range-adaptive wireless power transfer using multiloop and … · 2015. 9. 14. · kim and jeong:...

IEEE TRANSACTIONS ON INDUSTRIAL ELECTRONICS, VOL. 62, NO. 10, OCTOBER 2015 6233

Range-Adaptive Wireless Power Transfer UsingMultiloop and Tunable Matching Techniques

Jungsik Kim and Jinho Jeong, Member, IEEE

Abstract—In this paper, a range-adaptive wireless powertransfer (WPT) system is proposed to achieve high ef-ficiency over a wide range of distances by using tun-able impedance matching techniques. A multiloop topologyis employed to greatly reduce the variation in the inputimpedance of the WPT system with respect to the distance,where one of the four loops with a different size is selected,depending on the distance. It enables the design of a simpletunable matching circuit using a single variable capacitor.An algorithm is written to find the optimum loop and ca-pacitance in the matching network, based on the measuredinput return loss using a directional coupler and rectifiers.The fabricated WPT system shows a range-adaptive oper-ation with high efficiency over a wide range of distances.It attains 48% efficiency at a distance of 100 cm with amaximum efficiency of 92% at a distance of 10 cm.

Index Terms—Magnetic resonance, range adaptation,tunable impedance matching, wireless power transfer(WPT).

I. INTRODUCTION

W IRELESS power transfer (WPT) using magnetic res-onance coupling (MRC) can attain a high efficiencyfor a midrange of a few meters; thus, it can be applied forwireless charging of devices such as mobile phones, homeappliances, and biomedical implanted devices [1]–[9]. In thistechnology, electric power is transferred by the magnetic res-onance between coils with the same resonant frequency [10],[11]. Power transfer efficiency varies with the distance betweenthe transmitter (Tx) and the receiver (Rx). Maximum efficiencyis obtained at a distance where the impedance of the systemis perfectly matched [12]–[15]. However, the efficiency rapidlydrops outside this optimum distance. That is, it decreases at ashorter distance because of the frequency splitting effect, andat a longer distance because of weak coupling and impedancemismatches [2], [16]–[21].

WPT systems should maintain high efficiency even in thecase of misalignment and variable distance between Tx andRx coils for commercial applications [22]. For example, thecoupling between Tx and Rx coils in the wireless chargingof electrical vehicles can be easily affected by the improper

Manuscript received October 6, 2014; revised January 7, 2015 andFebruary 17, 2015; accepted March 10, 2015. Date of publicationApril 6, 2015; date of current version September 9, 2015.

The authors are with the Department of Electronic Engineering,Sogang University, Seoul 121-742, Korea (e-mail: [email protected]).

Color versions of one or more of the figures in this paper are availableonline at http://ieeexplore.ieee.org.

Digital Object Identifier 10.1109/TIE.2015.2420041

parking [23]. In biomedical implanted devices, the differentbody postures of the patients can change the coupling conditionand degrade the efficiency of WPT systems [7].

In order to solve these problems, there has been intensiveresearch on adaptive WPT systems [2], [7], [16]–[26]. In [2], afrequency tuning method was used to maintain high efficiencywith the distance, where the operating frequency was variedfrom 6.17 to 6.78 MHz over a distance range of 0.6 m. However,this method requires a wide bandwidth, which can be problem-atic in practical applications, because the system bandwidth istightly limited by the regulations [19], [20].

By contrast, tunable impedance matching techniques canattain high efficiency according to the distance using the sameoperating frequency. The input impedance of a WPT systemchanges with the distance, and thus, a tunable matching circuitcan be used to match the variable impedance with the distance[19], [20], [24]. In this technique, the tunable range of thematching circuit should be sufficiently large to accommodatethe variation in the input impedance of the WPT system withrespect to the distance. However, a widely tunable matchingcircuit can lead to increased losses with complex topology [27],[28]. In [19], a tunable matching circuit was designed for arange-adaptive WPT by using 21 relays, a capacitor bank with11 binary-weighted shunt capacitors and eight series capacitors,and two inductors.

We have proposed a multiloop WPT that maintains highefficiency over a wide range of distances [21]. Four loops withdifferent sizes were used, and one of the loops was selected,depending on the distance to match the impedances. Therefore,it achieved high efficiency at four different distances. How-ever, the efficiency drops at distances deviating from thesefour optimum distances, which are caused by impedance mis-matches. In addition, the loop was manually switched, depend-ing on the distance in [21].

