Transformer coil consisting of an insulating ribbon comprising electrically conducting patterns making it possible to produce paralleling of the patterns when this ribbon is accordion folded

Transformer coil consisting of an insulating ribbon comprising electrically conducting patterns making it possible to produce paralleling of the patterns when this ribbon is accordion folded. According to the invention, one side of the insulating ribbon (25) alternately comprises one face (34; 36) with pattern (26; 27) and one face (35; 37) without pattern, each pattern (26, 27) comprising two paralleling pads (28, 29; 30, 31) prolonging each of its extremities beyond a separation line (P1, P3) in order to overlap onto the face (35; 37) without pattern in such a way that the paralleling pads (28, 29; 30, 31) of each pattern (26, 27) come into electrical contact with the extremities of the neighbouring pattern (26, 27) when the ribbon (25) is accordion folded, in such a way as to produce paralleling of the patterns (26, 27). The invention applies especially to the production of high-frequency transformers used in switched-mode power supplies.

The field of the invention is that of transformer coils and more precisely 
that of coils consisting of strips of electrically-conducting material 
lying on an insulating ribbon. 
In a known way, miniature electrical coils, especially those employed in 
high-frequency transformers used in switched-mode power supplies, are 
generally fabricated from copper ribbons whose thickness is close to the 
depth of penetration of the electric currents in the conductors, that is 
to say to the skin thickness. These copper ribbons are arranged on a sheet 
of insulating material, accordion folding of which makes it possible to 
obtain a coil. Intimate imbrication of a sheet comprising ribbons 
constituting the primary of a transformer with a sheet comprising turns 
for the secondary makes it possible to confer good electrical efficiency 
on such a transformer. 
However, in the case where high currents are necessary in the secondary of 
the transformer, it is necessary to carry out paralleling of the turns of 
the secondary in order to limit the width of these turns and thus reduce 
the size of the transformer. 
In effect, taking the example of a transformer having 8 turns in series in 
the primary and 1 turn in the secondary, for an input voltage of 48 volts, 
the turn of the secondary will theoretically see 6 volts at its 
extremities. For a primary current drawn of 1 ampere, the current flowing 
in the secondary turn is thus 8 amperes. Hence it is not possible to 
configure the secondary turn like the primary turns. The secondary turn 
must be thicker and wider than each of the primary turns. 
In contrast, if the secondary consists of 8 turns in parallel, the current 
flowing in each turn is only 1 ampere, a much more reasonable value, 
whereas the secondary voltage is still 6 volts. 
Hence it is easy to understand the usefulness of a parallel connection of 
the secondary turns of a high-frequency transformer which has to meet 
minimum bulk and significant power conditions. 
Usually, paralleling of the turns of the secondary of a transformer of this 
type is produced by a remote connection, for example with the use of 
connecting wires such as shown in FIG. 1. This figure has been taken from 
ELECTRONIQUE DE PUISSANCE magazine No. 36, p. 46. 
FIG. 1 shows the principle of connecting primary and secondary turns of a 
transformer. The patterns 10 to 17 of the transformer consist of a 
continuous strip of copper. These patterns are in series and folding them 
makes it possible to obtain four turns in series. The patterns 10 and 17 
constitute the extremities and a primary voltage V1 can be applied to 
them. The secondary of the transformer consists of the individual turns 19 
to 22. Each of the turns of the transformer is traversed by a magnetic 
circuit whose axis is referenced 18. 
For the reasons set out above, the secondary turns of the transformer thus 
produced are connected in parallel with the use of pieces of wire 23, 24 
in order to reduce the current flowing in the turns 19 to 22. Thus is 
obtained a voltage V2 at the secondary when the primary and secondary 
turns are imbricated into one another. 
The main drawback of this type of transformer is that the paralleling of 
the turns of the secondary is produced by soldering and therefore limits 
the high-frequency performance characteristics. A wire connection 
furthermore gives rise to nonuniformity in the secondary current flows. 
Moreover, the parallel connection of the secondary turns is a delicate 
operation to carry out, given the size of these turns and the distance 
separating them once the insulating sheet is folded and fixed onto the 
former of the magnetic circuit. 
This nonuniform connection principle is also encountered in transformers 
consisting of open turns mounted in a housing constituting the magnetic 
core, the connections of the turns being produced with the aid of 
conducting tracks on an electronic card onto which is fixed the magnetic 
core. The part of the turns produced by the printed circuit is not in the 
same plane as the rest of the turn and its efficiency is thus affected by 
this. The fact of connecting one turn to the other, moreover, increases 
the length of the connections of the secondary of the transformer. 
The objective of the present invention is especially to alleviate these 
drawbacks. 
More precisely, one of the objectives of the invention is to furnish a 
transformer coil permitting simple paralleling of the turns of this coil, 
which reduces the connection lengths outside the useful areas of the 
turns, these connections possibly being produced homogeneously, that is to 
say without applying soldering or additional connections. 
