The 2014 International Power Electronics Conference

Contactless Power Transfer System
Suitable for Low Voltage and Large Current
Charging for EDLCs
Takahiro Kudo, Takahiro Toi, Yasuyoshi Kaneko, Shigeru Abe
Department of Electrical and Electronic Systems
Saitama University
Saitama, JapanVDC IDC

Abstract— The efficiency of a contactless power transformer
is determined by the output load. We propose a high-efficiency
contactless power transfer system for a low-voltage and largecurrent load. The proposed system is composed of a contactless
power transfer system with series and series resonant capacitors
and a current-doubler rectifier circuit. This paper presents thecharacteristic of the proposed system that charged electric double
layer capacitors as a low-voltage and large-current load, as wellas the experimental results.

Regulated
DC Power
SupplyVIN IIN V1 I2

V2ID

CS

IP

VL IL

CP

RL

Fig. 1. Contactless power transfer system with SP topology.

Keywords— Contactless power transfer system, current-doubler
rectifier, EDLCs, efficiency

I.

INTRODUCTION

A contactless power transfer system has many advantages,
including the convenience of being cordless and its safety
during high-power charging [1, 2]. Because no wear, spark, or
contact failure occurs in a contactless power transfer system, it
has been applied to practical use in home electric appliances[3, 4]. Charging of electronic devices has become easy using
the contactless power transfer system. Therefore, the capacityof a power storage device can possibly be reduced byincreasing the number of charging times.
The energy capacity of an electric double layer capacitor(EDLC) is smaller than that of a secondary battery, but thepower density of EDLCs is higher than that of the secondary
battery [5]. In addition, the lifetime of EDLCs is longer thanthat of the secondary battery because no chemical change
occurs during charging and discharging.
Conventionally, the battery used in an automated guided
vehicle (AGV) is a secondary battery. However, the method ofusing a contactless power transfer system with EDLCs has
recently attracted attention because it can solve the issues ofshort battery lifetime and of the charging time [6]. If a battery
is charged quite often by a contactless power transfer system
during luggage loading and unloading, the EDLCs can beapplied in the AGV. To increase the charging efficiency atlarge current, EDLCs with low internal resistance must beused.
In this study, we consider a circuit for a contactless powertransfer system with EDLCs. We also investigate theeffectiveness of this system by experiments.

©2014 IEEJ

Fig. 2. Contactless power transfer system with SS topology.

Two issues must be addressed in the contactless powertransfer system with EDLCs. First, the efficiency of acontactless power transformer reduces during the EDLC
charging. A contactless power transfer system has amaximum-efficiency load resistance RLmax. Because the EDLC
charging for the AGV has a low voltage and large current, theequivalent load resistance RL during charging is significantly
smaller than RLmax. Therefore, the efficiency of a contactless
power transformer significantly decreases during the EDLC
charging. Second, in charging the EDLCs, a constant current
is desired. In a constant-voltage charging of EDLCs, thetheoretical charging efficiency is a very low at 50%. Therefore,
a constant-current charging is desirable for the output of acontactless power transfer system.
To solve these issues, development of a system is required,
which combines a current-doubler rectifier and a contactless
power transfer system with primary and secondary series
resonant capacitors (SS topology) [7, 8]. RLmax of the SS
topology is k2 (k is a coupling factor) times smaller than thatof the conventional topology of the primary series andsecondary parallel resonant capacitors (SP topology) [9].
Furthermore, the equivalent load resistance RL, which
includes that of the current-doubler rectifier circuit, is 1/16
times smaller than that of a full-wave rectifier circuit. By
adopting a current-doubler rectifier circuit and the SS

1109

The 2014 International Power Electronics Conference
topology, the contactless power transfer system can achieve ahigh-efficiency EDLC charging.
In addition, the SS topology possesses the characteristic ofan immittance converter, and constant-current charging is easy.
Therefore, the SS topology is suitable for constant-current
charging of the EDLCs.
In this paper, we clarify the efficiency issues of acontactless power transfer system for a low-voltage and largecurrent load. Then, we propose a new method, which uses thecurrent-doubler rectifier circuit and SS topology in lieu of the
full-wave rectifier circuit and the SP topology. Further, weshow that the immittance converter characteristic of the SS
topology is suitable for constant-current charging of the
EDLCs. We investigate the characteristic of the proposed
method by experiments and demonstrate its usefulness andnovelty.
Section II describes the comparison between the SP and SS
topologies in a contactless power transfer system. Section III
describes the contactless power transfer system for the EDLC
charging. Section IV presents the comparison between thetheoretical and experimental values.
II.

