Abstract—Due to the convenience of using electronic devices,contactless energy transfer (CET) systems have garnered interestin various fields of industry. In this paper, a new design approachthat uses antiparallel resonant loops for CET systems is presented.Forward and reverse loops forming an antiparallel resonant struc-ture stabilize the transfer efficiency and therefore prevent it fromdramatic distance-related changes, a phenomenon that can occurin CET systems with nonradiative methods (or resonant methods).This paper proposes frequency-insensitive antiparallel resonantloops and the optimal design of these loops for uniform trans-fer efficiency according to the distance. The proposed techniqueachieves frequency variation that is one-sixth that of conventionalunidirectional loops, thus improving the power efficiency to a max-imum of 87%. The improved performance of data transmissionsfor near-field communication is also verified.
Index Terms—Antiparallel, contactless energy transfer (CET),frequency-insensitive, resonant loops.
I. INTRODUCTION
THE advent of electric power transmission has facilitatedthe widespread use of electricity since the late 19th cen-
tury using power cords in electric devices. With the increasingdeployment of wireless technologies, portable devices such aslaptops, Personal Digital Assistant (PDA), and cellular phonesplay essential roles of everyday life. Such devices require highmobility and a minimal size, and they typically use recharge-able batteries. As the available battery duration after a rechargecannot satisfy the required usage demand, users must incon-veniently recharge the batteries with power cords or replacethe batteries. In an effort to solve this problem, active studiesof contactless energy transfer (CET) technologies are ongoing[1]–[8].
As various wireless techniques are studied and applied inthe energy transfer field, CET technology can be classified intotwo main categories depending on the distance of the powertransfer. First, high-amplitude CET using the GHz ISM bandfor long distances (or far-field zones) has a long history, butit is also associated with difficulty in its commercializationdue to the excessive level of exposure to humans according
Manuscript received January 17, 2011; revised March 28, 2011 andSeptember 8, 2011; accepted November 15, 2011. Date of publicationNovember 30, 2011; date of current version September 6, 2012. This work wassupported under the Brain Korea 21 Project, by the Korea Advanced Instituteof Science and Technology (KAIST) 2011.
Color versions of one or more of the figures in this paper are available onlineat http://ieeexplore.ieee.org.
Digital Object Identifier 10.1109/TIE.2011.2177611
to the ICNIRP Guidelines [9] and the straight characteristicsof the wave [10]–[12]. Second, CET technologies for short-distance transmissions are divided into radiative methods andnonradiative methods (or resonant methods).
A typical application of the radiative method is the 900 MHzor 2.4 GHz radio frequency identification (RFID) technology.This technology communicates information through backscat-tered data by transferring power wirelessly to Integrated Circuit(IC) in the receiver. It has an operating range of approximately10 m. However, its power transfer efficiency is very low [13].
The nonradiative method (resonant method) invented byNikola Tesla over a century ago was referenced in 2007 byProf. Marin Soljacic from the physics research group at Massa-chusetts Institute of Technology (MIT) to recharge autonomouselectronic devices. Unlike the conventional far-field radiativemethod, this technology utilizes the near-field effect by trans-ferring wireless power over a short distance compared to thewavelength of the frequency. Furthermore, nonradiative CETtechnology capable of very high efficiency was developed,working by matching the resonant frequency. The publishednonradiative method (resonant method) has a maximum effi-ciency of approximately 50% by self-resonating at a distancebetween 1 and 2 m [14].
It is known that longer distances between the transmittingand receiving resonant coils in nonradiative CET approachesachieve a lower coupling coefficient between the resonant coils.To achieve greater energy transfer efficiency, the coil radiusrelated to the mutual inductance between the resonant coilsneeds to be larger. However, if the distance is close to the coildistance, the coupling coefficient between the transmitting andreceiving resonant coils increases. This phenomenon causes theseparation of the resonant frequencies and degrades the effi-ciency. Because the transfer efficiency can vary in nonradiativeCET methods according to the distance between the coils, shortdistances rapidly degrade in terms of efficiency [15].
