IEEE TRANSACTIONS ON POWER ELECTRONICS, VOL. 23, NO. 1, JANUARY 2008
455
Optimal Design of a Hybrid Winding Structure for
Planar Contactless Battery Charging Platform
Xun Liu, Student Member, IEEE, and S. Y. (Ron) Hui, Fellow, IEEE
Abstract—Planar contactless battery charging platform is
an emerging technology that has the potential of unifying the
charging protocols of portable consumer electronic products. In
this paper, a new hybrid structure which consists of a coil and a
spiral winding is proposed for improving the uniform magnetic
field distribution over the charging surface. An analysis into an
optimal design of the number of turns and the dimension of the
spiral winding is presented for a given concentrated coil. The
uniform magnetic field distribution of the designed prototype is
measured by an electromagnetic compatibility scanner and by an
energy-receiving coil. Based on circuit modeling and analysis, the
inverter circuit topology and particularly the resonant compensation tank is designed for maximizing power transfer for multiload
applications. A design procedure is proposed and verified by the
experiments. An efficiency of about 80% has been achieved for the
coupled structures when four loads are charged on the platform
simultaneously.
Index Terms—Coreless transformer, multiload application,
planar contactless battery charger.
I. INTRODUCTION
LANAR contactless battery charging platform is an
emerging technology that has the potential of unifying the
charging protocols of portable consumer electronic products
such as mobile phones, CD players and iPods etc. Through
near-field coupling, power should be able to transfer from
the charging platform to the energy-receiving winding for
charging the electronic equipment. Preferably, the planar
charging platform should be able to charge several electronic
devices simultaneously, regardless of their positions and orientations. Recently, two approaches have been proposed and are
documented in several patent documents [1]–[3]. These two
approaches are compared in [4] in detail. The first approach
[1] adopts a “horizontal flux” approach in which the line of
magnetic flux flows horizontally to the planar charging surface.
In order to sense enough ac flux, the cross-sectional area of the
energy-receiving winding vertical to the platform surface must
be large enough (thick and/or wide enough). Such requirement
P
Manuscript received February 9, 2007; revised May 9, 2007. This work was
supported by the Hong Kong Research Grant Council under Project (CityU
1141/05) and the City University of Hong Kong under Project 7001761.. Recommended for publication by Associate Editor D. Perreault.
X. Liu is with ConvenientPower (HK), Ltd., Hong Kong, China.
S. Y. R. Hui is with the Department of Electronic Engineering, City University of Hong Kong, Kowloon, Hong Kong, China (e-mail: eeronhui@cityu.edu.
hk).
Color versions of one or more of the figures in this paper are available online
at http://ieeexplore.ieee.org.
Digital Object Identifier 10.1109/TPEL.2007.911844
is not favorable for many modern consumer electronic equipments that must be thin and slim. Another planar inductive
battery charging platform based on a “perpendicular flux”
approach was proposed in [2], [3]. The lines of flux of this
charging platform flow “perpendicularly” into and out of the
charging surface. With the use of a patented electromagnetic
shield, the flux flows vertically into and out of the charging
surface like a “water fountain.” Such feature is very suitable
for slim design of the energy-receiving element because it
allows the energy transfer over the entire surface on which the
electronic equipment (to be charged) is placed [5].
As shown in [6] and explained in [3], the magnitude of the
magnetic field intensity over this platform that uses either a multilayer hexagonal winding array or a concentrated winding is not
ideally uniform. The magnetic field intensity drops down from
the periphery to the center of the charging surface, like the magnetic field distribution of a coil that is also mentioned in [1].
Such “concave” distribution means excessive current is needed
to provide enough charging power when the energy-receiving
unit is placed at the center of the charging platform.
Planar spiral winding, such as coreless printed circuit board
(PCB) transformers [7], [8] is another planar structure that can
achieve contactless power transfer using the vertical flux principle. In fact, it has been used for battery charging of cellular
phone [9]. As shown in [10], the magnetic field of the spiral
winding is non-linear in a “convex” manner that its magnitude
is highest in the central region of the spiral winding. Therefore,
the cellular phone in [9] must be placed in the specific location
in order to obtain optimal energy transfer.
