Article

Comparative Verification of Radiation Noise
Reduction Effect Using Spread Spectrum for
Inductive Power Transfer System
Keisuke Kusaka, Kent Inoue and Jun-ichi Itoh *Department of Electrical, Electronics and Information Engineering, Nagaoka University of Technology,
Niigata 940-2188, Japan; kusaka@vos.nagaokaut.ac.jp (K.K.); k_inoue@stn.nagaokaut.ac.jp (K.I.)* Correspondence: itoh@vos.nagaokaut.ac.jp; Tel.: +81-258-47-9563
Received: 10 April 2019; Accepted: 6 June 2019; Published: 10 June 2019

Abstract: This paper provides a comparative study on radiation noise reduction methods for
inductive power transfer systems using spread spectrum. In the spread spectrum methods, theradiation noise is reduced by continuously changing an output frequency of the inverter according
to pseudorandom numbers. The effects of the radiation noise reduction are evaluated with theinductive power transfer (IPT) system with series-parallel compensation and series-series
compensation. The results show that the peak values of the radiation noise around the fundamental
frequency are reduced by 7.8 and 8.1 dBμA in maximum with the series-series compensation
method and the series-parallel compensation method, respectively in comparison with the constantfrequency operation. From these results, the proposed methods are effective for both the seriesseries compensation and series-parallel compensation method. Moreover, the efficiency of the IPT
system with spread spectrum method is evaluated. The maximum DC-to-DC efficiency with the
spread spectrum is 94.1% and 92.0% with the series-series compensation and the series-parallel
compensation, respectively.
Keywords: inductive power transfer; radiation noise; spread spectrum

1. Introduction
In recent years, inductive power transfer (IPT) systems for electric vehicles (EVs) have been
actively studied and developed [1–7]. In particular, the practical realization of the IPT systems for
EVs or plug-in hybrid EVs (PHEVs) is highly desired because the IPT systems will improve theusability of the EVs and the PHEVs. However, the IPT systems emit radiation noise. Therefore, inorder to employ the IPT systems into practical applications, the radiation noise must be suppressedto satisfy existing standards, such as the well-known standard based on CISPR11, i.e., a standard
regulated by the International special committee in radio interference. Otherwise, the IPT systems
might affect other wireless communication systems or electronic equipment.
Figure 1 shows the maximum allowable radiation noise for the IPT systems of 7 kW or less for
EVs in Japan. In Japan, the allowable radiation noise has been legislated. The regulations are currently
under discussion for international standardization. An 85-kHz frequency band will be used as thetransmission frequency in the international standard. This regulation is basically corresponding to
CISPR 11 Group 2, Class B [8].

World Electric Vehicle Journal 2019, 10, 40; doi:10.3390/wevj10020040

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World Electric Vehicle Journal 2019, 10, 40

2 of 12Fundamental

79 kHz – 90 kHz

2nd

158 kHz – 180 kHz

3rd

237 kHz – 270 kHz

4th

316 kHz – 360 kHz

5th

395 kHz – 450 kHz

Figure 1. Maximum allowable radiation noise for inductive power transfer (IPT) system of 7 kW orless for use in electric vehicles (EVs) in Japan (under discussion).

