Coil Design and Shielding Methods for a Magnetic Resonant Wireless Power Transfer System

INVITED PAPER Coil Design and Shielding Methods for a Magnetic Resonant Wireless Power Transfer System This paper presents the basic principles of WPT based on magnetic field resonance with parametric effects, and discusses electromagnetic field noise from WPT, and related shielding and cancellation methodologies for different application scenarios. By Jiseong Kim, Member IEEE, Jonghoon Kim, Member IEEE, Sunkyu Kong, Hongseok Kim, In-Soo Suh, Member IEEE, Nam Pyo Suh, Dong-Ho Cho, Senior Member IEEE, Joungho Kim, Senior Member IEEE, and Seungyoung Ahn, Member IEEE ABSTRACT In this paper, we introduce the basic principles of wireless power transfer using magnetic field resonance and describe techniques for the design of a resonant magnetic coil, the formation of a magnetic field distribution, and electromagnetic field (EMF) noise suppression methods. The experimental results of wireless power transfer systems in consumer electronics applications are discussed in terms of issues related to their efficiency and EMF noise. Furthermore, we present a passive shielding method and a magnetic field cancellation method using a reactive resonant current loop and the utilization of these methods in an online electric vehicle (OLEV) system, in which an OLEV green transportation bus system absorbs wireless power from power cables underneath the road surface with only a minimal battery capacity. KEYWORDS Electromagnetic field (EMF) noise; magnetic field resonance; reactive resonant current loop; wireless power transfer (WPT) Manuscript received January 15, 2012; revised September 24, 2012; accepted February 10, 2013. Date of publication March 15, 2013; date of current version May 15, 2013. This work was supported by the National Research Foundation of Korea (NRF) funded by the Korean Government (MEST) under Grants 2011-0018253 and 2010-0029179. J. Kim, J. Kim, S. Kong, H. Kim, D.-H. Cho, and J. Kim are with the Electrical Engineering Department, Korea Advanced Institute of Science and Technology (KAIST), Daejeon 305-701, Korea (e-mail: js.kim@kaist.ac.kr). I.-S. Suh and S. Ahn are with the Cho Chun Sik Graduate School for Green Transportation, Korea Advanced Institute of Science and Technology (KAIST), Daejeon 305-701, Korea. N. P. Suh is with the Korea Advanced Institute of Science and Technology (KAIST), Daejeon 305-701, Korea. Digital Object Identifier: 10.1109/JPROC.2013.2247551 I. INTRODUCTION Wireless power transfer (WPT) technologies using inductive coupling have been used in a variety of applications, such as biomedical devices, automotive systems, industrial manufacturing and test systems, and in some consumer electronics [1] [6]. Many technical achievements have been made in various applications, and the primary technical drivers of WPT technology are convenience and the overall cost per watt for the charging system. The Wireless Power Consortium (WPC) [7] was recently organized, and the commercial market for the WPT technologies has begun to grow. Furthermore, WPT technology using strongly coupled magnetic resonance was proposed by Kurs et al. [8], and expectations for wireless power delivery over long distances have grown. Regarding the application of WPT systems to transportation vehicles, the California PATH program and a team at Auckland University (Auckland, New Zealand) have made important achievements. Per the PATH program reports, the wirelessly transferred power capacity was 60 kw, and the operational frequencies of the inductive power transfer were 400 and 8500 Hz, while achieving a maximal measured efficiency level of 60% with an air gap of 2 3 in [9] [11]. Wang et al. introduced a contactless energy-transfer system as a stationary electric vehicle charging system as a type of inductively coupled power transfer technology [2], [3]. Among the various WPT technologies, technology using magnetic field resonance offers not only the highest 1332 Proceedings of the IEEE Vol.101,No.6,June2013 0018-9219/$31.00 Ó2013 IEEE

