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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)
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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,
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6236 IEEE TRANSACTIONS ON INDUSTRIAL ELECTRONICS, VOL. 62, NO.
10, OCTOBER 2015
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
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KIM AND JEONG: RANGE-ADAPTIVE WPT USING MULTILOOP AND TUNABLE
MATCHING TECHNIQUES 6237
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.
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6238 IEEE TRANSACTIONS ON INDUSTRIAL ELECTRONICS, VOL. 62, NO.
10, OCTOBER 2015
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
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KIM AND JEONG: RANGE-ADAPTIVE WPT USING MULTILOOP AND TUNABLE
MATCHING TECHNIQUES 6239
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
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6240 IEEE TRANSACTIONS ON INDUSTRIAL ELECTRONICS, VOL. 62, NO.
10, OCTOBER 2015
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.
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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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