20169063

Reduction in Radiation Noise Level for
Inductive Power Transfer System with Spread Spectrum
Keisuke Kusaka 1) Kent Inoue 2) Jun-ichi Itoh 3)
1) Nagaoka University of Technology, Energy and Environment Science
1603-1 Kamitomioka, Nagaoka, Niigata, 940-2188, Japan (E-mail: kusaka@stn.nagaokaut.ac.jp)
2) Nagaoka University of Technology, Electrical, Electronics and Information Engineering1603-1 Kamitomioka, Nagaoka, Niigata, 940-2188, Japan (E-mail: k_inoue@stn.nagaokaut.ac.jp)
3) Nagaoka University of Technology, Electrical, Electronics and Information Engineering1603-1 Kamitomioka, Nagaoka, Niigata, 940-2188, Japan (E-mail: itoh@vos.nagaokaut.ac.jp)

Presented at the EVTeC and APE Japan on May 26, 2016

ABSTRACT: Reduction methods in radiation noise of inductive power transfer (IPT) systems for electrical vehicles (EVs) are proposed.
In the IPT system for EVs, noise reduction technologies are required because radiation noise from the IPT system have to be suppressedto satisfy the standards. By changing the output frequency of the inverter, the radiation noise from the transmission coils is spread in afrequency domain. From the experimental results, the harmonics components around fundamental frequency are suppressed by 33.1%
by using the proposed method in comparison with the system using a constant frequency.

KEY WORDS: inductive power transfer, radiation noise, spread spectrum, Stationary Wireless Power Transfer (E5)

1. INTRODUCTION

the fundamental frequency. References (9-12) have proposed the

In recent years, IPT systems for EVs or plug-in hybrid

suppression methods using a magnetic shield and a metal shield.

electrical vehicles (PHEVs) are actively studied and developed (1-The transmission coils are surrounded by the plates made by

6). The IPT system transmits power using a magnetic coupling

magnetic material or metal. The radiation noise can be

between a primary coil and a secondary coil. The fundamental

suppressed because these shields change a magnetic flux to an

principle of the IPT system is as same as a transformer. However

eddy current. However the eddy current increases the power loss

it is characterized that the coupling between the primary coil andof the IPT system. Moreover, the additional magnetic or metal

the secondary coil are weak such as 0.1–0.3. The practicalshields cause an increase of cost.

realization of the IPT system is highly expected because the IPT

Incidentally, a spread spectrum method is widely used in asystems are capable to improve a usability of the EVs and PH

motor drive system with a PWM inverter (13). In the part of theEVs.

motor drive system, the carrier frequency of the PWM inverter is

In order to put the IPT system into practical use, radiatedconstantly changed during the operation for the purpose of aelectromagnetic noise has to be suppressed to satisfy the

reducing acoustic noise. By changing the carrier frequency, thestandards(7-8),

frequency components of the acoustic noise caused by vibrations

which are published by CISPR are well-known

standards, because the IPT system never affects any radioof windings of the motor is spread in a frequency domain (13).

communication systems, or electronic equipment. As a

In this paper, two reduction methods on the radiation noise

conventional method, a suppression method using a filter circuit

using the spread spectrum are proposed. In the proposed system,has been proposed. The low pass filter is used at the input stage

the output frequency of the voltage-source inverter is changedof the primary coil. The filter circuit suppresses the harmonics

within 80 kHz to 90 kHz at each output period. The proposedcomponents of the current on the primary coil. However, themethod does not require additional components. Moreover, thefundamental components of the transmission coil cannot beeffect on the efficiency is small. In the chapter 3, two reduction

suppressed. Moreover the larger power loss on the filter circuit

methods are proposed. The proposed methods varied the output

occurs since the cut-off frequency of the filter circuit is closed tofrequency of the voltage-source inverter. In the first proposed

Copyright © 2016 Society of Automotive Engineers of Japan, Inc. All rights reserved

16mmmethod, the frequency is continuously changed. In contrast, theThen the proposed methods are demonstrated using a scale modelof the IPT system with an output power of 200 W in the chapter
4. In the chapter 5, the proposed method is implemented into theprototype with an output power of 3 kW. Finally, the effect on
the system efficiency is evaluated.

80
Radiation noise [dBuA/m]

frequency is changed in random in the second proposed method.

