Reduction on Radiation Noise Level
for Inductive Power Transfer Systems
with Spread Spectrum focusing on Combined
Impedance of Coils and Capacitors
Kent Inoue, Keisuke Kusaka, Jun-ichi Itoh
Nagaoka University of Technology
Department of Electrical, Electronics and Information
Nagaoka, Niigata, Japank_inoue@stn.nagaokaut.ac.jp, kusaka@vos.nagaokaut.ac.jp, itoh@vos.nagaokaut.ac.jp
Abstract—Two reduction methods on radiation noise of
inductive power transfer (IPT) systems are proposed andexperimentally demonstrated. In the IPT systems for electricalvehicles (EVs) or plug-in hybrid electrical vehicles (PHEVs), noise
reduction technologies are strongly required because the radiationnoise from the IPT system for EVs or PHEVs must not exceeds thelimits on standards; for example the regulation by CISPR is wellknown regulation. The proposed method suppresses the radiationnoise using spread spectrum technique. The radiation noise fromthe transmission coils of the IPT system is spread in a frequency
domain by changing the output frequency of an inverter at
random. The output frequency is selected according to pseudo
random numbers. The first proposed method; a spread spectrumwith a uniform distribution (SSUD), evenly selects the output
frequency within 80 kHz to 90 kHz. Another method; a spread
spectrum with a biased distribution (SSBD) is focusing on the
output current of the inverter. The possibility for the select of
output frequency is biased in proportion to a combined impedanceof the transmission coil and the resonance capacitors. In theexperiments with an output power of 3 kW, the fundamental
components are suppressed by 42.6% and 72.1% by applying the
SSUD and the SSBD in comparison with the conventional system,
which operates the inverter at a fixed frequency.

because the IPT system never affects any
communication system, or electronic equipment.

Keywords—inductive power transfer; wireless power transfer;spread spectrum; random carrier; radiation noise; EMI

Incidentally, spread spectrum is widely used in a motor drive
system with a PWM inverter [22–24]. This technique is also
called as a random carrier PWM. The carrier frequency of the
PWM inverter is constantly changed during the operation for the
purpose of a reducing acoustic noise. By changing the carrier
frequency, the frequency components of the acoustic noisecaused by vibrations of windings is spread in a frequency
domain [22–24]. The spread spectrum is also used in converters
for the purpose of a reducing conducted electromagnetic
interference (EMI) [25–27]. In both applications, the carrier
frequency is changed to suppress the acoustic noise orelectromagnetic noise. It means that the spread spectrum havenot been applied to a square wave inverter. For this reason, thespectrum spread has not been applied to the IPT system because
IPT system is generally driven by a square wave inverter.

I. INTRODUCTION
In recent years, an IPT system are actively studied anddeveloped [1–7]. Especially, a practical realization of the IPT
systems for EVs or PHEVs is highly expected because the IPT
system are capable to improve a usability of the EVs and PHEVs[8–13]. The IPT system transmits power using weak magnetic
coupling between a primary coil and a secondary coil via large
air-gap [14]. The fundamental principle of the IPT system is assame as a transformer. However, the weak coupling between the
primary coil and the secondary coil such as 0.1–0.3 is specificcharacteristic of the IPT system.
In order to put the IPT system into a practical use, radiatedelectromagnetic noise have to be suppressed [15] to meet thestandards (e.g. standard by CISPR is well-known regulation),

wireless

One of the reduction methods on the noise is a use of filter
circuit. A low-pass filter is connected to the input stage of the
primary coil. The filter circuit suppresses the harmonics
components of the current, which flows at the transmission coil.
However, the radiation noise with a fundamental component
cannot be suppressed. Moreover the power loss on the filter will
increase because the cut-off frequency of the filter circuit has to
be closed to the fundamental frequency. References [16–19]
have proposed the suppression method using the magnetic shieldor metal shield. The transmission coils are surrounded by plates
made by magnetic material or metal. The radiation noise can besuppressed because these shields change a magnetic flux to an
eddy current. However the eddy current increases the power lossof the IPT system. Besides, the aperture for magnetic path haveto be ensured in the IPT system. Thus, the shielding provides thelimited effect on the suppression of radiation noise. Ref. [20–21]
have proposed the noise reduction method by forming thecurrent, which flows in the coils, using the primary converter.
However, the additional switches are required to reduce theradiation noise.

