Method Addresses Noise of QiCompliant Wireless Chargers
Radiation emission (dB+V/m)
60
I
Noise Problems
Fig. 1 shows the wireless frequency
bands used by smartphones. Noise related to radiation emissions and receiver
sensitivity for wireless communications
have been confirmed in some frequency
bands.
There are cases where electromagnetic compatibility (EMC) regulations
of every country cannot be complied
with regarding radiation emission. For
example, the acceptable limits of radiation emission in 30MHz or higher
frequency bands are defined by the international standard CISPR22, which is
the base of EMC regulations for information technology equipment in each
High radiation
emission
40
Without noise
suppression measures
Without charging
CISPR22
max 46dB+V/m
30
20
10
0
10
100
Without noise
suppression measures
Without charging
15
10
5
Deterioration of maximum 14dB
0
920
930
940
950
960
600
800
1000
(b) Receiver sensitivity of audio/data transmission
country. When a noise suppression measure is not provided, peaks*2 exceeding
the CISPR22 limits can be observed in
the several tens of megahertz band, as
shown in Fig. 2-(a).
Regarding receiver sensitivity for
wireless communications, sensitivity
suppression of the receiver takes place
and communications or receiving of incoming calls is disabled during charging
in some cases. For example, many types
of wireless communications are used in
Japan, such as multimedia broadcasting
(200MHz band), TV broadcasting (400
Audio/data
transmission
1200
5
Deterioration of
maximum18dB
0
500
600
700
800
(c) Receiver sensitivity of TV broadcasting
Fig. 2: Noise problems occurring in wireless charger
Audio/data
transmission
400
10
Frequency (MHz)
Frequency
200
Without noise
suppression measures
Without charging
15
400
Frequency (MHz)
TV broadcasting
0
20
Sensitivity suppression
of the receiver (dB)
20
Sensitivity suppression
of the receiver
Multimedia
broadcasting
1000
Frequency (MHz)
(a) Radiation emission – Vertical polarization/peak strength
Sensitivity suppression
of the receiver (dB)
n recent years, products supporting
wireless power supply are increasing, led by smartphones. Wireless
power supplies have a number of
advantages, for example, easy charging
and convenience due to a waterproof
and dustproof design. On the other hand,
concerns about noise problems are present, like noise produced when DC power
is converted into high-frequency power
or leaking of electromagnetic waves because the power has to be transmitted
between coils that are set apart. There
are a number of wireless chargers compliant with Qi (inductive power standard)*1 already available in the market.
This article from Murata Manufacturing
Co., Ltd. introduces the noise suppression method of the Q1-compliant wireless charger.
50
1400
1600
1800
2000
2200 [MHz]
Fig. 1: Wireless frequencies used by smartphones and noise problems of wireless chargers
to 700MHz band), and audio/data transmission (700 to 900MHz band and 1800
to 2100MHz band). If noise suppression measures are not provided, noise
generated in a wireless charger overlaps
the wireless communication frequency
bands and a considerably high sensitivity suppression of the receiver (more than
10dB)*3 for audio/data transmission and
TV broadcasting using 900MHz band or
lower becomes present.
Noise Interference Mechanism
Fig. 3 shows the approximated noise
interference mechanism of a wireless
charger based on results of investigating
the noise source and radio wave propagation route. The noise source is an inverter that generates the high-frequency
power used for power transmission. Although the power transmission frequency (=switching frequency of inverter) is
about 100kHz, this harmonic component
is present in frequencies up to the several hundred megahertz band and causes
noise problems, such as sensitivity supAEI February 2014
Copyright©2014 Dempa Publications, Inc.
39
Power supply cable
Circuit board
Power
transmission coil
Inverter
Affects receiver Affects receiver
sensitivity
sensitivity
Causes radiation
emission
Noise
Power
transmission coil
Wireless charger
Receiver side (smartphone, etc.)
Fig. 3: Noise interference mechanism of wireless charger
Power supply cable
Inverter
Circuit board
Power
transmission coil
Y capacitor
CMCC
X capacitor
Power
transmission coil
CMCC
Y capacitor
Fig. 4: Noise suppression method for wireless charger
Noise Suppression Method
Fig. 4 shows an example of the noise
suppression method for a wireless charger. The effective suppression measures
of this method can be provided by installing the common mode choke coil
(CMCC) and across-the-line (X) and
line-bypass (Y) capacitors.
For radiation emission, the effective
countermeasures can be provided by
40
AEI February 2014
Copyright©2014 Dempa Publications, Inc.
installing CMCC (Murata DLW5BTM142SQ2) at the base of a power supply
cable.
Effective countermeasures against
sensitivity suppression of the receiver
for wireless communications can be
provided by installing CMCC (Murata
DLW5BTM251SQ2) and low-electrostatic inductance (ESL) type X capacitor (Murata LLL185R71H222MA01)
in the circuit immediately following an
inverter. If CMCC cannot be installed
10000
DLW5BTM142SQ2
Common-mode Impedance(ohm)
pression of the receiver for wireless
communications and radiation emission.
Noise observed as radiation emission
is emitted mainly from a power supply
cable. The dominant cause of the noise
that affects receiver sensitivity for wireless communications is radiation from
power transmission coils, but there are
cases where noise is emitted from circuit
boards. When the noise emitted from
these sources is received by a smartphone antenna, sensitivity suppression
of the receiver takes place.
Both normal-mode noise and common-mode noise are present as noise
modes. Therefore, noise suppression
measures are required for both normalmode noise and common-mode noise.
in the circuit immediately following an
inverter, noise could emit from a circuit
board. If the inverter and CMCC cannot be placed close together due to the
layout of a charger, this problem can be
resolved by simply adding a Y capacitor (Murata LLL185R71H222MA01)
in the circuit immediately following the
inverter.