To solve the problem of low efficiency with respect to thelocation of the receiving coil, CET systems generally achievemaximum power transfer efficiency via the addition of auto-matic frequency tuning circuits. Specifically, studies that seekto control impedance matching circuits have been conducted.In the transmitter, the transmitted power and reflected power,which are functions of the frequency, are continuously mea-sured using a directional coupler [16], [17]. Another studysuggests a CET system that decreases frequency splitting bychanging or tilting the coil configuration [18]. Further researchhas shown that, the transfer efficiency can be improved by afrequency tracking control component that consists of a currenttransformer, a differential amplifier, a phase compensator, and a
LEE et al.: CONTACTLESS ENERGY TRANSFER SYSTEMS USING ANTIPARALLEL RESONANT LOOPS 351
Fig. 1. The characteristics of contactless energy transfer using a unidirectionalresonant coil. (a) The frequency variation of the transfer coils regarding thedistance between the transmitting and receiving transfer coils. (b) Resonantfrequency variation and power transfer efficiency of coils at the distance(h1, h2) of (a).
phase-locked loop [19]. A tunable impedance matching circuitwas also implemented through varactor diodes [20].
In nonradiative CET systems, the use of automatic frequencytuning techniques complicates the system structure becausethe impedance matching circuit should be tuned according tothe transmitted power and the reflected power. Therefore, thispaper presents a CET system that use antiparallel (forward andreverse) resonant loops without the need for further systemconfiguration to solve the transfer efficiency variation accordingto the distance between the coils.
Section II explores the cause of the performance degradationwithin the operating range of the conventional unidirectionalloops and then presents the operating system principles re-garding the characteristics of the proposed antiparallel resonantloops. The design method of the antiparallel resonant loopsis presented in Section III, which includes the design con-ditions for maximum transfer efficiency. Section IV verifiesthe performance of the proposed loop structure by comparingthe theoretical analysis results with the experimental results.Finally, this paper concludes with Section V.
II. OPERATING PRINCIPLES OF ANTIPARALLEL
RESONANT LOOPS
As shown in Fig. 1, for nonradiative CET methods using con-ventional unidirectional resonant coils, the resonant frequencyand transfer efficiency change with respect to the distance be-tween the transmitting and receiving coils. Fig. 1(a) shows thatthe resonant frequencies (f0) of the transmitting and receivingtransfer coils are identical at a distance of (h2) and that thetransfer efficiency is high. However, if the distance betweenthe transmitting and receiving transfer coils approaches h1, theseparation phenomenon of the resonant frequency occurs, asdenoted by the gray line in Fig. 1(b), as the mutual inductancebetween the coils increases. In this case, the power transferefficiency in the resonant frequency (f0) decreases drastically.
Fig. 2 shows two coil structure types that consist of a forwardor a reverse loop. Fig. 2(a) illustrates the conventional coilstructure of the unidirectional resonant loops, whereas Fig. 2(b)presents the proposed coil structure with the antiparallel reso-nant loops. Small reverse loops in the proposed structure areplaced in the middle of the transmitting coils compared to thelocation of conventional resonant loops.
Fig. 2. (a) Coil structure (Type A) with the conventional unidirectionalresonant loops. (b) Proposed coil structure (Type B) with antiparallel resonantloops (in the two layer PCB, the black pattern is on the top layer, and the graypattern is on the bottom layer).
Fig. 3. (a) Mutual inductance transition with regard to the distance of theunidirectional resonant loops (Type A). (b) Mutual inductance transition withregard to the distance of antiparallel resonant loops (Type B).
The conventional unidirectional transmitting and receivingloops, as shown in Fig. 3, are forced to increase the mutualinductance as they move closer together. As mentioned above,the performance of a CET system drops significantly due toresonant frequency splitting. In the proposed antiparallel trans-mitting resonant loops, as the transmitting loop moves closerto a receiving coil, the reverse loop located in the center hasthe effect of suppressing the increase in the mutual inductance.In this case, it maintains mutual inductance at the same levelwithin the working range. Because the outer-forward and inner-reverse resonant loops are fixed in the transmitting coils, themutual inductance between the two is constant regardless ofthe distance from the receiving coils. This is a factor in the self-inductance of the transmitting coil.
III. DESIGN OF ANTIPARALLEL RESONANT LOOPS
This study concentrates on CET utilizing the 13.56 MHzfrequency band for use with HF RFID and NFC [21] targetingmobile applications. The configuration of the transmitting andreceiving coils and the structure and the layout of the coils areshown in Fig. 4(a). Fig. 4(b) shows the transmitting coil withthe proposed antiparallel resonant loops, whereas the receivingcoil designed in Fig. 4(c) follows the inlay model of the TITag-it HF-I transponder [22].
Fig. 5 shows the equivalent circuit models of Fig. 4. Mutualinductance between the resonant coils exists as a functionof the coil distance (h). The mutual inductance of the two
352 IEEE TRANSACTIONS ON INDUSTRIAL ELECTRONICS, VOL. 60, NO. 1, JANUARY 2013
Fig. 4. Schematic design of the proposed transmitting and receiving coilsat 13.56 MHz. (a) Configuration of the coils apart from distance (h).(b) Antiparallel transmitting coil (Tx, Type B). (c) Receiving coil (Rx).
