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Analysis and Mitigation of Coupling-Dependent
Data Flipping in Wireless Power andData Transfer System
Hao Qiu , Member, IEEE, Yuntao Jiang, Yi Shi, Member, IEEE, Takayasu Sakurai, Life Fellow, IEEE,
and Makoto Takamiya , Senior Member, IEEE
Abstract— Load shift keying (LSK) has been widely used ina wireless power and data transfer (WPDT) system, owing to
its low cost and power consumption. It was discovered thatthe demodulated data can flip when the coupling coefficient (k)between the transmitter (TX) and receiver (RX) coils becomes lessthan a critical value (kDF ) in a system with four basic compensation topologies (series–series, series–parallel, parallel–series,
and parallel–parallel). This problem is called coupling-dependent
data flipping (CDDF). Even more seriously, the transferred datacannot be recovered when k equals kDF . On the basis of a comprehensive circuit analysis of CDDF, a universal method applicable
to all four compensation topologies was proposed. By monitoring
the current through the TX coil rather than its voltage for data
demodulation, CDDF can be avoided. Furthermore, a WPDT
system was implemented in which the voltage information of theload resistance (RLoad ) was transferred to the TX side to control
the source voltage for load power ( PLoad ) regulation. Using
the conventional method, CDDF along with its correspondingkDF (0.35) was verified. On the other hand, using the proposed
method, the data was successfully transferred even when k isless than or equal to kDF . By a correct data transfer, PLoad has
been successfully regulated at around 1.1 W with a high systemefficiency of up to 60% under the variation in k from 0.09 to 0.45.
Index Terms— Coupling-dependent data flipping (CDDF), critical coupling coefficient, inductive coupling, load shift keying(LSK), uplink, wireless power and data transfer (WPDT).

I. I NTRODUCTIONW

IRELESS power transfer (WPT) based on inductive
coupling has been applied in a wide range of applications [1] such as electric vehicles (EVs) [2], [3], mobile
devices [4] and biomedical implanted devices [5], [6]. During
wireless charging, the coupling coefficient (k) between thetransmitter (TX) and receiver (RX) coils can change drastically
Manuscript received June 4, 2021; revised August 7, 2021; accepted
August 30, 2021. Date of publication September 15, 2021; date of current
version November 29, 2021. This work was supported in part by the Japan
Science and Technology Agency (JST) Exploratory Research for Advanced
Technology (ERATO). This article was recommended by Associate Editor
G. Di Capua. (Corresponding author: Hao Qiu.)
Hao Qiu, Yuntao Jiang, and Yi Shi are with the School of Electronic
Science and Engineering, Nanjing University, Nanjing 210023, China (e-mail:haoqiu@nju.edu.cn; yshi@nju.edu.cn).
Takayasu Sakurai and Makoto Takamiya are with the Institute of Industrial Science, The University of Tokyo, Tokyo 153-8505, Japan (e-mail:mtaka@iis.u-tokyo.ac.jp).
Color versions of one or more figures in this article are available athttps://doi.org/10.1109/TCSI.2021.3111100.
Digital Object Identifier 10.1109/TCSI.2021.3111100

and thus affects the load power (PLoad ) [7]. To monitor andregulate PLoad , it is necessary to establish a communication
link from the RX side to the TX side that is an uplink [8]–[10].
It is preferrable that the uplink design has a low implementation cost, consumes low power, and is robust against
the variation in k. Recently, many works have focused on
the communication link design in wireless power and datatransfer (WPDT) systems.
For the uplink design of WPDT systems, one intuitive idea
is to use an additional pair of coils for data transfer [11], [12].
However, it will result in a high implementation cost as
well as suffer from crosstalk with the pair of coils used for
power transfer. On the other hand, the simplest way is touse a single pair of coils to transfer both power and data.
To enable the data transfer simultaneously with power transfer,a modulation mechanism is necessary. In [13], a data transfer
method based on splitting frequencies was proposed. However,the data transfer range is limited to strong-coupled regions.
In [14], cyclic ON-OFF keying (COOK) was proposed tosupport a simultaneous WPDT. However, an accurate time
control is required and increases the cost of the system.
Compared with these methods, load shift keying (LSK) has
been extensively used owing to its simple architecture and low
power consumption [15]–[17]. As shown in Fig. 1, the data
is first modulated utilizing a shorting switch (SW) across theload resistance (RLoad ) on the RX side and then demodulated
by monitoring the voltage (VL ) across the inductance (L TX )on the TX side. The connection of compensation capacitanceson the TX side (CTX ) and RX side (CRX ) depends on thecompensation topology.
Thus, a WPDT system with LSK as its data modulation
mechanism is a good option. However, few works haveexamined its performance against the variation in k. Our
major contributions in this paper are summarized as follows:
1) A problem called coupling-dependent data flipping (CDDF)
was discovered, in which the uplink data can be incorrectly
demodulated when k is less than or equal to a criticalvalue (kDF ). 2) We performed a comprehensive circuit analysis
of CDDF in a WPTD system with four commonly-used compensation topologies [series–series (SS), series–parallel (SP),parallel–series (PS), and parallel–parallel (PP)] [18], [19].
3) For all these four compensation topologies, a universal
method to avoid CDDF was proposed. 4) In an implemented
WPDT system, we demonstrated that the uplink data can

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Fig. 1. Schematic of a WPDT system in which the power is transferred fromthe TX side to the RX side and the data is transferred in a reverse direction.
The data is modulated using LSK and demodulated by monitoring VL in aconventional system.

