Development of Heat-Resistant Triple-Insulated Winding Wire "TEX-F"
by Isamu Kobayashi *, Kunihiko Mori *, Naoyuki Chida *,Atsushi Higashiura *2, Tadashi Ishii *2 and Atsushi Taba *3

A triple-insulated winding wire with enhanced heat resistance, TEX-F, has beendeveloped enabling further downsizing and performance upgrading of transformers for switching mode power supplies. In terms of thermal class, TEX-F belongs to the class F of
155°C, while conventional TEX-E belongs to the class E of 120°C. The available conductor sizes
and insulation thickness are the same as for the conventional TEX-E, so that TEX-F winding
wires with conductor sizes ranging from 0.2 mm to 1.0 mm covered with a thin triple-insulation
layer of 100 µm (about 33 µm per layer) can be manufactured. As for safety standards, TEX-F
meets IEC60950 like TEX-E, having acquired certification from a number of countries.
Moreover, unlike TEX-E, TEX-F is provided with the superior property in high-frequency V-tcharacteristics. This gives the wire substantial long-term resistance against corona discharge thatis caused by excessive voltages on transformers, thus improving the reliability of transformers.

ABSTRACT1.

INTRODUCTION

Along with the downsizing of electric and electronic equipment in recent years, power supplies in such equipment
are required increasingly to be small in size. The mainstream for power supplies is switching mode type, inwhich switching devices are used to raise the frequency of
the input, so that the winding number of the transformer isreduced thus resulting in downsizing.
We had developed TEX-E, a triple-insulated winding
wire for switching mode power supplies, which has been
extensively accepted and used worldwide. Meanwhile, theheat generated in transformers has increased recently
associated with the growing need for downsizing, resulting
in users' requirements for a new triple-insulated winding
wire with higher thermal class.
Moreover, the electrical characteristics of winding wires
in the high-frequency band where these wires are actuallyused for switching mode transformers were not very wellinvestigated. Accordingly, we have explored the V-t characteristics of winding wires in the high-frequency bandusing a high-frequency rectangular pulse generator, thereby succeeding in establishing a resin composition that significantly improves the performance.
This report describes the development process of TEXF, a triple-insulated winding wire with enhanced heat
resistance and upgraded high-frequency V-t characteristics.

*

Development Sec., Technical Dept., Magnet Wire Div.
Polymer Engineering Center, Ecology and Energy Lab.
*3 Development Promotion Dept., Automotive Products Div.
*2

2.

IEC60950 AND TRIPLE-INSULATEDWINDING WIRESFirst, explanations will be given as to how switching modetransformers are constrained in terms of safety standards
and how and where triple-insulated winding wires areused.
One of the most important international standards concerning switching mode transformers is IEC60950 of theInternational Electrotechnical Commission, a standard on
Safety of Information Technology Equipment.
Major requirements of the standard for the transformers
are as follows:
1) Solvent-based enamel coatings are not considered
to be insulation.
2) Supplementary insulation or reinforced insulation
specified IEC60950 having a minimum distancethrough insulation of 0.4 mm shall be provided
between the primary and secondary windings.
3) Reinforced insulation comprises three layers ofmaterial for which all combinations of two layers
together shall pass the electric strength test. (Theinsulation thickness is not specified)4) A specified length of creepage distance --the shortest path between two conductive parts, or between aconductive part and the bounding surface of theequipment, measured along the surface of the insulation-- must be secured between the primary and secondary windings.
To comply with this standard, switching mode power
supplies using conventional enameled wires have been
designed to provide, as shown in Figure 1, an interleaved
insulation tape with three layers in addition to barrier tapes
securing a specified creepage distance. As can be seen,

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Interleaved insulation tape

Table 1Comparison of manufacturing methods of TEX-F.

Manufacturing
Tape
Enamel
Evaluation term
methodwrapping
coating
and importance

BobbinPowdercoating

Extrusion

Safety standard
Withstand voltage
Enameled
wire

Barrier tapeFigure 1

Structure of conventional transformers.