60

40
23.1

2nd

20

3rd

0
10

2. ALLOWABLE RADIATION NOISE LEVEL IN JAPANFig. 1 shows the standards of the radiation noise in Japan for

Fundamental

68.4

79 90

158 180

237 270

100
Frequency [kHz]1000

Fig. 1. Allowable Limits on Radiation Noise of IPT System

IPT systems. Note that, the standards are still under discussion

for EVs in Japan (under discussion).

for the standardization. In Japan, a use of a frequency range from
PHEVs. This standard is basically conformed to the CISPR11 (14).
The difference between the CISPR11 and the standards is anallowable limit on the transmission frequency and its low-orderharmonics. The allowable limits of the radiation noise within 79
kHz to 90 kHz will be mitigated to 68.4 dBA/m. Moreover, theallowable limits on low order harmonics components will be
mitigated by 10 dB. Note that, CISPR11 requires measuring the

Switching frequency [kHz]79 kHz to 90 kHz is considered for the IPT system of EVs or
90

80

o

0.25

0.5

Time [ms]noise by a quasi-peak detection method. In order to satisfy thestandards, the frequency components of the radiation noise(a) Proposed method I: The output frequency is varied

should be suppressed not only the fundamental component but

according to the triangle wave.

also the low-order harmonics component.

The radiation noise is mainly caused by the current, whichflows in the transmission coils. In the conventional system, theoutput frequency of the inverter is generally constant. Thus, theProbability distribution

3. PROPOSED NOISE REDUCTION METHODS

0.2

radiation noise contains the large output frequency components

domain by changing the output frequency of the voltage-source

0.1
0.05

650 Hz
90.1

80.0

0
78

and its low-order harmonic components.
In this paper, the radiation noise is spread in a frequency

0.15

80

82 84 86 88
Frequency[kHz]9092(b) Proposed method II: The frequency is chosen in random.
Fig. 2 Frequency Changing Pattern of Proposed Methods.

inverter in two manners. The output frequency is varied within
80 kHz to 90 kHz. In the one of the proposed methods, the output

frequency of the voltage-source inverter is changed in random.

frequency is continuously changed. Another method changes the

The output frequency is chosen according to a pseudo random

switching frequency of the inverter in random.

number. Thus the probability distribution of the output frequency

Fig. 2 shows the schemes of frequency changing. Fig. 2 (a) is

is approximately even. The pseudo random number is generated

the frequency changing pattern of the proposed method I. The

using a maximal length sequence (M-sequence) in the DSP. Note

output frequency of the inverter is discretely chosen according tothat, different pseudo random number generation methods can be

the triangular waveform. The frequency of an output frequency

used. However the generation method using an M-sequence ischanging is 2 kHz. Each frequency step is 650 Hz. Fig. 2 (b)chosen in this paper because a complex generation method of ashows the probability distribution of the output frequency when

pseudo random number is not suitable for an implementation to

the proposed method II is used. In this method, the output

the DSP.

Copyright © 2016 Society of Automotive Engineers of Japan, Inc. All rights reserved

Fig. 3 shows the generation method of pseudo random

LSB
Rz (1bit)

Rz-p Rz-p+1 ···· Rz-q ···· Rz-1

numbers based on an M-sequence. An M-sequence random
number is generated by (1).

X Z  X Z  p  X Z q

Sz

····

····

Xz-p Xz-p+1 ···· Xz-q ···· Xz-1

Xz

Sz-p

where Xz-p and Xz-q are the present value XZ delayed by p period
and q period, respectively (p > q). In this paper, p = 7, q = 1 areused. Moreover, the number of bits of pseudo random number isseven.

····

Sz-p+1 ···· Sz-q ···· Sz-1

····

(1)

MSB
(7bit)X Z  X Z  p  X Z q
4. EXPERIMENTS WITH SCALE MODEL

Fig. 3. Generation Method of Pseudo Random Numbers Baseon a Maximal Length Sequence.

In this chapter, the conventional method and the proposed
methods are experimentally evaluated using a scale model.
4.1. Experimental Setup

CS1

Fig. 4 and Table 1 show the circuit configuration of the
prototype and the specifications, respectively. The proposed
methods are tested with a scale model. In the scale model, the

CS2
k

+

i1

Vin

v1 L1

Co R
Vo

L2

output power is reduced from 3 kW to 200 W for simplicity. The
series-series (S/S) compensation method is used in order tocancel out the leakage inductance (15). Beside, silicon-carbide(SiC) MOSFETs and SiC diodes are used as switching devices.

Fig. 4. System Configuration.

The output frequency of the primary inverter is decided in the
DSP.

Table 1. Specifications of Prototype (Scale model).

