysis, the load impedance is simply considered as ZL’, and all mutual inductances between self-resonators are considered, although mutual
inductances between two self-resonators that are far away from one another can be negligible compared with them between two neighboring self-resonators. The mutual inductances brought about by certain cross coupling can affect the power transfer efficiency, for example, between the source and intermediate resonators and between the load coil and intermediate resonators. Kirchhoff’s voltage law is applied to determine the currents in each self-resonator in (1). Ip and Mpq denote the currents in the p self-resonator and the mutual inductance between p- and q- self-resonators when p q, respectively, where p and q are S, 1, m, 2, and L. VS Z S I S j 0 Z1 I1 j
p 1, q p
0 Z m I m j 0 Z 2 I 2 j 0 Z L I L j
M pq I p ,
p S ,q p
M pq I p ,
p m,q p
M pq I p ,
M pq I p ,
p 2, q p
p L,q p
(1)
M pq I p ,
where Z S Z 0 RS j LS 1 / j C S
, Z1 R1 j L1 1 / j C1 , Z m Rm j Lm 1 / j Cm Z 2 R2 j L2 1 / j C2 , , and Z L RL j LL Z L ' . When the currents at each self-resonator are obtained from (1), the input impedance (Zin) and the load voltage (VL’) are easily calculated as follows: Z in
VS Z 0 , VL ' Z L ' I L . IS
(2)
When the source and load impedance is 50 Ω, the transmission and reflection coefficients are expressed as follows:
S21
2 VL ' Z
0 ZL ' 50
VS
Z Z0 S11 in . Zin Z0
, (3)
J.W. Kim et al.: Optimal Design of a Wireless Power Transfer System with Multiple Self-Resonators for an LED TV TABLE I SPECIFICATIONS OF THE FABRICATED COILS
III. MUTUAL INDUCTANCE CALCULATION BETWEEN TWO RECTANGULAR COILS
z
W2 γ=0˚
Source coil Tx resonator Intermediate resonator Rx resonator
l2
1 ··· N2 H2
γ=90˚ Rx
Load coil
Type
Width (W)
Length (l)
Height (H)
Turns (N)
spiral
100 cm
10 cm
0.2 cm
7
helical
148 cm
18.5 cm
3.8 cm
15
spiral
149 cm
98.4 cm
0.2 cm
12
spiral
96 cm
18.5 cm
0.2 cm
22
spiral
60 cm
18 cm
0.2 cm
3
Tx
I1
(-W1/2,0,0)
777
(-l1/2,0,0)
TABLE II MEASURED ELECTRIC PARAMETERS OF THE FABRICATED RESONATORS
(W1/2,0,0) y (l1/2,0,0)
1
x
H1
RS
LS
Lumped CS
RL
LL
0.22 Ω
57.5 uH
7.63 nF
0.08 Ω
11.2 uH
R1
L1
Lumped C1
Cself,1
fr1
2.5 Ω
439.4 uH
901.8 pF
27 pF
250.63 kHz
Rm
Lm
Lumped Cm
Cself,m
frm
3.67 Ω
629.1 uH
628.0 pF
20 pF
249.75 kHz
R2
L2
Lumped C2
Cself,2
fr2
2.10 Ω
531.05 uH
763.5 pF
3 pF
249.61 kHz
N1 Fig. 3. Two rectangular coils of tilting angle γ=0˚, and γ=90˚ for mutual inductance calculation.
Fig. 3 shows a schematic drawing of two rectangular N-turn coils when tilting angle γ=0˚ and γ=90˚. The coils are available in spiral or helical types. N1 and N2 denote the number of turns of each coil. W and l depict the width and length of the coil, respectively. I1 is the current on a Tx coil. Referring to a mutual inductance calculation method [15], the mutual inductance can be determined for the case of Fig. 3 as follows: M12 0
dW dl I1
M12 90
dW dl I1
N1 N2 NW W2 / dW Nl l2 / dl
p 1 q 1
r 1
s 1
N1 N2 NW W2 / dW Nl l2 / dl
p 1 q 1
r 1
s 1
Bx p, q, r, s ,
(4)
Bz p, q, r, s .
