A 10kW Transformer with A Novel Cooling Structureof A Contactless Power Transfer System
for Electric Vehicles
Itaru Fujita*, Tomohiro Yamanaka*, Yasuyoshi Kaneko*, Shigeru Abe*, Tomio Yasuda***Saitama University, Saitama, Japan** Technova Inc., Tokyo, Japans12mm237@mail.saitama-u.ac.jp
Abstract— A contactless power transfer system for electricvehicles is required to have high efficiency, a large air gap,
good tolerance to misalignment in the lateral direction, and to
be compact and lightweight. A new 10kW transformer withdouble-sided winding for fast charging has been developed.
The proposed transformer has a novel cooling structure usingaluminum bars, aluminum junctions, and aluminum plates. In
this paper, the design concept, specifications, and test results of
the new transformer are described. The other feature of thistransformer is its interoperability with a 3kW transformer.
The interoperability test results are also shown.

I.

INTRODUCTION

Recently, due to environmental concerns, plug-in hybrid
electric vehicles (PHVs) and electric vehicles (EVs) have
gradually become more common. PHVs and EVs need to beconnected to power supplies by electrical cables to charge
their batteries. A contactless power transfer system would
have many advantages, including the convenience of being
cordless and safety during high-power charging [1].
A contactless power transfer system for EVs is required
to have high efficiency, a large air gap, good tolerance tomisalignment in the lateral direction, and to be compact andlightweight. We have revealed that a transformer withrectangular or H-shaped cores and double-sided winding has
many advantages applicable to the above specification incomparison with transformers with circular cores andsingle-sided windings [1, 2]. An H-shaped core transformer,
which is more compact and lightweight than a circular coretransformer, was developed [1, 2].
However, compact transformers have problems such as
the temperature rise caused by the charging loss incontinuous power transferring. A small-capacity transformer
with double-sided windings can dissipate the heat generated
at the winding by transferring heat to the aluminum plate in
the back of the transformer. A 3kW transformer for normal
charging with an H-shaped core has been developed, and it
can transfer 3kW power continuously without a temperatureproblem [2].

A 10kW transformer for fast charging with an H-shaped
core has also been developed in order to reduce the charging
time [3]. The test result showed that the temperature rise of
the ferrite core surrounded by the winding was a serious
issue, and so a more effective heat dissipation method must
be developed for 10kW transformers.
Inserting an aluminum plate between the core andwinding is effective for the heat dissipation of the ferrite
core. However, this method cannot be used for H-shapedcores because an eddy current might be generated in thealuminum plate.
Additionally, two transformers (a normal charging
transformer and a fast charging transformer) cannot be
installed on the same vehicle because of the increase in cost,weight, and space. Therefore, the fast-charging transformer
should be interoperable with the normal-charging
transformer.
In this paper, the design concept, specifications, and test
results of a new 10kW transformer with a novel coolingstructure for fast charging that is interoperable with a 3kWtransformer for normal charging are described. In order toimprove the heat dissipation of the ferrite core, aluminumbars are inserted in the core, and aluminum junctions areattached to connect the aluminum bars and aluminum platein the back of the transformer. The heat generated at the
ferrite cores is transferred to the aluminum plate throughthese aluminum bars and aluminum junctions. This newtransformer achieved a transfer of 10kW continuously for
150 minutes or more and maintained a high efficiency.
The new 10kW transformer was designed to have almost
the same dimensions and the same number of turns on the
primary and secondary windings as the 3kW transformer.
Consequently, both the 10kW and 3kW transformers have
similar parameters. Even when the primary or secondary
winding is replaced with that of the 3kW transformer, the10kW transformer can transfer and receive 3 kW with anefficiency of 91% or more.
Section II describes the contactless power transfer systemand the heat dissipation problem of the transformer with

Figure 1. Contactless power transfer system.

Figure 2. Detailed equivalent circuit.

double-sided winding. Sections III and IV show the designof the transformer with a novel cooling structure, thesimulation results, and the temperature rise test. Section V
presents the design with interoperability between normal
and fast charging and the corresponding test results.
CHARACTERISTICS OF CONTACTLESS POWER
TRANSFER SYSTEM FOR EVS

II.