In this paper, we propose a range-adaptive WPT system usingthe multiloop topology in [21] and a tunable matching circuit ata fixed operating frequency of 13.56 MHz. For this purpose,the multiloop WPT system is analyzed for a range-adaptiveoperation, including input impedance variation with respect tothe distance. Then, on the basis of the analysis of the multiloopWPT, a tunable matching circuit is designed with minimumtunable elements to reduce the matching losses. A searchingalgorithm is also developed to control the loop switching andtunable matching circuits, on the basis of the measured inputreturn loss (|S11|) of the system. We introduce and comparethe conventional and multiloop WPTs in Section II, focusingon the variation of input impedance and efficiency with respect

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6234 IEEE TRANSACTIONS ON INDUSTRIAL ELECTRONICS, VOL. 62, NO. 10, OCTOBER 2015

Fig. 1. (a) Conventional WPT system with four resonators. (b) Equiva-lent circuit of the WPT system.

to the distance. Then, a range-adaptive WPT is proposed inSection III, including its operating principle, the design of thetunable matching circuit, the (|S11|) measurement circuit, andthe automatic searching algorithm. The experimental results arepresented and compared with the previously reported adaptiveWPT systems in Section IV. Finally, WPTs with simplified Rxcircuits are discussed in Section V.

II. WPT USING MAGNETIC RESONANCE COUPLING

A conventional WPT using MRC consists of four resonators,as shown in Fig. 1(a) [2], [11]. Both Tx and Rx have asingle-turn loop and multiple-turn coil with the same resonantfrequency, i.e., ω0. The WPT system can be analyzed by usingan equivalent circuit model shown in Fig. 1(b) [15], [29],[30]. The loop is represented by self-inductance Ll, parasiticresistance Rl, and external capacitance Cl. The coil is modeled

by self-inductance Lc, parasitic resistance Rc, and parasiticcapacitance Cc. For the simplicity of the analysis, Tx and Rxresonators are assumed to be identical. The coupling coeffi-cients between coils and loops are denoted by kij . For thesymmetric WPT system, k12 = k34 and k13 = k24. The powersource in Tx is represented by the voltage source Vs andresistance R0, and the load in Rx by resistance R0. ApplyingKirchoff’s voltage law to the circuit in Fig. 1(b), we obtainthe following relation between the currents and voltages in theresonators at ω0:

⎡⎢⎢⎣I1I2I3I4

⎤⎥⎥⎦=

⎡⎢⎢⎣

R0 jω0M12 jω0M13 0jω0M12 Rc jω0M23 jω0M13jω0M13 jω0M23 Rc jω0M12

0 jω0M13 jω0M12 R0

⎤⎥⎥⎦

−1⎡⎢⎢⎣Vs000

⎤⎥⎥⎦

(1)

where Mij is mutual inductance, and ω0 is a resonant frequencygiven by ω0 = (1/

√LlCl) = (1/

√LcCc). In this relation, k14

is ignored, because it has a minimal effect on the performance.R0 +Rl is approximated to be R0, because quality (Q)-factorsof the loops are sufficiently high.

Power transfer efficiency η at ω0 (the power delivered tothe load divided by the available power from the source) canbe derived as (2), shown at the bottom of the page, whereQ1 and Q2 represent Q-factors of the loop and coil, respec-tively. It can be found from (2) that the efficiency varieswith respect to d23, the distance between transmitter and re-ceiver, because k23 is proportional to 1/d323 by the Neumannformula [21], [31]. This can be also explained by the vari-ation of input impedance of the WPT system depending ond23. From the equivalent circuit [see Fig. 1(b)], the inputimpedance of the WPT system, Zin, at ω0 can be calculatedto be (3), shown at the bottom of the page. It indicatesthat Zin changes with k23 (or d23) and it is matched to R0at only a single k23 (k23,matched), or d23 (d23,matched), for afixed k12. The k23,matched, where WPT exhibits the maximumefficiency, is given as (4)

k23,matched =

√(k212 − k213)

2Q21 − 1/Q22. (4)

The efficiency decreases as d23 deviates from d23,matchedbecause of impedance mismatches. However, the WPT systemshould maintain a high efficiency over a wide range of distancesfor practical applications. We can find from (4) that there canexist more than one k23 (or d23) that results in impedancematching, if k12 can be adjusted with respect to d23. In [18],k12 was adjusted by manually varying the spacing between theloop and coil (d12) with respect to d23, to satisfy (4) over a wide

η =|VL|2/R0|Vs|2/4R0

=

∣∣∣∣∣∣∣4Q21Q

22

(k212k23Q2 + 2jk12k13 + k

213k23Q2

)2(j((1 + k212Q1Q2)

2+ k223Q

22 + k

213Q1Q2 (k

213Q1Q2 − 2k212Q1Q2 + 2)

)+ 4k12k13k23Q1Q22

)2

∣∣∣∣∣∣∣(2)

Zin =R0Q1Q2

(Q1Q2k

412 − 2Q1Q2k212k213 + k212 − 2jQ2k23k12k13 +Q1Q2k413 + k213

)−2jQ1Q22k23k12k13 +Q22k223 +Q1Q2k212 +Q1Q2k213 + 1

(3)

KIM AND JEONG: RANGE-ADAPTIVE WPT USING MULTILOOP AND TUNABLE MATCHING TECHNIQUES 6235

Fig. 2. Proposed range-adaptive WPT system.

