Wireless Power for Mobile Devices
Eberhard Waffenschmidt
Philips Research Europe
Eindhoven, The Netherlands
eberhard.waffenschmidt@philips.com
Abstract— Wireless power transfer allows a convenient, easy to
use battery charging of mobile phones and other mobile
devices. No hassle with cables and plugs, just place the device
on a pad and that’s it. Such a system even has the potential to
become a standard charging solution. Where are the limits for
such a solution and which are the side conditions to consider?
What are the possibilities to realize such a system?
To make the whole idea a success, it is definitely necessary to
come to widely accepted standard. Therefore, in 2009 the
Wireless Power Consortium was founded with meanwhile
more than 80 international companies as members. The
consortium recently released the first worldwide standard on
wireless power for mobile devices of to 5W called “Qi”. The
contribution presents details of this standard and the rationale
behind.
RESONANT OPERATION
To investigate an inductive wireless power system, a
closer look to the system is necessary. Figure 1 shows a
typical arrangement consisting of a transmitter coil and a
receiver coil. An AC current in the transmitter coil generates
an alternating magnetic field, which induces a voltage in the
receiver coil used to power a load.
B
D2
Receiver
coil
z
Transmitter
coil
D
INTRODUCTION
Wireless power transmission based on inductive power
got into the focus of attention in the recent past. Since data
communication has become wireless, users expect similar
use comfort also for powering of their mobile devices.
These expectations are fed with large public relation effort
by some publications and experiments showing wireless
power transmission over several feet.
This publication aims in giving a review on
considerations about the feasibility and limits of such
systems. A major part of this section refers to a previous
detailed publication [1] on efficiency limits and cites from a
further one [2], but new aspects about resonance operation
and magnetic emissions are also added.
In a further part of this work, an inductive power
transmission pad is presented, which is intended to charge
devices like mobile phones. It was presented also before [1]
[2].
Finally, the Wireless Power Consortium [3] is presented,
which recently released the first industry standard for
inductive charging of mobile devices called “Qi”
(pronounce “chee”) [4], and which is based on the
considerations reviewed in this paper. The related section in
this publication presents details about the standard, similar
as in [5], from where several citations are taken.
Figure 1 Typical arrangement of a wireless inductive power
transmission system.
Since the early times of inductive power transmission by
Nicola Tesla, resonant operation is used to improve power
transmission. Resonant power transmission is more than
120 years old ! Figure 2 shows the input current (for a fixed
voltage) at the transmitter coil of a typical inductive power
system, where the receiver comprises a series resonant
capacitor. Two resonances can be observed.
1
Input current with different coupling
0.9
0.7
Input current / a.u.
I.
II.
0.5
0.1
Series resonance
= Max. power
k= 0.3
0.01
Parallel resonance
= optimal efficiency
0.001
50KHz
100KHz
150KHz
200KHz
250KHz
300KHz
Frequency
Figure 2 Input impedance spectra for different couplings
showing two typical resonances.
One has properties of a series resonance. It shifts with
the magnetic coupling of the coils. At this resonance
frequency the series stray inductivity caused by the weak
magnetic coupling is cancelled out by the series capacitor,
such that maximum power can be transmitted for a given
generator.
The coupling factor k determines the amount of
magnetic flux penetrating the receiver compared to the
whole generated flux. k varies between 0 and 1. For k = 0
the two coils are completely decoupled, while k→1 means
very well coupled coils. The coupling factor is determined
by the arrangement of the two coils.
The other is similar to a parallel resonance. It doesn’t
shift with the coupling. At this frequency, the capacitive
current in the receiver cancels out most of the inductive
magnetizing current in the transmitter. This resonance gives
the best power efficiency. For weak coupling these two
resonances get close together. But the figure shows that
maximum power transmission and optimal efficiency do not
match.