power transfer efficiency but also the higher wireless transmission power at near-field distances. Using resonant circuits in the transmitter and the receiver, magnetic field coupling is maximized and the highest power transfer capability at the resonance frequency is produced. In this type of WPT system with magnetic field resonance, the designs of the low-loss circuits, coils, matching circuits, and magnetic shielding structures are the key elements to be considered. By combining these design concepts, maximized power transfer and an optimal resonant magnetic field distribution can be achieved. When controlling the field distribution, the current phase, current amplitude, and coil shapes should be carefully designed. In this paper, we introduce the basic principles of WPT using magnetic field resonance and relevant models. We also describe the concepts and processes related to the design of the resonant magnetic coil, the formation of the magnetic field distribution, and the shielding methods used. The designed WPT systems are applied to lightemitting diode (LED) TV and automotive applications. In particular, we introduce an online electric vehicle (OLEV) system and show that an OLEV green transportation bus system can absorb wireless power from power cables underneath the road surface with a minimal battery capacity. Fig. 1. (a) Configuration of the WPT system. (b) The equivalent circuit model, power transferred to the load, and efficiency of the transferred real power; the resonance frequencies of the coils are tuned to 20 khz and the power transferred to the load is 50 W at the resonant frequency. II. BASIC PRINCIPLE OF WPT USING MAGNETIC FIELD RESONANCE A. Basic Principle of a Resonant WPT System A resonant WPT system consists of magnetically coupled transmitting and receiving coils as well as power electronic circuits, such as an inverter, a rectifier, and a voltage regulator. Because it is very important to improve the real power transfer efficiency in WPT systems, highly efficient power electronic technologies such as soft switching operation, class-e power amplifiers, and active rectifier devices have been studied [12] [18]. The resistance of the coil basically increases as the frequency increases due to the skin effect. Therefore, a coil design with a low amount of loss, which also helps to increase the quality factor, is also important. From a topological point of view, the magnetic resonance can be utilized with a capacitor to minimize the magnitude of the reactance and maximize the power transfer capability between magnetically coupled coils. The constant current source WPT system shown in Fig. 1(a) can be modeled with the simple passive elements depicted in Fig. 1(b), where L S and L R are, respectively, the inductances of the transmitting and receiving coils. When a transmitting coil is excited by the source, the transmitting and receiving coils are coupled magnetically (M is the mutual inductance and k is the magnetic coupling coefficient). For the maximum amount of power transfer, capacitors C S and C R are connected in series with each coil in the model. R S and R R are the internal resistances of each coil, and R L is the load resistance. From the model, the loop impedance of transmitter Z S and the loop impedance of receiver Z R can be calculated as Z S ¼ R S þ 1 þ j!l S j!c S (1) Z R ¼ R R þ R L þ 1 þ j!l R j!c R (2) where I S and I R are the source current and the load current, respectively. The transferred power P L and the real power transfer efficiency can be calculated using P L ¼ Re V L I! 2 M 2 R L I S I R ¼ S Z R Z R ¼ Re V L I R RefV S I Sg! 2 M 2 R L ¼ RefZ S Z R Z R þ! 2 M 2 Z RÞg (3) (4) respectively. Both the power transfer efficiency and the transferred power of the illustration case are shown in Vol. 101, No. 6, June 2013 Proceedings of the IEEE 1333

parasitic capacitance between each turn of the wire, has less loss than the LC resonant type, where the inductance of the coil L and the lumped capacitor C generate series resonance. The self-resonant type can transfer power over a long distance, whereas the LC resonance type is more controllable during the manufacturing process. Fig. 2. Two feeding types of transmitting and receiving coils. Fig. 1(b). The power transfer efficiency is a function of the passive circuit parameters and the operating frequency, and the power transfer efficiency is generally maximized at the resonance frequency. B. Power Feeding and Resonance Types of WPT Systems In the design of a WPT system, the transferred power, transfer efficiency, electromagnetic field (EMF) noise, system size, system weight, and cost are all design criteria candidates. Once the design criteria are determined, the appropriate power source, resonance topology, coil type, and control scheme should be determined to meet the system requirements. Among the design criteria, the coil design is the most important part of the physical implementation process of a WPT system for high efficiency and maximum power transfer. Fig. 2 shows the power feeding types of the transmitting and receiving coils [19]. In the indirect-fed coil type, the transmitting coils are separated and have high quality factor values because