TABLE IPARAMETERS OF THE FABRICATED RESONATORS

range of d23. However, it is not practical to manually move theloop. The multiloop WPT proposed by the authors in [21] usedfour loops with different diameters achieving four differentvalues of k12. Then, one of the four loops was manually selectedto achieve impedance matching depending on d23. Therefore,there were four d23 points satisfying (4). However, this multi-loop WPT still exhibits impedance mismatches except at thesefour distances, resulting in efficiency drops. Furthermore, theloop should be automatically switched depending on d23 in realapplications.

III. RANGE-ADAPTIVE WPT SYSTEM

A. Operating Principle

In this paper, a range-adaptive WPT system is proposedusing a multiloop topology and tunable impedance matching.Fig. 2 shows a proposed range-adaptive WPT system. Com-pared with the conventional WPT system, this system employsfour loops in the Tx and Rx, respectively [21]. Ll,n and Rl,nrepresent the self-inductance and parasitic resistance of theloop, respectively, and Cl,n represents the series capacitance,where n = 1− 4. Each coil and loop is designed to resonate atthe same frequency of f0 = 13.56 MHz.

Table I lists the dimensions of each resonator (coil and loop),which was fabricated by using a copper wire with a diameter of0.3 cm. It also includes the extracted parameters (inductance,capacitance, and resistance) of each resonator from the mea-sured data by a vector network analyzer. The diameters of loop 1,loop 2, loop 3, and loop 4 are determined so that impedancematching can be achieved at a distance d23 = 30, 50, 70, and90 cm, respectively, whereas d12 is fixed to 0.5 cm. Innerdiameter and pitch of the coils are 45 and 3 cm, respectively.

The capacitances of the loops in this table are the externallyconnected series capacitors Cl,n for each loop to resonate at f0.One of the loops is selected by using a single-pole four-throw(SP4T) switch, depending on the distance d23; that is, loop 1is selected for d23 ≤ 30 cm, loop 2 for 30 cm < d23 ≤ 50 cm,loop 3 for 50 cm < d23 ≤ 70 cm, and loop 4 for d23 > 70 cm.

The multiloop topology can be effectively used in the range-adaptive WPT. This fact is verified by the comparison of theperformance of the multiloop and the conventional WPTs. Onlyone loop, for example, loop 1, is used in the conventional WPT.Fig. 3(a) shows the simulated S-parameters at f0 = 13.56 MHzof the conventional and multiloop WPT systems as a functionof d23. The simulation was performed using the parameters ofthe fabricated resonators presented in Table I. As expected, theconventional WPT system shows a good impedance match onlyat d23 = 30 cm, with |S11| = −40 dB and |S21| = −0.4 dB(η = 91.2%). However, the efficiency drops as d23 deviatesfrom 30 cm. On the contrary, the multiloop WPT systemexhibits four impedance-matched distances (d23 = 30, 50, 70,and 90 cm). Therefore, it maintains high efficiency over a widerange of d23.

The advantage of the multiloop WPT can be more clearlyfound in the variation of the input impedance Zin, dependingon the distance, as shown in Fig. 3(b) and (c). The conven-tional WPT with a single loop shows an impedance match(Zin = 50 Ω) at d23 = 30 cm with very large variation of Zinfrom 5 to 850 Ω in real part and from −j18.7 to j192 Ω inimaginary part for d23 from 10 to 90 cm. This fact impliesthat a widely tunable matching circuit is required, which, ingeneral, can be designed by using a number of tunable elementswith a large variation in element value. This approach leads tohigher matching losses and an increase in circuit complexity,which, in turn, complicates the searching algorithm. On thecontrary, the Zin variation in the multiloop WPT is dramaticallyreduced, that is, from 5 to 50 Ω in real part and from −j18.7 toj2.8 Ω in imaginary part over the same range of d23. Therefore,the multiloop topology is well suited for the design of tunablematching circuits.