4.2. Operation waveforms

Symbol

Value

Vin

100 V

Fig. 5 shows the experimental results of the prototype. Fig. 5

Input DC voltage(a) is the waveforms with a constant-frequency. Fig. 5 (b) is the

Coupling coefficient

k

0.35

waveforms with a proposed method I: the output frequency is

Primary inductanceL1

176 uH

Secondary inductanceL2

180 uH

Primary capacitanceCS1

19.6 nF

Secondary capacitanceCS2

19.4 nF

Transmission distance

l

9 mm

varied according to the Fig. 2 (a). Fig. 5 (c) is the waveforms
with the proposed method II: the output frequency is changed in
random. In Fig. 5 (b) and (c), the output frequency of the inverter
is changed within 80 kHz to 90 kHz in every periods according
to pseudo random numbers. Due to the difference between theharmonics components around the fundamental frequency of 85

resonance frequency and the amplitude of the primary current i1

kHz by 37.2% and 57.0% in comparison with the conventional

is changed.

method, which operates the inverter at constant frequency. In

Fig. 6 shows the harmonics components of the primarysamemanner,the low-order harmoniccomponentsarecurrent i1. Fig. 6 (a) is the result with the conventional method.

suppressed in comparison with the conventional method. From

Fig. 6 (b) and (c) are the results with the proposed methods. In

the experiments with scale model, it is clear that the proposed

this paper, the harmonics components of the primary current are

method, which varied the output frequency in random, isevaluated instead of the radiation noise because the radiation

effective to suppress the radiation noise. Thus the proposednoise from a loop coil is proportional to the amplitude of current.

method is implemented into the second prototype, which has an

When the conventional method is used, the current with an

output power of 3 kW.

output frequency and low-order harmonics components are sharp.
In contrast, the proposed method I: the switching frequency isvaried according to triangle wave, and the proposed method II:
the switching frequency is varied in random, suppress the

Copyright © 2016 Society of Automotive Engineers of Japan, Inc. All rights reserved

Primary voltage v1 １00 V/div
0
Primary current i1 5 A/div0
Output voltage vo 100 V/div0

10 s/div(a) Constant frequency.

10 s/div(b) Varied frequency according to triangle wave.

10 s/div(c) Varied frequency in random.

Fig. 5. Operation waveforms with scale model. The output power is 200 W.
[dB]

0 dB=1.1 A

0 dB=1.1 A

0 dB=1.1 A

Primary current i1 [dB]

0

-40

-80

-120

100

850
350
600
Frequency f [kHz]

100

850
350
600
Frequency f [kHz](a) Constant frequency (b) Varied frequency according to triangle wave

100

850
350
600
Frequency f [kHz](c) Varied frequency in random

Fig. 6. Harmonics Analysis of Primary Current.

5. EXPERIMENTS WITH 3-KW PROTOTYPE
In this chapter, the proposed method is demonstrated with the3-kW prototype. The output frequency of the inverter is varied in
random.
5.1. Experimental Setup
Fig. 7 and Table 2 show the transmission coils, which are usedin the experiments and specifications of the prototype,respectively. Solenoid coils are used as the transmission coils.
The transmission distance is 150 mm.

Fig. 7. Transmission Coils for 3-kW Prototype.
Table 2. Specifications of Prototype.

5.2. Operation Waveforms

Symbol

Value

Vin

420 V

Fig. 8 shows the operation waveforms. Fig. 8 (a) is the

Input DC voltage

waveforms with a constant output frequency of 85.1 kHz. Fig. 8

Coupling coefficientk0.2(b) is the waveforms with the proposed method. The output

Primary inductanceL1

392 uH

Secondary inductanceL2

401 uH

Primary capacitanceCS1

8.96 nF

Secondary capacitanceCS2

8.78 nF

output power is 3 kW. When the proposed method is applied, theTransmission distancel

150 mmamplitude of the output current of the inverter is varied due to the

MOSFETs

ROHM: SCH2080KEC

Diodes

ROHM: SCS220AE

frequency of the voltage source inverter is varied within 80 kHz
to 90 kHz in random. In both of the operation waveforms, theCopyright © 2016 Society of Automotive Engineers of Japan, Inc. All rights reserved

Primary voltage v1 400 V/div
0
Primary current i1 10 A/div0
Output voltage vo 400 V/div0

10 s/div

10 s/div(a) Constant frequency(b) Varied frequency in random

Fig. 8. Operation Waveforms with 3-kW Prototype.
[dB]

0 dB=7.5 A

0 dB=7.5 A

Primary current i1 [dB]

0

-40

-80

-120

100

350

850
600
Frequency f [kHz]100(a) Constant frequency.