the resistances of the source and load are not included; hence, the WPT distance can be increased. However, as in other systems with a high quality factor, this system can be more sensitive to design parameters such as the inductance and the resonance frequency. Therefore, a direct-fed WPT system can be useful when stability is more important than the transfer distance. Fig. 3 shows two resonant coil types. The selfresonant coil, in which the spiral wire has its own resonance due to the inductance of the wire and the C. Load and Distance Variation Effects The load has been modeled simply with an equivalent resistor in Section II-A. In a practical system, however, the power consumption varies according to the load operating condition. Therefore, the applied voltage or current at the load will change as the load operating condition varies. An example of the transferred power and the efficiency as the load varies is shown in Fig. 4(a). The transferred power is maximized when the output impedance is equal to the load resistance in what is known as the impedance matching condition. If the effective load resistance is increased, the transferred power and the efficiency will be decreased. If the distance between transmitting and receiving coils is increased or if the coils are axially misaligned, the magnetic coupling coefficient k will decrease [20]. The transferred power is directly proportional to the value of k,asshowninfig.4(b).however,the efficiency is nearly constant when k is larger than 0.1 and is directly proportional to k when k is smaller than 0.01. III. EMF NOISE AND SHIELDING METHODOLOGIES A. EMF Noise From the WPT System When the WPT system transfers power from the transmitting coil to the receiving coil, the current through the coils generates an electromagnetic field, which is known as EMF noise, around the coils. As an example, EMF noise from a loop coil is shown in Fig. 5(a). Some Fig. 3. Two resonance types of transmitting and receiving coils. Fig. 4. The variation of transferred power and efficiency according to the effective resistance of the load and the distance between the transmitting and receiving coils. In this example, the resonance frequencies of the coils are tuned to 20 khz, and a 10-A constant current source supplies 50 W of power to the load. (a) The power efficiency and the transferred power when the load resistance varies, and (b) when the magnetic coupling coefficient kvaries. 1334 Proceedings of the IEEE Vol. 101, No. 6, June 2013

Fig. 5. (a) Magnetic field distribution and EMF noise with a diameter of 250 mm, a 14-turn loop coil with a 10-A current. (b) Reference levels for general public exposure to time-varying electric and magnetic fields in the ICNIRP guidelines. devices and the human body can be adversely affected by EMF noise; thus, the International Commission on Non- Ionizing Radiation Protection (ICNIRP) announced reference levels pertaining to general public exposure levels to time-varying electric and magnetic fields in 1998 and 2010, as described in Fig. 5(b) [21], [22]. For example, the guideline with respect to a magnetic field is 270 mg at 20 khz. If the B-field around the transmitting or receiving coil exceeds the guideline, the EMF noise should be suppressed until it is below 270 mg. B. Passive Shielding and Magnetic Field Cancellation Using a Reactive Resonant Current Loop Passive shielding refers to the use of a material to provide an alternate path for magnetic flux using ferrimagnetic materials or to produce a reverse magnetic field using conductive materials. Ferrite, which has high relative permeability (e.g., r > 1000) and a relatively low eddy current loss, can be used to guide the magnetic flux. Guiding the magnetic flux along a path close to magnetic field sources can improve the mutual inductance and selfinductances of magnetically coupledcoilsandeventually reduce the leakage magnetic field around magnetic field sources, consequently preventing energy losses in the surrounding conductive materials due to the induced currents [23]. With its advantages, however, ferrite also undergoes hysteresis losses which depend on the operating frequency and the intensity of the magnetic field; the hysteresis losses can be minimized if the peak amplitude of the magnetic field intensity is controlled such that it is far below the saturation region of the ferrite [23], [24]. Metallic shielding is presently popular as a useful means of shielding in radio-frequency applications, and its effectiveness has been proven in several studies [25], [26]. Within a conductive material exposed to a time-varying magnetic field, electric currents