Another objective of the invention is to simplify the fabrication process 
of such a winding and hence of a transformer using such a winding. 
A supplementary objective is to limit the insulation volumes of such a 
transformer, so as to reduce its bulk, while ensuring optimal imbrication 
of the primary and secondary turns. 
These objectives, as well as others which will appear below, are achieved 
by virtue of a transformer coil, of the type consisting of an insulating 
ribbon comprising, on one of its sides, patterns consisting of strips of 
electrically conducting material, the insulating ribbon being accordion 
folded, in order to constitute the coil, along equidistant separation 
lines delimiting faces of the insulating ribbon, each pattern lying 
between two separation lines constituting one turn of the coil, this coil 
being characterised in that this side of the ribbon alternately comprises 
one face with pattern and one face without pattern, each pattern 
comprising two paralleling pads prolonging each of its extremities beyond 
a separation line in order to overlap onto the face without pattern in 
such a way that the paralleling pads of each pattern come into electrical 
contact with the extremities of the neighbouring pattern when the 
insulating ribbon is accordion folded, in such a way as to produce 
paralleling of the patterns. 
This paralleling of the patterns is thus obtained by simple accordion 
folding of the ribbon and is accompanied by insulation between the turns, 
by virtue of the faces without patterns. 
The invention also relates to a transformer coil consisting of an 
insulating ribbon comprising alternately two faces carrying a group of two 
patterns in series and two faces without pattern comprising linking pads 
for the groups of patterns, the linking pads extending on either side of 
the separation line delimiting the two faces without pattern, each group 
of patterns comprising two paralleling pads prolonging each of its 
extremities beyond the separation line situated between the two patterns 
of the group, the paralleling pads of each pattern group coming into 
electrical contact with the extremities of the neighbouring pattern group 
by means of the linking pads when the insulating ribbon is accordion 
folded, in such a way as to produce paralleling of the groups of patterns. 
It is thus possible to produce paralleling of patterns in series. 
Advantageously, one of the coils as identified above constitutes one of the 
coils of a transformer, the other side of the insulating ribbon comprising 
patterns constituting the other coil of the transformer when the 
insulating ribbon is accordion folded. Thus is obtained optimal 
imbrication of the primary and of the secondary and minimised bulk. 
The invention also relates to a transformer produced from such a coil.

FIG. 1 has been described with reference to the state of the art. 
FIG. 2 shows an insulating ribbon comprising, on one of its sides, patterns 
of electrically conducting material, according to one embodiment of the 
invention. 
An insulating ribbon 25, only a part of which is shown, comprises, on one 
of its sides, patterns 26, 27 of electrically conducting material. These 
patterns are produced, for example, by a chemical etching method. The 
insulating ribbon 25 is, for example, made of Kapton and the patterns 26, 
27 of copper. The ribbon 25 is intended to be folded along equidistant 
separation lines P1, P2, P3. Each pattern 26, 27 corresponds to one turn 
of the winding produced by folding the ribbon 25, as will be detailed 
below. 
According to the invention, with the aim of producing paralleling of the 
patterns of the ribbon, paralleling pads 28, 29, 30, 31 each prolong 
patterns 26, 27 of the ribbon 25, this prolongation taking place up to 
beyond the separation lines P1 and P3, in such a way that the paralleling 
pads 28 and 29, prolonging the extremities of the pattern 26, come into 
contact, after the ribbon 25 has been folded, with the extremities 32 and 
33 of the pattern 27. This paralleling of the patterns will be better 
understood on reading the following description of FIG. 3. 
FIG. 3 shows the accordion folding of the ribbon of FIG. 2. 
Accordion folding of the insulating ribbon 25 is carried out along the 
folds P1 to P3, each folding taking place in the opposite direction to the 
preceding one. The face 34 comprises the turn 26, the face 35 the 
paralleling pads 28, 29 of the turn 26, the face 36 the turn 27 and the 
face 37 the paralleling pads 30, 31 of the turn 27. The faces 34, 36 
comprising patterns are alternated with the faces 35, 37 without pattern. 
During folding, carried out along a direction 38, face 35 comes into 
contact with face 36 and the paralleling pads 28 and 29 of turn 25 come 
into contact with the extremities 32 and 33 of turn 27. 
Paralleling of the turns is thus produced automatically, without it being 
necessary to add connecting wires after folding. Moreover, it is not 
necessary to insert an insulating ribbon between the faces during folding, 
this insulation being contributed by the absence of patterns on the faces 
situated between the faces comprising patterns. Thus is obtained a lower 
bulk for the coil than that shown by the coils of the state of the art. 
The paralleling pads 30 and 31 of turn 27 similarly come into contact with 
the extremities of a turn situated on a face with which face 37 comes into 
contact by folding. It is thus possible to produce paralleling of a large 
number of individual turns of a transformer coil. 
FIG. 4 is a side view of the insulating ribbon of FIGS. 2 and 3 entirely 
folded. 