A. Series and Parallel Resonant Capacitor Methods
Figure 1 shows a schematic diagram of the contactless
power transfer system with series and parallel resonantcapacitors (SP methods) [1-5]. A full-bridge inverter is used
as a high-frequency power supply. The cores are made offerrite, and the windings are Litz wires.
1) Equivalent Circuit
Figure 2 shows a detailed equivalent circuit. It consistsof a T-shaped circuit to which the primary series capacitor
CS, secondary parallel capacitor CP, and load resistance RL
have been added. The primary values are converted into
secondary equivalent values using the turn ratio a = N1/N2.
Since the winding resistances r'1 and r2 and the ferrite coreloss r'0 are considerably lower than the leakage reactance x'1
and x2 and the mutual reactance x'0 at the resonant
frequency, the winding resistances and the ferrite core loss
are ignored.
2) Characteristics of Series and Parallel Resonant
Capacitor Methods
The secondary parallel capacitor CP is determined toachieve resonance with the self-inductance of the secondary
winding L2 at the frequency f0 (=ω0/2π). CP is given by:1

ω 0C P= ω 0 L2 = x P = x 0′ + x 2(1)

The primary series capacitor CS (C'S denotes its secondary
equivalent) is given by:
x′ x

1
(2)
= x S′ = x1′ + 0 2
ω 0 C S′x 0′ + x 2
The input voltage V'IN and the input current I'IN can be
expressed by:x0′(3)′ = VIN / a = bV2 , I IN
′ = ID b , b =VINx0′ + x2

These equations suggest that the equivalent circuit of atransformer with these capacitors is the same as an idealtransformer with a voltage ratio of b at the resonant
frequency.
Ignoring the ferrite core loss (r0 = 0), the efficiency canbe approximated by:
RL I L2
RL
(4)
η=
=
2
RL I L2 + r1′I IN+ r2′I 22

RL +⎧⎪ ⎛ R ⎞ 2 ⎫⎪r1′
⎜ L⎟ ⎬
r+2 ⎨1 + ⎜x ⎟b2⎩⎪ ⎝ P ⎠ ⎪⎭The maximum efficiency ηmax is obtained when RL = RLmax.
1 r1′
1
(5)
R
=x
+ 1, η =L max

pb 2 r2

max

1+

1 r1′
+1
b 2 r22r2
xp3) Maximum Efficiency of Series and Parallel Resonant
Capacitor Methods using k and Q
The primary winding's quality factor Q1, and thesecondary winding's quality factor Q2 are represented by:
ωL
ωL
(6)
Q1 = 0 1 , Q2 = 0 2
r1
r2
Here, L1 and L2 are the self-inductances of the primary andsecondary winding, respectively, and ω0 is the resonant
angular frequency. If the coupling coefficient k is lower than
0.3 and Q1 ≅ Q2,
1 Q2>>1
k 2 Q1(7)

Then, equations (5) can be expressed using k and Q.
rQ
RLmax = 2 2
k

Q2
,
Q1

η max =

1

(8)

2
1+
k Q1Q2Equation (8) indicates that the maximum efficiency ηmax
depends simply on the winding's quality factor Q and thecoupling coefficient k if the core loss can be ignored [6].
B. Contactless Power Transformer
Contactless power transformers can have two different
types of structures: circular cores with single-sided winding[7, 8] and rectangular cores with double-sided windings [1-5,
9, 10]. Double-sided winding transformers have leakage
flux at the back of the core, and consequently, they have low
coupling coefficient k. However, this problem was solvedusing an aluminum plate attached to the back of thetransformer. Since the leakage flux is shielded by thealuminum plate, the coupling coefficient k can be large. For
the same coupling coefficient k, the core width (winding
width + pole width) of the double-sided winding needs to be
only half that of the single-sided winding. A transformer
with a double-sided winding can be made smaller than one
with a single-sided winding. Furthermore, the coupling
coefficient k of a single-sided winding transformer becomeszero when the lateral misalignment is approximately 40% of
the core diameter [8]. For the double-sided windingtransformer, if the secondary transformer is moved laterally
from the center of the primary transformer, the total flux
penetrating the secondary winding will decrease only
slightly, and the reduction in the coupling coefficient k will
also be small. Therefore, the double-sided windingtransformer has good tolerance to lateral misalignment.
However, the double-sided winding transformer haspoor heat dissipation from the ferrite core compared withthe single-sided winding transformer, because the singlesided winding transformer can be directly attached to a heatdissipation device on the back of the core, and it has a large
heat capacity due to its large size. On the other hand, thedouble-sided winding transformer’s core is covered with its
winding, which makes its heat dissipation disadvantageous.
C. Conventional 10kW Transformer for Fast Charging
In a previous study, a 10kW transformer that has
interoperability with a 3kW transformer (later shown in Fig.
5(b)) was developed. The transformer is shown in Fig. 3.
This transformer is small in size (280×300×45mm) and so
the heat dissipation area of the core is also small.
Additionally, the core is covered with the winding.
Moreover, the heat conductivity of ferrite (5W/m ･ K) is
much smaller than those of other metals like aluminum(237W/m ･ K) and copper (401W/m ･ K). Therefore, thetemperature rise due to iron loss and copper loss becomes aserious problem in the case of compact and large-capacitytransformers, which have a core made of ferrite.
Assuming actual charging, a contactless powertransformer is required for continuous operation. For
example, a 10kW transformer needs about two hours ofoperation to charge a 24kWh battery. An experiment ofcontinuous 10kW power transfer was performed in thecircuit shown in Fig. 1. In this test, the transformer
temperature was measured with thermocouples. The
operating frequency f0 was 30kHz, and it was constantduring the experiments. A full-bridge rectifier circuit and aload resistance RL were connected to a secondarytransformer. Sixty minutes later, the temperature of thesecondary core was about 100°C, and the coil was about
85°C (as shown by the dashed lines in Fig. 8). Thesetemperatures still show an upward trend. Considering atemperature limit of the Litz wire's insulating surface
coating, a solution for suppressing the temperature rise isnecessary.