In this paper, a new hybrid structure that combines the advantageous features of concentrated winding and spiral winding is
proposed and its optimal design theory is presented. This hybrid
winding structure can generate an improved near-uniform perpendicular magnetic flux distribution than previous approaches.
This patent-pending hybrid structure [3] consists of a coil and
a spiral winding. A mutual-inductance based method is used
for the design of this structure. The structure and the design
method are presented in Section II of this paper. In Section III,
the designed prototype is scanned by an electromagnetic compatibiltiy (EMC) scanner and measured with the help of a receiving coil. The calculated and measured results are compared
and they agree well with each other. In order to achieve optimal
power transfer for multiloads on the same charging platform, the
power inverter circuit topology and its resonant compensation
tank must be designed carefully for efficient power transfer. The
resonant compensation method has been utilized and analyzed
in some previous work [9], [11]–[18]. Based on the equivalent
circuit analysis, a systematic design procedure suitable for multiload application is proposed in Section IV. The experimental
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IEEE TRANSACTIONS ON POWER ELECTRONICS, VOL. 23, NO. 1, JANUARY 2008
Fig. 1. Sketch of the hybrid structure in (a) 3-D view, (b) cross-sectional view,
and (c) top-view.
spiral winding are represented by four concentric circles. Before the design, four assumptions are made:
a) the spiral winding is coaxial with the coil;
b) the spiral winding is connected in series with the coil,
which means the current in the spiral winding is the same
with that in each turn of the coil;
c) the spacing between the tracks, , as shown in Fig. 1(c),
is the same for a certain design;
d) the spiral winding that is made up of Litz wire, is treated
as a filament.
, the radius
The number of turns of the spiral winding,
of the first turn,
, and the spacing between the tracks, ,
decide the shape of the spiral winding, as shown in Fig. 1(c). In
the first step, these three parameters are fully searched and all
the possible results are presented. The minimum and maximum
values of the three parameters are decided by the dimension of
the coil and the available manufacturing technology.
In order to judge the validity of the possible results, a mutual
inductance based method is used as a tool for optimization. In
Fig. 1(c), the red dotted circle represents a testing coil above the
charging surface. If this testing coil (as energy-receiving unit)
has only one turn, the mutual inductance between the one-turn
testing coil and the hybrid structure is
Fig. 2. Two coaxial circular filaments.
(1)
TABLE I
SUMMARY OF THE DESIGNED SPIRAL WINDINGS FOR A
= 5,
= 75 mm)
GIVEN CIRCULAR COIL (
N
R
where
is the magnetic flux coupled to the one-turn testing
coil, is the current in the hybrid structure and is the area of
the one-turn testing coil. If the magnetic flux is uniform enough
in , (1) can be approximated to
(2)
For another one-turn testing coil with different radius, the
same relationship can also be established
results of a prototype are included to confirm the validity of the
theory.
II. HYBRID STRUCTURE AND ITS DESIGN
Fig. 1 shows the sketch view of the circular hybrid structure. This hybrid structure consists of a concentrated coil and
a spiral winding. The “concave” magnetic field distribution
generated by the concentrated coil can be compensated by the
“convex” magnetic field distribution generated by the planar
spiral winding, so that a more uniform flux distribution can
be achieved. A uniform magnetic flux distribution means that
different portable electronic devices (with inbuilt energy-receiving windings) can be placed and charged on the charging
platform, as represented by the small blue circles in Fig. 1(a).
The spiral winding can be placed on the same plane, below or
above the planar surface of the coil, as shown in Fig. 1(b).
As far as the design of the circular hybrid structure is concerned, one needs to design a spiral winding for a given concentrated coil (of a certain radius and number of turns). Therefore,
the number of turns and the dimensions of the spiral winding
have to be designed and optimized. Fig. 1(c) shows one example of the designed results, in which the four turns of the
(3)
From (2) and (3), if the magnetic flux is uniform enough over
the entire charging surface, the following equation or inequality
(4) must be applicable to any two one-turn testing coils placed
on the charging area
(4)
where is a small tolerance.