are induced due to Faraday s law of induction. These eddy currents have their own inductance and induce magnetic fields. These fields cancel the incident magnetic fields penetrating the material, thus reducing the net magnetic field at the opposite side of magnetic field source [23], [25]. To use a metal to suppress the leakage magnetic field from a WPT system, its effects on the electrical performance of a WPT system should be considered. Geselowitz et al. [27] show that metals in the vicinity of the coils cause a decrease in the self-inductance and mutual inductance and an increase in the effective series resistance of the coils, whereas if ferrite is placed between a coil andametallicshieldingand the thickness of the metal is greater than the skin depth, the variation of the circuit parameters can be minimized while the shielding effect is preserved. In addition to passive shielding methods using ferrimagnetic or conductive materials, spread spectrum technology and active magnetic field cancellation methods have been proposed [28] [30]. The magnetic field from the noise source can be canceled using an intentional current which generates ideally a magnetic field vector that has the same magnitude and a direction opposite to that of the original magnetic field vector. Although the active shielding allows the reduction of EMF noise without physical contact, it is necessary to use active components which require a power supply. The power supply for an active shield is not only a burden in terms of its size and weightbutitisalsodifficult to design because both the magnitude and phase of the active shield current should be determined. The reactive resonant current loop method, however, combines the advantages of passive and active shields and can be useful for specific applications of WPT technology. Like the active shield, the reactive resonant current loop generates a magnetic field which cancels the original magnetic field to minimize EMF noise. However, the reactive resonant current loop does not require any power source to generate the intentional fields because the cancelling magnetic field is generated from the original magnetic field noise. In an OLEV application, the generation of the cancelling magnetic field at a certain frequency is realized using a reactive current loop cable with capacitors and switch arrays to control the resonant frequency. Fig. 6 shows the magnetic field cancellation principle using the reactive resonant current loop method. When a reactive resonant current loop is placed between a magnetic field source and the measurement point, the original magnetic field induces voltage at the loop and the voltage generates current which provides the cancelling magnetic field, as expressed by V induced ¼ df Z dt ¼ d dt B source ds (5) I ¼ V induced Z loop (6) B total ¼ B source B induced : (7) Vol. 101, No. 6, June 2013 Proceedings of the IEEE 1335

Fig. 6. Magnetic field cancellation using a reactive resonant current loop. The magnetic field source induces voltage in the loop and the voltage generates current which provides the cancelling magnetic field. The induced voltage is determined by the magnetic field through the loop and the magnitude of the current can be controlled by changing the impedance of the loop. The design of a reactive resonant current loop is shown in Section V. IV. WPT SYSTEM DESIGN FOR CONSUMER ELECTRONICS In this section, the WPT system for a LED TV with the passive shielding method described in Section III-B is presented. A simplified equivalent circuit of the WPT system is shown in Fig. 7(a). The power inverter consists of the rectifier, direct current to direct current (dc dc) converter, and full-bridge inverter to generate a 30-kHz sinusoidal current. Following the power inverter is a set of two magnetically coupled resonators with a series-parallel resonant topology and an air gap of 200 mm. In addition to the designations of the circuit parameters shown in Fig. 1(b), R C and I LC denote the effective series resistance (ESR) of the capacitors and the current flowing through the parallel LC resonant loop, respectively. The approximate values of the capacitance for the series-parallel resonant topology can be determined, respectively, by [2] C R 1! 