V 'IN

V '1
r '1
- jx'Sjx'1

C 'S
I 'IN = I '1

V '0

jx2

V = VL
r2 2

I '0

r '0

I2

IL

CP
- jxP

jx'0

RL

IP

Fig. 3. Detailed equivalent circuit for the SP topology.

V 'IN

V '1
r '1
- jx'S1

jx'1

C 'S1
I 'IN = I '1

V '0

jx2

V2 VL
- jxS2

r2

I '0

r '0

CS2
I2 = IL

jx'0
Fig. 4. Detailed equivalent circuit for the SS topology.

x′ x
1= x S′ = x1′ + 0 2 .
ω 0 C S′x 0′ + x 2
V'IN and I'IN can be expressed as

COMPARISON OF THE SP AND SS TOPOLOGIES

A. Series and Parallel Resonant Capacitor Topology
Figure 1 shows the schematic diagram of a contactless
power transfer system with an SP topology. A full-bridge
inverter is used as a high-frequency power supply. A fullwave rectifier is used as a rectifier circuit at the secondary side
to increase the efficiency. The cores are made of ferrite, andlitz wires are used for the windings. Figure 2 shows theschematic diagram of a contactless power transfer system withan SS topology. A current-doubler rectifier is used as arectifier circuit on the secondary side as a low-voltage andlarge-current load to increase the EDLC efficiency.
Figure 3 shows the detailed equivalent circuit of the SP
topology. It consists of a T-shaped equivalent circuit in whichprimary series capacitor CS, secondary parallel capacitor CP,
and load resistance RL are added. The primary values areconverted into secondary equivalent values using the turnratio a = N1/N2. Because winding resistances r'1 and r2 andferrite core loss r'0 are considerably lower than leakage
reactances x'1 and x2 and mutual reactance x'0 at the resonant
frequency, the ferrite core loss is ignored. Here, M is themutual inductance (x'0 = ω0M/a). The rectifier is also omitted,
and the secondary circuit for the analysis consists of CP and
RL.
To achieve resonance at the input frequency f0 (= ω0/2π)
with the self-inductance of the secondary winding L2, which isequivalent to the addition of mutual reactance x'0 and leakage
reactance x2, the secondary parallel capacitor CP is given by1(1)= ω 0 L2 = xP = x0′ + x2 .
ω0CP
The primary series capacitor CS (C'S denotes its secondary
equivalent) is determined by

RL

(2)

x0′ .
(3)
x0′ + x2
These equations suggest that the equivalent circuit of atransformer with these capacitors is the same as that of an
ideal transformer at the resonant frequency.
By ignoring the ferrite core loss (r0 = 0), the efficiency canbe approximated by.
RL I L2
RL
(4)
ηSP =
=
2
2
2
′ = IL b , b =VIN′ = bV2 = b VL , I IN

RL I L + r1′IIN + r2 I 2

RL +⎧⎪ ⎛ R ⎞ ⎫⎪+ r2 ⎨1 + ⎜⎜ L ⎟⎟ ⎬2b⎪⎩ ⎝ xP ⎠ ⎪⎭2

r1′

The maximum efficiency ηmaxSP is obtained when RL = RLmaxSP.
RLmax SP = xP

1 r1′
+ 1 , η max SP =b 2 r2

1
2r
1+ 2
xP

1 r1′
+1
b 2 r2(5)

The coupling factor k, primary winding Q1, and secondary
winding Q2 are represented byM
ω L1
ω L2 .
(6)
k=

L1L2

, Q1 =

r1, Q2 =

r2

Here, L1 is the self-inductance of the primary winding (L1 =a2(x'1+x'0)/ω0).
Then, these equations can be expressed using k and Q.
.
(7)
rQ
RLmax SP = 2 2
k

Q2
Q1

ηmax SP =1
2
Q
1+
1+ 1 k2
Q2
k Q1Q2

If k is smaller than 0.3 and Q1 ≅ Q2, then
1 Q2
>>1 .
k 2 Q1

1110(8)

The 2014 International Power Electronics Conference
Thus, these equations can be expressed using k and Q, i.e.,rQ
RLmax SP = 2 2
k

Q2
Q1

ηmax SP ≅1
2
1+
k Q1Q2.

