20189178

Dynamic Contactless Power Transfer System using PS Topology
Considering Mutual Coupling of Transmitter Coils
Jun Yamada Yoshiki Shiozawa Yasuyoshi Kaneko 1)
1) Saitama University, Graduate School of Science and Engineering, Department of Science and Engineering
255 Shimo-Okubo, Sakura-ku, Saitama-shi, Saitama 338-8570, Japan (y.shiozawa.094@ms.saitama-u.ac.jp)

Presented at EVS 31 & EVTeC 2018, Kobe, Japan, October 1 - 3, 2018

ABSTRACT: The PS capacitor topology has been proposed as a method useful for dynamic contactless power transfer

systems. In the PS capacitor topology, the input current is suppressed during the no-receiver-coil condition. When multiple
transmitters are densely installed, mutual coupling between the coils must be considered. In this study, we analyzed thecircuit considering mutual coupling and evaluated its characteristics. In addition, we evaluated the characteristics when thecoil terminal not used for power supply was opened, and clarified the effect of mutual coupling.
KEY WORDS: dynamic contactless power transfer, electric vehicle, PS Capacitor topology, mutual coupling

1. INTRODUCTION
Electric vehicles are beginning to spread because ofenvironmental and energy concerns, but they have problems suchas short continuous cruising distance and long charging time. As amethod to solve these problems, a dynamic contactless powertransfer system with transmitter coils installed under the road asshown in Fig. 1 has been proposed(1). If the transmitter coils are
installed densely so as to continuously supply power, mutual

Fig. 1 Schematic of the dynamic contactless power transfer

coupling between transmitter coils must be considered.

system.

For a dynamic contactless power transfer system, the primaryparallel and secondary series resonance capacitors topology (PScapacitor topology(2)) is useful. With the PS capacitor topology,
the input impedance can be increased and the input current can besuppressed when there is a large misalignment or no receiver coil.
In power transfer to a traveling vehicle, a large misalignment
occurs only for a short time, so a PS capacitor topology that does

Fig. 2 PS capacitor topology (one-to-one).

not require a control mechanism is highly suitable.
Currently, in systems using PS capacitor topology, the power

Table 1 Capacitor values and characteristics of PS capacitor
topology (one-to-one).

transmission characteristics using multiple transmitter coils andTopology

PS Capacitor

clarified. In addition, although improvement in efficiency can be

Capacitor

11 L1 ,
 L2 1  k 2
C1
C 2

expected by connecting only the transmitter coil to be used, mutual

I/O

coupling exists between the transmitter coils, so the effect ofcharacteristics

improving the efficiency is unknown.

Input impedanceconsidering mutual coupling between the coils have not been

In this study, we evaluated the dynamic contactless power

VIN  aVOUT , I IN 

1
L
I OUT , a  1
a
M
2

L 
Z IN   1  RL
M 

transfer system considering mutual coupling between transmittercoils using PS capacitor topology. In addition, by focusing on the
Copyright © 2018 Society of Automotive Engineers of Japan, Inc. All rights reserved20189178

current of the transmitter coil, the characteristics were clarified

As shown in Equation (2), the primary capacitors Ca and Cb havewhen the terminal of the coil not used for power transfer is opened.

mutual inductance Mab between the primary coils. When Mab issufficiently smaller than L1, capacitors Ca and Cb have nearly the

2. PS CAPACITOR TOPOLOGY

same value as in the one-to-one contactless power transfer case,

2.1. Primary 1 : Secondary 1

and the power supply characteristics are nearly equal.

The circuit diagram of the PS capacitor topology is shown in
Fig. 2; the capacitor values and characteristics are shown in Table
1.

2.2.2. Primary [B] terminal opened
Primary [B] is little used for power transmission when primary

The primary parallel capacitor C1 is determined to achieve[A] and the secondary are completely facing each other. Therefore,

resonance with the primary self-inductance L1, and the secondary

it is better to open the primary [B] terminal as shown in Fig. 3(b)series capacitor C2 is determined so that the power factor is 1. The

and reduce the loss. The coil current Ib_open when the primary [B]expression of the input/output characteristics derived from theseterminal is opened is expressed byconditions shows that it is the same as the ideal transformer of theM a 2 M b 2  jM ab Z 2 
I OUT
(3)
M a 2 X b   M ab M b 2
Z2 is the impedance of the secondary, and Xb is the reactance ofvoltage ratio a’. Because of the mutual inductance M in the
denominator of the input impedance ZIN, if M decreases becauseof misalignment, then ZIN increases. Therefore, when the input

I b _ open 

primary [B]; they are respectively expressed by

1
 RL
jC 2
1
X b  Lb 
Cb

Z 2  jL2 

voltage VIN is constant, the input current can be suppressed.