(5)
Here, dW and dl denote the length of a subdivided section on the horizontal and vertical axis, respectively. NW and Nl are the number of subdivided sections in the horizontal and vertical axis; Bx and Bz are x- and z- directed magnetic flux density. Finally, p, q, r, and s are variables for the sum. IV. FABRICATION OF TX, RX, AND INTERMEDIATE RESONATORS FOR THE WIRELESS TV SYSTEM TABLE I shows the specifications of the fabricated coils. The Tx resonator is a helical type whereas the others are a planar spiral type. All coils except the Rx resonator are made of a copper litz wire of 200 strands. Each strand is a wire of 0.12 mm in diameter (overall diameter is 2 mm). The Rx resonator is made of a copper litz wire that consists of 100 strands of a wire of 0.12 mm diameter (overall diameter is 1 mm). TABLE II shows the measured resistance, inductance, capacitance, and resonant frequency of each coil. The resonant frequencies of the Tx, Rx and intermediate resonators are almost the same as 250 kHz. The capacitors used in the selfresonators have a high Q-factor of above a few thousands,
whereas capacitors which have low Q-factor are connected to the source coil. The load coil lacks a lumped capacitor in order to both reduce the complexity and enhance the power transfer efficiency of the system. In fact, a lumped capacitor of about 40 nF is needed for resonance of the load coil. However, the large lumped capacitor of 40 nF will have a low Q-factor (or higher ohmic loss than that of the load coil), and then the Q-factor of the load coil will be worse. Therefore, a low-Q capacitor at the load coil will decrease the power transfer efficiency in comparison with such a capacitor at the source coil. V. OPTIMAL POSITIONS OF THE COILS FOR MAXIMUM POWER TRANSFER EFFICIENCY
From the analysis results in Section II, when all the electric parameters of each resonator are fixed, the power transfer efficiency from the Tx resonator to the Rx resonator via the intermediate resonator is determined by D1m_z and D2m_z when D1m_x and D2m_x are fixed as 11 cm and 20 cm, respectively. It should also be noted that the impedance matching condition for maximum power transfer from the source coil to the load coil is determined by DS1 and D2L. Therefore, from the mutual inductance calculation method in Section III, the optimal values of the four parameters (D1m_z, D2m_z, DS1, and D2L) can be estimated.
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35
90 80
30
(uH) 1m
20
M
M
2m
(uH)
70 25
15
60 50 40 30
10 5
20 0
10
20 D
30 (cm)
40
10
50
0
5
D
2m_z
10 1m_z
(cm)
15
20
Fig. 4. Calculated mutual inductances. (a) M2m according to D2m_z and (b) M1m according to D1m_z.
Figs. 4(a) and (b) show the mutual inductance calculation results between the intermediate and Rx resonators and the Tx and intermediate resonators. In Fig. 4(a), the maximum value of M2m is 31.66 uH when D2m_z = 22 cm. Because M1m is larger than M2m, D2m_z is determined first. For maximum power transfer efficiency, M1m should be satisfied with the following condition as mentioned in the analysis of intermediate WPT [14]: M1m M 2m . (6) R1R m R 2R m
Fig. 5. Transmission coefficient according to calculated MS1 and M2L.
45 M mutual inductance (uH)
Therefore, M1m = M2m (R1/R2)0.5 ≈ 33 uH from both Table II and Fig. 4(a). Then, D1m_z can be easily predicted from Fig. 4(b) as 8 cm. In Fig. 4(b), a dotted line shows the optimal point for high power transfer efficiency in the system. As shown in Fig. 4(b), M1m changes much more drastically than M2m. Hence, the position of the Tx resonator should be determined carefully. MS1 and M2L should be determined for maximum power transfer efficiency. Fig. 5 illustrates the transmission coefficient at 250 kHz according to the calculated MS1 and M2L. The color bar is the transmission coefficient. The point marked by a dot denotes the maximum power transfer efficiency. It should be noted that a certain optimal point for the maximum power transfer efficiency exists in the figure. For example, maximum power transfer efficiency of 85% can be achieved when MS1 = 35 uH and M2L = 37 uH. These values also represent the optimal impedance matching conditions (MS1opt, M2Lopt). Fig. 6 shows the calculation results of mutual inductances MS1 and M2L according to the distances of DS1 and D2L, respectively. From Fig. 5, for maximum power transfer efficiency, the optimal distances to satisfy MS1 = 35 uH and M2L = 37 uH are when DS1 = 0 cm and D2L = 0.7 cm. It is found that D2L is more precisely determined than DS1 because M2L changes to a much greater extent than MS1 according to distance.