be correctly demodulated using the proposed method, whichguarantees the successful regulation of PLoad robust against thevariation in k. Compared with our earlier work [20], we haveadded the circuit analysis of CDDF in a WPDT system withthe compensation topologies of SP, PS, and PP, a universal
method to CDDF, and the implementation of a WPDT system.
The rest of the paper is organized as follows. In Section II,
the concept of CDDF is introduced, followed by a comprehensive circuit analysis in a WPDT system with four commonlyused compensation topologies. A universal method to avoid
CDDF is also proposed. In Section III, the implemented
WPDT system is described. Experimental results are presented
in Section IV. Finally, conclusions are given in Section V.
II. C IRCUIT A NALYSIS OF CDDF AND THE
U NIVERSAL M ETHOD TO AVOID I T
To obtain a deeper understanding of CDDF, we perform acomprehensive circuit analysis in a WPDT system with fourcommonly used compensation topologies – SS, SP, PS, and PP.
The relationships between kDF , the key parameter of CDDF,
and the component parameters in the WPDT system are also
discussed. Finally, a universal method to avoid CDDF which
is applicable to all four compensation topologies is proposed.
A. Discovery of CDDF
Fig. 2 shows the SW operations and the corresponding
waveforms of VL (indicated in Fig. 1) at different k. Here,the SS topology is taken as an example. A detailed analysis
will be given in Section II-B. Data “0” and “1” are modulated
when the SW is short and open on the RX side, respectively.
The state of the SW induces the change in VL on the TX
side. During data demodulation, large and small amplitudesof VL (|VL |) are defined as data “0” and “1”, respectively.
Thus, in the normal mode when k is large, the data can be
demodulated correctly. On the other hand, when k becomesless than kDF , the short and open states of the SW induces
small and large |VL |, respectively. This means that the originally modulated data “0” and “1” are incorrectly demodulated
and becomes “1” and “0”, respectively. We call this problem
CDDF. Moreover, when k is equal to kDF , it can be expectedthat |VL | will not change even when the state of the SWchanges. This means that the data cannot be recovered on theFig. 2. SW operations and corresponding VL waveforms in Fig. 1 at different
k. When 0 < k < kDF , CDDF occurs and the WPDT system operates in an
abnormal mode. SS topology is taken as an example.

TX side. Thus, CDDF is a critical problem in the conventional
WPDT system.
B. CDDF With Four Compensation Topologies
Figs. 3(a)-3(d) show the equivalent circuits of WPDT systems with the SS, SP, PS, and PP compensation topologies,
respectively. The TX side consists of a voltage source (VS )with an internal resistance (RS ), L TX , and CTX . The RX side iscomposed of an inductor (L RX ), CRX , and RLoad . In fact, both
L TX and L RX respectively have parasitic resistances RTX andRRX , which will affect kDF . However, L TX and L RX usually
have high quality factors (Qs) and thus the effects are small.
In the analysis, RTX and RRX are neglected for simplification.
The voltage across the TX coil is VTX and the current into the
TX coil is ITX .
According to Thevenin’s theorem, Figs. 3(a)-3(d) can beviewed as the universal topology in Fig. 3(e) with the corresponding expressions for VS ’, RS ’, RLoad ’ listed in Fig. 3(f).
VS ’, RS ’, RLoad ’ are the Thevenin VS , RS , and RLoad , respectively. In the equivalent circuit shown in Fig. 3(e), r is thereflected impedance from the RX side [21].
Applying Kirchhoff’s voltage law to Fig. 3(e), we obtainthe following equations at the resonance frequency ( f 0 ) [22]: RS + j ω0 L TX + r
VS = ITXITXr = − j ω0 M IRX





j ω0 M ITX= IRX
j ω0 L RX + RLoad

L TX CTX = L RX CRX = (ω0 )−2 ,(1)where j is the imaginary unit, 0 is the resonant angularfrequency and equals 2π f 0 , and M is the mutual inductance
between the TX and RX coils and equals k(L TX L RX )1/2 .
By solving (1), ITX ’ and IRX ’ can be obtained as follows.

ITX
=

1
ω2 M 2RS + j ω0 L TX + R  +0 j ω L
0 RX
Load


IRX
= 

RS + j ω0 L TX



VS

j ω0 MVS .

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(2)

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Fig. 3. Schematic of (a) SS, (b) SP, (c) PS, and (d) PP topologies. The four topologies can be considered as (e) a universal topology with (f) corresponding
expressions for VS ’, RS ’, and RLoad ’. Parasitic resistances of inductors are neglected for simplification.

Accordingly, r and VL can be obtained asj ω0 M IRX
r =−

ITX

VL = ITX ( j ω0 L TX + r ) .

(3)

According to the definition of kDF , it can be obtained byequalizing |VL | when SW is open with that when SW is short.
That is,|VL |SW open = |VL |SW short .

(4)

For the design of a WPDT system, there are four important component parameters: RS , RLoad , L TX , and L RX .
Using (1)-(4), the dependences of kDF on these parameters
can be calculated. In calculations, no restrictions are applied to
kDF and thus the calculated kDF can be any value. Considering
the physical meaning of kDF , if the calculated kDF is between
0 and 1, CDDF will occur in the designed WPDT system.
On the other hand, CDDF will not occur.
In this paper, we consider our target application as thewireless charging of mobile devices and select f 0 as 165 kHz.
Fig. 4 shows the calculated kDF surfaces under different L TX
and L RX in four compensation topologies. RS is assumed