II. ALLOWABLE RADIATION NOISE LEVEL IN JAPAN
Figure 1 shows the regulations of the radiation noise in Japanfor IPT systems with an output power of 7 kW or less [28]. Notethat the regulations are under discussion in Japan for the
standardization. In Japan, a use of a frequency range from 79
kHz to 90 kHz is considered for an IPT system of EVs. This
regulation is basically conformed to the CISPR11 Group 2,
Class B [29]. However the limits of the radiation noise within 79
kHz to 90 kHz will be mitigated to 68.4 dBμA/m. Moreover, thelimits on the following frequency bands will be mitigated by 10dB.
• 158 kHz – 180 kHz

80
68.4

Radiation noise [dBuA/m](at 10 m)

In this paper, two reduction methods of the radiation noisefor the IPT system based on a spread spectrum technique areproposed and demonstrated. The first proposed method; spread
spectrum with uniform distribution (SSUD), suppress theradiation noise using a spread spectrum. In this method, theoutput frequency is selected from a uniform probability
distribution. The second method; spread spectrum with biased
distribution (SSBD), selects the output frequency from a biased
probability distribution. The probability distribution is
proportional to the combined impedance of a compensation
capacitor and a transmission coil. In the rest of the papers, first,
the regulations on electromagnetic noise for the IPT system isshown. Then, two proposed methods are explained in the chapter
3. In the chapter 4, the proposed methods are implemented into
the prototype with an output power of 3 kW. Finally, the effecton the system efficiency is evaluated.

Fundamental

60
40
2nd

23.1

20

3rd

4th

5th

CISPR Class B
Group 2 + 10 dB

CISPR Class B
Group 2(CISPR/B/587A/INF)

0
-10
10

100
1000
Frequency [kHz]10000

Fig. 1. Allowable limits on radiation noise of 7-kW or less IPT system for
EVs in Japan (under discussion).

C1 r1
I1

k r2 C2
L1

L2

I2

R

V2DC

Fig. 2. Typical system configuration of IPT system for EVs.

• 237 kHz – 270 kHz• 316 kHz – 360 kHz• 395 kHz – 450 kHz
Besides, in the IPT system, the allowable limits within 9 kHz
to 150 kHz is added. The radiation noise on the frequency band
except the band within 79 kHz to 90 kHz have to be lower than
23.1 dBμA/m. Similarly, the allowable limits on the frequency
within 526.5 kHz to 1606.5 kHz is −2.0 dBμA/m because thisfrequency band have been used for amplitude modulation (AM)broadcasting.
Note that, CISPR11 prescripts to measure the noise using aquasi-peak measuring method. From the regulations, thefrequency components on the radiation noise should be
suppressed not only the fundamental component but also theharmonics component.

power. In order to solve above problems, the compensation
circuits such as a series–series compensation (S/S), a series–
parallel compensation (S/P) are widely used in order to cancelout the leakage inductance [31].
The primary current and the secondary current are calculated
as (1) and (2) when an input voltage V1 is applied into theprimary side. Note that the voltage V1 is the fundamental
component of the output voltage of the inverter.
1 

r2 + Req + j ωL2 −
C2 
ω

I1 =
V1



1 
1 
r2 + Req + j ωL2 − + ω 2 L2m
r1 + j ωL1 −ωC1 
ωC2 




(1)

III. PROPOSED NOISE REDUCTION METHODS
A. Compensation Circuits
Figure 2 shows the typical circuit configuration of the IPT
system with a series–series compensation (S/S) [30]. In the IPT
system for EVs, primary coils are buried in roads or parking. In
contrast, secondary coils are beneath the bottom of the cars. For
this reason, magnetic coupling between the primary coil and thesecondary coil is weak. The large leakage inductance attributed
to the weak magnetic coupling causes an increase of reactiveI2 =jω L m
V1





1  
1 
2 2
r2 + Req + j  ωL2 −  + ω Lm
r1 + j  ωL1 −ωC1 
ωC 2 




(2)

where Req is the equivalent load considering the rectifier, r1 isthe equivalent series resistance of the primary winding, r2 is theequivalent series resistance of the secondary winding, L1 is theprimary inductance, L2 is the secondary inductance, C1 is theprimary compensation capacitor, C2 is the secondary

compensation capacitor, Lm is the mutual inductance and ω is theangular frequency of the power supply. The equivalent load isexpressed by (3) using the analysis given in [30].
(3)

The compensation capacitors are generally selected to cancelout the reactive power at the input frequency. Thus, thecompensation capacitors can be calculated by (4) and (5).
C1 =

(4)

1
L2ω 2

(5)

(

650 Hz
90.1

80.0

0.05
0
78

80

82 84 86 88
Frequency[kHz]r2 + Req)r1 r2 + Req + ω 2 L2m

V1

(6).