The ohmic losses in the coils depend on the matching
between load resistance RL in the receiver and the coil
impedance ZC. An optimum ratio between both can be
found as Figure 3 shows. It is calculated for operation in the
parallel resonance, which provides optimal efficiency. If the
load resistance is too low, a high output current is needed to
provide the output power, which causes losses in the
receiver. If the load resistance is too high, a high output
voltage is needed, which requires a high magnetizing
current causing losses in the transmitter. Details see [1].
(1)
ωL
Q
R
The quality factor Q depends on the applied frequency
2πf = ω, the inductance value L and the resistance of the coil
R. For the whole system, the geometric average of the
transmitter’s and the receiver’s coil is relevant. This quality
factor Q can be influenced by the coil technology and their
sizes and shapes and the amount of conducting material
used. The higher Q is, the better the coils are. Technically is
difficult to obtain a quality factor above 1000. Values lower
than 10 are not very useful. For mass production values
around 100 can be expected.
100
Loss factor Ploss/Pout
0.03
k=
0.2
1
0.1
0.5
Optimum
High
0.01 current
0.001
0.9
High
voltage
0.01
0.1
1
2
Power efficiency
100%
Good efficiency
Distance
10%
1%
1
0.03
0.1
0.3
D2
= 0.01
D
Q = 1000
0.1
1
10
Figure 4 Power efficiency between loop inductors.
0.1
Qtx = 100
Qtr = 100
( k ⋅ Q)
(2)
2

⋅ 1 + 1 + ( k ⋅ Q) 
Axial position z / D
0.01
10
λopt
2
0.1%
0.01
Loss dependence on load matching
Bad
coupling
The optimum is a function of Q and k. The optimal loss
factor λopt, defined as ratio of losses to transferred power,
can be calculated as follows (equation derived from [1]):
Size ratio
LIMITATIONS DUE TO EFFICIENCY
Efficiency η
III.
To evaluate the efficiency, only the losses in the
magnetic system are investigated. Losses in the generator
and in the rectifier are comparable to any other switch mode
power converter. Radiation losses can be neglected because
of the low operating frequencies. Losses in the magnetic
system appear only as ohmic losses in the windings. They
are determined by the coil’s magnetic coupling factor k and
their quality factor Q:
10
Matching factor RL/ZC
Figure 3 Loss factor in the inductive power system as
function of the load matching.
To use this equation, the coupling factors for inductive
transmission structures with varying distance and size ratios
are calculated. Then a high quality factor of Q = 1000 is
assumed, leading to efficiency values, which can hardly be
exceeded. Using equation (2) the loss factor and from this
the efficiencies of these structures are calculated (see [1]).
The result is illustrated in Figure 4, which shows the
achievable efficiency as a function of the distance (scaled to
the diameter) of loop inductors with the size ratio of the
loops as parameter.
As a result, inductive power transmission in a larger
distance (e.g. a whole room) is very inefficient. Only low
power levels, which are not useful for consumer
applications like charging or illumination, can be
transmitted without wasting significant amount of power.
Possible applications could be industrial, e.g. for sensors,
which requires only low power (as proposed e.g. by [6]).
Contrary, Figure 4 shows that inductive power transmission
at a close distance (e.g. along a surface) can be efficient as
conventional power supplies.
IV.
POWER RESTRICTION DUE TO EMF LIMITS
A. Power extraction from a magnetic field
A further limitation is determined by the effect of
alternating magnetic field on human beings (EMF) and on
technical devices (EMI). Therefore, it is derived, how much
power can be drawn from a limited magnetic field.
Consider a loop inductor in a homogeneous magnetic
field BTx generated by the transmitter. The loop inductor is
penetrated by the magnetic flux φTx, as illustrated in
Figure 5a with the green arrow. If the turns of the coil N are
concentrated at the outer diameter, and if the magnetic flux
density BTx is homogenous over the area of the loop A, the
output voltage Uout is equal to the induced voltage:
U ind
(3)
N ⋅ A ⋅ j ω ⋅BTx
where ω = 2πf is the circular frequency of the magnetic
field.