as 10 . RLoad when SW is open and short are assumed as
10  and 0 , respectively. Here, the resistance of a short
SW is neglected. Fig. 5 shows the calculated kDF surfaces
under different RS and RLoad in four compensation topologies.
L TX and L RX are assumed as 50 μH. In the white regions
shown in Figs. 4(a), 5(a), and 5(c), the calculated kDF is
greater than 1. As discussed before, CDDF does not occur
in those design regions. However, in most cases, kDF is within
the range between 0 and 1 and thus CDDF is a common
problem in a WPDT system with all four compensation
topologies.
Table I shows the dependences of kDF on RS , RLoad , L TX ,and L RX . For the SS topology, an analytical expression for
kDF with respect to RS , RLoad , L TX , and L RX is derived inAppendix-A. From the above circuit analysis and calculation
results, kDF in a WPDT system can be easily estimated by(3) and (4) if specific component parameters, that is, L TX ,L RX , RS , and RLoad , are known. It can also be concluded that
one way to eliminate the detrimental effects of CDDF is tocarefully choose component parameters to design kDF to beoutside the k range of operation. For example, if k during
wireless charging ranges from 0.1 to 0.5, depending on thecompensation topology, specific component parameters can be

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Fig. 4.

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Calculated kDF surfaces in (a) SS, (b) SP, (c) PS, and (d) PP topologies under different values of L TX and L RX . RS and RLoad are 10 .

TABLE ITABLE IID EPENDENCES OF kDF ON RS , RLoad , LTX , AND LRX IN F OUR T OPOLOGIESO CCURRENCE OF CDDF IN F OUR T OPOLOGIES

chosen to make kDF greater than 0.5 or less than 0.1. To give
more design margins to the system, the calculated kDF caneven be designed to be less than 0 or greater than 1.
However, designing kDF corresponds to modifying thecomponent parameters in the system, which can add to theimplementation cost or even be unfeasible in most cases.
For L TX and L RX , which directly correspond to the physicalconfigurations of coils, they are usually determined in realapplications and can hardly be modified. RS and RLoad aredetermined by the output impedance of the power amplifier(PA) if the PA is added after the voltage source and thestatus of the load, respectively. Thus, these two parameters are
hardly to be designed. Though adding an impedance matching
network [23], [24] can be a way to change the equivalent RS or

RLoad , it would add to the implementation cost of the system.
Thus, a feasible and universal method that can be applied to all
four compensation topologies in a WPDT system is required.
C. The Universal Method to Avoid CDDF
Regarding the problem of data flip, two methods have been
well known. One is to use the non-return-to-zero-inverted(NRZI) code, in which a data bit is represented by the presenceor absence of a transition of the monitored signal. Take our
WPDT system as an example. Data “0” and “1” are firstmodulated when the SW on the RX side remains unchanged
and changes its state, respectively. On the TX side, VL ismonitored. If VL does not change, it is demodulated as data

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TABLE IIIE XPRESSIONS FOR IL AND |IL | IN F OUR T OPOLOGIES“0”. On the other hand, if VL shows a transition from LOW
to HIGH or vice versa, it is demodulated as data “1”. Since
NRZI code is insensitive to the direction of transition, data
can be correctly demodulated whether k is greater than or lessthan kDF . However, the problem of CDDF occurs when k isequal to kDF . On the RX side, data “1” is modulated whichcorresponds to the change of the state of the SW, whereas
VL on the TX side does not change that will result in anincorrectly-demodulated data “0”.
The other one is to add a known preamble sequence at the
beginning of the data to estimate whether the data flips or not.
However, in this method, it is required to measure k. Usually,k is measured by a vector network analyzer (VNA), which
is bulky and will make the system complex. Thus, the above
commonly used methods are not effective to avoid CDDF.
In this work, we proposed a simple method. It is also
universal to a WPDT system with four basic compensation
topologies. Here, the data transfer process is reviewed. The
data is first modulated on the RX side and then a parametershould be monitored on the TX side for data demodulation.
For the WPDT system shown in Fig. 3, in addition to theconventional VL , there are five other parameters that can bemonitored: the current through L TX (IL ), the voltage across
CTX (VC ), the current through CTX (IC ), VTX , and ITX . Whenone of these parameters is monitored for data demodulation,
whether CDDF occurs depends on kDF . If kDF is within therange from 0 to 1, CDDF can occur. The results are concludedin Table-II, where O and X respectively represent the casewithout and that with CDDF. By monitoring VL for data
demodulation, CDDF is a problem in all four compensation
topologies. On the other hand, by simply changing to monitor
IL for data demodulation, CDDF can be avoided and thismethod is effective for all four compensation topologies.
To further demonstrate that CDDF does not occur when IL
is monitored for data demodulation, the analytical expressions
for IL in all four compensation topologies, as shown in Figs.
3(a)-3(d), are summarized in Table-III. The amplitude of IL(|IL |) has a monotonic relationship with RLoad and thus it isexpected that the data can be correctly demodulated robust
against the variation in k.