40
M
S1 2L
(uH) (uH)
35 30 25 20
0
0.5
1
1.5 2 D , D (cm) 2L
2.5
3
S1
Fig. 6. Mutual inductance calculation results of MS1 and M2L according to distances of DS1 and D2L .
J.W. Kim et al.: Optimal Design of a Wireless Power Transfer System with Multiple Self-Resonators for an LED TV
VI. EXPERIMENTAL RESULTS AND VERIFICATION
47" LED TV
side view
Rx resonator
rectifier
Intermediate resonator (rear side)
source coil
B. Power transfer efficiency 0
S parameter (dB)
load coil
Tx resonator
779
-10
-20 measured S
11
measured S
-30
21
Fig. 7. Photograph of the fabricated wireless TV system.
calculated S
Here, the proposed WPT system is applied to a 47 inch LED TV of a maximum of 150 watt power consumption. Fig. 7 shows the fabricated wireless TV system. The LED TV is displaying a game screen. The WPT system for the LED TV is designed with a maximum power transfer efficiency of 85%. From the specification of the coils in Section IV and the result of the optimization of the geometry in Section V, the optimal values (D1m_z = 8 cm, D2m_z = 22 cm, DS1 = 0 cm, D2L = 0.7 cm) have been obtained.
calculated S
A. Mutual inductance TABLE III CALCULATED AND MEASURED MUTUAL INDUCTANCE Unit (uH)
MS1
M1m
M2m
M2L
MmL
Measurement
38.85
34.60
33.00
37.13
3.08
Calculation
35.19
32.77
31.66
36.41
3.28
Unit (uH)
MSm
MS2
MSL
M12
M1L
Measurement
4.19
1.01
0.36
0.5
0.18
Calculation
5.05
0.07
0.01
0.4
0.05
TABLE III shows the calculated and measured mutual inductances between the fabricated coils. The mutual inductances of MS1, M1m, M2m, and M2L between two adjacent coils are the most dominant factor in terms of the power transfer efficiency of the system. The measurement results are in good agreement with the calculation results. There is a slight difference between the calculated and measured results in MS1. This might be caused by inaccurate positioning between the source and Tx resonators. TABLE III also displays the mutual inductances between two nonadjacent coils (MmL, MSm, MS2, MSL, M12, and M1L), which might affect the power transfer efficiency. In particular, MSm and MmL are the second most dominant factors involved in the power transfer efficiency of this system, because these values are large enough to affect the power transfer efficiency. They are also in good agreement with each other. The others give negligible values because of their low mutual inductances. Due to the extremely low values, it is possible that these differences represent measurement error of the LCR meter.
11
-40 220
21
230
240 250 260 frequency (kHz)
270
280
Fig. 8. Calculated and measured S-parameters of the fabricated system.
Fig. 8 shows the calculated and measured S-parameters of the fabricated system. The square-marked line represents transmission coefficient (S21) whereas the circle-marked line denotes the reflection coefficient (S11). The measured results are achieved from vector network analyzer. The calculated results are achieved by substituting the measured parameters of both TABLE II and TABLE III into (1) to (3). There is a slight difference between the calculated and measured results of the reflection coefficient because the electric parameters are fixed values which are measured at 250 kHz. However, the transmission coefficients are in good agreement with each other. The transmission coefficient of the fabricated system is -0.72 dB at 250 kHz. That is, the fabricated system exhibits high power transfer efficiency of about 85%. Hence, it is noted that the input and output impedances are perfectly matched to 50 Ω because the system has only one transmission peak at an operating frequency as shown in the analysis of intermediate WPT [14]. It is also verified through measurement that the theoretical power transfer efficiency of 85% is achieved. C. Current, voltage, and impedance VTV VL’
D1
D2
D3
D4
CDC Z L’
(a)
DC capacitor
ZTV
diode
(b)
Fig. 9. (a) Rectifying circuit and (b) a photograph of the fabricated fullbridge rectifier.