Fig. 5. Equivalent circuits of the power transfer system with impedancematching and resonant conditions between the transmitting and receiving coils.
concentric coils depends on the geometry of the two resonantcoils, primarily the distance between the coils. The mutualinductances (Mf (h),Mr(h)) between the forward or reversetransmitting loops and the receiving loops are defined as afunction of the distance between the coils in all cases. Usingthe superposition theorem, the total mutual inductance (M(h))between the transmitting and receiving coils can be easilyobtained from the sum of Mf (h) and Mr(h). Additionally,the mutual inductance (Mfr) between the forward and reversetransmitting loops regardless of the distance from the receivingloops is held constant.
Details of the equivalent circuit models are explained below.Generally, the transmitting and receiving coils can be closelymodeled as series R and L components (Rtc and Ltc in Tx andRrc and Lrc in Rx), representing the loss (including the ohmicand radiation loss) and the inductance of the coil, respectively.Antiparallel transmitting coils have two RL components withthe forward and reverse resonant loops having the oppositedirections. Hence, the total resistance (Rtc) of the transmittingcoil is obtained from the sum of the forward and reverse coilresistance (Rtcf and Rtcr), and the total inductance (Ltc) ofthe transmitting coil is obtained by subtracting the mutualinductance (Mfr) between the forward and reverse transmittingcoils from the sum of each self-inductance (Ltcf and Ltcr)value in the forward or reverse loops as a result of the oppositedirection in the transmitting coil. The serial capacitor (Ctr), theresonant circuit, must create resonance in the transmitting coilat a particular frequency. To improve the efficiency of CET sys-tems, a T-type impedance matching circuit (Ctm1, Ctm2, Lm)
Fig. 6. Simplified equivalent circuit models for 50 Ω port transfer.
is added. This enables a matching condition in the relevantfrequency within the operating range of the receiving coil.
In the receiver, the IC to operate the receiver can be easilymodeled by series R (Ric) and C (Cic) components. A parallelcapacitor (Crr) is required to resonate between the IC andreceiving coil. The power and information from the transmittingcoil is magnetically transferred to the receiving coil
S21 =−2jωM
ω2Ctm2Crr2⎛⎝
[ZT Z2
LP + ZRZ2SP + ω2(ZS + ZL)M2
]+ 1
Z0
[Z2
SP Z2LP + ω2ZLZSM2
]+(ZT ZR + ω2M2)Z0
⎞⎠
(1)
where the related parameters are as follows:
Z2SP =ZT ZS +
1ω2C2
tm2
Z2LP =ZRZL +
1ω2C2
rr2
ZS =1
jωCtm1+
1jωCtm2
ZL = jωLpt +1
jωCrr2+
1jωCpt
ZT =Rtc + jωLtc + jωLtm +1
jωCtm2+
1jωCtr
ZR =Rrc + jωLrc +1
jωCrr1+
1jωCrr2
. (2)
Fig. 6 shows simplified equivalent circuit models for theanalysis of the transfer efficiency. A port impedance transfer,resulting in an output impedance of 50 Ω is used. Thus, LCcomponent modeling can be done. By developing formulasfrom the equivalent circuit models of Fig. 6, the S21 relatedto transfer efficiency can be derived, as shown in (1).
When the coils are close to each other relative to the coilsize, degradation of the transfer efficiency arises due to theimpedance mismatch with regard to the mutual inductancecoupling. That is, the resonant frequency gap is smaller orlarger due to the variable mutual inductance as the couplingbetween the coils decreases or increases. Consequently, thesystem efficiency worsens significantly, and operation errorsoccur in the NFC because the IC in the receiver does not receivesufficient voltage for operation.
A. Mutual Inductance Comparison
As mentioned above, a precise analysis of mutual inductanceenables an explanation of the power efficiency characteristics
LEE et al.: CONTACTLESS ENERGY TRANSFER SYSTEMS USING ANTIPARALLEL RESONANT LOOPS 353
Fig. 7. Mutual inductance between loops with concentric structure in a singleturn: (a) circular shape and (b) rectangular shape.