III. WPDT S YSTEM
Fig. 6 shows a schematic of a WPDT system in which
the voltage across RLoad (VLoad ) is to be regulated. In thiswork, we implement the WPDT system with the SS topology,
considering that it is the only one among four compensation
topologies in which f 0 is independent of k and RLoad [18] andwill make the whole system control simple. A sinusoid signal
is generated by the voltage source and then amplified by alinear PA. The PA drives the TX coil, which is coupled with
the RX coil. Rectification is achieved via a rectifier consistingof four diodes and a capacitor. The SW is connected to RLoadin parallel.
The uplink is for VLoad regulation. The short and open statesof the SW represent data “0” and data “1”, respectively. On theRX side, the data is generated through measurement of VLoad ,followed by analog-to-digital conversion. The data is recoveredon the TX side through waveform acquisition of IL , low-passfilter, and digital-to-analog conversion. The recovered data isused to control VS and thus PLoad . The control of the wholesystem was implemented in LabVIEW.
Fig. 7 shows the flow of the system control loop, in which
the goal is to regulate VLoad under the variation in k. First,
we assume the system is at a steady state where VLoad has
been at its target value of VLoad _T. Then k changes to anothervalue, so the change of VLoad follows. To regulate it back to
VLoad _T, VLoad is first digitized to a data pattern denoted as
DATA_A. Using the uplink in the system, it is transferred to
the TX side and recovered. Since the reference data patterndenoted to DATA_T that corresponds to VLoad _T has beenknow before, DATA_A can be compared with DATA_T in
the control unit. For example, if DATA_A > DATA_T, thatmeans VLoad is larger than VLoad _T and then VS is decreased
until that the condition DATA_A = DATA_T holds. Otherwise,
VS is increased to regulate VLoad . During control, the change
in VS can induce the change in IL , which may corrupt the
uplink. Using the method that the power level adjustment
event is scheduled at a different timing instant from the uplink
described in [25], the interference problem between the powerand data transfer can be solved.

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Fig. 5.

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Calculated kDF surfaces in (a) SS, (b) SP, (c) PS, and (d) PP topologies under different values of RS and RLoad . L TX and L RX are 50 μH.

Fig. 6. Schematic of the implemented WPDT system in which a PA and a rectifier are inserted on the TX and RX sides, respectively. The data are transferredfrom the RX side to the TX side to control VS for load regulation.

For the data demodulation in the above flow, IL is monitored. On the other hand, if VL is monitored for data demodulation, CDDF will occur when k is less than kDF . Take 4-bit
digitization as an example. DATA_A and DATA_T are “1101”
and “1000”, respectively. As discussed in Section II, when k isless than kDF , DATA_A will be incorrectly demodulated andbecome “0010”. Thus, the system will recognize that VLoad issmaller than VLoad _T, even though this is not the case, andthe system will also increase VS . That will make the control

loop far from its converging result, causing the regulation of
VLoad to fail. Thus, the proposed data demodulation method
based on monitoring IL is important for correct data transfer
and successful loop control for VLoad regulation in a WPDT
system.
The radius, thickness, number of turns, parasitic resistance,and quality factor of the TX coil are represented by rTX ,tTX , m, RTX , and Q TX , respectively. The radius, thickness,
number of turns, parasitic resistance, and quality factor of

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TABLE IVM EASURED PARAMETERS OF THE TX AND RX C OILS

Fig. 7.

System control for load regulation in the WPDT system.

Fig. 8.

Photograph of the implemented WPDT system.

the RX coil are represented by rRX , tRX , n, RRX , and Q RX ,respectively.
IV. E XPERIMENTAL R ESULTS
Fig. 8 shows a photograph of the implemented WPDT
system. A signal generator (Tektronix AFG3252) is used
to generate a sinusoidal wave signal at 165 kHz, which isthen amplified by a linear PA (NF Corp.) with a voltage
gain of 20. Here, the reason why a linear PA rather thana switching PA was chosen is explained as follows. For aswitching PA, the total harmonic distortion (THD) of its

output voltage is high and will introduce conducted noise andradiated noise [26]. Thus, a linear PA was chosen to alleviate
the detrimental effects of the EMI problem that would be
introduced by a switching PA on the data transfer in a WPDT
system [27].
The PA supplies the power to the TX coil, which isthen wirelessly transferred to the RX coil. On the RX side,
the four diodes in the rectifier use a DSEP60-12A Schottky diode from IXYS Corp.. CL and RLoad are a FG22 ×
7.1H685KRT06 capacitor (6.8 μF) from TDK Corp. and a
PF2205-10RF1 resistor (10 ) from Riedon Inc., respectively.
A IRLD024PBF SW from Vishay Siliconix Inc. is connected
to RLoad in parallel. All voltage and current waveforms arerecorded using an oscilloscope (Agilent D5054A). The data
pattern used to control the SW is generated by a digital waveform generator (National Instruments PXIe-6555).
Both the TX and RX coils are made of 18 AWG (RSPro) wire. Table IV shows their physical dimensions andtheir electrical parameters measured using a network analyzer(Keysight E5061B).
To measure the effect of k on data transfer, in this work,
we changed the distance (d) between the TX and RX coils.
Fig. 9 shows the measured dependence of k on d, obtainedusing the method described in [28]. The pair of coils are placedin parallel, and d is defined as shown in the inset. According
to [7], the effect of the misalignment between the TX andRX coils on k is similar to that of d. Thus, the effect of themisalignment on data transfer can be expected through thefollowing results.
Fig. 10(a) shows the measured k dependences of conventional |VL | when the SW is short and open. Data “0” anddata “1” correspond to the short and open states of the SW,
respectively. As discussed in Section II-A, these two curvesinterconnect at the point where k = kDF , whose value is0.35 in the WPDT system. The calculated kDF from the circuit
analysis in Section II-B, whose value is 0.28, is also given forcomparison.
In the calculation, the input impedance of the rectifier
was first measured by FRA5097 from NF Corp. and thenused as the equivalent RLoad . The inconsistency between
the measured and calculated kDF is explained as follows.

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Fig. 9. Measured relationship between k and d. The inset shows TX andRX coils in parallel.