2 0 L R (8) C S C RL 2 R L R L R M 2 : (9) Here, C R is the capacitance of the receiving coil, C S is the capacitance of the transmitting coil, and! 0 is the resonance frequency measured in radians per second. To convert high-frequency sinusoidal power to dc at the receiver and to minimize the ripple voltage at the load, Fig. 7. (a) The simplified equivalent circuit of a WPT system for a LED TV. (b) A pair of circular loop coils with ferrite and metal. a full-bridge rectifier with a capacitor is used. If a LED TV consumes constant power, this active load can be assumed as the passive resistance R dc and both the rectifier and the load can be modeled with resistance R L,asshownin R L R DC 2 (10) which will dissipate the same amount of sinusoidal power as the dc power in R dc [31]. Using (10), the nonlinearity arising from the rectifier can be omitted. Thus, the equivalent circuit of the WPT system becomes more intuitive and can easily be analyzed. Fig. 7(b) depicts the coil structure with a passive shield. The transmitting coil has a geometric mean radius (GMR) of 115 mm and ten turns of Litz wire made with 600 strands of American-wire-gauge (AWG) 36, and the receiving coil has the same GMR and nine turns of the same wire. The form factor and material characteristics of the passive shield are shown in Fig. 7(b). To investigate the effect of the passive shield on the system performance and the magnetic field distribution around a WPT system, the cases of loop coils only, coils with ferrite, and coils with both ferrite and metal are simulated. The simulated magnetic field distribution and the electrical characteristics of the 1336 Proceedings of the IEEE Vol.101,No.6,June2013

Fig. 8. Magnetic field distributions on the xz plane simulated under common conditions. (a) Case 1: loop coils only. (b) Case 2: with ferrite. (c) Case 3: with ferrite and metal. coil structures are extracted by the ANSYS Maxwell finite element method (FEM) solver and are presented in Fig. 8 and Table 1, respectively [32]. Fig. 8 shows that the simulated magnetic flux density directly above and below the shield is significantly lower due to the guiding and cancelling effects in case 3. However, along the x-axis, where humans watching TV are usually located, the differences in the magnetic flux densityaresubtle.thus, after implementing the coil structure with the ferrite and metal shield and the WPT system for a LED TV, we numerically compare the differences in the simulated magnetic flux density levels of the three coil structure cases and measurement results using a Narda EHP-200 electric and magnetic field analyzer along the measurement line, as shown in Fig. 9(a). Fig. 9(b) shows that there are clear differences in the magnitudes of the magnetic field and that the measurement results are in excellent agreement with the simulation results. Table 2 shows the SPICE simulation result with circuit models of each case [32]. Here, represents the coil-tocoil efficiency. Because the resistance and k vary with the coil structures, also varies with the coil structure. It was found that the efficiency of the coil structure with both Table 1 Simulated Electrical Characteristics of the Coil Structures Fig. 9. Implemented WPT system with ferrite and metal materials for a LED TV application. (a) Measurement setup of the leakage magnetic field and the measured electrical characteristics. (b) Simulation results and measurement results of the coil structure with both ferrite and metal. ferrite and metal is higher than that of the coil structure without any shielding material. However, the coil structure with only ferrite showed better coil-to-coil efficiency; thus, the power efficiency and shielding effectiveness should be considered together when the WPT system is designed. V. WPT SYSTEM DESIGN FOR AUTOMOTIVE APPLICATIONS In this section, we introduce the OLEV system and its noncontact power transfer mechanism, shown in Fig. 10, along with techniques to reduce the EMF noise from the power line and from the vehicle [33]. By applying a passive shield and magnetic field cancellation using the reactive resonant current loop method, the low-frequency EMF noise Table 2 Circuit Simulation Results Vol. 101, No. 6, June 2013 Proceedings of the IEEE 1337

Fig. 12. Vertical magnetic flux type of power lines and pickup module. (a) Cross-sectional view. (b) Perspective view. Fig. 10. The OLEV and the power transfer system using power lines placed underneath the road surface for wireless charging. was significantly reduced. Simulation and measurement results for vehicles currently in service are also given. A. Power Supply and Vehicle Architecture of the OLEV System For wireless dynamic charging while a vehicle is in motion on a road, a road-embedded power supply system and a power receiver system in a vehicle are required, as shown in Fig. 11 schematically. The power supply system can be a part of the public infrastructure of the road or highway system, while the power receiver device, or the power pickup