Windability
Cost
Overall judgement

Enameled wire

: Excellent: Good: Fair: Failure

BobbinTriple-insulated
wire

Figure 2

Structure of transformers using triple-insulated winding wire.

this structure makes downsizing and cost reduction oftransformers difficult.
Triple-insulated winding wire has been devised, therefore, in which the wire itself is provided with the function ofreinforced insulation. Such a winding wire also is stipulated in IEC60950, so that only those that meet the standard
are qualified for certification of triple-insulation.
Use of triple-insulated winding wires can modify the
structure of transformers as shown in Figure 2, i.e., theinterleaved insulation tape between the primary and secondary windings is eliminated, and insertion of barrier tapeis no longer necessary because the conductor is entirely
covered with the reinforced insulation thus resulting in no
emergence of creepage distance. The modified structure
enables downsizing of the transformer in addition to
reduction of the process in wrapping of the interleaved
insulation tape and insertion of the barrier tape.
Based on such concept Furukawa Electric has developed the TEX-E triple-insulated winding wire, which issupplied nowadays to a number of countries around theworld.

3.

DEVELOPMENT OF TEX-F

Recently, some of the transformer products are required
to reduce their size further, resulting in such structuresthat are high in heat generation or difficult to dissipate theheat. Thus, the temperature in the equipment in somecases rises from 120°C, which can be dealt with the classE wire, to 130°C (class B) or 155°C (class F).
3.1 Development Targets
We have decided therefore to develop a heat-resistantwinding wire TEX-F with upgraded heat resistance, aiming
at achieving the following development targets while maintaining the advantages of TEX-E.
1) To acquire the certification of safety standards fromselected countries
2) To withstand a voltage requirement of 3 kV for 1

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min, as required by safety standards, with a nominal
insulation thickness of 100 µm
3) To have an adequate level of windability for coilwinding with a nominal insulation thickness of 100 µm
4) To have the thermal class F, i.e., 155°C
3.2 Manufacturing Method
Four wire manufacturing methods of tape wrapping,enamel coating, powder coating, and extrusion were studied in comparison at the development of TEX-E. The
same evaluation terms were used to select the manufacturing method of the TEX-F winding wire with enhanced
heat resistance, and the results are shown in Table 1.
First, enamel coating was regarded unsuitable from thestandpoint of safety because, as mentioned before,
IEC60950 does not consider enamel coating as insulation.
In addition, both enamel coating and powder coating were
judged to be technically difficult in achieving the withstandvoltage characteristics of 3 kV for 1 min with an insulation
thickness of 100 µm, securely along the wire length, as
required by one of the development targets.
Thus, we had two options of tape wrapping and extrusion, which were supposed to result in not much difference in terms of electrical characteristics. In terms of
windability, however, tape wrapping raises concern about
irregularities at the lapped tape in addition to lifts of tapewhen the wire is worked by bending. Moreover, in terms of
manufacturing costs, tape wrapping seems to be inadequate for high-speed manufacturing due to the limit of line
speed. In particular, shorter wrapping pitches are needed
for smaller sizes, raising the cost further.
Consequently, we have decided to investigate the manufacturing method based on extrusion, the same as
before, in developing the TEX-F triple-insulated winding
wire with enhanced heat resistance.
3.3 Insulation Resin
Both modified polyester and polyamide, which wereselected as the insulation resin of TEX-E, were unsuitable
for the current development in which the thermal class targeted at was over 130°C. Using commercially available
heat-resistant resins, therefore, we carried out extrusion
experiments to make a comparative study taking into consideration the following terms:• Wire performance (electric characteristics, heat resistance, flexibility, appearance, and others)• Extrudability (workability, ease of high-speed extru-

Furukawa Review, No. 21

2002

Table 2
Resin
Evaluation term

Comparison of insulation resins for TEX-F.

Heat-resistantHeat-resistantpolyester (A)polyester (B)

280°C

350°C

Sulfone (A)Sulfone (B)Sulfone (C)

370°C

400°C

370°C

Heat-resistantFluorocarbon

polyamide

polymer

340°C

300°C

Electrical characteristics
Heat resistanceFlexibility
Appearance
Workability
High-speed extrudability
Extrusion temperature
Resin cost
Overall judgement: Good: Fair: Failure105

sion, and others)• Resin cost

104

Service life (hour)

Table 2 shows comparisons of heat-resistant resinsstudied at this time including:• Heat-resistant polyester group• Sulfone group• Heat-resistant polyamide group• Fluorocarbon polymer group