(9)

To increase the efficiency, Q and k must be increased, and we
can clearly see that a coil with a high Q value has a small loss.
B. Series and Series Resonant Capacitor Topology
Fig. 5. Experimental circuit for the EDLC charging.

Figure 4 shows the typical detailed equivalent circuit for
the SS topology. It consists of a T-shaped equivalent circuit inwhich primary series capacitor CS1, secondary series capacitor
CS2, and RL are added.
To achieve resonance with the self-inductance of the
primary winding L1 and the secondary winding L2, CS1 and CS2
are given by1
1(10)′ = x0′ + x1′= xS1= xS2 = x0′ + x2
′
ω0CS1
ω0CS2
and V'IN and I'IN can be expressed as
1(11)
V ′ = − jx ′ I
I′ = − j V .
IN

0 L

IN

x0′

TABLE ICOMPARISON OF THE CHARACTERISTICS
System

RLmax

Output of rectifier circuit

RL

Voltage VL

Current IL

SS and current-doubler
kr2 Q
rectifier circuit

1
VD
2 2

2 2I 2

kr2 Q
8

r2Q
k

2V 2

1
ID
2

2r2Q
k

SP and full-wave
rectifier circuit

L

These equations suggest that the equivalent circuit of atransformer with capacitors has the same characteristics as theimmittance converter at the resonant frequency.
Ignoring the ferrite core loss (r0 = 0), the efficiency can beapproximated by
η SS =R L I L2=R L I + r '1 I '12 + r2 I 22
2
L

RL
2

⎛R ⎞ .
R L + r2 + r '1 ⎜⎜ L ⎟⎟⎝ x'0 ⎠

(12)
Fig. 6. Transformer.

The maximum efficiency ηmaxSS is obtained when RL = RLmaxSS.
RLmax SS = x'0r2=,η
r1′ max SS

1
2 r2
1+
x '0

r1′
r2

(13)

TABLE II
TRANSFORMER SPECIFICATION
Transformer

Then, these equations can be expressed using k and Q.
.
(14)
1
RLmax SS = kr2 Q1Q2 ,η max SS =1+

2
k Q1Q2

ηmaxSS in (14) is almost equal to ηmaxSP in (9). In addition, RL of
the SS topology is k2 (k < 1) times smaller than that of the SP
topology when Q1 and Q2 are approximately equal. For
example, in the system with k = 0.3, RLmax of the SS topology
is approximately 1/10 times smaller than that of the SP
topology.
III.

Unit: mm
Power
Frequency
Gap
Primary
SecondaryLitz wire

CONTACTLESS POWER TRANSFER SYSTEM FOR EDLCCHARGING

A. Summary of the Proposed System using EDLCs
Figure 5 shows a contactless power transfer system for
EDLC charging. The contactless power transformer with the
SS topology is connected to a full-bridge inverter power
supply of a commercial power supply input as well as to a
current-doubler rectifier circuit with two inductors and two
diodes. The average voltage of the EDLC is VL, and its
charging power is PL. Here, the EDLC is represented by theequivalent load resistance RL (= VL2/ PL). When the inverter is

Aluminum-plate shield

6.0 kW
9.8 kHz
50 mm
27 turns (2p) 2.98 kg
13 turns(4p) 3.05 kg
0.1 mm Φ × 800
250 mm × 250 mm × 1
mmdriven at constant voltage, the contactless transformer with the
SS topology exhibits immittance converter characteristics.
Therefore, the current in the current-doubler rectifier circuit isconstant, and the EDLC is charged by a constant current.