B. Design of the Tunable Matching Circuit

On the basis of the simulation results aforementioned, wedesign a tunable matching circuit for a multiloop WPT system,


Fig. 3. Simulation results of conventional and multiloop WPT systems.(a) S-parameters. (b) Real part of Zin. (c) Imaginary part of Zin. Dottedand solid lines represent the conventional and multiloop WPT systems,respectively.

to achieve impedance matching even when d23 deviates fromthe four matched distances. Fig. 4 shows Zin variation on Smithchart as d23 increases from 10 to 90 cm with the correspondingloop selected. It can be found from this figure that tunableelements are required for the impedance matching according tothe distance. To minimize the number of tunable elements andsimplify the circuit topology, the loop capacitances in Table Iare adjusted as follows: Cl,1 = 150 pF, Cl,2 = 160 pF, Cl,3 =180 pF, and Cl,4 = 200 pF. Then, Zin with adjusted loopcapacitances exhibits an inductive part, as well as a resistivepart, as shown in Fig. 4. Therefore, a single shunt variablecapacitor is sufficient to match Zin to 50 Ω for the entire rangeof interest of d23.

Fig. 5(a) shows the designed tunable matching circuit using avaractor Cp, where Rb and Cb are used as a bias circuit for the

Fig. 4. Zin variation as d23 increases from 10 to 90 cm with thecorresponding loop selected.

Fig. 5. (a) Designed tunable matching circuit. (b) Zin trajectory as Cpincreases from 110 to 357 pF at various d23 ’s with loop 1 selected.

Fig. 6. Required Cp for the impedance matching as function of thedistance. Corresponding |S11| and |S21| at 13.56 MHz are included.

varactor. Capacitance of the varactor is controlled by voltageVd. Fig. 5(b) shows Zin trajectory when the capacitance Cpincreases from 110 to 357 pF at various d23’s, whereas loop 1is selected. It shows how Zin changes and matches to 50 Ωaccording to the value of Cp. Note that Cp’s at Tx and Rx arevaried with the same value. Fig. 6 shows the required Cp forthe impedance matching as a function of d23. It demonstrates


Fig. 7. |S11| measurement circuit. (a) Block diagram. (b) Schematic ofthe rectifier.

that a single tunable capacitor from 110 to 357 pF is sufficientto achieve the impedance matching for 10 cm ≤ d23 ≤ 90 cm.The capacitance variation ratio is only 1:3.25, which can beeasily obtained from a commercial varactor. The |S11| and |S21|of the multiloop WPT are simulated at 13.56 MHz using theselected capacitance, resulting in |S11| < −33 dB and |S21| <−2.3 dB for 10 cm ≤ d23 ≤ 90 cm.

C. Automated WPT System: |S11| MeasurementIn a range-adaptive WPT system, the optimum loop and

Cp achieving impedance matching should be automaticallyselected depending on the distance d23. This can be conductedby measuring |S11| of the system, and finding the optimumloop and Cp that minimize |S11| (see Fig. 2). Therefore, weneed to design a |S11| measurement circuit. In the proposedsystem, |S11| is measured by using a directional coupler andrectifiers, as shown in Fig. 7(a). Power levels at the coupled andisolated ports of the directional coupler are proportional to theincident and reflected power, respectively, under the assumptionof high directivity. In this paper, we used a direction couplerwith a directivity of 41.3 dB and coupling factor of 20.5 dB.The rectifiers convert these powers to dc voltages, which areread by the computer through a data acquisition (DAQ) board.Then, the computer estimates the incident and reflected powerfrom the dc voltages, and computes |S11|.

As shown in Fig. 7(b), the rectifiers are designed usinga diode (Skyworks SMS7621), a 50-Ω matching resistor, a100-nF capacitor, and a 10-MΩ load resistor. An accuraterelationship between input power and output dc voltage of therectifier is required to determine |S11|. For this purpose, thisrelationship is measured, as shown in Fig. 8, and is curve fittedby a seventh-order polynomial. Then, the inverse polynomialis used to determine the input power from the measured dcvoltage of the rectifiers. To verify the performance of the|S11| measurement circuit, |S11| values were measured with thecoupler terminated by several resistors [Ztm from 1 to 100 Ω inFig. 7(a)]. Fig. 8(b) shows a good agreement of the measuredand theoretical |S11|, where theoretical values were calculatedfrom (Ztm − 50)/(Ztm + 50).

D. Automated WPT System: Algorithm for Finding anOptimum Loop and Capacitance

Now, we need an algorithm for finding the optimum loop andcapacitance Cp to achieve impedance matching or to minimize

Fig. 8. (a) Measured output dc voltage (VDC) versus input power (Pin)of the rectifier). (b) Theoretical and measured |S11| versus terminationimpedance (Ztm).

|S11| for varying d23. Fig. 9 shows the simulated |S11| for eachloop as a function of d23 at Vd = 10, 3, 0.5, and 0 V. For loop 1,|S11| at Vd = 10 V exhibits a minimum value at d23 = 30 cm.As Vd decreases to 0 V, the distance exhibiting a minimum |S11|moves to d23 = 10 cm. We can make the similar observation of|S11| variation with respect to Vd for other loops. Therefore, theoptimum loop and capacitance can be searched by finding theinflection point of |S11| versus Vd while switching the loops asfollows.