module, can be a part of the vehicle. The pair of the power supply and the receiver system are tuned to maximize the transmission efficiency through an optimized magnetic field distribution and a properly tuned resonance frequency. This process is known as shaped magnetic field in resonance technology [34]. As shown in Fig. 11, the full-bridge type of power inverter introduces constant-current controlled electricity of 20 khz to the road-embedded power cables from the commercial electricity grid. The power cables can be composed of a set of segments which separately receive electricity via the switching control of the power inverter. A vehicle driving or parked on an energized segment will start to receive electric power to charge the on-board battery, if necessary. The vehicle has a completely integrated communication and control network to charge the battery or power the motor to drive the vehicle. B. Design of the Transmitting and Receiving Coils The transmitting coil carries 20 khz of current, which is generated from the full-bridge type inverter. For constant induced voltage at the receiving coil, we used a constant-current source while the inverter output voltage varies in a range of less than 500 V. Fig. 12(a) and (b) shows the structure of the vertical magnetic flux types of the transmitting and receiving coils. The transmitting coil carries a 200-A current, two magnetic flux path loops are generated, and the receiving coil catches the vertical magnetic flux. The width of the transmitting and receiving coils is 60 cm. The transmitting coil is underground, and the receiving coil is attached onto the bottom of the electric vehicle with an air gap of 20 cm between bottom of the receiving coil and the surface of the road. To transfer 100 kw of power, five 20-kW pickup modules are installed in the vehicle. In Fig. 13, the measured power transferred efficiency of the vertical magnetic flux type as a function of the output power is shown. The efficiency is reduced to 70% when the lateral distance is 15 cm; however, no significant Fig. 11. A schematic overview of the OLEV system architecture. Fig. 13. Measurement of the power transfer efficiency of the vertical magnetic flux type as a function of the output power of the regulator in a field environment [35]. 1338 Proceedings of the IEEE Vol.101,No.6,June2013

Fig. 14. Block diagram of the shielding method using a reactive resonant current loop. The magnetic field from the transmitter and receiver coils is cancelled out by the magnetic field from the reactive current loop. degradation of the power transfer efficiency occurs within adistanceof10cm[35]. C. EMF Shielding Design and Measurement Results Although the magnetic field distribution is determined by the coil design and the ferrimagnetic material, some form of shielding to reduce the EMF noise from the transmitting and receiving coils is required. Both passive and active shields can be implemented to ensure a low level of magnetic field noise. In an OLEV system, however, a field cancellation method with a reactive resonant current loop is used instead of an active shielding method to implement compact and cost-effective shielding with a minimal loss of power for shielding. Fig. 14 shows a block diagram of the shielding method using a reactive resonant current loop in which the original magnetic field is cancelled by the magnetic field generated by the resonant loop current. The magnitude and phase should be controlled by changing the capacitance of the reactive resonant loop to minimize the EMF. To control the capacitance optimally, a feedback system using a magnetic field sensing loop is implemented to determine the magnetic field intensity at the measurement position, and the processor in the controller block finds the optimal combination of the capacitors to minimize the total EMF by means of controlling switches. As the capacitor array includes 12 different values of capacitors connected in parallel, the precise total capacitance can be implemented by turning on and off the solid-state relay switch array. In the application of the reactive resonant current loop to reduce the EMF noise from the WPT system, the position and impedance of the loop are important factors. As five receiving coils are implemented in each vehicle, five reactive resonant current loops are attached to each sideofthevehicle.weusedtheansysmaxwell3-d Fig. 15. Application of the reactive resonant current loop for an OLEV. Placement of the reactive resonant current loop under the vehicle (a) as seen from the front lower position and (b) as seen from the side lower position. (c) Photograph of an