GB2128814A - Power transformers - Google Patents

Abstract

A small power transformer for direct mounting on printed circuit boards has an injection moulded plastics bobbin (1) and the connection tags of the transformer are supported by a separate heat resistant insulating strip (5) mounted on the bobbin cheek edge. In one embodiment (as shown) the tags are fixed to the heat resistant strip and have crimped portions for crimping a bundle of winding wire ends onto each tag. In an other embodiment each tag is actually formed by a bundle of the winding wires fed through a slot in the heat resistant strip and subsequently twisted together and dip soldered. In a third embodiment the winding wires are lead each to an individual key-hole slot in a heat resistant strip, in which each wire is a snap fit. In all embodiments the tags and wires can be dip soldered. <IMAGE>

Description

SPECIFICATION Power transformers This invention relates to small transformers for direct mounting or printed circuit boards.

Conventionally small transformers for mounting on printed circuit boards are either left with the winding wire ends flying for direct connection into the printed circuit board, or the winding spool which is a standard injection moulded item, has several connection tags in the cheek edges to which the wires are connected and these tags act as an interface between the wire ends on the one hand and the printed circuit board on the other.

In the first arrangement where there are a large number of flying leads, say more than six, then the time taken to connect the flying leads to the board is time consuming and errors in selecting the correct lead can be made. In the second arrangement problems arise where there are a large number of wires and the wires are of a substantial diameter; with recent developments in switched mode power supplies and direct-off-line (DOL) DC-DC converters, the need to get good coupling between the windings of the power transformer has become more critical requiring several high current multi-wire windings.

The wire is of the order 1.0 mm diameter and the connection of several such wires in common to a single terminal in a transformer with spool cheeks measuring 4 cm square or thereabout is difficult.

It is an object of the present invention to provide an improved small transformer for direct mounting on a printed circuit board, with multi-wire windings using a standard injection moulded spool.

According to the present invention a small transformer for direct mounting on a printed circuit board comprises an injection moulded plastics winding spool, at least one spool cheek having a beat resistant strip of insulating material fixed to one edge and carrying several connecting tags projecting in the same direction for direct insertion in the printed circuit board.

Preferably all the tags are formed by several wires in notches on the edge, the notches being formed in a strip heat resistant insulating material fixed to the edge of the pertaining cheek.

Preferably each notch is in the form of a slot leading to a circular hole.

According to another aspect of the invention a small transformer for direct mounting on a printed circuit board comprises an injection moulded piastics winding spool, each spool cheek having a heat resistant strip of insulating material fixed to one edge and carrying several connection tags, the windings being soldered to the connection tags, the tags projecting, in the same direction for direct insertion in the printed circuit board.

Preferably each tag has a portion which has been crimped onto the end of the winding wire attached to it. Preferably at least some of the tags are each connected to the several wires of a multi-wire winding.

According to another aspect of the invention there is provided a transformer comprising heatsoftenable moulded plastics winding spool having cheeks about 4 cm square and a plurality of windings on the spool of a wire having a diameter of the order of 0.9 mm, each winding having a termination on the transformer of two or more wires for direct soldering to a printed circuit board, the terminations being secured in a support member fixed to the spool, the support member being not appreciably softenable at normal soldering temperature up to 400O.

In order that the invention can be clearly understood reference will now be made to the accompanying drawings, in which: Figure 1 shows an end view and a side view of a transformer according to an embodiment of the present invention; Figure 2 shows schematically a typical winding arrangement of a transformer according to the present invention; Figure 3 shows a side view and an end view of the tag strip ofthe embodiment of Figure 1; Figure 4 shows a perspective view of one of the tags of Figure 3; Figure 5 shows a tag strip for a transformer according to second embodiment of the present invention; Figure 6 shows an end view of a transformer according to the second embodiment of the present invention; and Figure 7shows a tag strip suitable for use in a transformer according to a third embodiment of the present invention.

Referring to Figures 1 to 4 of the drawings, the transformer comprises a standard injection moulded plastics bobbin 1 having side cheeks 2 and 3 and a number of windings 4. The size of the side cheeks is about 4.5 cm square and the spacing between the side cheeks is about 3 cm. The winding 4 in fact consists of a number of separate windings and one example is shown schematically in Figure 2. This schematic is a view similar to the top half of the left hand side of Figure 1 showing the various layers of the winding. At the bottom is the primary winding 4a. This covered by a copper foil screen 4b and above this are five separate windings 4c to 49 inclusive.