Though CRX has been carefully selected for compensationof L RX , the unavoidable variation [29] in the value of CRX
can cause the reactance on the RX side at f0 . As a result,in measurements, the reactance on the RX side can affect theTX side through coupling and this effect becomes stronger
when the SW is short than when it is open. Thus, the wholesystem can deviate from its resonance state especially when k
is large and the SW is short. On the other hand, in calculations,the system is assumed at its resonance state in the entire k
range.
kDF separates the whole k range into two regions. At k =kDF (∼ 0.35), since |VL | is the same whether SW is short oropen, data cannot be demodulated on the TX side and thus theconventional WPDT system has an uplink failure. As definedin Section II-A, during demodulation, large |VL | and small |VL |are defined as data “0” and data “1”, respectively. Thus, in thenormal mode (k > kDF ), data “0” and data “1” can be correctly
demodulated. On the other hand, in the abnormal mode (k <kDF ), the modulated data “0” and data “1” will be incorrectly
demodulated and become data “1” and data “0”, respectively.
This also leads to an uplink failure in the conventional WPDT
system.
Fig. 10(b) shows the measured k dependences of proposed|IL | when SW is short and open. As also concluded in
Section II-C, in the entire measured k range, |IL | corresponding to a short SW is smaller than that corresponding to an open
SW. In this work, during demodulation, small |IL | and large|VL | are defined as data “0” and data “1”, respectively. Thus,in the entire k range, the proposed WPDT system supports an
uplink without failure.
As a proof of concept for data transfer, a data bit stream consisting of a repeating sequence of “10” at a data rate of 2 kb/s
was modulated by repeatedly controlling the state of the SW
between open and short.√According to [30], the maximum data
rate is determined by 2 ( f 0 /Q) and it can be potentially
improved by increasing f 0 or compensating Q. Moreover,rather than turning on the SW for tens to hundreds of cycles,by turning on the SW for a single cycle only, the data rate canbe further increased to f 0 /2 [14].
Figs. 11(a)-11(c) show the measured waveforms of IL and
VL at k = 0.45 (k > kDF ), k = 0.35 (k = kDF ), andFig. 10. Measured k dependences of (a) conventional |VL | and (b) proposed|IL | when the SW is open and short. The amplitude of VS is 1.35 V.

k = 0.19 (k < kDF ), respectively. Through the entire k range,
data can be correctly demodulated by monitoring IL . On theother hand, if VL is monitored, data “0” and data “1” cannot
be distinguished at k = kDF and can flip when k < kDF . Thus,
the proposed data demodulation method by monitoring IL can
avoid CDDF in the WPDT system.
Using the proposed method, we demonstrated a WPDT
system in which PLoad can be regulated robust against thevariation in k. The details of the system control have been
described in Section III. Since the focus of this paper is on
CDDF, and not the system control, the detailed results of thecontrol are not shown here.
By measuring VTX , ITX , and VLoad , as shown in Fig. 6,
PLoad and the system efficiency (ηSYS ) can be calculated as
2
PLoad = VLoad/RLoad ,PLoad,ηSYS =∗Re VTX ITX

(5)
(6)

∗ is the complex conjugate of I .
where ITX
TX
Fig. 12 shows the measured k dependences of ηSYS
and PLoad . Using the proposed data demodulation method,
the information of VLoad can be transferred to the TX
side correctly for VS control. Thus, irrespective of k, PLoadhas been regulated at around 1.1 W with a high ηSYS of

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Fig. 12. Measured k dependences of ηSYS and PLoad of the WPDT system.

as discussed in the introduction, most of the previous works
have focused on the data transfer performance at a constant k.
Secondly, in the limited discussion about the data transfer at
a different k [16], k was always less than kDF and thus CDDF
was not discovered. Besides, although IL has been monitored
for data demodulation in a few previous works [25], [31],no discussion on why IL was selected, let alone on CDDF,
was given.
V. C ONCLUSION

Fig. 11. Measured waveforms at (a) k = 0.45 (k > kDF ), (b) k = 0.35
(k = kDF ), and (c) k = 0.19 (k < kDF ). The current probe sensitivity waschosen as 2 mA/mV.

up to 60%. Both PLoad and ηSYS are calculated after the systemreaches the steady state.
More discussions are given. In the experiment, we also tried
other implementation ways of data modulation by putting the
SW at different positions, such as in [14], and our conclusions
remained the same (Data not shown here). Another thing
is about the basic assumption of CDDF that k decreases
below kDF . This is not rare in practical wireless charging
scenarios. For example, a high ground clearance of an EV
corresponds to a small k. For wireless charging of mobile
devices, it is preferred to hold them in hand and thus
k can be small. For biomedical devices, due to limited
size of coils in devices, k can easily decrease below kDF .
Thus, CDDF is a common problem in practical application
scenarios.
Moreover, LSK has been used as a modulation scheme fora long time and it may seem strange why CDDF has neverbeen reported before. There are mainly two reasons. Firstly,

We have discovered a problem called CDDF that the data
can flip when k is smaller than kDF in a WPDT system withfour basic compensation topologies (SS, SP, PS, and PP).
When k is equal to kDF , the data cannot even be recoveredon the TX side. On the basis of a comprehensive circuit
analysis of CDDF, a universal method applicable to all four
compensation topologies was proposed. CDDF can be solved
by monitoring the current through the TX coil rather thanits voltage for data demodulation. To verify the existence of
CDDF and the effectiveness of the proposed method, a WPDT
system has been implemented in which the status of RLoad wastransferred to the TX side to control VS for PLoad regulation.
Using the proposed method, CDDF was eliminated and thedata were successfully transferred even when k was less thanor equal to kDF (0.35). Using the correct data, PLoad in the
WPDT system was regulated at around 1.1 W with a high
ηSYS of up to 60% under the variation in k. In our future
work, a novel data modulation mechanism is to be exploredto solve the incongruent requirements for a high power transfer
efficiency and data rate with LSK modulation.
A PPENDIX
A. Analytical Expression of k D F in SS Topology
Taking the SS topology as an example, we qualitatively
and quantitatively explain why CDDF occurs if VL is monitored for data demodulation and this problem is solved bymonitoring IL .
Fig. 13(a) shows the schematic of a WPDT system with SS
topology. VL and IL can be expressed as:
VL = j ω0 L TX ITX − j ω0 M IRX
IL = ITX .