implemented reactive resonant current loop. (d) The EMF noise level with regard to the current in the reactive resonant current loop. magnetic field solver to find the best position of the reactive resonant current loop. The placement of the reactive resonant loop is shown in Fig. 15(a) and (b). Fig. 15(c) shows one of the implemented reactive resonant current loops with a capacitor array and a loop cable to reduce the EMF from a 20-kW pickup module. Because the five reactive resonant current loops are tightly coupled and affect each other, automatic control of the total capacitance in each loop is necessary. In contrast, the effect of the reactive resonant current loop cable on the receiving or transmitting cable can be neglected because Fig. 16. (a) The measurement positions based on the IEC 62110 standard in a front view. (b) Measurement positions in a side view. (c) Measured reduction of the EMF noise with and without the reactive resonant current loop. Vol. 101, No. 6, June 2013 Proceedings of the IEEE 1339

the coupling coefficient between them is about 0.0005, as the distance between them is about 55 cm. The power consumption of the reactive resonant current loop method, which is on the order of tens of watts, can also be neglected when considering a receiving power of 20 kw for each power pickup module. In Fig. 16(a), the measurement points of EMF noise for the OLEV system are shown. By adopting the IEC 62110 standard, which describes measurements of the magnetic field level generated by alternating current (ac) power systems [36], the average of the magnetic flux density at three points, P1, P2, and P3, is measured. By applying the reactive resonant current loop to the OLEV system, the magnetic field at the measurement point at each position in Fig. 16(b) is significantly reduced, as shown in Fig. 16(c). An acceptable human exposure level to EMF noise below international guidelines or the requirement published by ICNIRP, of which the suggested value is 62.5 mg at 20 khz [21], was achieved by minimizing the leakage magnetic field noise using the reactive resonant current loop. VI. CONCLUSION In this paper, we introduced the basic principles of a WPT scheme using magnetic field resonance and discussed the power efficiency and the maximum power transfer. A WPT system with different feeding and resonance types was also introduced, and passive shielding, active shielding, and magnetic field cancellation with a reactive resonant current loop to minimize EMF noise from the WPT system were described. For both TV and automotive applications, coupled coils were designed using field simulations, a magnetic field distribution was formed usingaferromagneticmaterial,andtheemfnoisewas suppressed using shielding technologies. h REFERENCES [1] P. Li and R. 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Fuina, ELF magnetic field mitigation by active shielding, in Proc. IEEE Int. Symp. Ind. Electron., Nov. 2002, vol. 3, pp. 994 998. [30] M. L. Hiles, R. G. Olsen, K. C. Holte, D. R. Jensen, and K. L. Griffing, Power frequency magnetic field management using a combination of active and passive shielding technology, IEEE Trans. Power Delivery, vol. 13, no. 1, pp. 171 179, Jan. 1998. [31] W. H. Ko, S. P. Liang, and C. Fung, Design of radio-frequency powered coils for implant 1340 Proceedings of the IEEE Vol.101,No.6,June2013

instruments, Med. Biol. Eng. Comput., vol. 15, pp. 634 640, Nov. 1977. [32] H. Kim, J. Cho, S. Ahn, J. Kim, and J. Kim, Suppression of leakage magnetic field from a wireless power transfer system using ferrimagnetic material and metallic shielding, in Proc. IEEE Int. Symp. Electromagn. Compat., Aug. 2012, pp. 640 645. [33] S. Ahn, J. Pak, T. Song, H. Lee, J. Byun, D. Kang, C. Choi, Y. Chun, C. Rim, J. Yim, D. Cho, and J. Kim, Low frequency electromagnetic field reduction techniques for the on-line electric vehicle (OLEV), in Proc. IEEE Int. Symp. Electromagn. Compat., Jul. 2010, pp. 625 630. [34] I. S. Suh, Application of SMFIR technology to future urban transportation, J. Integr. Design Process Sci., vol. 15, no. 3, pp. 3 12, Sep. 2011. [35] J. Shin, S. Shin, Y. Kim, S. Ahn, S. Lee, G. Jung, B. Song, S. Jeon, and D. Cho, Design and implementation of a shaped magnetic resonance based wireless power transfer system for roadway-powered moving electric vehicles, IEEE Trans. Ind. Electron., 2012. [36] International Electrotechnical Commission, Magnetic field levels generated by a.c. POWER SYSTEMSVMeasurement procedures with regard to public exposure, IEC 62110 Ed. 1, 2009. ABOUT THE AUTHORS Jiseong Kim (Member, IEEE) received the Ph.D. degree in electrical and computer