SPECIAL REPORT
New Energy-Related Technologies
Capacitive Coupling Powers
Transmission Module
This method, which was employed in designing the wireless transmission module,
promotes high positioning flexibility, high design flexibility, and high power efficiency.
M
urata Manufacturing Co.,
Ltd. advances the development of capacitive
coupling wireless power
transmission systems. It started mass
production of the LXWS Series of modules that provided wireless power transmission of 10W in August 2011. Used in
a wireless charging set for iPad2 by Apple Corp., the LXWS Series has already
been on the market. Murata Manufacturing has further increased the transmitted
power and developed a wireless power
transmission module that outputs 20W
of power or more.
This article summarizes the technology used in the modules developed by
Murata, as well as samples of their application to notebook PCs, and development steps in the future.
Capacitive Coupling Method
Murata’s
capacitive
coupling
wireless power transmission system
Fig. 2: Block diagram of the capacitive coupling wireless transmission system
is characterized by its two sets of
asymmetric dipoles consisting of active
and passive electrodes positioned on the
power transmitting and receiving sides.
Power is transmitted using an induction
Utilizing the electrostatic induction phenomenon generated by coupling two dipoles
High positioning
flexibility
Electrode (Passive Electrode)
Load
Power receiving side
Induction field
Electrode (active electrode)
Electric power
transmission
Power transmitting side
Oscillator
Electrode (passive electrode)
field generated by coupling these two
sets of asymmetric dipoles (See Fig.
1). This configuration achieves a highefficiency wireless power transmission
with high positioning flexibility.
Fig. 2 shows the block diagram of
the entire system. The power transmitting side consists of a power transmitting module and power transmitting
electrodes, and the power receiving side
consists of power receiving electrodes
and a power receiving module.
Among the advantages of the capacitive coupling method include (1) high
positioning flexibility; (2) an electrode
unit with high design flexibility; and
(3) an electrode unit that do not generate heat. The third advantage, that is, an
electric unit that do not generate heat, is
effective in achieving higher power. This
also allows the electrodes to be placed
near the battery in the equipment.
Patent No - PCT/FR2006/000614
Murata Manufacturing Co., Ltd. holds a patent for this configuration,and also
provides a wireless power transmission system based on this patent technology.
Fig. 1: Basic configuration of a power transmission system using the capacitive coupling
method
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AEI November 2012
Copyright©2012 Dempa Publications, Inc.
As Integrated in Notebook PCs
As shown in Fig. 3 and Fig. 4, a power
receiving module and power receiving
electrodes are incorporated in a notebook PC, and a power transmitting mod-
SPECIAL REPORT
New Energy-Related Technologies
Photo of the inside of the notebook PC
Photo of the bottom of the notebook PC
Active electrode on
the power receiving
side (50 x 100 mm)
Power receiving module
25mm
200mm
Passive electrode
on the power
receiving side
60mm
300mm
Fig 3: Incorporating the module and electrodes in a notebook PC (on the notebook PC side)
Fig 4: Incorporating the module and electrodes in a notebook PC (on the charging pad side)
ule and power transmitting electrodes in
a charging pad.
Electrode unit
Notebook PCs have been getting thinner over the years, and those with a thickness of less than 10mm are on the market.
This makes it difficult for the equipment
design to accept the increase in thickness
of notebook PCs due to incorporation of
wireless power transmission functions.
For this reason, a 0.1mm-thick metal
board is used as an electrode. One of the
characteristics of the capacitive coupling
method is that such a thin electrode can
be used. The distance between the active
electrode on the power transmitting side
and that on the power receiving side is
approximately 2mm.
On the power transmitting side, the active electrode with a size of 50 × 100mm
is surrounded by the passive electrode.
This structure can curb field emission
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Copyright©2012 Dempa Publications, Inc.
from the active electrode. On the power
receiving side, the active electrode with
a size of 50 × 100mm is surrounded by
the passive electrode, just like the power
transmitting side.
Power transmitting module
The power transmitting module consists of an inverter circuit that supports
an output power of approximately 30W,
a microcontroller for safety control, and
a step-up transformer. It is necessary to
use a step-up transformer with high volt-
age resistance because it boosts voltage
to 1kVp-p or so.
