FR2802762A1 - LOW THICKNESS INDUCTIVE COMPONENT - Google Patents

Abstract

Inductive component, in particular transformer, of low thickness comprising a stack (1) of several layers of insulating material, with end layers (11, 12) carrying conductive tracks (18, 19) and at least one intermediate layer, comprising an insulating plate (13) located between the end layers. The intermediate layer (13) has a recess (14, 26) in which is placed at least one coil (16) of enamelled conductive wire prepared separately, the axis of the coil being perpendicular to the general plane of the layers, and the thickness of the coil, in the direction of this axis, being less than the distance between the end layers, the ends (17) of the coil wire being connected to connection elements (23) accessible outside the component .

Description

Low thickness inductive component.

  The present invention relates to a thin inductive component, of the type comprising at least one winding and which can serve as an inductor but more commonly as an AC transformer. The applications particularly targeted by this product are transformers for switching power supplies and energy converters, in particular in the case where the input voltage is high. More generally, the field of application can extend to any type of inductive component, simple inductance or transformer,

small footprint.

  To make such components, it is notably known to use a technique called multilayer printed circuit. In this technique, the windings are obtained by stacking a plurality of insulating support layers for a printed circuit, on each of which are made one or a few turns formed by copper tracks etched according to the conventional technique of printed circuits. Each layer has for example a thickness of the order of 0.25 mm for the layers situated inside the stack, and of 0.4 mm for the end layers, and it is thus possible, for example, to stack 8 layers in total. The ends of the turns of different layers are conventionally connected to each other by the known technique of metallized holes, consisting in producing a conductive metallic deposit on the wall of a hole passing through one or more layers, also passing through an area of at least two tracks. copper engraved on two different layers. The connection between these two tracks is thus

  produced by the said metallic deposit.

  The production of windings or transformers by the technique of the multilayer printed circuit however presents a certain number of drawbacks. In particular, a large number of layers is necessary as soon as the number of turns required is large, which can lead to overall thicknesses that are too large for components for which we are just looking for a small footprint, especially in thickness. Also, compared to the same volume, the resistance of the windings is high, because the proportion of copper is low compared to the proportion of insulating material in a printed circuit. This high resistance results in poor performance and overheating

excessive room.

  The present invention aims to solve the above problems and aims in particular to provide an inductive component, in particular a transformer also using multilayer printed circuit technology, having a small thickness and which may include a large number of turns, with a good performance, high insulation between the windings, and good performance from the point of view of space

and heat losses.

  With these objectives in view, the invention relates to a thin inductive component comprising a stack of several layers of insulating material, with end layers carrying conductive tracks and at least one intermediate layer, consisting of a plate insulator located between the end layers, characterized in that said intermediate layer comprises a recess in which is placed at least one coil of enamelled conductive wire prepared separately, the axis of the coil being perpendicular to the general plane of the layers, and the thickness of the coil, in the direction of this axis, being less than the distance between the end layers, the ends of the coil wire being connected to connection elements

  accessible outside the component.

  It is specified that by enameled wire, one must also understand any conductive wire comprising a

  electrically insulating coating.

  To make a transformer, preferably at least one of the insulating layers comprises a conductive track arranged in a spiral, substantially coaxial with the coil. Advantage will be taken for this purpose of the tracks that can be produced on the outer faces of the end layers. These tracks will preferably be made substantially plumb with the coil, so as to allow easy implantation of a magnetic core around the assembly, as is the case.

will understand better later.

  The invention therefore makes it possible to produce transformers having a small height, part of the windings of which, for example the primary windings, is formed by the said coil or coils of enameled wire, and the other part, for example the coil or coils secondary, is constituted by the said spiral tracks. The invention makes it possible to ensure a high efficiency of the transformer thus produced, high insulation between the windings, good reproducibility of the electrical characteristics, and this with minimal bulk. By way of comparative example, a transformer produced in accordance with the invention makes it possible to obtain a drop in temperature rise of the order of 25% compared to an equivalent transformer produced using printed circuit technology.

  multilayer, equivalent volume.

  A particularly targeted application relates to transformers for switching power supplies and energy converters, but the invention can be applied more generally to any

  space-saving transformer.