1111

The 2014 International Power Electronics Conference
B. Comparison with Conventional Contactless Power
Transfer System (SP topology + Full bridge)
Figure 1 shows the configuration of a conventional
contactless power transfer system for electric vehicles. The
contactless power transformer with the SP topology isconnected to the full-bridge inverter power supply with acommercial power supply input as well as to the full-wave
circuit.
The EDLC voltage VL used in the AGV is low, and thecharging power PL is large. Therefore, the equivalent load
resistance RL (= VL2/PL) is very small. To drive a contactless
power transformer at high efficiency, decreasing the load
resistance RLmax at maximum transformer efficiency isnecessary. In the conventional system (Figure 1), the numberof turns in the secondary windings can be changed to adjust
RLmax. However, in the case of small RL of the EDLC,
adjusting RLmax is difficult. Table I lists the comparison of
RLmax of the proposed system (Figure 2) with that of theconventional system (Figure 1). Table I also shows RL at
maximum efficiency, which includes that of the rectifier
circuit.
First, we make a comparison of the capacitor topologies. If
Q1 and Q2 are represented as Q (Q1 = Q2), RLmax of the SS
topology is k2 (k < 1) times smaller than that of the SP
topology.
Second, we make a comparison of the rectifier circuits. To
drive a contactless power transformer at high efficiency, theload resistance, which includes that of the rectifier circuit,must be equal to RLmax. Therefore, we consider the load
resistance, including that of the rectifier circuit. Table I lists
the output voltage and output current of each rectifier circuit
using the fundamental-wave effective value of the input
voltage and input current. In the full-wave rectifier circuit, thepeak value of the input voltage is considered as an output
voltage. Therefore, the load resistance, which includes that of
the full-wave rectifier circuit, is 0.5 times smaller than theequivalent load resistance RL. When RL = 2RLmax, thetransformer efficiency is maximum.
On the other hand, in the current-doubler rectifier, theoutput current is twice that of the peak value of the input
current. Therefore, the load resistance, including that of thecurrent-doubler rectifier circuit, is eight times smaller than RL.
When RL = 1/8RLmax, the transformer efficiency is maximum.
In comparison with the combination of the capacitor
topology and the rectifier circuit, RL of the proposed system(SS topology + current-doubler rectifier circuit) is k2/16 times
smaller than that of the conventional system (SP topology +full-wave rectifier circuit). Therefore, driving a contactless
power transformer at high efficiency is possible when the
EDLC is charged.
IV.

f0 [kHz]r1 [mΩ]
r2 [mΩ]TABLE III
TRANSFORMER PARAMETERS
9.8
k
88.7
b
20.6
RLmaxSS [Ω]

0.32
0.32
1.12

l0 [μH]l1 [μH]

73.9
157.2

RLmaxSP [Ω]R'LmaxSS [Ω]

11.2
0.14

l2 [μH]
CS1 [μF]
CS2 [μF]

37.1
1.14
4.87

ηmax [%]
Q1
Q2

96.2
160
162

Fig. 7. Current-doubler rectifier with LC filter and EDLCs.
TABLE IV
CURRENT-DOUBLER RECTIFIER AND EDLC SPECIFICATIONSEDLCs*
Current-doubler rectifier circuit
LR1 [μH]

100

CL [F]

LR2 [μH]

100

DCR [mΩ]

175
16

Cf [μF]

108

Rleak [kΩ]**1.34

Lf [μH]

16

Rdivider [Ω]100*Low-resistance EDLCs made by NICHICON CORPORATION**Average value of one cell

windings. In addition, the leakage flux is shielded using an
aluminum plate attached to the back of the transformer. Table
III lists the transformer parameters. The maximum efficiency
ηmax and load resistance RLmax of the two topologies arecalculated from (9) and (14). We found that RLmaxSS is muchsmaller than RLmaxSP. The equivalent load resistance R'LmaxSS isvery small at 0.14 Ω by combining the current-doubler
rectifier circuit and the SS topology. Driving a contactless
power transformer at high efficiency is possible when the
EDLC is charged.

EXPERIMENTAL RESULT

A. Specification of the Contactless Power Transformer
Figure 6 shows the contactless power transformer for
EDLC charging. Table II lists the transformer specification.
The cores are made of ferrite, and litz wires are used for the

B. Specification of the Current-doubler Rectifier and EDLC
Figure 5 shows the experimental circuit for the EDLC
charging. Figure 7 shows the current-doubler rectifier circuitand the EDLC. Table IV lists their specifications.