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(A1)

QIU et al.: ANALYSIS AND MITIGATION OF CDDF IN WPDT SYSTEM

Fig. 13.

5191

Schematic of a WPDT system with SS topology. Dependences of related terms on RLoad .

Fig. 14. Calculated kDF surfaces in (a) SS, (b) SP, (c) PS, and (d) PP topologies under different values of L TX and L RX . Related parameters are selected
for biomedical applications.

RLoad dependences of related terms are summarized inFig. 13(b). When the state of the SW changes from short toopen, RLoad increases. ITX increases whereas IRX decreases.
Then the first term of VL increases whereas its second term
decreases. As a result, depending on the design parameter,
the amplitude of VL can stay the same. Thus, kDF can exist
if VL is monitored for data demodulation. On the other hand,with the increase of RLoad , IL , that is ITX , increases. Thismeans, if IL is monitored for data demodulation, data never
flips and CDDF can be avoided.
In addition, we derive the analytical expression for kDF withrespect to f 0 , RS , RLoad , L TX , and L RX .
By substituting (2) into (A1), the ratio of VL to VS at f 0
can be described as:
R Load j ω0 L TX + ω02 k 2 L TX L RX
VL
=
,
VS
RS RLoad + ω02 k 2 L TX L RX

(A2)

with its amplitude as

2

 
R 2Load ω02 L 2TX + ω02 k 2 L TX L RX
 VL  =.
V 
RS RLoad + ω02 k 2 L TX L RX
S
Through (4), kDF can be derived as

RLoad (ω02 L 2TX − RS2 )kDF =.
2RS ω02 L TX L RX

(A3)

(A4)

This also verifies the results shown in Table-I.
B. CDDF in A WPDT System for Biomedical Applications
In the body part of this work, we took the wireless charging
of mobile devices as an example and selected the corresponding parameters. In addition, we perform analysis in a WPDT
system for biomedical applications, in which LSK modulation
has also been widely used.

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5192

IEEE TRANSACTIONS ON CIRCUITS AND SYSTEMS—I: REGULAR PAPERS, VOL. 68, NO. 12, DECEMBER 2021

Corresponding circuit parameters targeting biomedical
applications are selected [8]–[10]. For example, f 0 , RS , andRL are selected as 13.56MHz, 50 , and 1k , respectively.
L TX and L RX are selected in the range between 100 nH and10 μH. Fig. 14 shows the calculated kDF surface in a WPDT
system with four compensation topologies. It can be seen that,except the white regions, CDDF will occur in a WPDT system.
Thus, CDDF is a common problem in a WPDT system fordifferent applications.
R EFERENCES[1] Z. Zhang, H. Pang, A. Georgiadis, and C. Cecati, “Wireless powertransfer—An overview,” IEEE Trans. Ind. Electron., vol. 66, no. 2,
pp. 1044–1058, Feb. 2019.
[2] F. Corti, A. Reatti, M. C. Piccirilli, F. Grasso, L. Paolucci, and
M. K. Kazimierczuk, “Simultaneous wireless power and data transfer:
Overview and application to electric vehicles,” in Proc. IEEE Int. Symp.
Circuits Syst. (ISCAS), Oct. 2020, pp. 1–5.
[3] F. Corti et al., “A secondary-side controlled electric vehicle wirelesscharger,” Energies, vol. 13, no. 24, pp. 6527(1)–6527(19), Dec. 2020.
[4] C.-G. Kim, D.-H. Seo, J.-S. You, J.-H. Park, and B. H. Cho, “Designof a contactless battery charger for cellular phone,” IEEE Trans. Ind.
Electron., vol. 48, no. 6, pp. 1238–1247, Dec. 2001.
[5] T. Campi, S. Cruciani, F. Palandrani, V. De Santis, A. Hirata, and
M. Feliziani, “Wireless power transfer charging system for AIMDs
and pacemakers,” IEEE Trans. Microw. Theory Techn., vol. 64, no. 2,
pp. 633–642, Feb. 2016.
[6] D. Jiang, D. Cirmirakis, M. Schormans, T. A. Perkins, N. Donaldson, and
A. Demosthenous, “An integrated passive phase-shift keying modulator
for biomedical implants with power telemetry over a single inductive
link,” IEEE Trans. Biomed. Circuits Syst., vol. 11, no. 1, pp. 64–77,
Feb. 2017.
[7] H. Qiu, T. Sakurai, and M. Takamiya, “Digital transmitter coil for
wireless power transfer robust against variation of distance and lateral
misalignment,” IEEE Trans. Microw. Theory Techn., vol. 68, no. 9,
pp. 4031–4039, Sep. 2020.
[8] A. Trigui, S. Hached, A. C. Ammari, Y. Savaria, and M. Sawan,“Maximizing data transmission rate for implantable devices over a single
inductive link: Methodological review,” IEEE Rev. Biomed. Eng., vol. 12,
pp. 72–87, 2019.
[9] B. Lee and M. Ghovanloo, “An overview of data telemetry in inductively
powered implantable biomedical devices,” IEEE Commun. Mag., vol. 57,
no. 2, pp. 74–80, Feb. 2019.
[10] H.-J. Kim, H. Hirayama, S. Kim, K. J. Han, R. Zhang, and J.-W. Choi,“Review of near-field wireless power and communication for biomedical
applications,” IEEE Access, vol. 5, pp. 21264–21285, Sep. 2017.
[11] M. Ghovanloo and S. Atluri, “A wide-band power-efficient inductive
wireless link for implantable microelectronic devices using multiple
carriers,” IEEE Trans. Circuits Syst. I, Reg. Papers, vol. 54, no. 10,
pp. 2211–2221, Oct. 2007.
[12] G. Wang, P. Wang, Y. Tang, and W. Liu, “Analysis of dual band
power and data telemetry for biomedical implants,” IEEE Trans. Biomed.
Circuits Syst., vol. 6, no. 3, pp. 208–215, Jun. 2012.
[13] J.-G. Kim, G. Wei, M.-H. Kim, H.-S. Ryo, P.-C. Ri, and C. Zhu, “A splitting frequencies-based wireless power and information simultaneoustransfer method,” IEEE Trans. Circuits Syst. I, Reg. Papers, vol. 65,
no. 12, pp. 4434–4445, Dec. 2018.
[14] S. Ha, C. Kim, J. Park, S. Joshi, and G. Cauwenberghs, “Energy
recycling telemetry IC with simultaneous 11.5 mW power and 6.78 Mb/s
backward data delivery over a single 13.56 MHz inductive link,” IEEE
J. Solid-State Circuits, vol. 51, no. 11, pp. 2664–2678, Nov. 2016.
[15] C.-C. Huang, C.-L. Lin, and Y.-K. Wu, “Simultaneous wirelesspower/data transfer for electric vehicle charging,” IEEE Trans. Ind.
Electron., vol. 64, no. 1, pp. 682–690, Jan. 2017.
[16] C. Huang, T. Kawajiri, and H. Ishikuro, “A 13.56-MHz wireless powertransfer system with enhanced load-transient response and efficiency
by fully integrated wireless constant-idle-time control for biomedical
implants,” IEEE J. Solid-State Circuits, vol. 53, no. 2, pp. 538–551,
Feb. 2018.
[17] X. Li, C.-Y. Tsui, and W.-H. Ki, “A 13.56 MHz wireless power transfer
system with reconfigurable resonant regulating rectifier and wirelesspower control for implantable medical devices,” IEEE J. Solid-State
Circuits, vol. 50, no. 4, pp. 978–989, Apr. 2015.