(a) A photograph of a conventional 10kWtransformer
unit : mm(b) Measurements
Figure 3. Conventional 10kW transformer for fast charging.

III.

A NEW 10KW TRANSFORMER WITH A NOVELCOOLING STRUCTURE

A. Cooling Methods
The transformer's windings, ferrite cores, and aluminum
plates are heat generation sources. The windings andaluminum plates can be cooled down by cooling fans andfins. However, air cannot access the ferrite cores which iscovered with winding, so the ferrite cores covered withwinding cannot be cooled that way. Therefore, a heat sink,
which can directly transfer heat away from the ferrite cores
covered with winding, is considered. However, it isnecessary to pay attention to the flow of the magnetic flux.
The heat sink shoud be attached in such a way that it cantransfer heat effectively and does not couple with themagnetic flux.
B. A Novel Cooling Structure
Figure 4 shows a new 10kW transformer with a novel
cooling structure constructed of aluminum bars andjunctions. The ferrite core was constructed with ferrite
blocks (50mm L × 80mm W × 5mm T). Therefore, thecore can be divided into three columns. Two aluminum bars
were inserted into both of the ferrite core sides, and twoaluminum bars were inserted between the ferrite cores (as(a) New 10kW transformer(b) Core without the winding

Figure 4. Photograph of new 10kW transformer with cooling structure.

unit : mm(a) New 10kW transformer for fast charging(b) 3kW transformer for normal charging

Figure 5. Dimensions of cores of transformers.

shown in Fig. 5(a); Fig. 5(b) shows a 3kW transformer core
without a cooling structure). Aluminum junctions wereattached on the back side of the magnetic poles. The heatgenerated at the ferrite cores is transferred to the aluminum
plate in the back of the transformer through these aluminumbars and aluminum junctions. Aluminum bars with a widthof 10mm were used.
These aluminum bars were inserted in a direction
perpendicular to the magnetic pole. In a double-sided
winding transformer, the flux inside the winding proceeds
between the pole through the ferrite core. Thus, the losses in
the aluminum bars are very small.
C. Specifications of the 10kW Transformer with Cooling
Structure
Table I lists the parameters of the conventional 10kWtransformer and the new 10kW transformer at their normal
position (no misalignment and a mechanical gap length of70mm). The resistance and inductance values were
measured with LCR meters and were calculated. The
inductance values were not significantly influenced eventhough the aluminum cooling structures were attached.
To see the flow of the main flux in the cores, theconventional 10kW transformer and the new 10kWtransformer with the cooling structure were simulated (using
the FEM software package JMAG). The simulation results
are shown in Fig. 6, and Table II lists the results for thelosses, power, and efficiency. As shown in Fig. 6, there is

TABLE I. PARAMETERS AT NORMAL POSITIONTypeConventional
for fast
charging

New proposal
for fast
charging

for normal
charging

Cooling
Structure×○×Power[kW]

10

10

3

r1[mΩ]

107

121

106

r2[mΩ]

9.1

8.9

9.3

l0[μH]

58.4

60.5

55.4

l1[μH]

110

110

115

l2[μH]

9.8

9.9

9.7

CS[μF]

0.19

0.188

0.189

CP[μF]

1.86

1.83

1.91

k

0.35

0.35

0.33

b

0.35

0.35

0.34

RLmax [Ω]

8.9

9.5

8.69

ηmax [%]

98

98

98

TABLE II.