Equation or inequality (4) provides the criterion to judge if
a uniform magnetic flux distribution has been achieved. The inequality is used in practice for optimization. For the convenience
of calculation, the two one-turn testing coils are chosen coaxial
with the spiral winding and the coil. One is fixed with the minimum radius
[see Fig. 1(c)] and the other is enlarged from
to
. If (4) is satisfied in the whole progress, this possible result is saved as a valid design. All the possible results
need such optimization process.
The one-turn testing coils are chosen to be coaxial with
the spiral winding and the coil because the mutual inductance
between two coaxial circular filaments can be calculated with
LIU AND HUI: OPTIMAL DESIGN OF A HYBRID WINDING STRUCTURE
457
Fig. 3. Top view of the rectangular hybrid structure.
simple expression as in (5) and is suitable for the iterative
searching process
(5a)
(5b)
where
and
are complete elliptic integrals of the first
and second kind, respectively [19]. The meanings of ,
and
are shown in Fig. 2. In the calculation, the two windings are
treated as filaments because they are made of thin Litz wire (
0.39 mm) for reduced power loss at high frequency. If other
wire such as printed-circuit board track is used, the formulae
presented in [20] can lead to more precise results.
The same method described above can also be applied to the
design of rectangular hybrid winding structure. Compared to the
circular structure, the rectangular windings are easier to make.
Given a rectangular coil of a certain turns number, as the first
step of the design, number of turns of the spiral winding,
,
half of the length of the first turn,
, and the spacing between
the tracks, , are fully searched and all the possible results are
presented, as shown by the example in Fig. 3. Then the possible results are judged by the mutual inductance based method.
The mutual inductance between two rectangular windings can
be calculated, using the method presented in [21].
III. DESIGN RESULTS AND EXPERIMENTAL VERIFICATION
A. Circular Structure
Using the above method, including full searching and optimization, for a given coil, the number of turns and the position
of each turn of the spiral winding can be designed. Table I lists
the design results when the number of turns of the coil,
equals 5, and the radius of the coil,
is equal to 75 mm.
For this design example, the initial searching criteria are given
as follows. The maximum value of
is slightly smaller than
the radius of the coil, while the minimum value of
is 3 mm.
The spiral winding with a radius smaller than 3 mm is not easy
, is asto make. The minimum spacing between each turn,
sumed to be 1 mm. The number of turns of the spiral winding,
, is searched from 2 to 10. Indeed, the initial criteria of these
three parameters can be set different for a cruder (but quicker)
or a finer searching. After all the possible results are presented,
the optimization tool introduced above is used to judge the validity of each result. In this example, the small tolerance, , in
(4) is set not larger than 10% of the average value.
In Table I, the first column is the number of turns of the spiral
winding,
, which is searched from 2 to 10. When
is
more than 5 (i.e. number of turns of the coil), no valid results
are found. The second column shows the radius of each turn, as
shown by
in Fig. 1(c). It is fully searched
from 72 mm to 3 mm in this example. The value in the third
column represents the intensity of the magnetic field generated
by the hybrid structure. It is equal to the mutual inductance between the hybrid structure and a one-turn testing coil of unit
area. In Fig. 1(b) the distance of the energy-receiving coil from
the energy-transmitting spiral winding and the energy-transmitting concentrated coil are
and
respectively. The self-inductance,
, and the dc resistance,
, of the hybrid structure are listed in the next two columns. The last column compares the ratio between the magnetic field intensity and the resistance. A higher value is preferred because it means that a higher
magnetic field is generated, accompanied by a relatively lower
power loss. This can also be seen in the last part of this paper
on energy efficiency discussion. Table II compares the design
results for given coils with different number of turns. For comparison, only the three-turn spiral winding is listed. As summarized in Tables I and II, a higher number of turns of coil or
spiral winding can generate a higher magnetic field. From the
efficiency point of view (last column of Tables I and II), it is
apoptimal when the number of turns of spiral winding
proaches the number of turns of coil
. This conclusion is
also applicable when
equals other values. Another interesting finding is that when
and
are equal to a same
value
4,5,6,7,8
, the results are summarized
in Table III. It can be seen that
is almost a constant value for all cases, while the mutual coupling
is increased. This finding indicates that more turns of windings
(while keeping
) can achieve better performance, if
the wire cost is not a major concern.
