GB2154374A - Miniature transformers and inductors - Google Patents

Abstract

A miniature transformer has a wound ferrite core (2) secured to a thick film circuit (1) which provides the external connection terminals for the transformer. Securement is made using a glass-filled nylon cup-shaped part (10). The wound core is housed in the cup-shaped part and the underside of the cup-shaped part is glued to the substrate (1). A closure lid (6) holds the wound core in the cup (10). The cup-shaped part eliminates magneto-strictive reaction which would otherwise be caused if the core (2) were glued directly to the substrate. Modifications are described to the housing. <IMAGE>

Description

SPECIFICATION Miniature transformers and inductors Miniature transformers and inductors normally use ferrite cores and modern ferrite cores can be obtained having a high permeability. This in turn gives high values of inductance in a relatively small volume.

To meet stringent requirements, in some instances low temperatures, for example temperatures as low as - 50 , it is necessary to ensure that magneto-strictive reaction is minimised if not eliminated because this tends to reduce the inductance of the device. In most applications it is necessary to mount these tiny devices on to a substrate which may for example be a thick film ceramic substrate, and the simpiest method we have found is to use a hot-cured resin to stick the core itself directly on to the substrate.

Surprisingly however we have discovered that, although the thick film circuit and the ferrite material of the core have substantially the same coefficient of expansion, nevertheless the interposition of a thin film of resin cured at high temperature results in a ferrite core which is under stress at normal ambient temperatures, and even higher stress at below-ambient temperatures, to the extent that the performance of the device is significantly impaired.

It is an object of the present invention to provide a miniature inductive component which includes a ferrite core mounted on a substrate in such a way that magneto-strictive reaction is minimised if not eliminated.

According to the present invention there is provided a wound inductive component incorporating a wound ferrite core of high permeability and secured to a substrate carrying the connection terminals of the transformer, wires from the wound core being connected to these connection terminals and the core being mounted on the substrate either indirectly by means of a housing including a cup-shaped element which contains the wound ferrite core, or directly by means of an adhesive which minimises if not eliminates stresses being set up in the ferrite core over a temperature range over at least 50"C.

In order that the invention can be clearly understood reference will now be made to the accompanying drawing in which: Figure 1 shows a two-part ferrite pot core for a transformer according to an embodiment of the present invention, Figure 2 shows shows a thick film circuit providing a substrate of the transformer according to the embodiment of the invention, Figure 3 shows a perspective view of a complete transformer according to an embodiment of the invention and corresponding to Fig. 7 in detail.

Figure 4 shows the transformer with a modified moulded plastics mounting cup and lid, according to another embodment of the invention, Figure 5 shows a transformer similar to Fig.

4 but with a deeper lid, Figure 6 shows a transformer similar to Figs. 3 and 7 but with no flange on the lid, Figure 7 shows the transformer of Fig. 3 along the line X, Y, Z, and Figure 8 shows a transformer similar to Fig.

6 but with slots in the cup and in the lid.

Referring to Fig. 3 of the drawings first, the final assembly of the transformer of Fig. 7 is shown in perspective view and comprises a thick film circuit substrate 1 with edge connection terminals A, B, C, D, E, F and G.

These connection terminals are formed by fired-on thick film conductive areas. The particular layout is shown in Fig. 2.

A wound ferrite core 2, shown in Fig. 1 and Fig. la, comprises two pot core halves 2A and 28, each as shown in Fig. 1 a secured together with adhesive and housing four windings shown generally by reference numeral 3.

As shown in Fig. la each pot core half has apertures 4 and 5 which are aligned and through which the terminal wires of the winding estend.

The wound pot core is housed in a two-part moulded plastics housing shown in Fig. 7 in section taken along the line XYZ of Fig. 3.

Referring to Fig. 7 one part of the housing is a moulded glass-filled nylon lid 6 which has a projecting flange 7. The other part is a cup 10 which is castellated to provide apertures 11 aligned with grooves in the flange.

The projecting flange 7 has underneath it a number of the grooves such as 8 and 9.

These grooves run along the underside of the flange 7 and locate the terminating wires 1 2 from the wound pot core so as to hold the wires 1 2 in position and locate them during assembly of the transformer.

Thus in manufacturing the transformer the pot core is wound in conventional manner with the wires extending from the apertures 4 and 5. The wound core is then placed in the cup 10 also made of glass-filled nylon so that the wires 1 2 protrude from apertures such as 9. Then the wires 1 2 are located under the flange 7, using a colour-coding technique, in respective ones of the grooves 8 and 9 so that this subassembly provides a number of terminating wires 1 2 projecting from the end of the grooves ready for connection to the connection areas A to G on the thick film substrate 1.