2.2. Primary 2 : Secondary 1

(4)
(5)

We consider the case of using two primary transmitters asshown in Fig. 3. The primary [A] transmitter and secondary
receiver are in a face-to-face condition. Each coil has mutual
coupling. We evaluated the characteristics according to theconnection method of the primary [B] transmitter.

(a) Outline.

(a) Parallel connection.

(b) Primary [B] terminal(b) Dimension.

open.

Fig. 4 Transformer outline and dimensions.

Fig. 3 Connection method of primary [B].

Table 2 Transformer parameters.
Topology

PS Capacitor

2.2.1. Parallel connection of primary coils
Fig. 3(a) shows the parallel connection of primary [A] and [B].
When expressed by Equation (1), each capacitor can beapproximated by Equation (2).

L1  La  Lb
1
11
  L1  M ab , L2 1  k a22
Ca Cb
C2





(1)

N1(a) / N1(b) / N2 [Turns]

12 T / 12 T / 24 T

ra / rb / r2 [mΩ]42.57 / 45.00 / 96.63

La / Lb / L2 [μH]22.81 / 23.16 / 96.63

Mab / Ma2 / Mb2 [μH]−0.637 / 11.42 / 1.222

Ca / Cb / C2 [μF]0.1562 / 0.1487 / 0.0388

RL [Ω]

16.0(2)

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20189178

Because Ib_open has reactance Xb, Ib_open greatly increases if primary

which is approximately 3.7 times that of the standard condition.

[B] is in a resonance condition. If mutual coupling Mab and Mb2

The reason that the input current without a secondary receiver iswith primary [B] are all 0, then Ib_open also becomes 0; otherwise,

suppressed is that the impedance has increased. The samethe reactance Xb greatly affects the efficiency. To improvecharacteristics as the one-to-one transformer were obtained.

efficiency, primary [B] should be placed at a position not affectedby mutual coupling, or reactance Xb must be increased by changing

3.2.2. Misalignment characteristics

capacitor Cb of primary [B].

With the primary transmitters connected in parallel, themisalignment characteristics in the x (progressing) direction were

3. EXPERIMENTS

measured. The power transfer experiment was performed at 30mm intervals from x = −120 mm to 360 mm. The standard

3.1. Conditions
The outline and dimensions of the transformer are shown in Fig.

condition was that the secondary receiver was placed at x = 0 mm.

4. H-shaped solenoid transformers of the same shape as the

The input voltage VIN for a load power PL of 1.5 kW in the standard

primary and secondary transformers were used. An aluminum

condition was taken as the reference voltage. The efficiency andplate for magnetic shielding was installed on the back of eachthe output power are shown in Fig. 7(a). The input power of thetransformer. The magnetic gap was 60 mm, and we placed primaryprimary transmitters is shown in Fig. 7(b). The input power factor[A] and the secondary at completely opposite positions (standardof the primary transmitters is shown in Fig. 7(c).

condition). The distance between primary transmitters was 120

From Fig. 7(a), the efficiency becomes a maximum at the

mm. The transformer parameters are shown in Table 2, and thepositions (x = 0 mm, 240 mm) where the primary transmitters andexperimental circuit diagram is shown in Fig. 5. In order tosecondary are completely opposite; it is 93.7% at x = 0 mm andevaluate at the fundamental frequency f0 = 85 kHz, an LC filter

93.5% at x = 240 mm. The efficiency becomes a minimum at thewas used.
Table 3 Experimental results of parallel connection
Without
Secondary coil
With
248.6
VIN [V]
248.6

Fig. 5 Experimental circuit.
3.2. Parallel connection
3.2.1. With and without secondary
The primary transmitters were connected in parallel, and the
input voltage and current were measured under the condition with

IIN [A]

7.20

1.97

pfIN

0.95

0.13

IIN(a) / IIN(b)[A]

5.97 / 1.71

1.21 / 0.90

pfIN(a) / pfIN(b)0.97 / 0.66

0.11 / 0.21

Ia / Ib[A]22.6 / 20.9

VD [V]

158.3

17.8 / 16.9-

ID [A]

11.1-

PIN [W]

1703

65.5

PIN(a) [W]

1438

33.2

PIN(b) [W]

266

32.3

PD [W]

1597-

and without the secondary receiver. The input voltage VIN for a

PL [W]

1500-

load power PL of 1.5 kW in the standard condition was taken as

ηTR

93.75

-

the reference voltage. The power transfer results are shown in

ZIN [Ω]

34.5

126.0
400

40

standard condition is 0.95, which is close to 1. Because Mab is

200

20

200

20

0

0

0

0

-20

-200

-40
30

-400
0

-20
VIN
IIN
-40
30

smaller than La and Lb, the effect on power factor is small. As
shown in Fig. 6(b), the input current without a secondary receiver-200

is suppressed compared with the standard condition. The-4000

impedance in the standard condition was 34.5 Ω, which is small.
Further, the impedance without a secondary receiver is 126.0 Ω,

VIN
IIN
10

20

IIN [A]40

VIN [V]400

VIN [V]

shown in Fig. 6. From Fig. 6(a), the input power factor in the

10

20
Time [s]Time [s](a) With secondary coil.

(b) Without secondary coil.