Step 1) Initially, Vd is set to be 10 V, that is, Cp is at thelowest value of 110 pF. Then, |S11| values are mea-sured while switching the loops, and the loop with aminimum |S11| is selected for the Vd sweep.

Step 2) For a selected loop, |S11| values are measured whilesweeping Vd from 10 to 0 V in steps of 1 V. If themeasured |S11| values are concave with respect to Vd,a fine tuning for Vd (in steps of 0.2 V) is performedaround an inflection point to find a minimum |S11|.If the minimum |S11| is less than 0.1, the algorithmends.

Step 3) Step 2 is repeated by switching the loops until aminimum |S11| is less than 0.1. If |S11| is not lessthan 0.1 for any combination of the loop and Vd, thealgorithm selects the loop and Vd at which the |S11|is a minimum, and the algorithm ends.


Fig. 9. Simulated |S11| as a function of d23 for each loop. (a) Vd =10 V. (b) Vd = 3 V. (c) Vd = 0.5 V. (d) Vd = 0 V.

Note that Tx and Rx have identical loop switching andtunable matching circuits, as shown in Fig. 2. In the afore-mentioned algorithm, the optimum loop and Cp are searched,whereas the same loop and the same Vd to Tx and Rx areselected.

Fig. 10. (a) Photograph of the fabricated range-adaptive WPT system.(b) Fabricated circuit board consisting of the |S11| measurement circuit,tunable impedance matching circuit, and SP4T switch.

IV. EXPERIMENTAL RESULTS

The performance of the proposed range-adaptive WPT sys-tem was verified by the experiments. Fig. 10(a) shows thefabricated WPT system. A signal source (Rohde & SchwarzSML03) is used to generate an input power of 17 dBm at13.56 MHz. The output waveform is measured by an oscil-loscope (Agilent DSO-X2012A). The computer reads the dcoutput voltages of the two rectifiers through a DAQ board,and computes |S11|. Then, it runs the algorithm to find anoptimum loop and varactor control voltage (Vd). Finally, itemits the control voltages for the SP4T switches and varactorsthrough an arbitrary waveform generator. The S-parameters ofthe fabricated WPT system were also measured by a usingvector network analyzer.

Fig. 10(b) shows the fabricated circuit board consisting of the|S11| measurement circuit, tunable matching circuit, and SP4Tswitch. The tunable matching circuit was implemented by usinga fixed capacitor of 80 pF and four varactors in parallel. Eachvaractor (Skyworks SMV1212) exhibits a capacitance variation


Fig. 11. (a) Selected loop and varactor control voltage (Vd). Dottedlines represent the optimum Cp obtained from the simulation (seeFig. 6). (b) The measured S-parameters of the WPT system. Solid anddotted lines represent measurement and simulation, respectively.

from 4.7 to 72.4 pF for Vd from 10 to 0 V. It has a parasiticseries resistance of 0.8 Ω. The SP4T switch is composed ofthree relays (Panasonic ARE10A06) with an insertion loss of0.02 dB. The size of the circuit in Fig. 10(b) is 6 cm × 4 cm.

The performance of the fabricated WPT system was mea-sured while increasing the distance d23 in 5-cm incrementsfrom 10 to 100 cm. At each distance, the system automaticallymeasures |S11| and finds the optimum loop and Vd for min-imizing the |S11|. Fig. 11(a) shows the selected loop and Vdby the developed algorithm as a function of the distance d23.The corresponding Cp is also included in this figure, which iscalculated from the relationship between the capacitance andthe bias voltage of the varactor. This figure shows that thedeveloped algorithm can properly find the optimum loop andCp for 10 cm ≤ d23 ≤ 100 cm.

Fig. 11(b) shows the measured S-parameters of WPT system.It shows that the system was well matched over a wide rangeof the distance, that is, |S11| is below −32 dB for 10 cm ≤d23 ≤ 90 cm. The measured |S11| and |S21| agree well with theoptimized performance predicted by the simulation.