The input voltage of the power
transmitting module is set to 19V so that
existing AC adopters can be used. The
power transmitting module is based on
the design of the already mass-produced
product capable of transmitting 10W
of power and uses high-voltage, highcurrent components to increase electric
power. The power transmitting module
measures 30 × 68 × 10(t)mm.
Power receiving module
The power receiving module consists
of a step-down transformer, which steps
down AC voltage of approximately
1kVp-p to 30Vp-p, followed by a
rectifying circuit and a DC/DC converter.
The voltage of output to the charging
circuit of a notebook PC is set to 19V.
In order to install it in a notebook
PC, the module needs to be smaller
and thinner. To this end, the configuration of the step-down transformer has
been changed as one of the measures
to achieve a lower profile. Although the
existing module capable of power transmission of 10W includes a relatively
large transformer, the newly developed
module includes two small transformers.
This has achieved a smaller and thinner
power receiving module with a size of
97 × 11 × 4.6(t)mm.
Highest Power Transmission
The specifications of the newly
developed system are shown in Table
1, and its characteristics of power
transmission in Figs. 5 and 6.
As shown in Fig. 5, the power transmission efficiency is designed to become the highest when 10 to 20W of
power is transmitted. The highest efficiency of 73 percent is ensured when
the notebook PC is placed at the center of the charging pad. In addition, a
positioning flexibility of ±35mm is
achieved. As shown in Fig. 6, the power
transmission efficiency varies only 1 to
Table 1: Specifications
Item
Input DC Voltage
Input Current
Output Voltage
Spec.
Min
Unit
Typ.
Max
18.5
19
19.5
-
1.9
2.2
DCA
18.75
19
19.25
DCV
W
DCV
Output Power
-
25
-
Efficiency (DC in~DC out)
-
73
-
%
510
525
540
kHz
Operation frequency
80
80
75
Transmission efficiency [%]
Transmission efficiency [%]
75
70
65
60
55
70
Output
power
10W
65
18W
22W
60
50
55
45
50
24W
0
40
0
5
10
15
20
25
30
Fig 5: Power transmission characteristics in terms of output power
30
40
50
80
Aiming for a power trans75
mission efficiency of 80
percent or more, Murata
70
Output
will promote the developpower
ment of such a module by
10W
65
introducing a new method
18W
to achieve higher efficien22W
60
cy, making full use of the
24W
characteristics of the ca55
pacitive coupling method,
as well as reduces power
loss of the transformer,
50
0
10
20
30
40
50
rectifying circuit, and
Position gap (Y direction) [mm]
DC/DC converter units.
Although some notebook Fig. 8: Power transmission in the Y-direction
PC models consume 50W
of electricity or more, the first target is to
Along with the development of these
support up to 50W of power.
technologies, Murata will work on ensuring mutual connectivity by means of
standardization, which will be required
for the widespread use of wireless systems. The company has already started
standardization efforts for the capacitive
coupling method, developing technology standards and business bases. An important point, in terms of technology, is
to develop a charging system that can be
used for a variety of devices. Taking advantage of the characteristics of the capacitive coupling method, Murata aims
to develop a charging system usable for
devices of different amounts of power
and different sizes.
Transmission efficiency [%]
Future Development Efforts
Murata Manufacturing intends to focus on achieving the following three
points to develop wireless power transmission systems capable of transmitting
high power in the future: (1) higher efficiency; (2) higher power (from 25 to
50W); and (3) module size reduction.
In order to develop a higher-power
and smaller module, it is essential to improve the power transmission efficiency.
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Fig. 7: Power transmission in the X-direction
Output power [W]
2 percent as long as the notebook PC
is placed on the charging area of the
charging pad. As for heat generation,
the surface temperature of the notebook
PC is reduced to 40°C or less when
power is transmitted.
10
Position gap (X direction) [mm]
Fig. 6: Power transmission characteristics in terms of the position of a notebook PC
About This Article:
The author, Shinji Goma, is from the
Business Development Section 3, New
Business Development Department,
Technology and Business Development
Unit at Murata Manufacturing Co., Ltd.
AEI November 2012
Copyright©2012 Dempa Publications, Inc.
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