Fig. 6 Input waveform.

Copyright © 2018 Society of Automotive Engineers of Japan, Inc. All rights reserved

IIN [A]

Table 3, and the input voltage and input current waveform are20189178

position (x = 120 mm) where the secondary is placed at the middle

3.3. Primary [B] terminal openedof the primary transmitters; the efficiency is 92.6%, which is

We evaluated the characteristics when the primary [B] terminal

approximately 1% lower than the maximum efficiency. The

was opened, in comparison with the results of the parallel

average efficiency is 93.2% between primary [A] and primary [B]connection. The load power PL was 1.5 kW in the parallel(x = 0~240 mm), so it was found that power can be transferredconnection. The input voltage with the terminal opened was thewith nearly constant efficiency. The output power becomes asame input voltage as in the parallel connection. The experimental

minimum (1121 W) at x = 120 mm; the decline rate is 25.3%. The

results are shown Table 4.

average output power between primary [A] and primary [B] is1317W and the reduction rate is 12.2%.
From Fig. 7(b), the input power of the primary transmitters
becomes a maximum at x = 0 mm, 240 mm, and the input power

From the results, we found that when the primary [B] terminalis opened, the coil current Ib increases from 20.9 A to 28.5 A andthe efficiency decreases from 93.75% to 85.68%, because the coiland the capacitor of primary [B] resonate.

of each transmitter is nearly equal at x = 120 mm.
From Fig. 7(c), the input power factor of primary [A] is 0.97 at
x = 0 mm and that of primary [B] is 0.98 at x = 240 mm. They are

Table 4 Experimental results with primary [B] terminal opened
Open
Primary [B]Parallelalso close to 1. The input power factor of the primary transmitters

VIN [V]

248.6

248.6

becomes a maximum at x = 0 mm, 240 mm; and it is 0.95 at x = 0

IIN [A]

7.2

3.7

pfIN

0.95

0.97

IIN(a) / IIN(b)[A]

5.97 / 1.71

(3.7) / -pfIN(a) / pfIN(b)0.97 / 0.66

(0.97) / -Ia / Ib[A]22.6 / 20.9

VD [V]

158.3

21.6 / 28.5
110.1

ID [A]

11.1

7.7

PIN [W]

1703

895

mm and 0.94 at x = 240 mm. The minimum input power factor is0.86 at x = 120 mm. The average input power factor is 93.2%
between primary [A] and primary [B].

(a) Output power and efficiency.

PIN(a) [W]

1438

(895)

PIN(b) [W]266-

PD [W]

1597

767

PL [W]

1500

717

ηTR

93.75

85.68

ZIN [Ω]

34.5

66.7

3.4 Increased interval of the primary transmitters
The power transfer experiment was performed by changing theinterval of the primary transmitters and the capacitor Cb. Primary[B] was moved from x = 120 mm to 360 mm at an interval of 60
mm to feed 1.5 kW. In addition to the standard capacitors that were0.1487 μF, two kinds of capacitors of 0.15012 μF and 0.15439 μF(b) Input power.

were prepared. The results are shown in Fig. 8. Fig. 8(a) shows thecurrent Ib flowing through the coil of primary [B], and Fig. 8(b)shows the efficiency. The broken line in Fig. 8(a) is the value
estimated on the basis of the Ib value at 1.0-kW power transfer.
Because the broken line in Fig. 8(b) exceeded the upper limit of
the coil current at 35 A, the data for 1.0-kW power transfer are
posted as a reference. The relationship between capacitor Cb andthe resonance frequency is shown in Table 5.

(c) Power factor.
Fig. 7 Experimental results of misalignment.