Fig. 12(a) shows the measured efficiency of the proposedWPT system as a function of the distance d23. The previousmultiloop WPT system in [21] shows efficiency drops at thedistances deviating from 30, 50, 70, and 90 cm. As shown inthis figure, these efficiency drops were recovered in [21] by

Fig. 12. Measured efficiency of the proposed WPT system. (a) Effi-ciency versus distance d23. (b) Efficiency versus the normalized dis-tance with coil diameter.

manually tuning the frequency of the transmitter from 10.60 to13.56 MHz. However, the frequency tuning should be automat-ically carried out in real applications, so that it requires thecircuit to detect the distance, algorithm to find the optimumfrequency, and the circuit to control the frequency of Tx. Inaddition, it needs wide bandwidth of 2.96 MHz. Unfortunately,only 14-kHz bandwidth is permitted in the 13.56-MHz indus-trial, scientific and medical band [14]. On the contrary, theWPT system in this work operates at a fixed frequency of13.56 MHz and accomplished an automatic range adaptation byusing tunable impedance matching, so that it is more practical.It maintains high efficiency across the entire distance from 10 to100 cm, as shown in Fig. 12(a). The efficiency was calculated asthe power delivered to the load divided by the power availablefrom the source. Note that the efficiency in Fig. 12 includes lossof every component of the system: |S11| measurement circuit(directional coupler), loop selection circuits (relays), and tun-able impedance matching circuits (varactors), resonators (loopsand coils), and impedance mismatches. It shows a maximumefficiency of 92% at d23 = 10 cm, which corresponds to a 52%increase compared with the multiloop WPT system in [21]. Italso attains an efficiency of 48% at d23 = 100 cm. At d23 = 30,50, 70, and 90 cm, the proposed WPT systems exhibits about3% lower efficiency, which is caused by the losses of thetunable matching circuits.

The efficiency of the proposed WPT system is also comparedwith other range-adaptive WPT systems in [19], [20], as shown


TABLE IICOMPARISON OF REPORTED ADAPTIVE WPT SYSTEMS AT 13.56 MHZ

in Fig. 12(b). For fair comparison, the efficiency is plotted withrespect to the normalized distance with coil diameter. It can befound from this figure that the proposed WPT system maintainshigh efficiency over a wider range of the normalized distance.This improvement of the range adaptation was accomplished bycombining the multiloop and tunable matching techniques.

Table II compares the reported range-adaptive WPT systems.The WPT systems in [19], [20] employed switching capacitorsto obtain wide tunability from the impedance matching circuit,where many switches (relays) and capacitors are used to ob-tain a capacitance variation ratio higher than 1:1000. On thecontrary, the tunable matching circuit in the proposed systemscould be designed by using a varactor with a capacitancevariation ratio of only 1:3.25, because the multiloop topologygreatly reduces the variation of the input impedance. Therefore,the searching algorithm to maximize the efficiency was alsosimplified in the proposed system. It took less than 1.2 s tofind the optimum loop and capacitance and settle the system.The simplified tunable matching circuits and algorithm arebeneficial in real applications.

V. MODIFIED WPTS WITH SIMPLIFIED RX CIRCUIT

The proposed WPT system in the aforementioned requiresa radio-frequency (RF) communication channel between Txand Rx, to control the switch in the loop selection circuit andvaractor in tunable matching circuit of Rx. Although WPTstandard such as Rezence by Alliance for Wireless Powerincludes an RF communication channel operating at 2.4 GHz[32], there still exists a need of a simplified Rx circuit for morepractical, reliable and robust operations. In order to see if theproposed WPT system can handle this issue, the simulation iscarried out for the modified WPT systems with a simplified Rx.

Two modified WPTs with simplified Rx are considered inthe simulation: modified WPT 1 with single loop and tunablematching Rx, and modified WPT 2 with single loop and fixedmatching Rx. Four Tx loops in modified WPTs are resized fromoriginal ones in Table I to achieve best efficiency performance.Note that loop 3 is only used in Rx of the modified WPTs 1and 2. That is, k34 is fixed to 0.10. In the modified WPT 2, thevaractor capacitor Cp in Rx is fixed to 80 pF.

The efficiency of each WPT system is simulated by using anequivalent circuit shown in Fig. 2. Fig. 13 shows the simulatedefficiency as a function of d23. The modified WPT 1 recoversthe efficiency across the distance range of interest, even though

Fig. 13. Simulated efficiency of WPT systems.

the efficiency slightly drops at the distances (between 10 and50 cm), which were covered by loops 1 and 2 in the originalWPT system. This efficiency drop is more serious in the mod-ified WPT 2, as shown in Fig. 13, because there are no tuningor control elements in Rx of WPT 2. However, the efficiencyis still high (higher than 76.6%) at this range of distances. Thissimulation result verifies that the proposed WPT system canmaintain relatively high efficiency across a wide range of dis-tances without any control of Rx. Note that Tx tunable matchingcircuit in WPT 2 was modified to improve the tunability; thatis, a series varactor Cs was added to a parallel varactor Cp,whereCs was varied from 150 to 175 pF, whereasCp from 80 to670 pF.