To verify the design results, two experiments are conducted.
In the first experiment, a small signal (
10 mA, at 400 kHz)
is injected into the hybrid structure and the generated magnetic
field is scanned by NoiseKen EPS-3000 EMC scanner with
Probe-A100 k (frequency range: 100 kHz 100 MHz). Fig. 4
is the measured results of the 4-turn prototype chosen from
Table I, from which it can be seen that the flux distribution
of the hybrid structure is more uniform than the other two
approaches. In the second experiment, a practical receiving
coil is placed and moved above the charging surface, as shown
in Fig. 5(a), and the mutual inductance between them is measured point by point. For simplicity, the hybrid structure of the
charging platform is represented by a large circle in Fig. 5(a).
The receiving coil has a radius of 15 mm and five turns, without
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IEEE TRANSACTIONS ON POWER ELECTRONICS, VOL. 23, NO. 1, JANUARY 2008
TABLE II
SUMMARY OF THE DESIGNED THREE-TURN SPIRAL WINDINGS FOR GIVEN CIRCULAR COILS (
R
TABLE III
SUMMARY OF THE DESIGNED CIRCULAR STRUCTURES WHEN
N
= 75 mm, N
= 4, 5, 6, 7)
=N
Fig. 4. Magnetic field intensity scanned by EMC scanner: (a) coil only, (b)
spiral winding only, and (c) coil and spiral winding <Circular>.
Fig. 6. Calculated and measured mutual inductance between the receiving coil
and the circular hybrid structure that consists of: (a) five-turn coil and two-turn
spiral winding and (b) five-turn coil and four-turn spiral winding.
Fig. 5. Sketch of the experimental set-up of (a) circular structure and (b) rectangular structure.
any magnetic core. Fig. 6(a) and (b) show the calculated and
the measured results for the two-turn and the four-turn prototype, respectively. The measured results agree well with the
theoretical ones. In Fig. 6, “ ” represents the misalignment
between the energy-receiving coil and the platform, as shown
in Fig. 5(a).
B. Rectangular Structure
The design results of the rectangular structure are summarized in Table IV, when the rectangular coil has four turns
4 , and a 126 mm 97 mm dimension. The meaning
of each column is the same with that of Table I. To verify
the design results, a four-turn coil, three-turn spiral winding
prototype is chosen for demonstration. Fig. 7(a)–(c) show
the scanned magnetic field plots of the rectangular coil, the
LIU AND HUI: OPTIMAL DESIGN OF A HYBRID WINDING STRUCTURE
TABLE IV
SUMMARY OF THE DESIGNED SPIRAL WINDINGS FOR A GIVEN RECTANGULAR
4,
63 mm,
48.5 mm)
COIL (
N
= a
=
b
=
459
IV. CIRCUIT DESIGN FOR MULTILOAD APPLICATION
One apparent merit of the hybrid structure is that many secondary energy-receiving units (i.e. multiloads) can be placed on
the platform and be charged simultaneously. To fulfill this purpose, besides the uniform magnetic flux generated by the hybrid
structure, the circuit topology, especially the resonant compensation tank, must also be designed carefully for efficient power
transfer. It is shown in [18] that the series-connected capaciand ) added at both the primary side and the sectors (
ondary side, as shown in Fig. 10(a), is an effective compensation
method [15]–[17], although some other methods such as using
series compensation on the primary side only [9], [11], [12] or
on the secondary side only [13], [14] have been employed.