If the loop is shorted (Figure 5b), a short circuit current
flows, which generates a flux φRx (red arrow) in opposite
direction to the transmitter flux φTx. Ideally, it just cancels
out the transmitter flux inside the loop.
φTx
φTx
φRx
b A
IL
φTx
φRx
c) A
Ires
CS
Uout
a) A
Figure 5. Magnetic flux in a loop. a) Open loop. b) Shorted
loop. c) Resonant loaded loop.
IL LRx
RS
UL
UR
Uind
CS
UC R
L
Iout
Uout
Figure 6. Equivalent circuit of a loaded resonant inductor
coil.
If the loop is part of a loaded resonance circuit, the
magnetic flux of the receiver may even exceed the
transmitter flux (Figure 5c). Figure 6 shows the related
equivalent circuit. The voltage source represents the induced
voltage Uind according to equation (4). The voltage drop at
the receiver inductor LRx corresponds to the magnetic flux of
the receiver φRx. In case of resonance, the inductor and
capacitor voltages cancel each other. The current is only
limited by the series resistance of the resonant circuit RS and
the load resistance RL. The inductor voltage UL may exceed
the induced voltage by a factor equal to the loaded quality
factor of the resonance circuit. Thus, also the receiver flux
φRX is that much higher than the transmitter flux φTx.
Furthermore, the flux density in the vicinity of the receiver
increases over the initial homogeneous flux density. The
intrinsic quality factor Q of the resonance circuit is
determined by the series resistor RS according to equation
(1) with L = LRx and R = RS. In case of resonance, maximum
power can be taken, if RL is matched to RS, that is RS = RL.
Then the maximum output power is:
Pmax
Q⋅
(
Uind
)
(4)
2
4ω ⋅LRx
Defining an AL value as the inductance scaled to one
turn with LRx = N2 . AL, the maximum output power can be
directly calculated from the magnetic flux density BTx by
combining equations (3) and (4):
Pmax
Q⋅
ω
4
⋅
( A⋅
BTx
)
2
(5)
AL
The inductance AL value can be calculated similar as in
the previous chapter according to [1]. Depending on the
assumption of the winding width w, the inductance for a
ring coil inductor may vary slightly. As a further inductance
value, an approximation is calculated according to the
equation given in [7]:
(6)
L
N² ⋅0.35
µH
m
⋅dout ⋅10
din
dout
, valid for din/dout ≤ 0.9,
where dout is the outer and din is the inner diameter. A
loop is approximated with a diameter ratio din/dout = 0.9.
Assuming a reasonable quality factor of Q = 100, the
resulting possible output power is calculated for a typical
case with a flux density of 1 µT at 1 MHz. The results are
shown Figure 7 in dependence on the loop diameter.
The graph can be scaled linearly with the frequency and
the quality factor where as it scales with the square of the
magnetic flux density according to equation (5). It shows a
cubic dependence of the
density. This means,
proportional to the loop
have one large receiver
covering the same area.
output power on the magnetic flux
the output power scales overarea of the receiver. It is better to
instead of many smaller receivers
Maximum output power of a loop
100
10
0.1
Above 10 MHz the assumption of a constant resistance is
certainly no longer valid. Furthermore, at these high
frequencies the system can no longer be considered as only a
magnetic system, but it will begin to radiate electromagnetic
waves. Therefore, the calculated values are doubtful above
10 MHz and plot only as dashed lines.
0.01
1 .10
BTx = 1 uT
3
1 .10
Estimated maximum output power, limited by EMF-limits
Q = 100
4
1 .10
f = 1 MHz
1E+3
5
0.01
0.1
1
Loop diameter d / m
Figure 7. Maximal output power for a resonant loop receiver
as a function of the size of the receiver loop.
B. Limitations due to EMF standards
The ICNIRP (International Committee on Non-Ionizing
Radiation Protection) derived guidelines for electromagnetic
field levels with respect to the exposure of humans to it [8].
For a general case, the reference limits for public exposure
and occupational exposure to magnetic fields are applicable.