VI. CONCLUSION

In this paper, a range-adaptive WPT has been proposed,achieving high efficiency over a wide range of distances. Mul-tiloop topology was utilized to simplify the design of a tunablematching network and a searching algorithm. The optimumloop and capacitance were determined by using the measuredinput return losses. The fabricated WPT system showed arange-adaptive operation with high efficiency greater than 48%up to a distance of 100 cm.

REFERENCES[1] B. Choi, J. Nho, H. Cha, T. Ahn, and S. Choi, “Design and implementa-

tion of low-profile contactless battery charger using planar printed circuitboard windings as energy transfer device,” IEEE Trans. Ind. Electron.,vol. 51, no. 1, pp. 140–147, Feb. 2004.

[2] A. P. Sample, D. T. Meyer, and J. R. Smith, “Analysis, experimen-tal results, and range adaptation of magnetically coupled resonators for


wireless power transfer,” IEEE Trans. Ind. Electron., vol. 58, no. 2,pp. 544–554, Feb. 2011.

[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. A. Taylor, Z. N. Low, J. Casanova, and J. Lin, “A wireless powerstation for laptop computers,” in Proc. IEEE Radio Wireless Symp., 2010,pp. 625–628.

[5] T. Imura, H. Okabe, and Y. Hori, “Basic experimental study on heli-cal antennas of wireless power transfer for electric vehicles by usingmagnetic resonant couplings,” in Proc. IEEE VPPC, Sep. 7–10, 2009,pp. 936–940.

[6] J. Shin et al., “Design and implementation of shaped magnetic-resonance-based wireless power transfer system for roadway-powered moving elec-tric vehicles,” IEEE Trans. Ind. Electron., vol. 61, no. 3, pp. 1179–1192,Mar. 2014.

[7] P. Si, A. Hu, S. Malpas, and D. Budgett, “A frequency control method forregulating wireless power to implantable devices,” IEEE Trans. Biomed.Circuits Syst., vol. 2, no. 1, pp. 22–29, Mar. 2008.

[8] A. K. RamRakhyani, S. Mirabbasi, and M. Chiao, “Design and opti-mization of resonance-based efficient wireless power delivery systems forbiomedical implants,” IEEE Trans. Biomed. Circuits. Syst., vol. 5, no. 1,pp. 48–63, Feb. 2011.

[9] D. Ahn and S. Hong, “Wireless power transmission with self-regulatedoutput voltage for biomedical implant,” IEEE Trans. Ind. Electron.,vol. 61, no. 5, pp. 2225–2235, May 2014.

[10] A. Karalisa, J. D. Joannopoulosb, and M. Soljacic, “Efficient wirelessnon-radiative mid-range energy transfer,” Ann. Phys., vol. 323, no. 1,pp. 34–48, Jan. 2008.

[11] A. Kurs et al., “Wireless power transfer via strongly coupled magneticresonances,” Science, vol. 317, no. 5834, pp. 83–86, Jul. 2007.

[12] T. Imura and Y. Hori, “Maximizing air gap and efficiency of magneticresonant coupling for wireless power transfer using equivalent circuitand Neumann formula,” IEEE Trans. Ind. Electron., vol. 58, no. 10,pp. 4746–4752, Oct. 2011.

[13] M. Zargham and P. G. Gulak, “Maximum achievable efficiency in nearfield coupled power-transfer systems,” IEEE Trans. Biomed. CircuitsSyst., vol. 6, no. 3, pp. 228–245, Jun. 2012.

[14] T. C. Beh, T. Imura, M. Kato, and Y. Hori, “Basic study of improvingefficiency of wireless power transfer via magnetic resonance couplingbased on impedance matching,” in Proc. IEEE ISIE, Jul. 4–7, 2010,pp. 2011–2016.

[15] S. Cheon et al., “Circuit-model-based analysis of a wireless energy-transfer system via coupled magnetic resonances,” IEEE Trans. Ind. Elec-tron., vol. 58, no. 7, pp. 2906–2914, Jul. 2011.

[16] J. Park, Y. Tak, Y. Kim, Y. Kim, and S. Nam, “Investigation of adaptivematching methods for near-field wireless power transfer,” IEEE Trans.Antennas Propag., vol. 59, no. 5, pp. 1769–1773, May 2011.

[17] N. Y. Kim, K. Y. Kim, J. Choi, and C.-W. Kim, “Adaptive frequency withpower-level tracking system for efficient magnetic resonance wirelesspower transfer,” Electron. Lett., vol. 48, no. 8, pp. 452–454, Apr. 2012.

[18] T. P. Duong and J. W. Lee, “Experimental results of high-efficiency reso-nant coupling wireless power transfer using a variable coupling method,”IEEE Microw. Wireless Compon. Lett., vol. 21, no. 8, pp. 442–444,Aug. 2011.