1112

The 2014 International Power Electronics Conference

VDC

100

40

80

30
20

IL

50

100

Efficiency[%]

VDC[V] , IL[A]

VL

50

VL[V]

150

10

60

ηTR Experimental value
ηREC Experimental value
ηTR Calulated value
SS and Current doubler

40

ηTR Calulated value

20

SP and Full wave

0
0

10

20

30
40
Time[s]

0
60

50

0
0

Fig. 8. Charging voltage and current for EDLCs.

10

IL

VIN

IIN

50

2
1

100V

Power[kW]Voltage[V] , Current[A]

PL

100A

I2
0

25

30

35

40

40

VIN

3

100

30

Fig. 10. Efficiency of each topology.

150
VD

20
VL[V]

0

IIN

300V

VD

50A

I2

VL[V]

Fig. 9. Experimental results for transformer.

0

0.1
0.2
Time[ms]

0.3

Fig. 11. Waveforms of transformer (CS2 = 4.87μF).

The current-doubler rectifier circuit and the EDLC aredesigned by considering that the inductor current is one-half ofthe output current, and the diode current is equal to the output
current. In addition, to protect the EDLC from heating by thecurrent ripple, an LC filter is connected to the rectifier circuit
output.
The EDLC rated voltage is 2.5 V, and the rated capacity of
one EDLC is 1750 F. The EDLCs have a low resistance andare connected in a 20-series, 2-parallel configuration, for a
total of 40 EDLCs. Therefore, the capacity of the EDLC CL is
175 F, and the internal resistance (direct-current resistance) is
16 mΩ. To prevent voltage unbalance due to variations in theleakage resistance and capacitance, a dividing resistor (Rdivider= 100 Ω) is connected parallel to each cell.
A full-bridge inverter is used as a high-frequency power
supply at 9.8 kHz. The capacitance of the connected capacitor
is calculated from (9). The charging voltage VL of the EDLC is
24–42 V, and the charging current IL is 75 A.

VIN
100V
100A

IIN

300V

VD

50A

I2

0

0.1
0.2
Time[ms]

0.3

Fig. 12. Waveforms of the transformer (CS2 = 3.7μF).

C. Experimental Result for EDLC Charging
The experimental results are shown below. Figure 8 shows
the time course of the charging voltage and current of the
EDLCs. Figure 9 shows the various voltages and currents ofthe transformer and charging power PL. Figure 10 shows theefficiency of each part during the charging. Figure 11 shows
the waveforms of the transformer at VL = 42 V (CS = 4.87 μF)
Figures 8 and 9 show that charging current IL of the EDLC
is 75 A, and rapid charging is completed in approximately 35 s.
When the input voltage of the inverter VDC is constant, IL is1113

TABLE VFUNDAMENTAL POWER FACTOR OF THE TRANSFORMERINPUT AND OUTPUT
Transformer input
Transformer output
CS2 [μF]ηTR [%]
power factor
power factor
4.87
3.7

0.863
0.944

0.746
0.75

90.3
92.2

constant. This contactless power transfer system exhibits
immittance converter characteristic, which achieves constantcurrent output using constant-voltage driving.
Figure 10 shows the transformer efficiency and the rectifier
circuit efficiency during charging. The dashed lines are

The 2014 International Power Electronics Conference
obtained from (4) and (11). RL is the load resistance, includingthat of the rectifier circuit. Figure 10 shows that the proposedsystem has high efficiency for a low-voltage and large-current
load EDLC.
From the experimental results, the average transformer
efficiency is 91.7%, and the average rectifier circuit efficiency
is 88.9%. When the EDLCs are charged by the conventional
system (SP topology + full-wave rectifier circuit), significant
reduction in the transformer efficiency is apparent.