Due to the compensation circuit, the input impedance fromthe view point of the output of the power supply is relatively
low. Thus the input current contains large fundamental
component. Note that, the input current contains the low-orderharmonic components depending on the coupling between the
primary and the secondary coils.
The radiation noise is mainly caused by the current, whichflows in the primary coil and the secondary coil. For otherconverters, the shielding with magnetic material and metal are
effective to suppress radiated noise. However, in the IPT system,
the aperture for magnetic path have to be ensured. Thus, theshielding provides the limited effect on the suppression ofradiation noise.
B. Proposed Noise Reduction Method
In this paper, the radiation noise is spread in a frequency
domain by changing the output frequency of the voltage-source
inverter in two manners. The output frequency is selected at
random within 80 kHz to 90 kHz. In the SSUD, the output
frequency is selected from a discrete uniform probability
distribution. By selecting the output frequency of the voltage
source inverter from the uniform distribution, the harmonics
components of the voltage is evenly spread. On the other hand,
SSBD selects the output frequency of the voltage source inverter
from a biased probability distribution. The probability
distribution is biased to be proportional to a combined
impedance of the transmission coil and the compensation
capacitor. Due to the biased probability distribution, theharmonic components of the current, which is output from theinverter, is spread. The spread spectrum increases the reactive

9092(a) Proposed method I: spread spectrum with uniform distribution (SSUD)

0.2

The primary current with the compensation is expressed by

I1 =

0.1

0.15
650 Hz

0.1
80.0

0.05
0
78

80

82 84 86 88
Frequency[kHz]

40
35
30
90.1
25
20
15
10
5
0
90 92

Combined impedance of
coil and capacitor Z [Ω]

C2 =

1
L1ω 2

0.15

a

where V2DC is the secondary DC voltage and P2 is the output
power.

Probability distribution

8 V22DC
π 2 P2

Probability distribution

Req =

0.2

(b) Proposed method II: spread spectrum with biased distribution (SSBD)
Fig. 3. Probability distributions for spread spectrum.

current in comparison with the operation under the resonancecondition. However, the decrease of noise by the spread
spectrum is larger than the increase of noise caused by thereactive current.
Figure 3 shows the probability distribution of the output
frequency of the inverter. Fig. 3 (a) is the probability distributionof SSUD. The probability distribution is discrete uniform
distribution within 80 kHz to 90 kHz. It means that the each
output frequency is evenly selected. Note that, the parameter isdiscrete because the carrier for the inverter is generated in the
FPGA. The output frequency is renewed at every periods. By
selecting the output frequency of the voltage source inverter
from the uniform distribution, the harmonics components of thevoltage is evenly spread. Fig. 3 (b) is the probability distributionof SSBD. In the IPT system, the input impedance of the IPT
system depends on the frequency. Thus the probability
distribution is proportional to the combined impedance of thecoil and the compensation capacitor. By selecting the output
frequency of the voltage source inverter from the biased
distribution, the harmonics components of the current is evenly
spread.
Table I shows the assignment of the pseudo random numbers
for the output frequency. Table I (a) is for the proposed method
focusing on the inverter output voltage. Table I (b) is for theTABLE I.

ASSIGNMENT OF OUTPUT FREQUENCY

Rz-p Rz-p+1 ···· Rz-q ···· Rz-1

LSB
Rz (1bit)