TABLE IPARAMETERS OF COIL STRUCTURE
related to the frequency splitting. The mutual inductance be-tween two loops with a single turn is given by the doubleintegral Neumann formula
M12 =ψ2
I1
=μ0
4π
∮C1
∮C2
dl1 · dl2R12
(3)
where μ0 is the magnetic constant (4π × 10−7 H/m), C1, andC2 are the curves spanned by the loops, and R12 is the distancebetween two points. This indicates that the mutual inductanceis related to the shapes of the two loops and their orientationwith respect to each other.
In the mutual inductance of the two shapes (circular andrectangular), which have the same area as the loop, Fig. 7shows the coils that are separated by distance h on the sameaxis. The coil sizes are given in Table I. The mutual inductance(Mc(r1, r2, h)) between the circular one-turn coils in Fig. 7 canbe obtained using
Mc(r1, r2, h) = μ0√
r1r22k
[(1 − k2
2
)K(k) − E(k)
](4)
where
k(r1, r2, h) =
√4r1r2
(r1 + r2)2 + h2(5)
and where K(k) and E(k) are complete elliptic integrals of thefirst and second kind, respectively, as tabulated by Dwight [23].
At the symmetric position in the z-axis, the mutual induc-tance (Mr(r1, r2, h)) of the rectangular coils with a single turn
Fig. 8. Comparison of the mutual inductance between circular loops andrectangular loops. To obtain the same loop area, the equivalent diametersbetween the loop types (circular and rectangular) are set.
is given by the following expression:
Mr(a1, a2, b1, b2, h)
=μ0
2π
[2asln
[as + abss
as + abst
]+ 2atln
[at + abtt
at + abts
]
+ 2bsln[bs + abss
bs + abts
]+ 2btln
[bt + abtt
bt + abst
]
+ (at − as)ln[bs + h2
bt + h2
]
+(bt − bs)ln[as + h2
at + h2
]](6)
where the parameters are defined as follows: as = a2 − a1,at = a2 + a1, bs = b2 − b1, and bt = b2 + b1 in the rectangu-lar loops. As shown in (7), the parameters (abss, abtt, abst, andabts) are defined as shown below
abss =√
a2s + b2
s + h2 abtt =√
a2t + b2
t + h2
abst =√
a2s + b2
t + h2 abts =√
a2t + b2
s + h2 (7)
For the same loop area between circular loops andrectangular loops, the mutual inductances of the loops(Mc(r1, r2, h),Mr(a1, a2, b1, b2, h)) can be equivalent exceptfor a small amount of error (less than 4%) when the distance(h) is close to 0, as shown in Fig. 8. This demonstrates thatthe mutual inductance with the same loop area is equivalentregardless of the loop shape.
B. Uniform Mutual Inductance for an Optimal Design
The mutual inductance (M(rt, rr, h)) changes as a functionof the radii and the number of turns or the distance betweenthe transmitting and receiving coils. As the power transferefficiency (S21) is a function of the mutual inductance and fre-quency according to (1), constant mutual inductance is crucialfor an optimal design.
Using unidirectional loops in the transmitting coil, it is notpractical to make the mutual inductance uniform by chang-ing the number of turns or varying the loop size in Fig. 9,
354 IEEE TRANSACTIONS ON INDUSTRIAL ELECTRONICS, VOL. 60, NO. 1, JANUARY 2013
Fig. 9. Mutual inductance variation on the relative position between thetransmitting and receiving coils by changing the number of turns or radius inthe forward loops in the transmitting coil.
because numerous turns or a large loop size would be required.Therefore, the use of a reverse loop becomes necessary. Fig. 10shows mutual inductances having various numbers of turnsof forward or reverse loops in the transmitting coil. In thiscase, the receiving coil condition (loop size and turns) is thesame. The loop size and the number of turns for an optimaltransmitting coil are determined by the range of the uniformmutual inductance.
The total mutual inductance (MT (rt, rr, h)) between thetransmitting coil with antiparallel loops and the receiving coilcan be simplified by
MT (rt, rr, h)
=nf
t∑i=1
nr∑j=1
Mc
(rit, r
jr, h
)−
nrt∑
i=1
nr∑j=1
Mc
(rit, r
jr, h
)
≈nft · nr · Mc
(raft , ra
r , h)−nr
t · nr · Mc (rart , ra
r , h) (8)
where rft and rr
t are the radii of the forward or reverse loops inthe transmitting coil. Additionally, raf
t and rart are the average
radii in the forward and reverse loops in the transmitting coil,respectively. In the receiving coil, ra
r is the average radius.For an optimal design with frequency-insensitive character-
istics, the mutual inductance with regard to the loop distance(h) must be uniform. To control the mutual inductance innear distances, a transmitting coil against a receiving coil mustconsist of adequate turns in the antiparallel loops according tothe loop size of the coils shown in Fig. 10. In this case, theproposed loops enable fixed energy efficiency and a resonantfrequency that is insensitive to the distance between the coilswithout auto matching circuits.