[18] C.-S. Wang, O. H. Stielau, and G. A. Covic, “Design considerations for acontactless electric vehicle battery charger,” IEEE Trans. Ind. Electron.,
vol. 52, no. 5, pp. 1308–1314, Oct. 2005.
[19] Y. H. Sohn, B. H. Choi, E. S. Lee, G. C. Lim, G.-H. Cho, and C. T. Rim,
“General unified analyses of two-capacitor inductive power transfer
systems: Equivalence of current-source SS and SP compensations,” IEEE
Trans. Power Electron., vol. 30, no. 11, pp. 6030–6045, Nov. 2015.
[20] H. Qiu, T. Sakurai, and M. Takamiya, “Coupling-dependent data flippingin wireless power and data transfer system,” in Proc. IEEE Int. Symp.
Circuits Syst. (ISCAS), May 2019, pp. 1–5.
[21] M. Fu, H. Yin, X. Zhu, and C. Ma, “Analysis and tracking of optimalload in wireless power transfer systems,” IEEE Trans. Power Electron.,
vol. 30, no. 7, pp. 3952–3963, Jul. 2015.
[22] T. Imura, Wireless Power Transfer: Using Magnetic and Electric Resonance Coupling Techniques. Singapore: Springer, 2020.
[23] T. C. Beh, M. Kato, T. Imura, S. Oh, and Y. Hori, “Automatedimpedance matching system for robust wireless power transfer via
magnetic resonance coupling,” IEEE Trans. Ind. Electron., vol. 60, no. 9,
pp. 3689–3698, Sep. 2013.
[24] Y. Lim, H. Tang, S. Lim, and J. Park, “An adaptive impedance-matching
network based on a novel capacitor matrix for wireless power transfer,”
IEEE Trans. Power Electron., vol. 29, no. 8, pp. 4403–4413, Aug. 2014.
[25] Z. Tang, B. Smith, J. H. Schild, and P. H. Peckham, “Data transmission
from an implantable biotelemeter by load-shift keying using circuit
configuration modulator,” IEEE Trans. Biomed. Eng., vol. 42, no. 5,
pp. 524–528, May 1995.
[26] M.-L. Yeh, W.-R. Liou, H.-P. Hsieh, and Y.-J. Lin, “An electromagnetic
interference (EMI) reduced high-efficiency switching power amplifier,”
IEEE Trans. Power Electron., vol. 25, no. 3, pp. 710–718, Mar. 2010.
[27] E. N. Baikova, S. S. Valtchev, R. Melicio, and V. M. Pires, “Electromagnetic interference impact of wireless power transfer system on data
wireless channel,” in Proc. Doctoral Conf. Comput., Electr. Ind. Syst.
(DoCEIS), Apr. 2016, pp. 293–301, doi: 10.1007/978-3-319-311654_29.
[28] K. Hata, T. Imura, and Y. Hori, “Simplified measuring method of kQ
product for wireless power transfer via magnetic resonance couplingbased on input impedance measurement,” in Proc. IECON, Oct. 2017,
pp. 6974–6979.
[29] F. Lu, H. Hofmann, J. Deng, and C. Mi, “Output power and efficiency sensitivity to circuit parameter variations in double-sided LCCcompensated wireless power transfer system,” in Proc. IEEE Appl.
Power Electron. Conf. Expo., Mar. 2015, pp. 507–601.
[30] A. Djemouai and M. Sawan, Prosthetic Power Supplies, vol. 17.
New York, NY, USA: Wiley, 1999.
[31] G. Wang, W. Liu, M. Sivaprakasam, and G. A. Kendir, “Design andanalysis of an adaptive transcutaneous power telemetry for biomedical
implants,” IEEE Trans. Circuits Syst. I, Reg. Papers, vol. 52, no. 10,
pp. 2109–2117, Oct. 2005.