A. Analysis
The equivalent circuit of the primary side with multireflected impedance is shown in Fig. 10(b). The multi reflected
impedances are connected in series because they are excited
by the same current in the platform windings. Analyzed from
, can be
Fig. 10(a) and (b), the voltage gain of the th load,
expressed by
(6)
Fig. 7. Magnetic field intensity scanned by EMC scanner: (a) coil only, (b)
spiral winding only, and (c) coil and spiral winding <Rectangular>.
spiral winding and the hybrid winding structure, respectively.
The hybrid structure offers the best uniform flux distribution.
In the second measurement, a five-turn 50 mm
50 mm
energy-receiving coil is placed and moved on the charging
surface, as shown in Fig. 5(b). The mutual inductance between
the energy-receiving coil and the platform is recorded along (a)
-axis, (b) -axis, and (c) the diagonal, and compared with
the calculated results in Fig. 8(a)–(c), respectively. Again they
are in good agreement.
where
is the root-mean-square (rms) value of the fundamental component of the input square wave, ; is the total
number of secondary units;
is the impedance of the pri;
is the
mary side and equals
impedance of the ith
1,2,
secondary side and is equal
;
is the equivalent reto
sistor of the load stage which may consist of rectifier, regulator
and the battery.
For efficient power transfer, the system is operated at the secondary resonant frequency [18], that is
(7)
C. Influence of the Height Distance Between the Receiving
Coil and the Platform
In the previous sections, the distances of the energy-receiving
coil from the energy-transmitting spiral winding and the concentrated coil are
3.5 mm and
2.5 mm, as shown
in Fig. 1(b). Indeed, these two parameters can be adjusted in the
design and fabrication easily. Fig. 9(a) shows the influence when
is solely increased, from 2 to 6 mm, while keeping
2
mm. It only depresses the magnetic field intensity near the periphery. Fig. 9(b) is the result when
is solely increased, i.e.
the spiral winding is placed under the coil surface. Although the
magnetic field intensity drops a little, it becomes prominently
more uniform. So the quasi-coplane structure which places the
spiral winding layer under the coil layer can achieve potentially
better effects, even compared to the results in Fig. 9(c) when
and
are changed together.
If (7) is satisfied,
is equal to
. Because the
resistance of the secondary winding,
, is normally much
smaller than
,
is further approximated to
. Then
(6) can be simplified to
(8)
The ratio between the reactive part and the active part of
in (8) is defined as
and expressed by (9). In this paper, only
the
1
is considered because it guarantees the
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IEEE TRANSACTIONS ON POWER ELECTRONICS, VOL. 23, NO. 1, JANUARY 2008
Fig. 8. Calculated and measured mutual inductance between the rectangular hybrid structure and the receiving coil which is moved: (a) along
-axis, and (c) along diagonal.
Y
operating of the system above primary resonant frequency and
therefore zero-voltage switching in the inverter [17]
X -axis, (b) along
is 2, 0.949 when
is 3,
where is equal to 0.895 when
and 0.970 when
is 4. So
influences the stability of the
system, i.e., the higher
the more stable of the system. But it
should also be noted that a higher
brings a lower
.
B. Design Procedure
(9)
From (8), it can be seen that when more loads are added
1 , the voltage gain will inevitably drop. But if the value of
is high enough, the voltage gain is almost a constant value for
all the load conditions
(10)
In the design of the circuit, the given parameters are the operating frequency, , the input voltage,
, the information
about the platform,
,
and
(see Tables I–III),
and the requirements of the loads,
,
and . Two points
should be noted. First, for charging LI-ION batteries, the voltage
source characteristics demonstrated by series compensation at
secondary side [22] is needed, and the output voltage (8 V in
this example) is higher than the minimum requirement of input
voltage of common regulators such as 6 V for LT1374. The
equivalent resistance of each load,
, is then decided by the
charging power. The value of
in Table V, 32 , represents the initial charging power, 2 W. When the battery is fully
charged,
tends to very large, and the reflected impedance
LIU AND HUI: OPTIMAL DESIGN OF A HYBRID WINDING STRUCTURE
461
TABLE V
GIVEN AND DESIGNED CIRCUIT PARAMETERS
Fig. 9. Influence of the height distance change of: (a) d only, (b) d only, and
(c) d and d together.