  According to a particular arrangement, several insulating plates are used between the end layers, each plate comprising a conductive spiral track, and the tracks of the different plates and / or end layers being connected together to form a winding, of similar to the technique of multilayer printed circuits. According to another arrangement, several coils are placed in the recess, coaxially, superimposed in the axial direction, or concentric, one around the other. Many configurations are possible by adapting as needed the number and arrangement of the coils of enameled wire and those formed according to the technique of the multilayer circuit, and by assigning the primary or secondary function to one or

the other type of winding.

  The recess constituting the housing of the coil or coils can be produced in the form of a groove machined in the plate or plates of the intermediate layer, said groove being able to pass through over the thickness of the intermediate insulating plate, or only machined over part of the thickness of the plate, thus providing a groove bottom which can serve as a temporary support for the coil at the time of manufacture. However, to best minimize the overall thickness, it may be preferable to use the entire thickness of the plate or plates

  insulators to place the coil (s) there.

  The intermediate plate or plates and the end layers are preferably made of glass, epoxy, or other material having characteristics

similar electrical insulation.

  Since the coil of enameled wire is placed between the end insulating layers, it is necessary to provide for its connection to connection means accessible from the outside. We could for example simply let protrude the ends of the enamelled wires out of the stack of insulating plates and layers, on the side of the stack, and weld them flat on a surface

  copper or on a pin arranged for this purpose.

  Preferably, the ends of the wires will be placed in grooves provided for this purpose in the intermediate layer, and the connection of each wire will be ensured with a connection track produced on an external face of at least one of the end layers, at the by means of a metal layer deposited on the internal surface of a hole with a diameter greater than that of the wire, the hole being drilled through the component so as to cut the end of said wire, at least partially. The diameter of the hole is preferably greater than the width of the groove so as to be sure that, if the hole is centered on the groove, the wire is well cut, taking into account the fact that the wire may not itself be centered in the groove, its diameter being deliberately sufficiently smaller than the width of the groove. Depending on the nature of the wire enamel, it may be necessary to tin the ends of the wire, at the places of drilling, before placing the coil in the recess, to ensure a good connection during

metallization of the hole.

  It should be noted that this connection technique is in itself original, and can be applied in other cases where one would seek to ensure a connection between a conductive wire embedded in a piece of insulating material and a conductive track on the surface of said piece. Indeed, the technique of metallized holes is known for ensuring connections between the tracks of the different layers of a multilayer printed circuit of conventional type, but in this case, the holes which subsequently receive the metallization are made through the tracks of copper with a diameter less than the width of the track at the level of the hole, so that the copper track keeps its continuity. In practice, a widening of the track is commonly carried out at the planned level of the hole, to form a sort of pad at the center of which the hole is drilled and then metallized. The small thickness of the copper layer is compensated by the fact that the connection is provided over the entire periphery of the hole, to ensure a sufficient area of the

  link between track and metallization.

  Contrary to this, according to the invention, the hole drilled has a diameter much greater than that of the wire, in particular greater than the width of the groove in which the wire is placed, to be sure to cut the wire, even in the event of an offset. between the axis of the wire and the axis of the hole. The hole could for example have a diameter of 1 mm, for a wire of 0.2 mm in diameter, placed in a groove of 0.6 mm wide, the width of said groove being sufficiently greater than the diameter of the wire to facilitate placing the wire in said groove. The connection between the wire and the metallization of the

  hole is provided on the area of the wire section.

  Once the wire constituting the coil is connected with the outside of the component according to the invention, the connection of the component itself with a support or another circuit can be ensured by any conventional means, using for example suitable tracks of the circuit. print consisting of one of the end layers, or pins welded into the metallized holes, etc. The coil or coils of enameled wire can be kept in their housing by gluing or preferably by coating in a resin filling the recess formed in the intermediate plate. It is the same for the maintenance of

  ends of the wires in their specific grooves.

  Also, a central cutout is preferably provided in the different insulating layers, and the external shapes of these layers can be adapted, so as to allow the establishment of a magnetic core having for example a branch axially traversing the whole of the coil and printed turns, and two branches passing laterally

from either side.