The DLW5BTM142SQ2 recommended for radiation emission suppression is
a very small (5 × 5mm) and low profile
(t=0.235mm) device. Despite its small
size, this CMCC provides extremely high
noise reduction performance in frequency bands that require radiation emission
suppression measures. As shown in Fig.
5, the common-mode impedances of this
CMCC are 530Ω and 1,650Ω at 30MHz
and 100MHz, respectively.
The DLW5BTM251SQ2 recommended for countermeasures against sensitivity suppression of the receiver for wireless communications is also a very small
(5 × 5mm) and low-profile (t=0.235mm)
device. Similarly, despite its small size,
this CMCC achieves a high rated current (5A) and at the same time provides
a high noise reduction performance in
all frequency bands that require countermeasures against sensitivity suppression of the receiver. As shown in Fig. 5,
the common-mode impedances of this
CMCC are 400Ω and 200Ω at 200MHz
and 1000MHz, respectively.
The LLL185R71H222MA01 recommended for countermeasures against
sensitivity suppression of the receiver
for wireless communications is a lowESL type capacitor called the LW reverse capacitor. Although this capacitor
is very small (0.8 × 1.6mm) and low
profile (t=0.5mm), it maintains a low-
DLW5BTM102SQ2
1000
DLW5BTM501SQ2
DLW5BTM251SQ2
DLW5BTM101SQ2
100
10
Frequency bands that
require radiation emission
suppression measures
1
1
10
100
Frequency (MHz)
Fig. 5: DLW5BT Series
Frequency bands that
require countermeasures
against sensitivity
suppression of the receiver
1000
Conclusion
As described in this article, the society is faced with noise problems with
wireless chargers related to radiation
emission and receiver sensitivity for
wireless communication, and Murata’s
CMCC and X and Y capacitors are very
effective as noise suppression measures
for wireless chargers. Murata believes
that these noise suppression measures
can be applied to electromagnetic induction type wireless chargers that transmit
power at about 100kHz band such as
PMA-compliant*4 chargers besides Qicompliant chargers.
Radiation emission (dB+V/m)
50
40
30
Without noise
suppression measures
After providing noise
suppression measures
Without charging
CISPR22
Improvement of
maximum 21dB
20
10
0
10
100
1000
Frequency (MHz)
Without noise suppression
measures
After providing noise
suppression measures
Without charging
15
10
5
0
920
Improvement of
maximum 13dB
930
940
950
960
Frequency (MHz)
(b) Receiver sensitivity of audio/data transmission
20
Sensitivity suppression
of the receiver (dB)
20
Without noise
suppression measures
After providing noise
suppression measures
Without charging
15
10
5
0
400
Improvement of
maximum 17dB
500
600
700
800
Frequency (MHz)
(c) Evaluation result of receiver sensitivity of
TV broadcasting
Fig. 7: Effects of noise suppression measures
In the coming years, Murata intends to
develop noise suppression technologies
for wireless power supplies that comply
with the A4WP standard*5 as well as the
Qi Mid Power standard*6 and new noise
suppression solutions will be proposed.
Footnotes:
*1
: One of the interface standards developed by the Wireless Power Consortium for electromagnetic induction type
wireless power supply. The power to be
supplied is up to 5W. Qi-compliant products are available worldwide.
*2
: The limit value of CISPR22 is a
quasi-peak value and the measurement
100
Common-mode Impedance(ohm)
60
(a) Radiation emission – Vertical polarization/peak strength
Sensitivity suppression
of the receiver (dB)
impedance characteristic up to high
frequencies and provides a high noise
reduction performance in all frequency
bands that require countermeasures
against sensitivity suppression of the
receiver. As shown in Fig. 6, the common mode impedances of this capacitor
are 0.30Ω and 0.55Ω at 200MHz and
1000MHz, respectively.
The effects of the above noise suppression measures are summarized in
Fig. 7. It has been confirmed that the
peak strength of radiation emission is
reduced by up to 21dB and the peak
strength value can be suppressed to less
than the limit value of CISPR22 Class B.
Moreover, an improved receiver sensitivity of audio/data transmission by
up to 13dB has been confirmed and it is
possible to achieve receiver sensitivity
equal to the level as when charging is not
performed. Similarly, the receiver sensitivity of TV broadcasting is improved by
up to 17dB and it is possible to achieve
receiver sensitivity equal to the level as
when charging is not performed.
Frequency bands that require
countermeasures against
sensitivity suppression
of the receiver
10
1
result is the peak value.
*3
: Although this information is not
mentioned here, we have confirmed that
receiver sensitivity is suppressed in the
same way for audio/data transmission
at 800MHz band. On the other hand, we
did not confirm any significant sensitivity
suppression of the receiver in frequency
bands higher than 1800MHz.
*4
: Electromagnetic induction type
wireless power supply adopted by the
Power Matters Alliance. PMA-compliant products are sold mainly in North
America.
*5
: Magnetic resonance type wireless power supply adopted by the Alliance for Wireless Power. As of October
23, 2013, no A4WP-compliant products
have been released.
*6
: The standard of electromagnetic
induction type wireless power supply developed by the Wireless Power Consortium. The Qi Mid Power standard products can supply power of up to 15W. As
of October 23, 2013, no Qi Mid Powercompliant products have been released.
0.1
0.01
1
10
100
Frequency (MHz)
Fig. 6: LLL185R71H222MA01
1000
About This Article:
The author, Hiroyuki Takatsuji, is
from the Application Engineering Section, Product Promotion Department,
Murata Manufacturing Co., Ltd.
AEI February 2014
Copyright©2014 Dempa Publications, Inc.
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