Depending on the configuration of the transmitting coils withantiparallel loops and the receiving coils with unidirectionalloops, the optimal condition for uniform mutual inductance iscalculated via differentiation in (4). When the value from theleft of (9) is determined by the distance (h1) between the coils,
Fig. 10. Comparison of the mutual inductance variation of two concentricloops in the CET: the mutual inductance with various turns of (a) forwardloops or (b) reverse loops in the transmitting coil, and (c) the mutual inductancebetween various turns of forward and reverse loops in the transmitting and thereceiving coil. In the (a) and (b), current direction in the transmitting loops isopposite.
LEE et al.: CONTACTLESS ENERGY TRANSFER SYSTEMS USING ANTIPARALLEL RESONANT LOOPS 355
Fig. 11. Using the influence of reverse loops in the transmitting coil, uniformmutual inductance against the distance and the number of turns in the antipar-allel coil are determined using the optimum design condition.
TABLE IICOIL PARAMETERS TO ANALYZE MUTUAL INDUCTANCE
the number of turns in the antiparallel resonant loops for theoptimum design is fixed. (See (9) at the bottom of the page.)
Using the radii for the transmitting coil (including the for-ward and reverse loops) and the receiving coil, k(raf
t , rar , h1)
and k(rarr , ra
r , h1) are obtained using (5). Equation (9) deter-mines the number of appropriate turns for the optimum design.As shown in Fig. 11, the optimum mutual inductance of theproposed coils, the size and the radii of the coils in Table II, canbe calculated.
IV. VERIFICATION
From the coil structure using the dimensions presented in Ta-ble II, the equivalent circuits (refer to Fig. 5) can be representedby lumped elements when the transmitting coil is matched toa specific distance using LC components. Table III shows thevalues of the RLC components, which consist of a transmittingand a receiving coil, resonant circuits, matching circuits for
TABLE IIIANALYZED RLC COMPONENTS OF EQUIVALENT CIRCUIT
MODELS BETWEEN TRANSMITTER AND RECEIVER
Fig. 12. Measurement setup of S21 between a transmitting coil (Type B) anda receiving coil using a vector network analyzer.
specific distances between the coils, and equivalent modelsfor the IC in the receiver. The parallel capacitor (Crr) in thereceiver is obtained using the resonance condition at 13.56 MHzafter the receiving coil, and the ICs in the receiver are modeledby series RL and series RC components, respectively. On theother hand, the transmitting coil creates resonance using theseries capacitor (Ctr) when the receiver is within the operationdistance from the transmitter. Then, T-type matching circuitsare applied for maximum energy transfer so that a 50 Ω cablecan serve as the connection without an impedance mismatch.
In particular, the transmitting and receiving coils are imple-mented using a PCB for the ease of manufacturing and realisticNFC communications, as shown in Fig. 4. Fig. 12 shows thatS21 (the CET efficiency) is readily measured using a two-port50 Ω measurement system (a vector network analyzer).
As shown in Table III, the value of the coil resistance(Rtc, Rrc) is large as a result of the small cross section(1.0 mm × 0.035 mm), which causes the overall system powerefficiency to be low. The following section explains the rela-tionship between the power efficiency and the coil resistance.
k(raft ,ra
r ,h1)√raf
t
[K
(k
(raft , ra
r , h1
))−
(1− 1
2 k2(raft ,ra
r ,h1)1−k2(raf
t ,rar ,h1)
)E
(k
(raft , ra
r , h1
))]k(rar
t ,rar ,h1)√
rar
[K (k (rar
t , rar , h1)) −
(1− 1
2 k2(rart ,ra
r ,h1)1−k2(rar
t ,rar ,h1)
)E (k (rar
t , rar , h1))
] =nr
t
nft
(9)
356 IEEE TRANSACTIONS ON INDUSTRIAL ELECTRONICS, VOL. 60, NO. 1, JANUARY 2013
Fig. 13. Relationship between S21, distance between coils and resonantfrequency characteristics. (a) Between the unidirectional transmitting coil(Type A) and the receiving coil. (b) Between the proposed antiparallel trans-mitting coil (Type B) and the receiving coil.
Fig. 14. Comparison of the frequency splitting characteristics in the CET.(a) Between the unidirectional transmitting coil (Type A) and the receivingcoil. (b) Between the proposed antiparallel transmitting coil (Type B) and thereceiving coil.