  Other characteristics and advantages will appear

  in the description which will be made of a transformer

  thin made in accordance with the invention.

  Reference will be made to the accompanying drawings in which: - Figure 1 is a cross-sectional view of the transformer, - Figure 2 is an exploded perspective view of the transformer, - Figure 3 is a perspective view showing the mounting of a core on the transformer, - Figure 4 is a detail view in section of the end zone of a coil wire, during the manufacturing after assembly of the different constituent layers of the transformer, - Figure 5 is a view similar, after drilling a hole cutting the end of the wire, - Figure 6 is a similar view after metallization of the hole, - Figure 7 is a perspective view of a

  variant embodiment of the transformer.

  The transformer shown in Figures 1 and 2 consists of a stack 1 of two end layers 11, 12 made of insulating material, for example made of epoxy glass, enclosing an intermediate layer made of a plate 13 also made of insulating material. An annular groove 14, of substantially square shape, is hollowed out in the insulating plate 13. In the example shown, the groove 14 is hollowed out over only part of the thickness of the plate 13, thus leaving a bottom serving as a support for the coil. But the groove could also pass through the entire thickness of the plate. Two secondary grooves 15, of smaller section than the

  annular groove, open into it.

  The end layers 11, 12 are formed in the manner of printed circuit boards, and carry on their outer faces copper tracks 18, 19, shaped in spirals, and forming the secondary windings of the transformer. It will be noted that similar tracks could also be made on the internal faces of the plates 11, 12, that is to say at the interface between said plates and the insulating plate 13, in particular for increasing the number of turns of the secondary, or form an additional separate winding. Likewise, it is also possible to use the technology of multilayer circuits to produce the intermediate layer 13, and to obtain an even greater number of turns. In general, all the layers are assembled in the known manner to assemble the layers of a conventional multilayer printed circuit, in

  possibly using vacuum pressing.

  The conductive tracks, such as 18 and 19, forming the different turns can be connected together, in various configurations depending on the desired characteristics, through the insulating plates, preferably by the hole technique

  metallized previously mentioned.

  A prefabricated coil of enamelled wire, wound without a carcass, is placed in the annular groove 14, the ends 17 of the wire constituting the coil 16 being placed respectively in the secondary grooves 15, after having been optionally tinned beforehand. The coil 16, which constitutes the primary of the transformer, is produced beforehand on a suitable manufacturing tool, and without a carcass, according to a technique known per se. The turns of the coil are preferably bonded together before removing the manufacturing tool, for example by a process using the principle of thermo-adhesion, that is to say that the wires of the coil are previously covered with a material which polymerizes with heat, then, after winding, the coil is subjected to a strong electric current or hot air, to heat the polymer material and thus ensure the assembly of the turns between them. Other principles for bonding the turns together, known to those skilled in the art, can

also be used.

  During manufacture, epoxy resin or the like is poured into the groove to fill it and rigidly hold the coil therein, which is thus found embedded in the said resin, as well as the ends 17 of the wires, and forms a rigid monobloc piece. with the plates 11, 12 and 13, after these are also assembled together by the conventional method of assembling the layers of a multilayer printed circuit, such as bonding, to form the stack

1 visible in Figure 1.

  By way of non-limiting example of a transformer according to the invention, the size of the

  different layers is of the order of 45 X 22 mm.

  The thickness of the plate 13 is 1.6 mm, and that of the end plates 11, 12 0.4 mm. The coil comprises 36 turns of wire of 0.2 mm in diameter and an overall section of approximately 1.5 mm in diameter, for a dimension of the order of 20 X 11 mm. The annular groove has a width of about 2.5 mm and the secondary grooves of

0.6 mm.

  As seen in Figure 3, a central cutout 2 is formed in the stack 1 to allow the passage of a central leg 5 of a magnetic core formed of the two half-cores 3, 4, then assembled by gluing, the legs end 6, 7 passing on either side of the stack 1. Cutting 2 can also be carried out before

  assembly of the different layers.