TEX-F

103

TEX-E

102

Two types of heat-resistant polyester group were studied, but one of them resulted in poor heat resistance andthe other insufficient flexibility of the wire.
Next, three types of sulfone group were studied. Every
resin had a high value of viscosity, raising concern about
manufacturing of small sized wires. Besides, the resin
cost was rather high. However, practically no problem waspresented in terms of workability despite their extrusion
temperatures that were slightly high, and it was entirely
feasible to conduct high-speed extrusion with all the resins
excluding one type. Among these, Sulfone (C) was confirmed to be superior in appearance when it was extruded
to form wires.
Then a type of heat-resistant polyamide group wasstudied. It was found that the resin was high in viscosity,
raising problems of high-speed extrusion.
Lastly a fluorocarbon polymer group was studied.
Although it was proved that the resin caused no problem
with regard to wire performance including electrical characteristics, it emitted decomposed fluoric gases resulting
in inferior workability; it had poor appearance when
extruded at high speed; and it created cost problems
because the resin was considerably expensive.
As the result of the study, we concluded to selectSulfone (C) as the insulation resin for the TEX-F heatresistant triple-insulated winding wire.
Figure 3 shows the heat resistance --determined inaccordance with IEC60172-- of the TEX-F triple-insulated
winding wire manufactured using Sulfone (C) in comparison with that of TEX-E. It has thus been confirmed thatthe high-temperature service life --converted to 20000
hours of service-- of TEX-F is well over the class F of
155°C.

10
100

140

180

220

260

300

Temperature (°C)

Figure 3

4.

High-temperature service life of TEX-E and TEX-F.

IMPROVEMENT IN HIGH-FREQUENCYCHARACTERISTICS

Heretofore, the characteristics of TEX-E under conditionsof use in switching mode power supplies have not been
carefully studied, including in particular, its high-frequency
electrical characteristics. In view of this, we have measured the partial discharge inception voltage and the V-tcharacteristics of TEX-E as well as those of TEX-F using
a high-frequency rectangular pulse generator shown in
Photo 1. V-t characteristics refer to the relationshipbetween applied voltage (V) and duration of voltage application (t) until breakdown occurs.
Table 3 shows the measured results of partial discharge
inception voltages at high frequency using rectangularpulses for polyurethane wire (UEW), TEX-E, and TEX-F. It
can be seen that TEX-F shows a little lower result than
TEX-E regarding the partial discharge inception voltage athigh frequency, i.e., 10 kHz in this case. This may be
accounted for by a theory that the partial discharge inception voltage is proportional to the insulation distance andinversely proportional to the relative permittivity of theresin used, reflecting the fact that the relative permittivity,
i.e., 3.5 of Sulfone (C) is slightly larger than that, i.e., 3.3
of modified polyester/polyimide used for the insulation of
TEX-E.

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1000
Frequency: 100 kHz

Time (min)100

Photo 1

Inorganic filler
addedMeasuring equipment for high-frequency V-t characteristics.
Inorganic filler
not added

Table 30Comparison of partial discharge inception voltage of
UEW, TEX-E, and TEX-F.

3.5

4.0

Voltage (kV)
1-UEW

TEX-E

TEX-F

/

/

/

1-UEW

TEX-E

TEX-F

1.66

3.34

3.20

SamplePartial discharge inception voltage (kV),at 10 kHz, 8 µs pulse width

Table 4High-frequency V-t characteristics of TEX-E and TEXF.
SampleMeasuringconditions
High-frequency V-t (min),
20 kHz, 10 µs, 3.0 kV
High-frequency V-t (min),
20 kHz, 10 µs, 3.5 kV

TEX-E

TEX-F

/

/

TEX-E

TEX-F

100

430

5

63

Improvement of high-frequency V-t characteristics of
TEX-F by the addition of inorganic filler.

temperature, and the high heat resistance is effective in
improving the V-t characteristics.
• Phenomenon: The addition of inorganic filler improves
the high-frequency V-t characteristics.
Reason: Electrons attack the insulation when discharge
occurs, abrading the insulation. It is likely that the attack isrelieved by the inorganic filler.

5.

Next, Table 4 shows the measured results of V-t characteristics using rectangular pulses. It has been confirmed
that, according to Table 4, TEX-E suffers breakdown in a
relatively short time at high-voltage ranges where partial
discharge occurs, and that TEX-F developed at this timeoutperforms TEX-E in this regard.
It is known that addition of inorganic filler to the coating
improves the high-frequency V-t characteristics, so that
we investigated the effects of inorganic filler addition to
the insulation of TEX-F. We chose an inorganic filler thatis superior in mutual solubility with Sulfone (C) and tends
to cause little degradation of electrical characteristics aswell as flexibility.
Figure 4 shows the change in the high-frequency V-tcharacteristics due to the addition of the inorganic filler,from which the V-t characteristics are seen to have substantially improved by the addition of the filler.
The reasons for this remarkable phenomenon may be
explained as follows.
• Phenomenon: Sulfone (C) with the high heat resistance of insulation is superior in high-frequency V-tcharacteristics.
Reason: It is supposed that the current concentrates on
the conductor surface at high frequencies raising the wire