Hao Qiu (Member, IEEE) received the B.S. degree
in materials science and the M.Eng. degree in electrical engineering from Nanjing University, Nanjing,
China, in 2010 and 2013, respectively, and the
Ph.D. degree in electrical engineering from The
University of Tokyo, Tokyo, Japan, in 2016. He wasa Researcher with The University of Tokyo from2017 to 2020. In 2021, he joined the School of Electronic Science and Engineering, Nanjing University,
as an Assistant Professor. His research interests
include integrated power management circuits for
wireless powering and energy harvesting, and nano-systems based on lowdimensional materials. He served as a Research Fellow for Japan Society for
the Promotion of Science (JSPS) from 2015 to 2017 and a Representative forJapan Society of Applied Physics (JSAP) from 2016 and 2018. He received the2016 IEEE Electron Devices Society (EDS) Japan Chapter Student Award andthe 2017 Chinese Government Award for Outstanding Self-Finance Students
Abroad.

Yuntao Jiang, photograph and biography not available at the time of
publication.

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QIU et al.: ANALYSIS AND MITIGATION OF CDDF IN WPDT SYSTEM

Yi Shi (Member, IEEE) received the B.S. degreein physics from the National University of Defense
Technology, China, in 1983, and the M.A.Sc. and
Ph.D. degrees in physics from Nanjing University,
China, in 1986 and 1989, respectively. In 1989,
he joined Nanjing University as an Assistant Professor, where he became an Associate Professor in 1993, and a Professor in 1996. He has
published extensively on advanced electronic andoptoelectronic materials and devices, and nanotechnology in professional journals (more than 600) andconferences (more than 50). His current research interest is focused on
semiconductor materials and devices.

Takayasu Sakurai (Life Fellow, IEEE) received
the Ph.D. degree in electrical engineering from The
University of Tokyo in 1981. In 1981, he joined
Toshiba Corporation, where he designed CMOS
DRAM, SRAM, RISC processors, DSPs, and SoC
Solutions. From 1988 to 1990, he was a Visiting Researcher with the University of California
at Berkeley, where he conducted research in thefield of VLSI CAD. Since 1996, he has been a
Professor with The University of Tokyo, working
on low-power high-speed VLSI, memory design,interconnects, ubiquitous electronics, organic IC’s, and large-area electronics. He is currently a Professor Emeritus. He has worked extensively oninterconnect delay and capacitance modeling known as Sakurai model andalpha power-law MOS model. He has published more than 700 technicalpublications, including 200 invited presentations and several books, and filed
more than 200 patents. He served as an Executive Committee Member for
ISLPED; and a Program Committee Member for ISSCC, CICC, A-SSCC,
DAC, ESSCIRC, ICCAD, ISLPED, and other international conferences.
He is an IEICE Fellow. He was a recipient of the 2010 IEEE Donald O.
Pederson Award in solid-state circuits, the 2010 IEICE Electronics SocietyAward, the 2009 and 2010 IEEE Paul Rappaport Award, the 2009 IEICE
Achievement Award, the 2005 IEEE ICICDT Award, the 2005 P&I Patentof the Year Award, the 2004 IEEE Takuo Sugano Award, and four productawards. He served as the Conference Chair for the Symposium on VLSI
Circuits and ICICDT; the Vice Chair for ASPDAC; and the TPC Chair for
A-SSCC and VLSI symposium. He is also the Executive Committee Chair of5193

the IEEE VLSI Symposia and the Steering Committee Chair of IEEE
A-SSCC. He delivered keynote speech at more than 50 conferences, including
ISSCC, ESSCIRC, and ISLPED. He was an Elected Administration Committee Member of the IEEE Solid-State Circuits Society. He was an IEEE CAS
Distinguished Lecturer and an IEEE SSCS Distinguished Lecturer. He is alsoa Domain Research Supervisor of nano-electronics area with Japan Scienceand Technology Agency.

Makoto Takamiya (Senior Member, IEEE) received
the B.S., M.S., and Ph.D. degrees in electronicengineering from The University of Tokyo, Japan,
in 1995, 1997, and 2000, respectively. In 2000,
he joined NEC Corporation, Japan, where he wasengaged in the circuit design of high speed digital
LSI’s. In 2005, he joined The University of Tokyo,
where he is currently a Professor of the Institute ofIndustrial Science. From 2013 to 2014, he stayed at
the University of California at Berkeley, as a Visiting
Scholar. His research interests include the integrated
power management circuits for wireless powering and energy harvesting
for wearable and the IoT applications, and the digital gate driver IC for
power electronics. He is a member of the Technical Program Committeeof the IEEE Symposium on VLSI Circuits. He received the 2009 and2010 IEEE Paul Rappaport Award and the Best Paper Award from the2013 IEEE Wireless Power Transfer Conference. He formerly served on thetechnical program committees for the IEEE International Solid-State Circuits
Conference (ISSCC) from 2015 to 2020 and the IEEE Custom Integrated
Circuits Conference from 2006 to 2011. He was a Far East Regional Chairof ISSCC 2020. He is also a Distinguished Lecturer of the IEEE Solid-State
Circuits Society.

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