Step 1: Estimate : The relationship between the active
power, the reactive power [see (9)] and the apparent power at
primary side is expressed by
(11)
where
is the total active power absorbed by all the loads.
In this example, at most four loads are supposed to be placed
and charged on the platform simultaneously and
equals 8
W, as calculated from data in Table V. In principle,
can be
chosen for higher output power. When
is chosen as 12 W
(50% higher than 8 W) and
is set at 4, the primary current,
, is estimated as 2.2 A, by solving (11).
Step 2: Design
: Because
is high enough (set at 4 in
this example), the primary current, , which was solved in step
1, is decided by the equation in
Fig. 10. Equivalent circuit modeling of: (a) coupling between the platform and
the ith load and (b) primary circuit with multi reflected impedance.
has little influence to the primary side [see (6)]. Secondly, for
a clear view of the open-loop power transfer ability, the operating frequency and the input voltage are fixed in the operation,
although they can also be controlled with the feedback signal
[14]. The objective is to design the secondary windings and the
resonant tanks so that the required power can be transferred to
each load, even when all the loads are charged simultaneously.
In the following discussion, a five-step procedure is illustrated
in a design example. A hybrid structure which has the same dimension with the prototype used in Section III, i.e., four-turn
rectangular coil with three-turn spiral winding is chosen for this
design example. Without losing generality, all the loads are assumed to be the same. The given parameters are summarized in
Table V.
(12)
By solving (12),
is chosen as 25 nF in this example. The
voltage rating of the capacitor can be estimated by the amplitude
of
times the impedance of the capacitor.
Step 3: Design
: The mutual inductance is determined
by the required output voltage of each load. By solving (10),
(
1, 2, 3, 4) is chosen as 1.39 H. It must be noted
that if the required output voltage of each load is different,
must also be different from each other.
Step 4: Design the Secondary Windings: The area of the secondary winding is restricted by the dimension of different electronic products. In Tables I–III,
is the mutual inductance between the hybrid structure and a one-turn receiving coil
of unit area. It has the relationship with
as expressed by
(13)
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Fig. 11. Photograph of the multiload experiment.
IEEE TRANSACTIONS ON POWER ELECTRONICS, VOL. 23, NO. 1, JANUARY 2008
Fig. 13. Calculated and measured efficiency of coupling windings, Eff , with
the number of loads changing.
The power loss in the system exists both in the high frequency
power conversion circuit and in the coupled windings. The energy efficiency of the system can be expressed by
Fig. 12. Measured waveforms when (a) one load or (b) four loads is/are charged
on the platform. (Ch1: input voltage, V , 25 V/div; Ch4: primary current, I ,
5 A/div; Ch2: output voltage of the first load, V , 10 V/div; Ch3: output
voltage of the 4th load, V , 10 V/div).
where
is the area of the energy-receiving coil and
is its
number of turns. By solving (13), the required number of turns
of the energy-receiving coil is decided. This method does not
lose generality because it is also applicable to secondary units
with different areas.
Step 5: Design
: The self-inductance,
, and the resistance,
, of the energy-receiving coil can be easily calculated
[19]–[21] or measured. Then
is chosen to meet (7).
C. Experimental Results and Discussions
The proposed design method has been verified by experiments. Fig. 11 is a photograph showing four loads on the platform. The experimental parameters are the same as those in
Table V. The measured waveforms for charging one load and
four loads on the platform are shown in Fig. 12(a) and (b), respectively. It can be seen that the required power is transferred
to each load even when all the loads are charged simultaneously.
Another finding is that the current in the primary winding, ,
is almost invariable because
is high enough. In the experiment, the input square wave
is generated by a full-bridge
inverter.
(14)
where
is related to the loss in the power conversion which
mainly comes from the switching loss in the inverter at the primary side and in the rectifier at the load stage;
is related to
the loss in the coupled windings, which mainly come from the
ohmic loss of the windings. This study focuses on the latter part
because the winding structure is the main concern. The detailed
expression of
is also given in (14) if all the secondary parts
are assumed to be the same
1,2,
. Fig. 13 shows
the calculated and measured
when the number of loads
changes. The energy efficiency increases to about 80% when
four loads are charged simultaneously.