350

850
600
Frequency f [kHz](b) Varied frequency in random.

Fig. 9. Harmonics Analysis of Primary Current.
difference between the resonance frequency and the inverter
output frequency.
Fig. 9 shows the harmonics components of the primary
current i1. Fig. 9 (a) and (b) are the result without the proposed
method and the result with the proposed method. In this paper,
the harmonics components of the primary current is evaluatedinstead of the radiation noise because the radiation noise from aloop coil is proportional to the amplitude of a current. The
components around the fundamental frequency are suppressed by
33.1% by applying the proposed method. In same manner, thelow-order harmonic components are suppressed in comparison
with the operation without the proposed method.
5.3. Efficiency Comparison
Fig. 10 shows the DC-to-DC efficiency characteristics. Notethat the efficiency is defined as the ratio of the output DC power
to the input DC power. Both curve show similar characteristic.
The maximum efficiency is 94.9% at an output power of 3.0 kW

Efficiency (DC-to-DC) h [%]100
98
Without proposed method(constant frequency)

96
94

With proposed method(frequency in random)

92
90
88
86

0

500 1000 1500 2000 2500 3000 3500
Output power [W]

Fig. 10. Efficiency characteristic.
when the proposed method is used. The decrease of efficiency is
due to the difference of the resonant frequency and the output
frequency of the inverter. However, the decrease in efficiency is
0.84%. The effect of the spread spectrum on the efficiency isenough small to be ignored.

when the inverter is operated at constant frequency. In contrast,
the maximum efficiency is 94.1% at an output power of 3.0 kW

Copyright © 2016 Society of Automotive Engineers of Japan, Inc. All rights reserved

6. CONCLUSION

for Electric Vehicles, IEEE Energy Conversion Congress &Expo 2014, Vol. , No. , pp. 1762-1769 (2014)

In this paper, the reduction methods in a radiation noise of the
IPT systems for EVs are proposed. In the IPT system for EVs,
noise reduction technologies are required because the radiationnoise from the IPT system has to be suppressed to satisfy thestandards. By changing the output frequency of the inverter, theradiation noise from the transmission coils is spread in afrequency domain. In this paper, two reduction methods based on
the spread spectrum are proposed. One of the proposed methods
varies the output frequency of the inverter according to thetriangle wave. Another varies the output frequency in random.
First, the conventional method and the proposed methods areexperimentally evaluated with a scale model at an output powerof 200 W. The results shows that the reduction rate of theharmonics components of the second proposed method, which
varies the output frequency in random, is larger than the first(6) H. Takanashi, Y. Sato, Y. Kaneko, S. Abe, T. Yasuda: A
Large Air Gap 3 kW Wireless Power Transfer System forElectric Vehicles, IEEE Energy Conversion Congress &Expo 2012, Vol. , No. , pp. 269-274 (2012)(7) H. Shoki: Trends of Wireless Power Transmission
Technologies and Approaches for Commercialization,
IEICE Technical Report, No. WPT2010-07, pp. 19-24(2010)(8) J. Kim, H. Kim, M. Kim, S. Ahn, J. Kim, J. Kim: Analysis
of EMF Noise from the Receiving Coil Topologies for
Wireless Power Transfer, Asia-Pacific Symposium on
Electromagnetic Compatibility (APEMC) 2012 , pp.645-648(2012)(9) M. Jo, Y. Sato, Y. Kaneko, S. Abe: “Methods for Reducing
Leakage Electric Field of a Wireless Power Transfer System

proposed method, which varies the frequency according tofor Electric Vehicles”, ECCE US, pp.1762-1769 (2014)

triangle wave. Then the second method is implemented into the(10) H. Kim, J. Cho, S. Ahn, J. Kim, J. Kim: “Suppression of

3-kW prototype. From the experimental results, the harmonics
components around fundamental frequency are suppressed by
33.1% by using the proposed method in comparison with thesystem using a constant frequency. In addition, the decrease of
the transmission efficiency at the rated power is 0.84%.

Leakage Magnetic Field from a Wireless Power Transfer
Systemusing

Ferrimagnetic

Material

and

Metallic

Shielding”, IEEE International Symposium on EMC, 9781-4673-2061-0，pp.640-645 (2012)(11) T. Campi, S. Cruciani, M. Feliziani: “Magnetic Shielding of

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Leakage Electric Field of a Wireless Power Transfer System

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