SIMULATION RESULTS
Conventionaltransformer

Primary
Copperloss[W]

Iron loss[W]

140
Temperature [℃]

120

32.8
50.4

-

8.0

36.8

36.6

Al plate

51.7

52.8

Al bar

-6.6

Primary

74.1

55.6

Secondary

93.6

77.0

POUT [W]

10084

10124

ηTR [%]

96.7

96.9

Secondary core
(Conventional transformer)

100

TABLE III.
Secondary core
(Proposed transformer)

80

EXPERIMENTAL RESULTS

POUT [kW]6010Normal position40

Gap length
η[%]

20
0
0

33.7
51.0

Coil

(a) New core

Figure 6. Flow of the main magnetic flux.

Coil
Al plate
Al barSecondary(a) Conventional core

Proposedtransformer

Tolerance tomisalignment

50

100Time [min]

95.1
100mm

93.7

x

60mm

94.2

y

150mm

94.0

150

Figure 7. Simulation results for the temperature of the ferrite cores.

little interlinkage flux with aluminum bars. In Table II, theprimary and secondary aluminum bar losses were 14.6 W,
which were less than 5% of the total loss. Therefore, theeddy current loss in the aluminum bars is very small
because the aluminum bars were inserted in the same
direction as the main flux.
Table III lists the experimental results of the new 10kWtransformer for the 10kW transfer at the normal position,with misalignments (±60mm in the forward direction x and±150mm in the lateral direction y), and with a +30mm
change in the gap length. The operating frequency f0 was
30kHz, and it was constant during the experiments. A fullbridge rectifier circuit and a load resistance RL wereconnected to a secondary transformer. The new transformer
efficiency at the normal position was 95.1%, even withmisalignments, and when a change in the gap length
occurred, the efficiency was 93.7% or more.
IV.

TEMPERATURE RISE TEST RESULTS

A. Simulation Results
Figure 7 shows a 10kW simulation result for heat
conduction, given that the heat generation of the ferrite core,
the coil, the aluminum plate, and the aluminum bars withthe loss values are uniform. The loss values in Table II wereused in this simulation. Three hours later, the ferrite-coreaverage temperature for the new transformer was more than
30°C lower than the conventional one. This result indicatesthat the aluminum cooling structure is able to improvecooling performance.
B. 10kW Temperature Rise Test
A 10kW temperature rise test at the normal position was
performed in the circuit shown in Fig. 1. In this test, thetransformer temperature was measured with thermocouples.
Figure 8 shows the results for the temperature rise of thesecondary ferrite core and the aluminum plate. The
conventional 10kW transformer without the coolingstructure is shown in Fig. 8 with dashed lines. The new
10kW transformer is shown with it in Fig. 8 with solid lines.
The conventional transformer temperature showed an
upward trend over about 100°C. On the other hand, the new
10kW transformer's secondary ferrite core reached atemperature of 80°C in 60 minutes. One hundred and fifty
minutes later, the temperature rose up to about 100°C. Then,the new transformer was at thermal equilibrium. This newtransformer was able to maintain a high efficiency of morethan 95%. Additionally, the temperature of the newtransformer’s aluminum plate was higher than thetransformer without the cooling structure because thealuminum bars and the aluminum junctions were able totransfer the heat generated at the ferrite core to the120
Secondary core (without Al bar)Temperature [℃]100

Secondary core80
Secondary Al sheet

60
40

Secondary Al sheet (without Al bar)

20

Room

0
0

Room (without Al bar)

50
100Time [min]150

Figure 8. Experimental results for temperature rise.

aluminum plate. Therefore, these results show that the novel
cooling structure provides heat dissipation for a transformer
with a double-sided winding.
In this study, aluminum bars are used as a heat sink.
However, it is possible to improve the cooling performanceby modifying the metallic material or changing the heat sink
to something such as a heat pipe.
V.

(a) Case A: normal to fast(b) Case B: fast to normal

Figure 9. Interoperability between normal and fast.