A. Theoretical Analysis
Based on (1) as derived in Section III, the theoretical resultsof the designed transmitting and receiving coils are presented inFig. 13 by plotting the power transfer efficiency regarding theloop distance and the resonant frequency characteristics.
Fig. 15. Simulated and measured results between the transmitting coil(Type A or Type B) and the receiving coil (matching distance = 5 cm).
Fig. 16. S21 comparison between the transmitting coil (Type A or Type B)and the receiving coil regarding the matching distances.
TABLE IVVOLTAGE LEVELS AND NORMALIZED RESULTS
RECEIVING FROM THE TRANSMITTER IN THE TAG
In Fig. 14, when the distance between the coils changes, S21
pertaining to the resonant frequency is plotted in the frequencydomain to compare the frequency-insensitive characteristics
LEE et al.: CONTACTLESS ENERGY TRANSFER SYSTEMS USING ANTIPARALLEL RESONANT LOOPS 357
Fig. 17. Signal waveforms of the power transmission from the reader with a Type A to the tag.
between the transmitting coil (Type A or Type B) and thereceiving coil. Frequency splitting is clearly shown when the re-ceiving coil approaches Type A. However, the effect of the res-onant frequency does not appear in Type B because the reverseloop of Type B inhibits the growth in the mutual inductance.
From this phenomenon, the transferring efficiency deterio-rates at the initial resonant frequency (refer to f0 in Fig. 1);thus, several studies [16], [17] and [19] have proposed fre-quency tracking techniques to improve the efficiency of theCET system. Consequently, the systems become more com-plex due to the additional frequency tracking control circuitssuch as couplers, amplifiers, and control algorithms. However,the proposed coils have a frequency-insensitive characteristicand, as a result, improve CET efficiency without the need forautomatic frequency techniques. The proposed CET systemswith antiparallel resonant coils are attractive and practical fora simple CET system design.
B. Simulated and Measured Results in the Implementation
In Fig. 4, the transmitting and receiving coils are imple-mented using a 0.5 mm FR4 substrate (εr = 4.5). The termina-tion of the coils is connected to a 50 Ω coaxial cable to measureS21 in Fig. 12 or the signal waveforms in Figs. 17 and 18.
From the designed transmitting and receiving coils, the LCcomponents are determined to have resonance and impedancematching conditions in the distance between the coils, as shownin Table III. Using known parameters, S21 is simulated againstthe respective loop distance using the method of moments.
Fig. 15 shows a comparison between the simulation and mea-surement for a contactless power transfer using the proposedcoil structure. The simulated results are 5% higher than themeasured results due to implementation error or measurementuncertainty. Moreover, the frequency-insensitivity performancebetween Type A and Type B is clearly distinguished from 1 cmto 5 cm. The antiparallel resonant loops (Type B) maintain
358 IEEE TRANSACTIONS ON INDUSTRIAL ELECTRONICS, VOL. 60, NO. 1, JANUARY 2013
Fig. 18. Signal waveforms of the power transmission from the reader with a Type B to the tag.
uniform efficiency within the operating range. The gap of thetwo resonant frequencies in Type B is one-sixth of that in TypeA. Regarding efficiency, Type B remains unchanged within theworking range (1 cm–5 cm) and is up to two times higher.
Fig. 16 shows the influence of the matching distance, whichdetermines the uniform efficiency in the coil size. In the vicinityof the loop radius, the influence of the mutual inductance isshown. For this reason, it is difficult to obtain good performance(efficiency and frequency insensitivity) at a distance far fromthe radius of the loops.
In near-field communication from the reader to the tag,sufficient voltage is required from the reader in order to operatethe tag IC. Using the proposed system structures, the signalwaveforms in the tag are analyzed by supplying the signal withdata from the reader. The transmitting coil in the reader is con-nected to the signal generator with a 1-mWatt magnitude andpulse modulation (pulse period = 16 μs, pulse width = 8 μs)at 13.56 MHz. The signal waveforms of the tag are measured
using an oscilloscope. The voltage levels with regard to eachdistance can be obtained as shown in Table IV. Figs. 17 and 18show the voltage levels obtained from the reader according tothe transmitting coil type (Type A or Type B). If the receivedpowers of Type A at 5 cm normalize those of Type B, aCET efficiency rating of 87% results at 1 cm in the proposedantiparallel resonant loops (Type B).