The glass filled nylon moulded lid 6 is an interference fit over the top of the cup 10 so as to close the cup but leaving the apertures 11 open to allow passage of the wires.

The subassembly is then provided on the underside of the lid or flange 7 with a dab of resin, either hot-curing or cold-curing, and the subassembly is placed on the thick film sub strate so that the core corresponds generally with the dotted line shown at 1 3 in Fig. 2. In this way the projecting wires from the grooves 8 and 9 lie on or very close to the surfaces of the connection terminals A to G as shown in Fig. 3 and it is then a matter of using a conductive resin such as conductive Araldite (Trade Mark), to electrically connect the wires to the conductive areas.

We have found that it is best to arrange that the size of the cup 10 in relation to the external diameter of the wound ferrite core needs to be such that the core is a sliding fit inside the cup 10, Then at the lowest expected working temperature such as - 50"C, this fit should become just an interference fit.

The above arrangement eliminates any magneto-strictive reaction effects which would otherwise be caused if the ferrite core were directly bonded to the ceramic substrate.

Referring now to Fig. 4 of the drawings the transformer is similar to the one just described except that the lid 6a has no flange and the cup 11 a has no apertures. The wires 1 2 follow an 'S' path and are located generally by the close fit of the cup and the lid and could be prepositioned in a jig as the cup and lid are fitted together.

The lid has fairly shallow side walls and the embodiment of Fig. 5 differs from the embodiment of Fig. 4 only in that the lid has deeper side walls 6'a which bring the wires 12 out along the substrate 1.

Fig. 6 shows an embodiment which is similar to Fig. 4 except that here the cup 10 has apertures 11 in the same way as the embodiment of Fig. 7. Fig. 8 shows an embodiment similar to Fig. 7 except that the lid 6b does not have the flange 7 although it does still have apertures 7a which act to guide the wires 1 2 in the same way as the grooves 8 and 9 in the flange 7 of Fig. 7 embodiment.

In all the embodiments like reference numerals indicate like parts, the method of assembly being the same for all embodiments.

As an alternative to the embodiments described, it would also be possible to bond the ferrite core directly to the thick film substrate provided the bonding material is flexibble so that over a wide temperature range this fiexibility will prevent stresses being set up in the ferrite core thus producing magneto-strictive reaction.

As yet another alternative it might be possible to select a cold-curing resin which is cured at a temperature roughly centrally disposed within the range of temperatures anticipated in use of the transformer.

The conductive areas on the thick film substrate are gold plated.

The permeability of the ferrite core is fairly high, for example 5000.

The dimensions of the wound core discribed is about 5 mm in diameter and 2.5 mm high but we anticipate that this invention can be applied to inductive components in which the wound ferrite core occupies a cube measuring 10 mm along each side.

The thickness of the wires used in the transformer winding are 0.071 mm in diameter and the thickness of the plastic lid 6 and cup 10 is about 10 thou, giving a low profile product.

The gaps between parts shown in the drawings are for illustration only and not to scale.

In the embodiments the lid 6 would be secured with an epoxy resin either to the cup 10 or direclty to the substrate 1.

Claims (9)

1. A wound inductive component incorporating a wound ferrite core of high permeability and secured to a substrate carrying the connection terminals of the transformer, wires from the wound core being connected to these connection terminals and the core being mounted on the substrate either indirectly by means of a housing including a cup-shaped element which contains the wound ferrite core, or directly by means of an adhesive which minimises if not eliminates stresses being set up in the ferrite core over a temperature range over at least 50"C.

2. A component as claimed in claim 1, wherein the housing comprises the cupshaped element and a closure lid, the wires from the core projecting generally radially from the housing towards the connection terminals of the substrate.

3. A component as claimed in claim 2, wherein the housing has a flange-like member providing location channels extending between the core on the one hand and the connection terminals of the substrate on the other hand.

4. A component as claimed in any preceding claim, wherein the cup has apertures in it side through which the wires of the wound ferrite core extend.

5. A component as claimed in claim 1 or claim 2, wherein the wires from the core extend radially over the rim of the cup and under the rim of the lid following a generally 'S' shaped path.

6. A component as claimed in claim 1 or claim 2, wherein the cup and the lid each have apertures in the overlying walls through which the wires pass and in which the wires are located.

7. A component as claimed in claim 1, wherein the wound ferrite core is directly connected to the substrate by means of a flexible adhesive.

8. A component as claimed in claim 1, wherein the wound ferrite core is direclty secured to the substrate by an adhesive which has cured at a temperature disposed centrally in the range of temperatures for which the component is designed in use.

9. A wound component substantially as hereinbefore described with reference to and as illustrated in Figs. 1, 2, 3 and 7, modified or not by Fig. 4, Fig. 5, Fig. 6 or Fig. 8 of the accompaning drawings.