[19] T. Beh, M. Kato, T. Imura, S. Oh, and Y. Hori, “Automated impedancematching system for robust wireless power transfer via magneticresonance coupling,” IEEE Trans. Ind. Electron., vol. 60, no. 9,pp. 3689–3698, Sep. 2013.

[20] B. H. Waters, A. P. Sample, and J. R. Smith, “Adaptive impedancematching for magnetically coupled resonators,” in Proc. PIERS, Moscow,Russia, Aug. 2012, pp. 694–701.

[21] J. Kim, W. Choi, and J. Jeong, “Loop switching technique for wirelesspower transfer using magnetic resonance coupling,” Progr. Electromagn.Res., vol. 138, pp. 197–209, 2013.

[22] B. H. Waters, A. P. Sample, P. Bonde, and J. R. Smith, “Powering aventricular assist device (VAD) with the free-range resonant electricalenergy,” Proc. IEEE, vol. 100, no. 1, pp. 138–149, Jan. 2012.

[23] S. Aldhaher, P. C.-K. Luk, and J. F. Whidborne, “Electronic tuning ofmisaligned coils in wireless power transfer systems,” IEEE Trans. PowerElectron., vol. 29, no. 11, pp. 5975–5982, Nov. 2014.

[24] Y. Lim, H. Tang, S. Lim, and J. Park, “An adaptive impedance-matchingnetwork based on a novel capacitor matrix for wireless power transfer,”IEEE Trans. Power Electron., vol. 29, no. 8, pp. 4403–4413, Aug. 2014.

[25] W. Lee, H. Lee, K. Oh, and J. Yu, “Switchable distance-based impedancematching networks for a tunable HF system,” Progr. Electromagn. Res.,vol. 128, pp. 19–34, 2012.

[26] Z. Pantic and S. M. Lukic, “Framework and topology for active tuning ofparallel compensated receivers in power transfer systems,” IEEE Trans.Power Electron., vol. 27, no. 11, pp. 4503–4513, Nov. 2012.

[27] W. C. E. Neo et al., “Adaptive multi-band multi-mode power amplifier us-ing integrated varactor-based tunable matching networks,” IEEE J. Solid-State Circuits, vol. 41, no. 9, pp. 2166–2176, Sep. 2006

[28] H. Nemati, C. Fager, U. Gustavsson, R. Jos, and H. Zirath, “Design ofvaractor-based tunable matching networks for dynamic load modulationofhigh power amplifiers,” IEEE Trans. Microw. Theory Techn., vol. 57,no. 5, pp. 1110–1118, May 2009.

[29] L. Chen, S. Liu, Y. Zhou, and T. Cui, “An optimizable circuit structurefor high-efficiency wireless power transfer,” IEEE Trans. Ind. Electron.,vol. 60, no. 1, pp. 339–349, Jan. 2013.

[30] J. Garnica, R. A. Chinga, and J. Lin, “Wireless power transmission: fromfar field to near field,” Proc. IEEE, vol. 101, no. 6, pp. 1321–1331,Jun. 2013.

[31] S. Raju, R. Wu, M. Chan, and C. P. Yue, “Modeling of mutual couplingbetween planar inductors in wireless power applications,” IEEE Trans.Power Electron., vol. 29, no. 1, pp. 481–490, Jan. 2014.

[32] “Alliance for wireless power,” Rezence, Altamonte Springs, FL, USA,2014. [Online]. Available: http://www.rezence.com

Jungsik Kim received the B.S. degree inwireless communications engineering fromKangwoon University, Seoul, Korea, in 2011. Heis currently working toward the Ph.D. degreein electronic engineering at Sogang University,Seoul, Korea.

His research interests include wireless powertransfers, monolithic microwave integrated cir-cuits, and terahertz integrated circuits.

Jinho Jeong (S’00–M’05) received the B.S.,M.S., and Ph.D. degrees in electrical engi-neering from Seoul National University, Seoul,Korea, in 1997, 1999, and 2004, respectively.

From 2004 to 2007, he was with the Uni-versity of California at San Diego, La Jolla,CA, USA, as a Postdoctoral Scholar, where hewas involved with the design of high-efficiencyand high-linearity radio-frequency power ampli-fiers. In 2007, he was with the Department ofElectronics and Communications Engineering,

Kwangwoon University, Seoul. Since 2010, he has been with the Depart-ment of Electronic Engineering, Sogang University, Seoul. His researchinterests include monolithic microwave integrated circuits, terahertzintegrated circuits, high-efficiency/high-linearity power amplifiers andoscillators, and wireless power transfers.

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