V. CONCLUSION
In CET systems using antiparallel resonant loops, appro-priate loop designs permit uniform mutual inductance andinsensitive resonant frequency characteristics. The antiparallelresonant loops provide improved efficiency of a maximumof 87% and six times the frequency insensitivity comparedto unidirectional resonant loops within the transferring rangewithout the need for additional automatic resonant circuits.
LEE et al.: CONTACTLESS ENERGY TRANSFER SYSTEMS USING ANTIPARALLEL RESONANT LOOPS 359
To implement a realistic NFC environment, magneticallycoupled coils with thin traces were fabricated on a PCB, whichresulted in the high equivalent resistance of the coils, degradingthe system efficiency overall. However, this verified that theproposed coils for the CET systems could lead to improvedrange-independent efficiency in the theoretical analysis if thewire coil resistance is reduced.
As portable devices with an NFC function require the con-figuration of simple structures for wireless power transfers, theproposed design concept for antiparallel resonant loops offersconsiderable benefits as it improves the transfer efficiencywithout supplementary circuits.
REFERENCES
[1] C.-J. Chen, T.-H. Chu, C.-L. Lin, and Z.-C. Jou, “A study of looselycoupled coils for wireless power transfer,” IEEE Trans. Circuits Syst. II,Exp. Briefs, vol. 57, no. 7, pp. 536–540, Jul. 2010.
[2] I.-J. Yoon and H. Ling, “Realizing efficient wireless power transfer usingsmall folded cylindrical helix dipoles,” IEEE Antennas Wireless Propag.Lett., vol. 9, pp. 846–849, Aug. 2010.
[3] B. L. Cannon, J. F. Hoburg, D. D. Stancil, and S. C. Goldstein, “Magneticresonant coupling as a potential means for wireless power transfer tomultiple small receivers,” IEEE Trans. Power Electron., vol. 24, no. 7,pp. 1819–1825, Jul. 2009.
[4] J. J. Casanova, Z. N. Low, and J. Lin, “A loosely coupled planar wirelesspower system for multiple receivers,” IEEE Trans. Ind. Electron., vol. 56,no. 8, pp. 3060–3068, Aug. 2009.
[5] J. O. Mur-Miranda, G. Fanti, Y. Feng, K. Omanakuttan, R. Ongie,A. Setjoadi, and N. Sharpe, “Wireless power transfer using weaklycoupled magnetostatic resonators,” in Proc. IEEE ECCE, Sep. 2010,pp. 4179–4186.
[6] Z. N. Low, J. J. Casanova, and J. Lin, “A loosely coupled planar wire-less power transfer system supporting multiple receivers,” Adv. PowerElectron., vol. 2010, pp. 1–13, 2010.
[7] I. Awai and T. Komori, “A simple and versatile design method ofresonator-coupled wireless power transfer system,” in Proc. ICCCAS,Jul. 2010, pp. 616–620.
[8] Z. N. Low, J. J. Casanova, P. H. Maier, J. A. Taylor, R. A. Chinga, andJ. Lin, “Method of load/fault detection for loosely coupled planar wirelesspower transfer system with power delivery tracking,” IEEE Trans. Ind.Electron., vol. 57, no. 4, pp. 1478–1486, Apr. 2010.
[9] “Guideline for limiting exposure to time-varying electric, magnetic, andelectromagnetic fields (up to 300 GHz),” Health Phys., vol. 74, no. 4,pp. 494–522, Apr. 1998.
[10] W. C. Brown, “The history of power transmission by radio waves,”IEEE Trans. Microw. Theory Tech., vol. MTT-32, no. 9, pp. 1230–1242,Dec. 1984.
[11] W. C. Brown, “The history of wireless power transmission,” Sol. Energy,vol. 56, no. 1, pp. 3–21, Jan. 1996.
[12] J. O. McSpadden and J. C. Mankins, “Space solar power programs andmicorwave wireless power transmission technology,” IEEE Microw. Mag.,vol. 3, no. 4, pp. 46–57, Dec. 2002.
[13] S. A. Ahson and M. Ilyas, RFID Handbook: Applications, Technology,Security, and Privacy. Boca Raton, FL: CRC Press, 2008.
[14] A. Kurs, A. Karalis, R. Moffatt, J. D. Joannopoulos, P. Fisher, andM. Soljacic, “Wireless power transfer via strongly coupled magnetic res-onances,” Science, vol. 317, no. 5834, pp. 83–86, Jul. 2007.
[15] T. Imura and Y. Hori, “Maximizing air gap and efficiency of magneticresonant coupling for wireless power transfer using equivalent circuit andNeumann formula,” IEEE Trans. Ind. Electron., vol. 58, no. 10, pp. 4746–4752, Oct. 2011.