VDC IDC
Regulated DC
Power Supply＋－

VIN IIN V

I2 V2 ID

1

CS

VL IL
＋
－

CP

RL

2kW HF Inverter

P1

η

P2

Pout

(a) Circuit in Case A

INTEROPERABILITY BETWEEN 10KW FAST CHARGING
AND 3KW NORMAL CHARGING TRANSFORMERS

A. Transformer Design for Interoperability
A contactless power transformer is desirable to haveinteroperability between fast charging and normal charging,as shown in Fig. 9. When designing the 10kW transformer
for fast charging, the number of winding turns had to be thesame and the distance between the magnetic poles had to benearly the same to ensure interoperability with the 3kWtransformer for normal charging. When there is nearly thesame distance between poles in the 10kW and 3kWtransformers, the inductances and load resistance thatmaximize the efficiency (=RLmax) are the same [3]. The
characteristics of the 3kW, conventional 10kW, and the new
10kW (aluminum structure) transformers were compared interms of interoperability. Table I lists the transformer
parameters. Table I indicates that these transformers have
similar inductances and values for RLmax, even whenaluminum bars are inserted.
B. 3kW Interoperability Test Results
Interoperability tests were performed assuming thefollowing: For case A, a fast-type (10kW) transformer isinstalled on the EVs, and a normal-type (3kW) transformer
is laid on the ground, as shown in Fig. 9(a); for case B, anormal-type (3kW) transformer is installed on the EVs, anda fast-type (10kW) transformer is laid on the ground, as
shown in Fig. 9(b). The 3kW transformer and the new
10kW transformer were used. The normal position is taken(b) Circuit in Case B

Figure 10. Contactless power transfer system.

to have a mechanical gap length of 70mm with nomisalignment. The transformer characteristics were
measured for a gap length of 70±30mm, misalignments inthe forward direction x of ±60mm, and misalignments in thelateral direction y of ±150mm.
The operating frequency f0 was constant at 30kHz
throughout the tests. Figure 10(a) shows the schematic
diagram for case A; a full-bridge rectifier circuit and a load
resistance RL=15 Ω are connected to the secondarytransformer. Figure 10(b) shows a schematic diagram forcase B; a double-voltage rectifier circuit and load resistance
RL=80 Ω are connected to the secondary transformer. In thistest, the transfer power is limited to the lower rated power of
the two transformers. Therefore, the transfer power was
3kW.
Figures 11 and 12 show the parameters andcharacteristics for cases A and B, respectively. TheseFigure 11. Experimental results in case A.

Figure 12. Experimental results in case B.

experimental results show that both transformers haveapproximately the same characteristics.
Table IV lists the transformer parameters for different
gap lengths and positions of misalignment. In case A, thetransformer efficiency was 95.8% at the normal position;even when the mechanical gap was increased to 100mm, theefficiency was 93.1%. In the misalignment experiments, thetransformer efficiency was always more than 93.6%. On theother hand, in case B, the transformer efficiency was 94.4%
at the normal position; even when the mechanical gap wasincreased to 100mm, the efficiency was 91.9%. In themisalignment experiments, the transformer efficiency was
always more than 92.1%.
These results show that even if a transformer withdouble-sided windings has an aluminum cooling structure,having the same number of winding turns and nearly thesame distance between the magnetic poles ensuresinteroperability between the normal charging and fast
charging transformers.

TABLE IV.

EXPERIMENTAL RESULTS.

Case
Primary
SecondaryTransformer
Power (kW)Normal position

Average
Gap length(40-100mm)
100mm
Averageη[%]xTolerance to misalignment

Averagey

VI.

REFERENCES

[2]

[3]

[4]

[5]

(0-150mm)
150mm

[6]

CONCLUSION

A new 10kW transformer with a novel cooling structure
for fast charging has been developed. The cooling structure
is made of aluminum bars, aluminum junctions, andaluminum plate and can transfer the heat of the ferrite core
without decreasing the transformer efficiency. The test
results show that the transformer can transfer 10kW
continuously for 150 minutes with a core temperature of
100°C.
This 10kW transformer is also designed to haveinteroperability with the 3kW transformer. It is required thatthe 10kW transformer and the 3kW transformer have thesame number of turns and nearly the same distance between
the poles. The 3kW interoperability test results show thehigh efficiency and good tolerance to misalignment in thelateral direction between the primary windings of the 3kWtransformer and the secondary windings of the 10kWtransformer as well as the inverse combination.
[1](0-60mm)
60mm

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Case A
3kW-type
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3.0
95.8

Case B
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3kW-type
3.0
94.4

95.3

94.0

93.1

91.9

95.2

93.7

94.0

92.3

95.0

93.5

93.6

92.1

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