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Figure 4

CHARACTERISTICS

Table 5 compares the major wire characteristics for TEX-F
and TEX-E of conventional type. As shown, it is seen that
TEX-F is superior to TEX-E with regard to insulation
breakdown voltage and abrasion resistance. Moreover, it
is confirmed that TEX-F passes Annex U and Modified
Annex C of IEC60950 that specify safety standards evenunder the condition of class F. In terms of solderability,
however, the heat-resistant resin used in TEX-F does not
permit direct soldering, necessitating removal of the insulation layer. In addition, it is known as one of the shortcomings of the resin that Sulfone (C) suffers cracking
against some resins and solvents, and thus is unsuitablefor use with styrene resins in particular.
Table 6 shows the compatibility of TEX-F with commercially available varnishes. As shown in the Table, TEX-F isseen to be insufficiently compatible with impregnating varnishes based on styrene, while its compatibility with thevarnishes based on water-soluble solvents and gasolines
virtually creates no problems presenting no degradation incharacteristics. In any case, it seems that the compatibility
has to be confirmed when an impregnating varnish is to
be used with TEX-F.

Furukawa Review, No. 21

2002

Table 5Comparison of wire characteristics for TEX-F and
TEX-E.
Item

Table 7

List of safety codes that certify TEX-F.

Organization

CountryUL

U.S.A.

CSA

CanadaAcquired standards

TEX-E

TEX-F

Conductor diameter (mm)

0.400

0.400

Overall diameter (mm)

0.600

0.600

Insulation thickness (mm)

0.100

0.100

CSA C22.2 No.1 950

Breakdown voltage (kV)

23.4

28.6

IEC60950

Solderability at 420°C (sec)

3.0

Unsolderable

Spark test at 3 kV (count/30 m)

0, 0, 00, 0, 0Resistance to softening (°C)

231

260

HD 195 S6Unidirectional scrape test (N)

19.31

20.87

IEC60950

Reciprocating scrape test (times)

285

1000<

Static friction coefficient

0.125

0.250

Chemical resistance Xylene

3HH(pencil hardness)

3H

NG

Styrene monomer

UL1950IEC60950

VDE

Germany

IEC60065

EN60950
IEC60065
DIN VDE 0860

TUV

Germany

DIN EN 60950 VDE 0805
IEC60065
DIN EN 60065 VDE 0860
IEC60950

BSI

Annex U.2.1

U.K.

BS EN60950
IEC60065
BS EN60065

Annex U.2.2
Annex U.2.3, E-class

NEMKO

Annex U.2.3, B-class

NorwayIEC60950
IEC60065

Annex U.2.3, F-class
Annex U.2.4
Modified Annex C, E-class7.

Modified Annex C, B-class
Modified Annex C, F-class: GoodTable 6

: Fair: Failure

Compatibility of TEX-F with impregnating varnishes.
Residual breakdown voltage (%)

Main insulating
material
Original

Solvent

No

-

100

Polyester group

Water

105

Alkyd group

Gasoline

110

Alkyd group

Xylene80

Solventless

37

Unsaturated
polyester / Styrene

6.

130°C

155°C

180°C

heating × 7 days × 7 days × 7 days
107

101

104

95

92

99

106

112

78

90

90

90

30

24

13

SUMMARY

We have developed a triple-insulated winding wire TEX-F
with enhanced heat resistance (class F of 155°C), andhave succeeded in acquiring safety standard certificationfrom a number of countries. TEX-F has been successful inimproving the high-frequency V-t characteristics, so thatthe wire is likely to advance the safety of transformers.
Remaining issues that we plan to address in future are
acquisition of UL1446 for insulation systems including thebobbin material and insulation tape, in addition to development of a new wire with improved performance in solvent
resistance and solderability.

Manuscript received on November 2, 2001.

ACQUISITION OF SAFETY CERTIFICATION

TEX-F succeeded in acquiring certification of safety standards from a number of countries as shown in Table 7. It
should be noted that the certified rating temperature isclass A of 105°C for the UL, and is class F of 155°C forother organizations. The UL stipulates that the wire aswell as the bobbin material, insulation tape, and impregnating varnish have to be approved according to UL1446
independently as an insulation system constituted by thematerials, when they are used at an ambient temperature
exceeding class A of 105°C. Meanwhile, the certified
working voltage is 700 Vrms or 1000 Vpeak for VDE, and is1000 Vrms for other organizations.

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