In previous studies, suppression methods using a magnetic material or metal shield have been
proposed [9–12]. In order to suppress the radiation noise, transmission coils are surrounded by plates
composed of magnetic material or metal plate. The radiation noise can be suppressed because theseshields convert the magnetic flux from the radiation noise into eddy currents. However, the eddy
current increases the power loss of the IPT systems. Moreover, the additional shields (magnetic or
metal) lead to an increase in cost. So far, the authors have proposed two radiation noise reduction
methods, in which the power is transmitted with changing switching frequency of the inverter
continuously; it is also known as spread spectrum [13,14]. The proposed methods do not needadditional components. Moreover, the effect on the efficiency by using the proposed method is small
at the rated power [14]. In [14], the radiation noise reduction effect of these two proposed methods
have been evaluated with the IPT system with a series-series (SS) compensation topology. The SS
compensation topology is one of the dominant options for the standardization of International
electrotechnical commission (IEC) because of their efficiency characteristic. However, thecompensation topology should be discussed from a diversified standpoint including the radiationnoise. Thus, this manuscript focuses on a comparison of the radiation noise between the SS
compensation and series-parallel (SP) compensation. In [15], the radiation noise reduction technique
is implemented for SP compensation. The performance of the radiation noise reduction between the
SS compensation and the SP compensation is compared in [15]. However, the effect on transmission
efficiency by employing the spread spectrum has not been evaluated.
In this paper, the proposed radiation noise reduction technique is employed for the SS
compensation and the SP compensation. Then, the radiation noise reduction performance and theeffect on the DC-to-DC efficiency are evaluated.
2. Radiation Noise Reduction Method
2.1. Generation Method of Pseudorandom Numbers
In the proposed method, the radiation noise is spread over the frequency domain from 80 kHz
to 90 kHz by changing the switching frequency of the inverter in two different ways. The switchingfrequency is changed according to pseudorandom numbers, which are generated by using amaximum length sequence (M-sequence) shown as Figure 2. The M-sequence random number isgenerated as:
𝑋 =𝑋

⊕𝑋

(1)

where Xz-p and Xz-q are the present values of Xz delayed by periods of p and q, respectively (p > q). In
this paper, p = 7 and q = 1 are used. Note that the number of bits of the pseudorandom number isseven.

World Electric Vehicle Journal 2019, 10, 40

3 of 12Figure 2. Generation method of pseudorandom numbers based on maximal length sequence.

2.2. Probability Distributions for Reduction Method
Figure 3 shows the probability distribution of the switching frequency of the inverter in two
proposed radiation noise reduction methods. Figure 3a shows the probability distribution of spread
spectrum with a uniform distribution (SSUD) [2]. The probability distribution is a discrete uniform
distribution from 80 kHz to 90 kHz. Therefore, the spectrum of the voltage applying to thetransmission coil is uniformly spread. Figure 3 (b) shows the probability distribution of spread
spectrum with a biased distribution (SSBD) [3]. The input impedance depends on the switchingfrequency. Thus, the probability distribution is proportional to the impedance of the resonancecircuit.
20

40

40

20

35
15

35

30

30

15

25
10

25

20

20

10

15

15
10

5

10

5

5

5

0

0

0
79

80

81

82

83

84

85

86

87

88

89

90

91

0
79

80

81

82

83

(a)

84

85

86

87

88

89

90

91

(b)

Figure 3. Probability distributions for proposed methods. Changing range of switching frequency of
inverter is from 80 kHz to 90 kHz: (a) is the proposed method I (SSUD), and (b) is the proposed
method II (SSBD).

3. Design of Transmission CoilFigure 4 shows the equivalent circuit for designing the IPT system. Figure 4a,b show the SS
compensation topology and the SP compensation topology, respectively.
3.1. SS Compensation Method
From the circuit equations of the equivalent circuit, which are shown in Figure 4a, the primary
current and the secondary current are calculated as:
𝐼 =

𝐼 =

𝑟 +𝑅
𝑟 + 𝑗 𝜔𝐿 −1
𝜔𝐶

+ 𝑗 𝜔𝐿 −
𝑟 +𝑅

1
𝜔𝐶

+ 𝑗 𝜔𝐿 −

1
𝜔𝐶

+𝜔 𝐿

+ 𝑗 𝜔𝐿 −

1
𝜔𝐶

+𝜔 𝐿

𝑗𝜔𝐿
1
𝑟 + 𝑗 𝜔𝐿 −
𝜔𝐶

𝑟 +𝑅

𝑉

𝑉

(2)

(3)

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4 of 12

where V1 is the primary voltage, Req is the equivalent load resistance, r1 is the equivalent series
resistance of the primary winding, r2 is the equivalent series resistance of the secondary winding, L1
is the primary inductance, L2 is the secondary inductance, C1 is the primary compensation capacitor,
C2 is the compensation capacitor on the secondary side, Lm is the mutual inductance, and ω is theangular frequency of the power supply. Note that the voltage V1 is the fundamental component of
the output voltage of the inverter.