Accordion folding ensures paralleling of turns 26 and 27. Access to the 
coil turns is easy, given that at the site of folds P1 and P3 the 
conducting tracks are visible. 
The turns of the ribbon can either be held in contact by pressure in a 
transformer, or soldered after folding in order to ensure optimum contact 
between the turns. 
According to a preferential embodiment, the insulating ribbon is made of 
Kapton and measures between 50 and 75 .mu.m thick and the copper has a 
thickness of about 75 .mu.m. 
Needless to say, the parallelled turns are not necessarily individual. 
Hence, for one configuration of the different turns, it is possible to 
parallel groups of several turns. FIG. 5 is an exemplary embodiment of a 
paralleling of groups of two turns in series. 
The insulating ribbon 25 comprises silk screen printed patterns consisting 
of two turns in series. Hence, faces 50, 51, 54 and 55 each comprise one 
turn, respectively referenced 56, 57, 58 and 59. Turns 56 and 57 are in 
series, as are the neighbouring turns 58 and 59, this series connecting of 
the turns being provided by conducting tracks. Faces 52 and 53 of the 
ribbon 25 comprise only linking pads 60, 61 extending on either side of 
the fold P6. 
Turns 56 and 57 form a group of turns whose extremities are prolonged by 
paralleling pads 62 and 63 extending beyond the fold P4 in opposite 
directions. 
During folding of the ribbon 25, the paralleling pad 62 comes into contact 
with the part of the linking pad 60 situated on face 52 and the part of 
the linking pad 60 situated on face 53 comes into contact with the 
extremity of turn 58. The same applies for the paralleling pad 65 of face 
54 which comes into contact with the extremity of turn 57 via linking pad 
61. 
The folding carried out, associated with a specific configuration of the 
turns, hence makes it possible to parallel groups of two turns in series. 
The paralleling pads 63 and 64 come into contact with linking pads 
situated respectively above and below the turns shown. 
Needless to say the number of turns in series in a group is not limited to 
two. Different configurations of the turns make it possible to parallel 
groups consisting of a large number of turns in series. 
In this embodiment, insulation between the turns is also automatically 
obtained by accordion folding the ribbon, since faces 52 and 53 do not 
comprise a pattern constituting a turn. 
The non-referenced orifices pierced in the centre of each face permit a 
magnetic circuit to be passed through. These orifices are also present, 
but not shown on the first embodiment (FIGS. 2 and 3). 
Needless to say, extending the paralleling pads over the fold can also 
serve for connecting turns in series. This embodiment also permits a 
reduction in the lengths of the conductors. The patterns, however, exhibit 
more complex shapes. It is then necessary to insulate some copper surfaces 
in order avoid short circuits, production of the coil being for this 
reason more complex. 
As described above, the ribbon 25 comprising the patterns constituting 
turns is intended to be imbricated with another ribbon. This other ribbon 
may, for example, comprise turns in series, and constitute the primary of 
a transformer, the secondary being produced by paralleling turns in 
accordance with the invention. 
This known embodiment, however, exhibits the drawback of exhibiting 
variable efficiency, according to whether the ribbons are more or less 
well imbricated. 
For this reason the primary and the secondary of the transformer are 
preferentially produced on the same insulating ribbon. One side of the 
insulating ribbon comprises the turns constituting the primary winding and 
the other side those constituting the secondary winding. During 
fabrication of the ribbon, it is then easy to arrange the turns in such a 
way that optimal primary-secondary imbrication is ensured. 
Moreover, the orifices for the passage of the magnetic circuit need be 
produced only once. 
FIGS. 6 and 7 show the two sides of such an insulating ribbon. 
On a first side of the ribbon 25, represented in FIG. 6, the patterns are 
put in series by prolonging the conducting strips from one pattern to the 
next. Each pattern consists of a turn which will be traversed by a 
cylindrical bar constituting the magnetic circuit. The ribbon 25 is 
intended to be folded along folds P10 to P15. 
The opposite side of the ribbon 25 comprises the patterns represented in 
FIG. 7. These patterns are intended to be connected in parallel by folding 
and consist of individual turns. 
It will be noted, with regard to FIG. 6, that the folds P10 to P15 produced 
permit the paralleling pads prolonging the extremities of the patterns of 
FIG. 7 to overlap. Moreover, as shown by the broken lines 70, the patterns 
of FIG. 7 are opposite a pattern for every other pattern of FIG. 6. 
When the folded insulating ribbon 25 is inserted into a magnetic circuit, 
the folds P10, P12, P13 and P15 are accessible from the outside of the 
transformer, especially in order to solder the turns if the ribbon is not 
sufficiently compressed, so as to ensure sufficient and permanent contact 
of the superimposed turns. 
The transformer coil according to the invention thus permits maximum 
reduction in the lengths of the conductors, which is essential when 
working frequencies are high, absence of soldered connections when the 
folded ribbon is sufficiently compressed, simplification in assembly of 
the transformer and limitation of the insulation volumes.