Further national recommendations or standards exist, e.g. a
German workplace standard (BGV B11) [9]. The limits are
graphically shown in Figure 8 as function of the frequency.
As the most and “worst” relevant case, the ICNIRP public
exposure limit (red curve) is taken for further investigations.
Figure 8 EMF-Limits: Magnetic flux density limits
according to INCNIRP guidelines [8] and German
workplace standard BGV B11 [9].
Given this magnetic flux density as a limiting value, the
maximum possible power reception is calculated according
to equation (5) and using equation (6), which already takes
Maximum output power Pout / W
Power P / W
1
w=0.1mm
w=0.2mm
w=0.5mm
w=1mm
w=2mm
Approx.
resonant operation into account. The results are shown in
Figure 9 for different receiver sizes. A frequency dependent
quality factor of the resonant circuit is assumed for the
calculations, based on a frequency independent coil
resistance. This resistance is selected to achieve a quality
factor of 100 at 1 MHz. Thus, the assumption would further
lead to a quality factor 1000 at 10 MHz.
100E+0
10E+0
1E+0
Loop diameter / m
1
0.6
0.1
0.04
0.01
1
0.6
0.1
0.04
0.01
ICNIRP public exposure
100E-3
10E-3
1E-3
100E-6
10E-6
Q (1MHZ ) = 100
1E-6
1E+3
10E+3
100E+3
1E+6
10E+6
100E+6
1E+9
Frequency f / Hz
Figure 9 Maximum output power estimation for a receiver
in a magnetic field room. Limited by EMF-Limits according
to INCNIRP guidelines.
The calculated levels must be considered as estimation.
They have a high uncertainty and represent more or less only
the order of magnitude of the power level. A better quality
factor can yield higher power levels. On the other hand, the
calculation is based on the homogenous field of the
transmitter at the position of the receiver. However, in reality
the magnetic field decays with the distance. In order to
maintain the safety limit for the user in the area closer to the
transmitter, the power level has to be decreased.
Furthermore, the resonant operation leads to a field level
increase in the vicinity of the receiver. If the user has access
to this area, the power level has to be decreased further. At
last, the estimations assume a receiver matching for
maximum power transmission. This matching criterion is
different from the optimal efficient criterion (as derived in
the previous chapter) and always has a power efficiency of
less than 50%. The matching for optimal efficiency requires
a further reduction of the transmitted power.
With a loop diameter of 1 m power levels in the order of
some 10th of Watt can be received (blue curve). If the loop
diameter is shrunk to 4 cm to fit in mobile device (light green
curve) like a mobile phone, only less than 10 mW are
received. Thus, the probability for a wireless power space
like e.g. W-LAN for power seems quite low.
An application like a wireless power space requires a
magnetic field at any arbitrary position in that space. But
then the whole body of the user is exposed to the magnetic
field, without any time restriction. Therefore, the strict
ICNIRP guidelines for public exposure have to be applied
without any relaxes.
This looks different for a wireless power surface,
especially for locally activated operation. In this case the
magnetic field is concentrated in the area between the
transmitter and the mobile device. The user is only exposed
to the strongly decaying stray fields. Furthermore, only the
extremities of the user are exposed to the magnetic field (e.g.
the hand), and the exposure time is in many applications
limited for a short period (e.g. during placing a device in the
power area). This allows orders of magnitude higher
magnetic flux levels for the power transfer.
Figure 10 Illustration of the FEM calculation of induced
currents in the user‘s hand.
be shown that an arrangement as described in the Qistandard for mobile devices (see later chapter) with coil sizes
of about 4 cm with appropriate shielding could meet the
basic restrictions even at 5 W power transfer. This is far
beyond the power level, which can be expected for open
space power transfer.
Concluding, also from an EMF point of view, power
transfer in an open space is not recommended, but at a
surface, reasonable wireless power transfer is possible.
V.
APPLICATION
According to these findings, inductive wireless power
transfer is promising along a surface. A possible application
can be the charging of mobile devices. Here, wireless
communication already has become standard, and the
consumer expects that charging would also be possible
without the hassle of cables and plugs.