(a)

(b)

Figure 4. Equivalent circuits for designing the IPT systems: (a) is the SS compensation method, and(b) is the SP compensation method.

The parameters of the transmission coil are designed with the equivalent circuit. The resistance
Req indicates that equivalent load resistance considering the full-bridge rectifier. Then the equivalentload resistance is given by [16]:
𝑅

=

8 𝑉 ,
𝜋 𝑃

(4)

where VDC,2 is the DC voltage on the secondary side and P2 is the output power.
The inductances of the primary and the secondary coils are designed according to the following
equations:
𝐿 =

𝑅 𝑉
𝜔𝑘 𝑉

𝐿 =

,

(5)

,

𝑅
𝜔𝑘

(6)

where VDC,1 is the DC voltage on the primary side and k is the coupling coefficient.
The compensation capacitors are selected in order to cancel out the reactive power at the input
frequency. Thus, the value of the compensation capacitors is calculated as:
𝐶 =

1
𝜔 𝐿

(7)

𝐶 =

1
𝜔 𝐿

(8)

3.2. SP Compensation Method
The circuit equations of the equivalent circuit, which is shown Figure 4b are calculated as:
𝑟 + 𝑗 𝜔𝐿 −
𝐼 =

1
𝜔𝐶

𝑅

−𝑗

1
𝜔𝐶

𝛥

𝐼 =

𝑗𝜔𝐿

𝑅

−𝑗
𝛥

𝐼 =

1𝐿
𝑉
𝛥𝐶

1
𝜔𝐶

𝑉

+

1
𝜔 𝐶

𝑉

(9)

(10)

(11)

World Electric Vehicle Journal 2019, 10, 40

5 of 12𝑟 + 𝑗𝜔 𝐿 −
𝛥=

1
𝜔 𝐶

−𝑗𝜔𝐿

−𝑗𝜔𝐿

𝑟 + 𝑗𝜔 𝐿 −

0

𝑗

0
1
𝜔 𝐶

1
𝜔𝐶

𝑗

1
𝜔𝐶

𝑅

−𝑗

(12)

1
𝜔𝐶

Note that the voltage V1 is the fundamental component of the output voltage of the inverter.
The parameters of the transmission coil are designed with the equivalent circuit. The resistance
Req indicates the equivalent load resistance considering the full-bridge rectifier. Then the equivalentload resistance is given by [16]:
𝑅

=

𝜋 𝑉 ,
8 𝑃

(13)

The inductances of the primary and the secondary coils are designed according to the following
equations:
𝐿 =𝐿

8 𝑉
𝜋 𝑘𝑉

𝐿 =

𝑅
𝑘
𝜔 √1 + 𝑘

(14)

,
,

(15)

The compensation capacitors are selected in order to cancel out the reactive power at the input
frequency. Thus, the values of the compensation capacitors are calculated as:
𝐶 =1
𝜔 𝐿 1−𝑘

(16)

1
𝜔 𝐿

(17).

𝐶 =

4. Experimental Results
4.1. Experimental Setup
Figure 5 and Table 1 show the circuit configurations for the prototype and specifications,
respectively. The self-inductance of the primary side and secondary side in Table 1 was measured
from the prototype of the transmission coils. Silicon carbide MOSFETs were used as the switching
devices in the inverter. Also, silicon carbide schottky barrier diodes were used in the rectifier.
Figure 6 shows the primary and secondary coils for the prototype. The common solenoid-typecoils were used for both the compensation topologies. In order to obtain the common output power,
the number of turns of the secondary coil was different between the SS compensation and SP
compensation topology. In this paper, it is assumed that the transmission coils were well aligned.
The effect on the spread spectrum performance by coil misalignment is reported in [17].

C1
Vin

+

i1

v1

C2

k
L1

C1
R

L2

Vin
vo

+

i1

v1

k
L1

R
L2

C2

vo

World Electric Vehicle Journal 2019, 10, 40

6 of 12(a)(b)

Figure 5. Circuit configuration of prototype: (a) is the series-series (SS) compensation method, and (b)is the series-parallel (SP) compensation method.
Table 1. Specifications of prototype.