It can be seen from (14) that the efficiency may be further
improved with the following measures:
a) increase the operating frequency, , provided the
switching losscan be kept low;
b) increase the mutual inductance, , between the load and
the platform, as well as
, (see the comparison listed
in Tables I–IV). But this approach is accompanied by the
increase of self-inductance of the primary side, which may
need a tradeoff with the system penalty factor [12];
c) decrease the equivalent resistor of each load,
. For example, when it is decreased from 32 to 8 ,
is predicted to rise to around 70% when one load is charged or
near 90% when four loads are charged. In other words, the
energy efficiency will improve with a higher total output
power.
LIU AND HUI: OPTIMAL DESIGN OF A HYBRID WINDING STRUCTURE
V. CONCLUSION
In this paper, a new hybrid winding structure is proposed for
a planar contactless battery charging platform. It consists of a
coil and a spiral winding. For a given coil, the spiral winding
is designed and optimized for the appropriate number of turns
and the position of the tracks. The magnetic field distribution
of the prototype is measured by an EMC scanner and by an energy-receiving coil. The measured results agree well with the
theoretical values and confirm the improved uniform magnetic
field distribution. For multiload application, besides the uniform magnetic flux generated by the hybrid structure, the circuit
topology, especially the resonant compensation tank, is also designed for efficient power transfer. A design procedure is proposed and verified by the experiments. The efficiency of the coupling structure reaches about 80% when four loads are placed on
the platform simultaneously. Some suggestions for improving
efficiency are also given for future work.
ACKNOWLEDGMENT
The authors would like to thank P. W. Chan for his help in
making the platform and the inverter circuit for the experiments.
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Xun Liu (S’04) was born in China in 1978. He received the B.S. and M.S. degrees in electrical engineering from Tsinghua University, Beijing, China, in
2001 and 2003, respectively, and the Ph.D. degree
from the City University of Hong Kong, in 2007.
He is a Technology Manager with ConvenientPower (HK), Ltd., and responsible for the R&D
of a new generation of universal wireless charging
platform for a wide range of consumer electronic
products His main research interests include planar
integration in power electronics, applied supercon-
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[1] P. Beart, L. Cheng, and J. Hay, “Inductive Energy Transfer System
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ductivity, and EMC/EMI.
S. Y. (Ron) Hui (F’03) was born in Hong Kong in
1961. He received the B.Sc degree (with honors) from
the University of Birmingham, Birmingham, U.K. in
1984, and the D.I.C. and Ph.D degrees from the Imperial College of Science and Technology, University
of London, London, U.K., in 1987.
He was a Lecturer in power electronics at the
University of Nottingham, Nottingham, U.K., from
1987 to 1990. In 1990, he went to Australia and took
up a lectureship at the University of Technology,
Sydney, where he became a Senior Lecturer in 1991.
He joined the University of Sydney in 1993 and was promoted to Reader of
Electrical Engineering in 1996. Presently, he is a Chair Professor of Electronic
Engineering at the City University of Hong Kong. From 1999 to 2004, he was
an Associate Dean of the Faculty of Science and Engineering at CityU. He
has published over 200 technical papers, including over 120 refereed journal
publications. He holds over 20 patents.
Dr. Hui received the Teaching Excellence Award in 1999 and the Grand Applied Research Excellence Award in 2001 from the City University of Hong
Kong, and the Best Paper Award from the IEEE IAS Committee on Production
and Applications of Light in 2002. He is a Fellow of the IEE and has been an
Associate Editor of the IEEE TRANSACTIONS ON POWER ELECTRONICS since
1997. Since 2007, he has been an Associate Editor of the IEEE TRANSACTIONS
ON INDUSTRIAL ELECTRONICS. He has been an At-Large member of the IEEE
PELS AdCom since October 2002. He has been appointed as an IEEE Distinguished Lecturer by IEEE PELS for 2004–2007.