The LED TV is connected to the fabricated WPT system as a load. A 47 inch LED TV can consume a maximum 150 W of power. The DC power is connected directly to a power module input of the LED TV. The impedance matching condition is the same as that in Fig. 8. Fig. 9(a) shows the full bridge rectifying circuit used in the system; Fig. 9(b) is a
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IEEE Transactions on Consumer Electronics, Vol. 58, No. 3, August 2012
photograph of the fabricated rectifier. The rectifier consists of four diodes and DC capacitors of 377 uF. A 125 V square wave with an amplitude of 159 V at a fundamental frequency of 250 kHz is used to supply the voltage, while the input current is 2.21 A. Hence, the input power at 250 kHz is 175.7 W, and the input impedance is about 73 Ω. After rectifying the AC power, DC voltage of 108 V is supplied to the LED TV. The impedance of the LED TV ZTV is about 83.5 Ω. The LED TV dissipates 139.8 W on average as shown in Fig. 7. As a result, the fabricated wireless TV system has the overall power transfer efficiency of 80%. It can be said that the 5% difference in the power transfer efficiency, compared with the target efficiency of 85%, may be caused by AC to DC conversion loss and change in the impedance matching condition by the rectifier.
[8] [9] [10] [11] [12] [13]
[14]
VII. CONCLUSION This paper demonstrated an optimal design procedure based on mutual inductance calculation and a circuit analysis for maximum power transfer efficiency in a wireless TV system with multiple, asymmetric self-resonators. It showed that the optimal position of each self-resonator and proper impedance matching conditions can be determined by this optimal design procedure. It is certain that the optimal design procedure can be employed in WPT design with multiple self-resonators. From the viewpoint of applications, it should be mentioned that power transfer efficiency of above 90% can be achieved if the optimized self-resonators are used for high mutual inductance and low conducting loss. In addition, the power module of the LED TV can be simplified since the output of the WPT system is DC power. ACKNOWLEDGMENT The authors would like to acknowledge the support of LG Innotek Components & Device Lab. and LG Electronics Home Entertainment. REFERENCES [1] [2]
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T. Yoo and K. Chang, “Theoretical and experimental development of 10 and 35 GHz rectennas,” IEEE Transactions on Microwave Theory and Techniques, vol. 40, pp. 1259-1266, June 1992. J. A. Hagerty, F. B. Helmbrecht, W. H. McCalpin, R. Zane, and Z. B. Popovic, “Recycling ambient microwave energy with broad-band rectenna arrays,” IEEE Transactions on Microwave Theory and Techniques, vol. 52, no. 3, pp. 1014-1024, Mar. 2004. H. Jabbar, Y. S. Song, and T. T. Jeong, “RF energy harvesting system and circuits for charging of mobile devices,” IEEE Transactions on Consumer Electronics, vol. 56, no. 1, pp. 247-253, Feb. 2010. A. Kurs, A. Karalis, R. Moffatt, J. D. Joannopoulos, P. Fisher, and M. Soljacic, “Wireless power transfer via strongly coupled magnetic resonances,” Science, vol. 317, pp. 83−86, July 2007. A. Kumar, S. Mirabbasi, and M. Chiao, “Resonance-based wireless power delivery for implantable devices,” 2009 IEEE Biomedical Circuits and Systems Conference, BioCAS, pp. 25−28, 2009. F. Zhang, X. Liu, S. A. Hackworth, R. J. Sclabassi, and M. Sun, “In vitro and in vivo studies on wireless powering of medical sensors and implantable devices”, 2009 IEEE/NIH Life Science Systems and Applications Workshop, LiSSA, pp. 84−87, 2009. T. Imura, T. Uchida, and Y. Hori, “Flexibility of contactless power transfer using magnetic resonance coupling to air gap and misalignment for EV,” World Electric Vehicle Journal (03/2009), S. 24−34, 2009.