[16] A. P. Sample, D. A. Meyer, and J. R. Smith, “Analysis, experimentalresults, and range adaptation of magnetically coupled resonators for wire-less power transfer,” IEEE Trans. Ind. Electron., vol. 58, no. 2, pp. 544–554, Feb. 2011.
[17] T. C. Beh, T. Imura, M. Kato, and Y. Hori, “Basic study of im-proving efficiency of wireless power transfer via magnetic resonancecoupling based on impedance matching,” in Proc. IEEE ISIE, Jul. 2010,pp. 2011–2016.
[18] Y. Tak, J. Park, and S. Nam, “Mode-based analysis of resonant charac-teristics for near-field coupled small antennas,” IEEE Antennas WirelessPropag. Lett., vol. 8, pp. 1238–1241, Nov. 2009.
[19] W. Fu, B. Zhang, and D. Qiu, “Study on frequency-tracking wirelesspower transfer system by resonant coupling,” in Proc. IEEE IPEMC,May 2009, pp. 2658–2663.
[20] D.-H. Won, H.-S. Kim, and B.-J. Jang, “13.56 MHz wireless power trans-fer system using loop antennas with tunable impedance matching circuit,”J. Korea Electromagn. Eng. Soc., vol. 21, no. 4, pp. 519–527, May 2010.
[21] S. Ortiz, Jr., “Is near-field communication close to success?,” Computer,vol. 39, no. 3, pp. 18–20, Mar. 2006.
[22] Texas Instruments, “Tag-it HF-I standard transponder inlays large rectan-gle,” TI-RFID2005, Dec..
[23] C. R. Paul, Inductance—Loop and Partial. Hoboken, NJ: Wiley, 2010.
Wang-Sang Lee received the B.S. degree fromSoongsil University, Seoul, Korea, in 2004, and theM.S. degree in electrical engineering from the Ko-rea Advanced Institute of Science and Technology(KAIST), Daejeon, Korea, in 2006. Since 2010, heis currently working toward the Ph.D. degree inelectrical engineering from KAIST.
From 2006 and 2010, he joined the Electromag-netic Compatibility (EMC) Testing Center of theDigital Industry Division of Korea Testing Labora-tory, Ansan, Korea, where he worked on the mod-
eling and simulation of Printed Circuit Board (PCB) circuits, photovoltaicsystems, RF/microwave transceivers, and antennas. His current research in-terests focus on the near-field communication and powering systems, mi-crowave/millimeter wave circuits (hybrid), electromagnetic interference/EMC,and radio frequency identification/Ubiquitous Sensor Network (USN).
Wang-Ik Son received the B.S. and M.S. degreesin electrical engineering from the Korea AdvancedInstitute of Technology, Daejeon, Korea, in 2006 and2008, respectively, where he is currently workingtoward the Ph.D. degree.
During his studies for the M.S. degree, he focusedon the compact circularly polarized antenna. His re-search interests are global positioning system anten-nas, microwave/millimeterwave circuits (monolithicmicrowave integrated circuits, hybrid), and wirelesscommunication systems.
Kyoung-Sub Oh received the B.S. degree fromChonbuk National University, Jeonju, Korea, in1994, and the M.S. and Ph.D. degrees in electricalengineering from the Korea Advanced Institute ofScience and Technology (KAIST), Daejeon, Korea,in 1997 and 2004, respectively.
From 2004 to 2005, he worked at Hyundai Motor.He also served Maltani Lighting and Samung Elec-tronics from 2005 to 2008 and from 2008 to 2010,respectively. Since 2010, he has been an assistantprofessor in electrical engineering in KAIST and
is currently a research associate professor in KAIST. His research interestsemphasize microwave imaging, inverse scattering, wireless power transfer,wireless communication system, and radio frequency identification/UbiquitousSensor Network (USN).
Jong-Won Yu (M’05) received the B.S., M.S., andPh.D. degrees in electrical engineering from theKorea Advanced Institute of Science and Technology(KAIST), Daejeon, Korea, in 1992, 1994, and 1998,respectively.
From 1995 to 2000, he worked at Samsung Elec-tronics. He also served Wide Tecom Head and Tel-son, from 2000 to 2001 and from 2001 to 2004,respectively. In February 2004, he joined as an Assis-tant Professor with electrical engineering at KAIST,where since February 2006, he has been an Associate
Professor. His research interests emphasize microwave/millimeter wave circuit(hybrid), wireless power transfer, wireless/near-field communication system,and radio frequency identification/Ubiquitous Sensor Network (USN).
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