Sz-p Sz-p+1 ···· Sz-q ···· Sz-1

Sz(a) For proposed method I: SSUD
Psuedo random
numberFrequency [kHz]
⁞
8

⁞

80.00

0001000⁞
72⁞
1001000

85.47

0001001

80.00
80.65

9
⁞
16

73

1001001

86.21

⁞

80.65

0010000

80.65

⁞
80

⁞
1010000

86.21

17001000181.30

811010001

86.96

⁞
24

⁞
0011000

81.30
81.30

⁞
88

⁞
1011000

86.96

250011001

81.97

891011001

87.72

⁞
32

⁞
0100000

81.97⁞
1100000

87.72

81.97

⁞
96

87.72

33

010000182.64

971100001

88.50

⁞
40

⁞
0101000

82.64
82.64

⁞
104

⁞
110100088.50

410101001

83.33

105

1101001

89.29

⁞
48

⁞
0110000

83.33
83.33

⁞
112

⁞
111000089.29

490110001

84.03

113

1110001

90.09

⁞
56

⁞
0111000

84.03⁞
1111000

90.09

84.03

⁞
120

57

0111001

84.75

⁞

⁞

84.75

64

1000000

84.75

85.47

86.21

88.50

89.29

90.09

Psuedo random
numberFrequency [kHz]010101085.47

10000001

80.00

42

⁞
11

⁞

80.00
80.00
80.65

⁞
47

⁞
010111185.47

120001011000110048

0110000

86.21

⁞
20

⁞
0010100

80.65

⁞
55

⁞
011011186.21

21001010181.30

56

0111000

86.96

⁞
27

⁞
0011011

81.30
81.30

⁞
65

⁞
100000186.96

28

0011100

81.97

66

1000010

87.72

⁞
32

⁞
0100000

81.97
81.97

⁞
77

⁞
100110187.72

33

0100001

82.64

781001110

88.50

⁞
35

⁞
010001182.64⁞
1011011

88.50

82.64

⁞
91

88.50

36

0100100

83.33

92

1011100

89.29

80.65

Xz-p Xz-p+1 ···· Xz-q ···· Xz-1

Xz

85.47

86.21

86.96

87.72

37

010010184.03

38

0100110

84.75

⁞
108

⁞
1101100

89.29

89.29

⁞
41

⁞
0101001

84.75

109

1101101

90.09

84.75

⁞
127

⁞
1111111

90.09

90.09

MSB
(7bit)X Z = X Z − p ⊕ X Z −q86.96(b) For proposed method II: SSBD
Psuedo random
numberFrequency [kHz]

····

65

····

80.00

····

0000001····

1Psuedo random
numberFrequency [kHz]1000001
85.47

Fig. 4. Generation method of 7-bit pseudo random numbers based on a
maximal length sequence.

proposed method focusing on the inverter output current. The
output frequency is selected according to generated 7-bit pseudo
random numbers.
Figure 4 shows the generation method of pseudo random
numbers. The pseudo random numbers are generated using amaximal length sequence (M-sequence) [25] [32] in the DSP.
Note that, the different pseudo random number generation
methods can be used. However the generation method using an
M-sequence is chosen in this paper because a complex
generation method of a pseudo random number is not suitable
for an implementation of an algorithm for the DSP. An Msequence random number is generated by (7).
X Z = X Z − p ⊕ X Z −q(7)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. The pseudo random number is calculated by an exclusive
or of the XZ-p and XZ-q.
IV. EXPERIMENTAL RESULTS
A. Experimental Setup
Figure 5 and Table II show the configuration for the
prototype and the specifications, respectively. In thisexperiments, the 420-V DC voltage power supply is used. As
switching devices, silicon-carbide (SiC) MOSFETs and SiC
diodes are used. The SiC- MOSFETs are controlled by the
FPGA and the DSP.
The inductances of the primary coil and the secondary coil
are designed according to the following equation [12] assuming
the effect of the spread spectrum can be ignored where ω0 is thecenter frequency of the frequency range, which is used for the
spread spectrum.

L1 =

L2 =

Req V12DC

ω0 k0 V22DC
Req

ω0 k0

(8)

C1

(9)

+

Vin

i1

C2

k

v1 L1

R
Vo

L2

The compensation circuit can be calculated by (4) and (5)using the center frequency ω0. It means that the resonance circuit
is designed to resonate at 85.1 kHz.
Figure 6 shows the primary coil and the secondary coil forthe prototype. Solenoid-type coils [33] are used as thetransmission coils. Note that the transmission distance is 150
mm assuming the transmission from a road to a bottom of theEVs or PHEVs.
B. Operation Waveforms
Figure 7 shows the operation waveforms. In the all of theoperation methods, the output power is 3 kW. Fig. 7 (a) is the
waveforms with a conventional method. The output frequency
is fixed at 85.1 kHz. Fig. 7 (b) is the waveforms with SSUD. The
output frequency of the voltage source inverter is selectedaccording to Table I (a) at random. Fig. 7 (c) is the waveforms
with SSBD. The output frequency is selected according to Table
I (b). In Fig. 7 (b) and (c), the output frequency is changed within
80 kHz to 90 kHz in every periods according to pseudo random
numbers. When the proposed methods are applied, theamplitudes of the primary current i1 varies. However, theconstant output voltages are obtained in spite of the operation
methods.
C. Harmonics Components Analysis
Figure 8 shows the harmonics components of the primary
current i1. Fig. 8 (a) is the result with the conventional method.
Fig. 8 (b) and (c) are the results with the proposed methods;
SSUD and SSBD, respectively. The lower figures shows theresults focusing on the fundamental components. In this paper,
the harmonics components of the primary current are evaluatedinstead of the radiation noise because the radiation noise from aloop coil is proportional to the amplitude of current. When theconventional method is used, the fundamental component andlow-order harmonics components sharply appear. In contrast,
when the proposed method is used, the maximum value on thefundamental and low-order harmonics are suppressed. The
harmonics components around the fundamental frequency issuppressed by 42.6% and 72.1% by using the SSUD and SSBD
in comparison with the conventional method, which operates theinverter at fixed frequency. In same manner, the low-orderharmonic components are suppressed in comparison with theconventional system. Both the proposed methods are valid tosuppress the components. In the operation with SSUD, however,
harmonics components peak at 85.1 kHz. It is caused by thefrequency characteristic of the IPT system. In the IPT system theimpedance from the view point of the power supply takes
minimum value at the resonance angular frequency ω0. Thus, thecurrent harmonics has peak at the resonance angular frequency