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P. Si, A. P. Hu, S. Malpas, and D. Budgett, “A frequency control method for regulating wireless power to implantable devices,” IEEE Transactions on Biomedical Circuits and Systems, vol. 2, no. 1, pp. 22−29, Mar. 2008. H. Sugiyama, “Optimal designs for wireless resonant energy link based on nonradiative magnetic field,” IEEE 13th International Symposium on Consumer Electronics (ISCE2009), pp. 428−429, 2009. R. E. Hamam, A. Karalis, J. D. Joannopoulos, and M. Soljačić, “Efficient weakly-radiative wireless energy transfer: An EIT-like approach,” Annals of Physics, vol. 324, pp. 1783−1795, 2009. F. Zhang, S. A. Hackworth, W. Fu, C. Li, Z. Mao, and Mingui Sun, “Relay effect of wireless power transfer using strongly coupled magnetic resonances,” IEEE Transactions on Magnetics, vol. 47, no. 5, May 2011. W. Zhong, C. Lee, and S. Hui, “Wireless power domino-resonator systems with non-coaxial axes and circular structures,” IEEE Transactions on Power Electronics, 2011. R. Koma, S. Nakamura, S. Ajisaka, and H. Hashimoto, “Basic analysis of the circuit model using relay antenna in magnetic resonance Coupling position sensing system,” 2011 IEEE/ASME International Conference on Advanced Intelligent Mechatronics (AIM2011) Budapest, Hungary, pp. 25−30, July 2011. J. Kim, H.-C. Son, K.-H. Kim, and Y.-J. Park, “Efficiency analysis of magnetic resonance wireless power transfer with intermediate resonator,” IEEE Antennas and Wireless Propagation Letters, vol. 10, pp. 389−392, 2011. J. Kim, H.-C. Son, D.-H Kim, K.-H. Kim, and Y.-J. Park, “Efficiency of magnetic resonance WPT with two off-axis self-resonators,” 2011 IEEE MTT-S International Microwave Workshop Series on Innovative Wireless Power Transmission (IMWS-IWPT), pp. 127−130, May 2011. BIOGRAPHIES
JinWook Kim (S’11) received his B.S. degree in Electronic Engineering from Ajou University, Suwon, in 2009. He is currently in an integrative program at the Department of Power Electrical Equipment Information and Communication Engineering, University of Science and Technology (UST), Korea. He received Best Paper Award from IEEE MTT-S IMWS-IWPT2011 in 2011. His research interests include wireless power transmission and artificial microwave materials. Hyeon-Chang Son received his B.S. degree in Electronic Engineering from Hanyang University, Ansan, in 2010. He is currently in master’s course at the Department of Power Electrical Equipment Information and Communication Engineering, University of Science and Technology (UST), Korea. His research interests include wireless power transmission. Do-Hyeon Kim received his B.S. degree in Electronic Engineering from Yeonse University, Seoul, in 2009. He is currently in an integrative program at the Department of Power Electrical Equipment Information and Communication Engineering, University of Science and Technology (UST), Korea. His research interests include microwave antennas and radars. Young-Jin Park (M’03) received the B.S. degree from Chungang University, Seoul, Korea, in 1997 and the M.S. degree in electrical engineering from KAIST, Taejon, Korea, in 1999. He received Dr.-Ing. (Ph.D.) at the Institut fuer Hoechstfrequenz-technik und Elektronik (IHE) at Universitaet Karlsruhe (currently Karlsruhe Institut fuer Technologie), Karlsruhe, Germany in 2002. From Mar. 2002 to Oct. 2002, he worked as a research associate at the IHE. From Nov 2002, he joined at Korea Electrotechnology Research Institute (KERI) where he is working as a principal researcher currently. From Mar. 2005, he is working as an adjunct professor at University of Science and Technology (UST). His research interests include high resolution impulse radio based-UWB sensors (UWB RTLS, GPR, TDR), wireless power transfer based on magnetic resonance and microwave, and mm-wave antennas and propagation for automotive radar.