  The connection of the ends 17 of the wires of the coil 16, embedded in the stack 1, with the outside of this stack, is carried out in the following manner. As seen in Figure 4, the end 17 of each wire is embedded in the thickness of the stack 1 after assembly of the various constituent layers. A hole 21 is drilled through the stack 1 right through, so as to intersect the wire 17, as seen in FIG. 5, the hole being of diameter significantly greater than that of the wire, and greater than the width of the groove 15, to be sure that it will cut the wire well, the position of the latter not being ensured very precisely because of the play made voluntarily between the wire and the groove 15 where it is housed, to facilitate its setting place during assembly. Then the hole 21 is coated internally by metallization, to form a layer of conductive metal 22 welded on the section 24 of the cut wire, and ensuring the electrical connection with a track or a conductive patch 23, on the surface of one or the other end layer. From there, the electrical connection with other circuits or components can be made in a manner known per se, for example with pins or

  SMD inserts (surface mounted component).

  The production technique which has just been described allows an easy mass production of the transformers or windings according to the invention, by manufacturing them by boards comprising a plurality of windings or transformers, each then being

  cut from the board obtained.

  FIG. 7 schematically illustrates an alternative embodiment in which a recess 26, of dimensions slightly greater than the external dimensions of the coil 16, is formed in the intermediate plate 13, over the entire thickness of the latter. Then He the coil is placed in this recess is fixed there and maintained by a resin molding carried out during the assembly of the various layers, after having placed in the core of the coil and in the cutouts 27 of the end plates 11 , 12 a molding core

provisional, not shown.

  The invention is in no way limited to transformers which have just been described solely by way of example. As has already been indicated, many types of inductive components and transformers can be produced in a similar manner, combining in very many possible configurations one or more coils of enamelled wires and one or more coils made of turns made according to the technique of multilayer printed circuits.

Claims (10)

  1. Inductive component of low thickness comprising a stack (1) of several layers of insulating material, with end layers (11, 12) carrying conductive tracks (18, 19) and at least one intermediate layer, comprising a plate insulator (13) located between the end layers, characterized in that said intermediate layer (13) has a recess (14, 26) in which is placed at least one coil (16) of enamelled conductive wire prepared separately, l the axis of the coil being perpendicular to the general plane of the layers, and the thickness of the coil, in the direction of this axis, being less than the distance between the end layers, the ends (17) of the coil wire being connected to elements of   connection (23) accessible outside the component.   2. Inductive component according to claim 1, characterized in that at least one of the insulating layers (11, 12) comprises a conductive track arranged in a spiral (18, 19), substantially coaxial with the coil (16). 3. Inductive component according to any one of   claims 1 or 2, characterized in that several   insulating plates are used between the end layers (11, 12), each plate having a conductive spiral track, and the tracks of the different plates and / or end layers being connected   between them to form a winding.   4. Inductive component according to claim 1, characterized in that several coils (16) are placed in the recess.   5. Inductive component according to claim 1, characterized in that the recess constituting the housing of the coil (s) (16) is produced in the form of a groove (14) machined in the plate or plates of the intermediate layer (13 ). 6. Inductive component according to claim 5, characterized in that the recess (26) is through on   the thickness of the intermediate insulating plate (13).   7. Inductive component according to claim 1, characterized in that the ends (17) of the wires of the coil (s) (16) are arranged in grooves (15) arranged for this purpose in the intermediate layer, and the connection of each wire is ensured with a connection track (23) produced on an external face of at least one of the end layers (11, 12), by means of a metal layer (22) deposited on the internal surface of a hole (21) of diameter greater than that of the wire, the hole being drilled through the component so as to cut, at least partially, the end (17) of said thread.   8. Inductive component according to claim 7, characterized in that the diameter of the hole (21) is   greater than the width of the groove (15).   9. Inductive component according to claim 1, characterized in that the coil or coils (16) of enameled wire are held in their housing by gluing or by coating in a resin filling the recess   formed in the intermediate plate.   10. Inductive component according to claim 2, characterized in that a central cutout (2, 27) is provided in the different insulating layers, and the external shapes of these layers are adapted, so as to allow the establishment of a magnetic core (3,4) having a branch (5) axially passing through the whole of the coil (16) and printed turns (18, 19), and two branches (6,7) passing laterally on both sides.