engineering from the University of Texas at Austin, Austin, TX, USA, in 2000. He is currently a Research Professor at Korea Advanced Institute of Science and Technology (KAIST), Daejeon, Korea, with main research interests in modeling, analysis, and design of highspeed digital systems, high-speed serial interfaces, and wireless power transfer (WPT) technologies. In-Soo Suh (Member, IEEE) received the Ph.D. degree in mechanical engineering from the Massachusetts Institute of Technology (MIT), Cambridge, MA, USA. He is currently an Associate Professor in the ChoChunShik Graduate School for Green Transportation, Korea Advanced Institute of Science and Technology (KAIST), Daejeon, Korea. His expertise and research fields are electric vehicle systems with wireless and conductive charging infrastructure strategies. Jonghoon Kim (Member, IEEE) received the Ph.D. degree in electrical engineering from Korea Advanced Institute of Science and Technology (KAIST), Daejeon, Korea, in 2003. In 2003, he joined the Memory Division of Samsung Electronics, Korea. He moved to the Department of Electrical Engineering, KAIST, in 2010, where he is currently a Research Professor. His current research interests include signal integrity (SI), power integrity (PI), and the design of wireless power transfer (WPT) considering electromagnetic interference/ electromagnetic field (EMI/EMF) noise. Sunkyu Kong received the M.S. degree in electrical engineering from Korea Advanced Institute of Science and Technology (KAIST), Daejeon, Korea, in 2012, where he is currently working toward the Ph.D. degree in electrical engineering. His research interests include electromagnetic interference/electromagnetic compatibility (EMI/ EMC) issues in analog digital mixed-mode system with chip-package hierarchical structures. Nam Pyo Suh received the S.B. and S.M. degrees in mechanical engineering from the Massachusetts Institute of Technology (MIT), Cambridge, MA, USA, in 1959 and 1961, respectively, and the Ph.D. degree in mechanical engineering from Carnegie Mellon University, Pittsburgh, PA, USA, in 1964. He is the President of Korea Advanced Institute of Science and Technology (KAIST), Daejeon, Korea, and a Cross Professor, Emeritus, of the Massachusetts Institute of Technology (MIT), Cambridge, MA, USA. In 1984 1988, he was at the National Science Foundation (NSF) as the AD for Engineering. His major contributions include the delamination theory of wear, axiomatic design theory, and microcellular plastics. Hongseok Kim received the B.S. degree in electronic and electrical engineering from Sungkyunkwan University, Suwon, Korea, in 2011. Currently, he is working toward the M.S. degree in the Division of Future Vehicle, Korea Advanced Institute of Science and Technology (KAIST), Daejeon, Korea. His current research interest is the design of wireless charging system for electric vehicles. Dong-Ho Cho (Senior Member, IEEE) received the Ph.D. degree in electrical engineering from Korea Advanced Institute of Science and Technology (KAIST), Daejeon, Korea, in 1985. From 1987 to 1997, he was a Professor in the Department of Computer Engineering, Kyunghee University,Seoul,Korea.Since1998,hehasbeena Professor in the Department of Electrical Engineering, KAIST. His research interests include wired and mobile communication networks and protocols, online electric vehicles (OLEVs) based on wireless power transfer (WPT) and construction-it convergence, and bio-it convergence. Vol. 101, No. 6, June 2013 Proceedings of the IEEE 1341

Joungho Kim (Senior Member, IEEE) received the Ph.D. degree in electrical engineering from the University of Michigan at Ann Arbor, Ann Arbor, MI,USA,in1993. Since 1996, he has been a Professor in the Department of Electrical Engineering, Korea Advanced Institute of Science and Technology (KAIST), Daejeon, Korea. Since joining KAIST, his research has centered on electromagnetic compatibility (EMC) modeling, 3-D integrated circuit (IC) design and measurement methodologies, system-in-package (SiP) technologies, and wireless power transfer (WPT) technologies. Seungyoung Ahn (Member, IEEE) received the Ph.D. degree in electrical engineering from Korea Advanced Institute of Science and Technology (KAIST), Daejeon, Korea, in 2005. He is currently an Assistant Professor in the ChoChunShik Graduate School for Green Transportation, KAIST. His research interests include high-efficiency wireless power transfer (WPT) system designs and electromagnetic compatibility designs for digital systems applications. 1342 Proceedings of the IEEE Vol. 101, No. 6, June 2013

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