Fig. 5. Experimental setup.

TABLE II.

Input DC voltage

SPECIFICATIONS OF PROTOTYPE.
Symbol

Value

Vin

420 V

Coupling coefficientk

0.2

Primary inductanceL1

392 uH

Secondary inductanceL2

401 uH

Primary capacitanceC1

8.96 nF

Secondary capacitanceC2

8.78 nF

Transmission distancel

150 mmMOSFETs
DiodesFerrite plates

SCH2080KEC(Rohm)SCS220AE(Rohm)PC40(TDK)

Secondarywinding

Ferrite plate
Primary
winding
gap: 150 mm

Fig. 6. Transmission coils with a rated power of 3 kW. The solenoid typeis chosen.

even if the output frequency of the voltage is spread in thefrequency domain.

Primary voltage v1 400 V/div
00Primary current i1 10 A/div0

Output voltage vo 400 V/div
10 μs/div

10 μs/div(a) Constant frequency (85.1 kHz)10 μs/div(b) Proposed method I: SSUD

(c) Proposed method II: SSBD

Fig. 7. Operation waveforms of IPT system.

[dB]
00 dB = 6.8 A

0 dB = 6.8 A

0 dB = 6.8 A

-40

-80
-120

100

350

600

[dB]
0

850

[kHz]

100

350

600

0 dB = 6.8 A

850

[kHz]

100

350

600

0 dB = 6.8 A

850

[kHz]

0 dB = 6.8 A

-40

-80
-120

80.0

82.5

85.0

87.5

[kHz]

(a) Constant frequency (85.1 kHz)

80.0

82.5

85.0

87.5

[kHz]

(b) Proposed method I: SSUD

80.0

82.5

85.0

87.5

[kHz]

(c) Proposed method II: SSBD

Fig. 8. Harmonics components on the prmary current.

D. Efficiency Evaluation
Figure 9 shows the DC-to-DC efficiency characteristics.
Note that the efficiency is defined as the ratio of the input DC
power to the output DC power. All of the curves show similar
characteristics. The maximum efficiency is 94.9% at an output

power of 3.0 kW when the inverter is operated at fixed
frequency. In contrast, the maximum efficiency is 94.1% at an
output power of 3.0 kW when the SSUD is used. The decreaseof efficiency is attributed to the increased reactive current due tothe difference between the operating frequency and theresonance frequency. The reactive current increases the copper

V. CONCLUSION
In this paper, two reduction methods on radiation noise of
the inductive power transfer system are proposed andexperimentally demonstrated. The radiation noise from thetransmission coils for an IPT system is spread in a frequency
domain by changing the output frequency of the voltage source
inverter at random. Therefore, additional components such as anoise shield and a filter circuit are not required in the proposed
methods. The first proposed method; spread spectrum withuniform distribution (SSUD) selects the output frequency of thevoltage source inverter within 80 kHz to 90 kHz from thediscrete uniform probability distribution. The second proposed;
spread spectrum with biased distribution (SSBD), selects theoutput frequency from the biased discrete probability
distribution. The probability distribution is proportional to the
combined impedance of the transmission coil and thecompensation capacitor considering the frequency characteristicof the IPT system. Owing to the bias, the frequency componentsof the output current, which flows in the transmission coils, iseven within 80 kHz to 90 kHz. From the experimental resultswith an output power of 3.0 kW, the primary current around afundamental component are suppressed by 42.6% and 72.1% byapplying the SSUD and SSBD, respectively. Therefore SSBD ismore effective to suppress the noise than SSUD and theconventional method unless it works in the light-load region. In
the light-load region, SSUD may increase the no-load loss,
which dominates the loss. Therefore, SSUD should be used in
the light-load region.

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95

Proposed method I(SSUD)

Proposed method II(SSBD)

90

85
1000

Conventional(Constant frequency)

1500

2000

2500

3000

Output Power [W]
Fig. 9. Efficiency characteristics.

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