Input DC voltage
Rated power
Coupling coefficient
Primary inductance

SymbolVin
Pk
L1

Secondary inductanceL2

Primary capacitance

C1

Secondary capacitance

C2

Transmission distance

l

Value
420 V
3.0 kW
0.20
392 H
401 μH (SS)24.2 μH (SP)8.96 nF
8.78 nF (SS)145 nF (SP)150 mmFigure 6. Transmission coils with rated power of 3 kW. Lower side is transmitter coil, upper side isreceiver coil. White winding on the secondary side is used as SS compensation method. Red winding
on secondary side is used as SP compensation method.

4.2. Operation Waveform
Figures 7 and 8 show the operation waveform of the IPT system with SS and SP compensation,
respectively. In all of the operation methods, the output power was fixed at 3.0 kW. Figure 7a shows
the waveforms obtained using the conventional method with the output frequency fixed at 85.1 kHz.
Figure 7b,c show the waveforms obtained using SSUD and SSBD, respectively. The output frequencyof the voltage source inverter was randomly selected. When the spread spectrum methods were used,
the amplitude of the primary current i1 varies. However, constant output voltages were obtained forall operation methods. In the waveforms with SP compensation, a similar characteristic was obtained
as shown in Figure 8. Besides, when the operating frequency was higher than the resonant frequency
due to the operation of spread spectrum, zero-voltage switching (ZVS) was achieved because thecurrent flowed in the direction to discharge the parasitic capacitance of the MOSFETs, which would
turn-on after the dead time, during the dead time. In contrast, ZVS was not achieved when thetransmission frequency was lower than the resonant frequency.

World Electric Vehicle Journal 2019, 10, 40

7 of 12

(a)

(b)

(c)
Figure 7. Operation waveforms with SS compensation: (a) is the constant frequency operation, (b) is
the spread spectrum with a uniform distribution (SSUD), and (c) is the spread spectrum with a biased
distribution (SSBD).

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8 of 12

(a)

(b)

(c)
Figure 8. Operation waveforms with SP compensation: (a) is the constant frequency operation, (b) is
the SSUD, and (c) is the SSBD.

4.3. Radiation Noise Measurement Conditions
Figure 9 shows the setup of the probe used for the measurement of the radiation noise. The
radiation noise was measured at two points. Each distance from the edge of the transmission coils
and to each measurement position was 500 mm.

World Electric Vehicle Journal 2019, 10, 40

9 of 12(a)(b)

Figure 9. Measurement environment of radiation noise: (a) is the measurement point A, (b) is themeasurement point B. The distance from the edge of the transmission coils to each measurement
position is 500 mm.

Figure 10 shows the radiation noise with the SS compensation method. Figure 10a shows theresult with a constant switching frequency operation, whereas Figure 10b,c show the results using
the spread spectrum with SSUD and SSBD, respectively. As a result, by using the SSUD and SSBD,
the radiation noise on the fundamental frequency was reduced by 4.8 dBμA and 7.8 dBμA in
comparison with the conventional method. In addition, the effect of reduction was confirmed at loworder harmonics components, similarly.
[dB A]
60

Reduced by 4.8 dB A

40

Reduced by 7.8 dB A

20
0

-20
100

225

350

(a)

475 [kHz]

100

225

(b)

350

475 [kHz]

100

225

350

475 [kHz](c)

Figure 10. Radiation noise at measurement point A in y-z plane with SS compensation: (a) is theconventional method with constant frequency, (b) is the proposed method I: SSUD, and (c) is the
proposed method II: SSBD.