For such an application, pads that charge wirelessly are
proposed. One example is the Powerpad (see Figure 11),
which was presented by Philips Research [1] [2]. It provides
very simple functionality to the user: Just place the mobile
somewhere on the pad and it will charge.
The Powerpad consists of an array of transmitter cells,
as proposed before by Ron Hui [10]. However, the
Powerpad provides a local activation feature. Each cell
comprises a detector, which activates the cell, if a device is
placed on it. This way the magnetic field emissions are
limited and the efficiency is improved. The pad can transmit
up to 1.2 W with one transmitter cell and operates at
500 kHz. The size is 20cm x 26cm and it contains 52
transmitter cells of 40 mm diameter each. The pad is
realized in printed circuit board (PCB) technology. The
receiver circuits are also made from PCB such that they are
less 1 mm thick with similar diameter. It is not compatible
to the Qi standard (see following chapter).
Analyzing the exposure needs more effort in this case.
ICNIRP defines reference levels for the field only for
homogeneous fields and exposure of the whole body.
Localized inhomogeneous fields, which affect only
extremities, require the application of the basic restrictions
defining limits for induced currents and dissipated heat
(SAR) in the body. These values cannot be measured, but
must be simulated with an appropriate method like the Finite
Element Method (FEM), for which simplified models e.g. of
a hand are defined. Figure 10 illustrates such a calculation.
These calculations cannot be generalized, but have to be
performed for any particular case. Several cases were
investigated in the frequency range between 100 kHz and
500 kHz. The limiting factor turned out to be the induced
current. The SAR value was negligible in all cases. It could
Figure 11 Inductive Powerpad for the charging of mobile
devices [1].
VI.
STANDARDISATION
A. The Wireless Power Consortium
The application of wireless power technology in
consumer devices such as the presented Powerpad will
benefit greatly from the availability of a standard that is
widely accepted in the industry. The primary target of such
standard is to establish interoperability between wireless
power devices and wireless power chargers.
To address this issue, an international consortium of
companies founded the “Wireless Power Consortium” [3] in
2009 to create a standard definition for wireless power
transfer of mobile devices. Figure 12 shows the regular
members in mid 2010, when the consortium released the
first standard. Meanwhile, the consortium has more than 90
regular and associated members, which support the
standard. The actual number and names change nearly daily
and is provided on the consortium website [3].
interoperability could be guaranteed in all cases. Therefore,
the design freedom for the transmitter is limited to three
standard transmitter types, as Figure 13b illustrates. The
design of receivers is left free to the manufacturers, because
here the pressure on cost and size is much higher than for
transmitters.
Tx 2
Tx 2
Reference Tx
Tx 1
+
Certification
Rx 1
a)
Rx 3
Rx 2
Certification
b)
Tx 3
Operation ?
Rx 1
Rx 2
Rx 3
Tx 2
Tx 1
or
Certification and
operation
Rx n
Rx 2
=
Reference Rx
Std. Std. Std.
Tx1 Tx2 Tx3
Rx 1
Tx 1
Tx 3
Tx n
Certification and
operation
Standardized Rx
Figure 13 Possibilities to obtain interoperability:
a) Specification with reference transmitters (Tx) and
receivers (Rx). b) Definition of standard Rx and Tx.
Free Positioning
(Moving Coil)
y
Guided Positioning
(Magnetic Attraction)
M
Figure 12 Regular members of The Wireless Power
Consortium in mid 2010.
The consortium has defined a standard for providing up
to 5W of power to low power mobile devices, like mobile
phones, batteries and camera’s [4]. It aims in providing free
positioning. In a later state, higher power levels will be
addressed as well.
x
a)
b)
Free Positioning (Coil Array)
The logo “Qi” (pronounced as “chee”, see Figure 12)
was created, meaning “vital” energy in Asian philosophy an intangible flow of power as a sign for compatible
wireless power devices. The Qi sign indicates that a device
is compatible to the wireless consortium standard and
complies with its requirements on interoperability and
performance. It also means that the device needs to fulfill
the international and regional regulations on safety and
electromagnetic interoperability.