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20189178

Table 5 Capacitor and resonance frequency.
Cb [μF]
Frequency [kHz]0.148785.8

0.1501285.4

0.15439

84.1

3.5 Connection method of the primary [B] terminal
Comparative experiments were performed to ascertain the effectof efficiency by connection method for primary [B]. For the
primary transmitters, a power transfer experiment was conductedby two kinds of connection methods, as shown in Fig. 9. The load

From the results, when the distance between the primarytransmitters is small, the current Ib flowing through primary [B]increases and the efficiency decreases. When the capacitor waschanged from Cb = 0.1487 μF to 0.15012 μF, Ib increased by nearlya factor of 2, and the efficiency greatly decreased. In addition,
when Cb was changed to Cb = 0.15439 μF, conversely Ib decreased
and efficiency improved. It is considered that the current Ib
decreased because of the increase in the reactance of primary [B],and the efficiency was affected. From the data in Table 5, when Cb
is 0.15012 μF, the resonance frequency is closest to the power

power PL was 1.5 kW. The results are shown in Table 6. For
reference, the results where the primary transmitters wereconnected in parallel are also included.
From the results, when the primary [B] terminal is opened,because current does not flow through the coil, the efficiency isimproved. An efficiency improvement of approximately 1% was
confirmed compared to the parallel condition. When the primary[B] terminals are shorted, the reactance of primary [B] increases
because the coil does not resonate and little current Ib flows,
according to Equation (5).

transfer frequency of 85 kHz. Because the reactance reaches aminimum because of the resonance with Lb, the current Ib greatly
increases according to Equations (5). As a result, the efficiency
declines. On the contrary, when Cb is 0.15439 μF, the reactance ofprimary [B] reaches a maximum and efficiency improves because
the resonance frequency is farthest from 85 kHz.

Cb = 0.14870 F
Cb = 0.15012 F
Cb = 0.15439 F

4030

Ib [A](a) Open.
(b) Short.
Fig. 9 Connection state of primary [B] terminal.

parallel connection(Ib = 20.9A)

20
10
0
100

200
300
misalignment(x) [mm]400(a) Current Ib.

94

TR [%]

92
90
88
86
100

parallel connection(TR = 93.7%)

Cb = 0.14870 F
Cb = 0.15012 F
Cb = 0.15439 F

200
300
misalignment(x) [mm]400(b) Efficiency η.
Fig. 8 Experimental results when increasing the interval of the
primary transmitters.

Table 6 Experimental results of the primary [B] terminal
connection method.
Open
Short
Parallel
Type
290.1
288.7
VIN [V]
248.6
IIN [A]

5.9

5.9

7.2

pfIN

0.99

0.99

0.95

IIN(a) / IIN(b) [A](5.9) / -(5.9) / -5.97 / 1.71

pfIN(a) / pfIN(b)(0.99) / 15.8

(0.99) / 1.0

0.97 / 0.66

25.2 / 0.12
158.4

22.6 / 20.9

VD [V]

25.3 / 0.10
158.5

ID [A]

11.1

11.1

11.1

Vb [V]
Ia / Ib [A](248.6)
158.3

PIN [W]

1684

1683

1703

PIN(a) [W](1684)(1683)

1438

PIN(b) [W]

-

-

266

PD [W]

1598

1598

1597

PL [W]

1501

1500

1500

ηTR

94.91

94.97

93.75

ZIN [Ω]

49.4

49.0

34.5

4. CONCLUSION
In this study, we examined the dynamic power transfer systemusing the PS capacitor topology with multiple transmitter coils,
and confirmed the mutual coupling effect. We found that if the

Copyright © 2018 Society of Automotive Engineers of Japan, Inc. All rights reserved

20189178

mutual coupling between the transmitter coils is small enoughcompared with the self-inductance, the characteristics in theparallel connection are nearly equal to those of the one-to-one
transformer.
In the misalignment characteristic, the average efficiency is
93.2%, and the fluctuation in efficiency is approximately 1%,
which is almost constant. However, the fluctuation of output
power is large; it becomes a minimum (1121 W, a decline of
25.3%) at the position (x = 120 mm) where the secondary wasplaced at the middle of the primary transmitters. The average
reduction rate of the output power between primary [A] andprimary [B] is as large as 12.2%, and measures to suppress
fluctuation are necessary for improving the output
When the terminals of primary transmitters that are rarely used
for power transmission are released, the reactance of the primarytransmitters is small; it was found that current flowed through thecoil even when there was a small mutual coupling, causing
efficiency reduction. When short-circuiting the coil terminals of
the released primary transmitters, the reactance increased and thecurrent flowing through the coil was suppressed. As a result, it was
confirmed that efficiency was improved.
When power is transferred, it is necessary to design parallel oropen connections of the primary transmitters that do not contributeto power transfer, considering the influence of mutual coupling
and reactance, such as widening the arrangement interval or
shorting the coil terminal.

REFERENCES(1) N. Oba, T. Yasuda, M. Sato: Proposed Dynamic Contactless
Transfer System, JSAE Annual Congress (Autumn), 29720145827 (2014)(2) J. Yamada, K. Tsuda, R. Kobayashi, Y. Kaneko: Circuit
Analysis and Characterization of Contactless Power Transfer
System with Variable Impedance, IEEJ Trans. IA, Vol. 137,
No. 11, pp. 815–826 (2017) (in Japanese)

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