REFERENCES
[1]

[2]
[3]
[4]

D. Influence of Inductor in the Current-doubler Rectifier
The current-doubler rectifier circuit has two inductors.
Therefore, we investigate the effects of the inductor on theentire circuit. Because the inductor component is included in
the load of the proposed system, it becomes an inductive load.
Therefore, the phase of the current is advanced relative to thatof the voltage at the transformer input by approximately 28.2°(Figure 11). The advanced phase of the current causes failureof the inverter power supply. Changing the value of thesecondary series capacitor CS2 will prevent excessive advanceof the current phase. CS2 is then changed from 4.87 to 3.7 μF.
Figure 12 shows the experimental results of the transformer
waveform with VL = 42 V (CS2 = 3.7 μF). These waveforms
are subjected to a discrete Fourier transform processing, andthe extracted fundamental-wave power factor is 9.8 kHz.
Table V lists the results when CS2 is changed. Figure 12 showsthat the advanced phase of the current decreases to 14.1° from
28.2°. The fundamental wave power factor also improves to0.944 from 0.863. The average transformer efficiency
improves to 93.3%, and the average rectifier circuit efficiency
improves to 91.2% with the improvement in the fundamentalwave power factor.
From the above results, the effects of the inductor must be
considered when a current-doubler rectifier circuit is used as aload of the contactless power transfer system. The power
factor at the transformer input needs to be monitored.
However, this problem is solved by changing the value of thesecondary series capacitor CS2.
V.

[5]
[6]

[7]
[8]

[9]

CONCLUSION

When EDLCs are to be charged at low voltage and large
current, the current-doubler rectifier circuit and contactless
power transfer system with an SS topology are suitable. In the
proposed method, we have demonstrated that the contactless
power transfer system has high efficiency. The constantcurrent charging of EDLCs is easier than that of theconventional system (SP topology + full-wave rectifier circuit).
When a current-doubler rectifier circuit is used, a problem
arises in that the phase of the current is advanced comparedwith that of the voltage at the inverter output. However, by
adjusting the value of the secondary series capacitor CS2 of thecontactless power transfer system, the problem can be solved.
We have validated the performance of the proposed systemby rapid-charging experiments of EDLCs. A 175-F EDLC wascharged in 35 s at a constant current of 75 A by a contactless
power transformer with a gap length of 50 mm and a 6-kW
rating. As a result, the achieved average transformer efficiency
was as high as 93.3%.

1114

M. Chigira, Y. Nagatsuka, Y. Kaneko, S. Abe, T. Yasuda, and A.
Suzuki, “Small-size light-weight transformer with new core structure forcontactless electric vehicle power transfer system,” in Proc. IEEE ECCE
2011, pp. 260-266, 2011.
K. Kusaka and J. Itoh, “Input impedance matched AC-DC converter incontactless power transfer for EV charger,” ICEMS2012, LS2A-2, 2012.
G. A. J. Elliott, S. Raabe, G. A. Covic, and J. T. Boys, “Multi-phasepick-ups for large lateral tolerance contactless power transfer systems,”
IEEE Trans. Ind. Electron., vol. 57, no. 5, pp. 1590-1598, 2010.
A. Zaheer, M. Budhia, K. Kacprzak, and G. A. Covic, “Magnetic designof a 300W under-floor contactless power transfer system,” in Proc. 37th
Annu. Conf. IEEE Ind. Electron. Soc., Melbourne, Australia, Nov. 7–10,
pp. 1343-1348, 2011.
M. Okamura, “Battery system using electric double-layer capacitor,” J.
IEE Jpn., vol. 120, no. 10, pp. 610-613, 2000.
T. Tabata, T. Yamamoto, S. Hori, and N. Inagaki, “Trial production andefficiency evaluation of wireless charging system for AGV,” in Proc.
IEICE Technical Committee on Wireless Power Transfer, WPT2012-15,
2012.
H. Nakano, Y. Higuchi, and K. Hirachi, “Current doubler rectifier,”
IEEJ Trans. IA, vol. 116, no. 10, pp. 1081-1082, 1996.
T. Imura, T. Uchida, and Y. Hori, “Basic experimental study on helicalantennas of contactless power transfer for electric vehicles by using
magnetic resonant couplings,” in Proc. IEEE Vehicle Power and
Propulsion Conference, pp. 936-940, 2009.
T. Tohi, Y. Kaneko, and S. Abe, “Maximum efficiency of contactless
power transfer systems using k and Q,” in Proc. IEEJ Technical Meetingon Semiconductor Power Converter, SPC11-179, 2011.