Figure 14 Three standard transmitters (Tx) for free
positioning: a) Guided positioning. b) Moving Tx coil.
c) Coil array.
B. Standard transmitters
To guarantee interoperability, a precise specification of
the magnetic interface would be necessary. However,
specifying of a free positioning interface would require
reference transmitters and receivers (see Figure 13a). But
with this solution the consortium is not confident that
The Qi specification contains 3 basic transmitter types
representing 3 different techniques to achieve good
alignment between transmitter and receiver coils: Guided
positioning, free positioning moving coil and free
positioning coil matrix. Figure 14 illustrates the transmitter
types.
c)
The coil array transmitter contains a matrix of primary
coils distributed over the surface area (Figure 14c). It
supports free positioning by activating the primary coil(s)
that are at the receiver’s location. The coils are arranged in
three overlapping layers. The transmitter activates three
overlapping primary coils, generating a similar magnetic
field as the single coil. Localization of a receiver can be
realized by “scanning” the primary coils on the presence of
a receiver.
D. Data communication
1) Physical layer
Data communication is provided only in one direction
from the transmitter to the receiver. It is realized by load
modulation. The receiver changes its impedance
periodically by switching e.g. a resistor or capacitor. This
leads to a modulation of the coil current or coil voltage in
the transmitter, which is detected and demodulated by a
suitable circuit. The implementation in the receiver is not
standardized. However, a minimum modulation depth of the
coil current of 15 mA or coil voltage of 200 mV in the
standard transmitter is specified in the standard. Figure 16
shows possible realizations. It is recommended to realize the
modulator by switching a capacitor between the receiver
coil and the rectifier. Experiments showed problems when
switching a (resistive) load connected at the output of the
rectifier due to the low internal resistance of the battery.
C. Power transmission
Transmitter
The standard defines a power transmission of 5 W.
Nominal operating frequency is 110 kHz, but it may vary up
to 205 kHz for the purpose of power control. Resonant
operation is used to optimize power transfer and to allow
frequency control as derived in the previous chapter and
also proposed by further consortium members, e.g. [11].
Power control can be realized by frequency control or by
amplitude control or by pulse width control. The system is
shown in Figure 15.
Transmitter
Receiver
Cp
-
Cs
Lp
Power
Ls
Cd
C
Load
+
Receiver
Cs
Cp
+
-
Lp
Comm.
I p Vp
Ls
The design of a receiver is largely free including the
design of the secondary coil LS. It must comprise a series
resonant capacitor CS for optimized power transfer, which
must be in resonance with LS at 100 kHz. An additional
parallel capacitor CP provides a parallel resonant circuit at
Modulation
Cm
Cd
C
Rm
Power
Figure 16 Communication by load modulation
2) Data Format
The standard applies a simple packet format for
communication (see Figure 17). A packet contains a preamble (≥ 11 bit) for bit synchronization, a header (1 byte) to
indicate the type of the message, the message contents
(1...27 byte) and a checksum (1 byte). Each byte is coded
with a start-bit, 8 bit data, a parity bit and a stop bit. Bits are
bi-phase encoded and have a period of 0.5 ms (2 kB/s).
0.5ms
Figure 15 Power transmission system.
1
Start
The transmitter consist of a switching stage, e.g. a half
bridge, connected to a series resonant circuit. The
parameters Cp and Lp (transmitter coil) as well as the DC
supply voltage are well defined for the different transmitter
types. The physical dimensions of the transmitter coils are
also standardized in detail.
Modulation
0
1
0
1
1
0
0
b0 b1 b2 b3 b4 b5 b6 b7
Preamble
Header
Message
Stop
In a further transmitter type, free positioning is
supported by locating the receiver and moving the primary
coil mechanically towards the receiver’s location
(Figure 14b). The transmitter contains an additional facility
(not shown) to detect and locate a receiver on the
transmitter.