Figure 11 shows the radiation noise with the SP compensation method. Figure 11a shows theresult with a constant switching frequency operation, whereas Figure 11b,c show the results obtainedusing the SSUD and SSBD, respectively. As a result, by using the SSUD and SSBD, the fundamental

World Electric Vehicle Journal 2019, 10, 40

10 of 12frequency was reduced by 4.8 dBμA and 8.1 dBμA in comparison with the conventional method. In
addition, the effect of reduction was confirmed at low-order harmonics components, similarly. As aresult, the proposed radiation noise reduction methods were effective to reduce the fundamental andlow-order harmonics components for both the SP and SS topology. However, the use of spread
spectrum had a by-product. The continuous frequency change caused the increase of the sideband.
The increase of the sideband could be adjusted to satisfy the standard by narrowing the frequency
band for spread spectrum and decreasing number of frequency changes considered. These
parameters should be adjusted with a 10m test range anechoic chamber. This adjustment should bedone for each standard on the radiation noise. Therefore, this paper does not mention the adjustmentof the effect of the sideband and only shows the comparison with and without the spread spectrum.

(a)

(b)

(c)

Figure 11. Radiation noise at measurement point A in y-z plane with SP compensation: (a) is theconventional method with constant frequency, (b) is the proposed method I: SSUD, and (c) is the
proposed method II: SSBD.

4.4. Efficiency Evaluation
The effect on the efficiency of the proposed spread spectrum was evaluated. Figure 12 shows the
DC-to-DC efficiency characteristics of the IPT systems with or without the spread spectrum. The
efficiency from the primary DC side to the secondary DC side was measured by the power analyzer(Yokogawa, WT1800). This means that the efficiency took into account the power loss of the inverter
and the rectifier. The output power was changed by changing load resistance. In the SS compensation
shown in Figure 12a, the maximum efficiency was 94.9%, 94.1%, and 93.8% without the spread
spectrum, with SSUD, and with SSBD, respectively around an output power of 3 kW. Using the
spread spectrum slightly reduced efficiency because the IPT system was operated at a slightly
different frequency from the resonant frequency. Similarly, the maximum efficiency was 92.5%,
92.0%, and 91.6% without the spread spectrum, with SSUD, and with SSBD when the SP
compensation was employed. Using the spread spectrum caused a difference between the operating
frequency and the resonant frequency. The reactive current caused by the difference increased incopper loss of the transmission coils and the secondary coils. Moreover, it also increased the
conduction loss of the inverter. Therefore the DC-to-DC efficiency dropped when the spreadWorld Electric Vehicle Journal 2019, 10, 40

11 of 12

spectrum was implemented. Additionally, the SSBD selected the transmission frequency far from theresonant frequency with high probability. Thus, the efficiency drop of SSBD was higher than SSUD.
The maximum efficiency was lower than the SS compensation because circulating current of the SP
compensation on the secondary coil was larger than the SS compensation. It caused a larger copper
loss on the secondary coil.
100

100

95

95

90

90

85

85

80
1000

1500

2000

2500

3000

80
1000

(a)

1500

2000

2500

3000

(b)

Figure 12. Efficiency characteristic with (a) SS compensation, and (b) SP compensation.

Note that the prototype was not optimized in terms of efficiency. Thus, the transmission
efficiency will be improved by optimization.
5. Conclusions
The radiation noise reduction technique for IPT systems using the spread spectrum has been
proposed by the authors. The radiation noise is reduced by continuously changing the output
frequency of the inverter. In this paper, the radiation noise reduction effects of two proposed methods(the SSUD and the SSBD) were experimentally demonstrated with the IPT system of 3 kW prototype
employing SS and SP compensation methods. In addition, the reduction effect is evaluated with the
SS and SP compensation methods. As a result, the harmonics components around the fundamental
frequency of the radiation noise with the SS compensation method were suppressed by 4.8 and 7.8
dBμA by applying the SSUD and the SSBD, respectively. On the other hand, in the SP compensation
method, the harmonics components around the fundamental frequency of the radiation noise were
suppressed by 4.8 and 8.1 dBμA by applying the SSUD and the SSBD, respectively. From the results,
a radiation noise reduction effect is expected using the two proposed methods with IPT systems
employing the SP compensation method as well as the SS compensation method.
Author Contributions: K.K. provided the concept of the proposed radiation noise reduction technique. J.I.
supervised the work and contributed to the design of the prototype. K.I. performed the implementation andinvestigation. Then he mainly wrote the paper with input from all authors.
Funding: This research received no external funding.
Conflicts of Interest: The authors declare no conflict of interest.

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