1 MHz, which can be used for the detection of the receiver.
The receiver must comprise a full wave rectifier and a
switch must be provided, which connects the load only, if
the power control is stable and without error. Several
example receiver designs are presented in the standard.
Parity
With guided positioning (Figure 14a), the user gets
tactile feedback for aligning the receiver to the transmitter
by the use of magnetic attraction. For this transmitter, a
magnet is placed at the center of the primary coil. A receiver
can be equipped with a magnetic attractor at the center of
the secondary coil, which could be another magnet, or any
material that provides an attractive force to a magnet.
Checksum
Figure 17 Packet format (bottom) and byte and bit encoding
(top).
E. Power Control
The receiver is in control of the power transfer. It
measures its output value and calculates the difference to its
desired output value. The output value itself is not specified.
It may be an output voltage or current or power or any other
value.
Transmitter
Desired
Receiver
Control Error
Interpret
Message
Control
Error
Calculate
Desired
2) Ping phase
Actual
Load
Actual
Adapt
Power Conversion
Power Pick-up
Power
Figure 18 Power control overview
Then the receiver transmits this difference as so called
control error to the transmitter at least every 1.5 s. The
transmitter increases or decreases its output power
according to the control error within 20 ms. The control
error value specifies a percentage change of the actual
transmitter coil current.
F. Initialization
To initialize the power transmission, three phases need
to be passed (see Figure 19).
Transmitter
Receiver
Signal
Select
Select
ID&C
Ping
Signal
Identification
Required Power
ID&C
End Transfer / Signal Lost
End Transfer / Error / Timeout
Signal Strength
Rx
Detected
Configured
PT
Control Data
End Power
In the ping phase, the transmitter checks if a detected
object is a receiver and if it has any need for power. Then,
the transmitter provides a power signal on the primary coil
for max. 65 ms. The receiver must react within this time by
load modulation and send the strength of the received power
signal. If detected, the transmitter continues to deliver
power to proceed to the next phase.
3) Identification and configuration phase (ID&C)
In the identification and configuration phase, the power
receiver provides information to prepare the transmitter for
power transfer. First the receiver communicates an identifier
to help the transmitter to recognize the version of the
receiver, to localize the receiver unambiguously, or for any
other proprietary application. In addition the receiver
communicates configuration parameters to the transmitter,
like the maximum required power. Finally, the transmitter
decides to accept or refuse to power the receiver.
4) Power transfer phase (PT)
Object
detected
Ping
detected e.g. by a change of the transmitter’s resonance, by
capacitive detection or by detecting a receiver’s resonance.
For the latter, a parallel resonance must be provided in the
receiver (see section C. Power transmission). Usually, this
start phase will be implemented such that it complies with
the requirements for low standby power of the standard. If a
transmitter detects any change in the presence, it will enter
the ping phase.
PT
Adapted
Figure 19 System Control phases.
1) Selection phase
If a user places a receiver on a transmitter, the
transmitter must indicate within 0.5 s that it has recognized
a receiver. The implementation is left open for the designer,
but the standard provides some examples. An object may be
In the following power transfer phase, the receiver
controls the power transfer as described before. The power
transfer finishes, if the receiver sends an “end of power”
message or if no message are received within a timeout of
1.5 s. Then, the system state goes back to the selection
phase.
G. Further topics
The Qi standard consist of three parts:
Part 1 “Interface Definition” is available for the general
public since mid 2010 and can be downloaded from the
website of the Wireless Power Consortium [3] after a cost
free registration. It describes all topics to guarantee
interoperability and includes in detail the specification
presented here in this publication. If contains further
information, e.g. the data protocol and physical dimensions
of inductors.
Part 2 “Performance Requirements” describes further
requirements, which Qi devices must meet. They relate e.g.
to standby power, efficiency, magnetic emissions and safety
aspects, such as an unexpected power loss in the
transmission path.
Part 3 “Compliance Test Specification” describes how to
certify a Qi compliant device and the related measurement
procedures.
The latter parts are available only for members of the
consortium. An application form for membership can be
found on the consortium website [3]. Compliance with all
three parts is required for the use of the Qi logo.
Meanwhile, first Qi compliant products are available on
the market. Please, check the consortium’s website [3] for
actual details and further information.
VIII. REFERENCES
[1]
Eberhard Waffenschmidt and Toine Staring,
"Limitation of inductive power transfer for consumer
applications", 13th European Conference on Power
Electronics and Applications (EPE 2009), Barcelona,
Spain, 8.-10.Sept. 2009, paper #0607.
[2]
Eberhard Waffenschmidt, "Wireless power for
mobile devices", VDE-Kongress 2010, Leipzig,
Germany, 8.-9.Nov.2010.
[3]
http://www.wirelesspowerconsortium.com/
index.html
[4]
“System Description Wireless Power Transfer,
Volume I: Low Power, Part 1: Interface Definition”,
Version 1.0.2, Wireless Power Consortium, April
2011.
[5]
Dries van Wageningen, Toine Staring, "The Qi
Wireless Power Standard", 14th International Power
Electronics and Motion Conference (EPE-PEMC
2010), Ohrid, Macedonia, 6.-8. Sept. 2010.
[6]
Kathleen O’Brian, “Inductively Coupled Radio
Frequency Power Transmission System for Wireless
Systems and Devices”, PhD Thesis, TU Dresden,
5.12.2005, Shaker Verlag Aachen 2007, ISBN 9783-8322-5775-0.
[7]
E. Waffenschmidt, B. Ackermann, "Size advantage
of coreless transformers in the MHz range", EPE
2001 (European Power Electronic Conference), Graz,
Austria, 27.- 29. Aug. 2001.
[8]
International Commission on Non-Ionizing Radiation
Protection (ICNIRP), "Guidelines for limiting
exposure to time-varying electric, magnetic, and
electromagnetic fields", Health Physics April 1998,
Volume 74, Number 4.
[9]
Berufsgenossenschaft der Feinmechanik und
Elektrotechnik, "Unfallverhütungsvorschrift
Elektromagnetische Felder Berufsgenossenschaftliche Vorschrift für Sicherheit
und Gesundheit bei der Arbeit", BGV B11 (VBG
25), 1.6.2001
[10]
R.Hui, W. Ho, “A new generation of universal
contactless battery charging platform for portable
consumer electronic equipment,” 35th IEEE power
electronics specialists conference 2004
VII. CONCLUSION
From an efficiency point of view, wireless inductive
power transfer is feasible for general power applications
only if transmitter and receiver coils are in close proximity
to each other. Inductive power transfer in a larger space is
not feasible due to the very low efficiency. An inductive
power pad to charge mobile devices is presented, which
allows arbitrary positioning and local detection. To trigger
the market introduction of this technology to charge mobile
devices, the international Wireless Power Consortium with
meanwhile more than 90 industry members released the first
standard named “Qi” (pronounce “chee”) for devices up to
5 W.
The wireless power consortium provides an industry
standard between power transmitters and receivers based on
inductive coupling at proximity with well aligned coils. The
standard provides a high design freedom for receivers and
the means to control power transfer; this allows meeting the
requirements of various mobile device applications both
commercially as well as functionally. Confidence in
achieving interoperability is achieved by limiting the design
freedom for transmitters in the early phases of releasing the
standard.
ACKNOWLEDGMENT
Thanks to my colleague Dries van Wageningen, who
participated the Wireless Power Consortium for Philips
Research and who allowed me to use material from his
publication. Further thanks to all contributors of the
specification workgroup of the wireless power consortium
[11] X. Liu, W. M. Ng, C. K. Lee and S. Y. R. Hui,
"Optimal operation of contactless transformers with
resonance at secondary circuit", conference
proceedings APEC'08, USA, Feb. 2008, pp. 645-650.