Primary winding 4a comprises thirty four turns of a single wire of 0.9 mm diameter and is connected to terminals a and bin Figure 1. The screen 4b is a single turn of copper foil and is connected to a flying lead (not shown in Figure 1). The secondary winding 4c comprises two turns of ten wires each of 0.9 mm diameter and this winding is formed of two groups of five wires each, one group of five extending from terminal b to terminal e and the other group of five extending from terminal c to terminal f. Winding 4d comprises two-and-a-half turns of four wires each of 0.9 mm diameter and this winding extends from terminal g to terminal j on the same side cheek and similarly winding 4e, also comprising two-and-a-half turns of four wires, each wire of 0.9 mm diameter, extends from terminal k to terminal n on the same side cheek.

Winding 4f comprises four-and-half turns of three wires, each of 0.9 mm, and extends from terminal h to terminal i and finally the winding 49 also compris ing four-and-a-half turns of three wires, each wire 0.9 mm, extends from terminal 1 to terminal m on the same side cheek.

Thus it can be seen, referring back to Figure 1, that each connection tag has to be secured to a bundle of relatively heavy gauge wires and for this purpose a resin bonded strip 5 carrying four connection tags is provided secured to the edge of each bobbin cheek.

The strip 5 is secured to the cheek 2 or 3 by means of a small connection tag, such as 2a which is embedded within the cheek of the winding spool and is bent over at 2b to hold the strip 5 on the cheek. There are four such strips 5 on the four corresponding edges of the cheeks of the bobbin. These strips carry terminals a, b, c, d, on one edge of cheek 2 and connection tags e and f on the opposite edge of cheek 2, a third strip carries connection tags, g, h, i, on one edge of cheek 3, and finally a fourth strip carries terminals k, I, m, n, on the opposite edge of cheek 3. Thus the transformer has fourteen connection tags connected to the wiring as described earlier.

Each connection tag is the same as the connection tag a shown in Figure 4. It comprises a spade 6 opposed shoulder parts 7 and 8 which prevent the spade being pushed through the corresponding slit in the tag strip 5, a U-shaped portion 9 and crimp portions 10 and 11 which can be crimped around the bundle of wires from the winding, prior to soldering.

It is also to be noted on the left hand side of Figure 1 that the wires held in the respective crimped portions 10 and 11 of the connection tags are located outboard of the bobbin cheeks, so that the crimped wires in the connection tags can be dip-soldered by dipping first one side of the wound transformer into a solderbath, corresponding to the side defined by cheek 2, and then turning the transformer over and dipping the other crimped portions of the terminals in the solderbath, corresponding to the cheek 3.

In this way the substantial amount of heat generated by the solderbath and maintained by the large number of relatively heavy duty wires and the relatively heavy duty connection tags, can be sustained by the resin bonded fibre tag strip 5 which is heat resistant at such soldering temperatures. In contrast the injection moulded plastic bobbin would not be so resistant to the soldering temperatures and one could not in fact dip-solder a number of connection tags held directly in the edges of the bobbin cheeks since these bobbins are made of a heat softenable injection moulded plastics material.

Significant distortion of the cheeks would occur, causing significant misalignment of the tags so that subsequent direct insertion into a printing circuit board would not be possible. Furthermore this distortion is aggravated by tensions in the wires caused when the wires are wrapped around the conventional connection tag leaving a net stress on the tag. In contrast these crimped portions of the heavy duty tags in tag strip 5 shown in Figures 3 and 4, have little or no net stress since the wires are not pulled around the terminal but are, in contrast, laid in between the portions 10 and 11 which are subsequently crimped together.

As an alternative to the embodiment described in Figures 1 to 4, there is shown in Figures 5 and 6 a wound transformer which corresponds to the transformer described in Figures 1 and 2, except that instead of the tag strip 5 supporting heavy duty connection tags, the tag strip 21 shown in Figure 5 has no connection tags on it at all, but merely a number of wire slots 21 a to 21 d. As shown in Figure 6the multiple wires of each winding are led into the slots 21 a to 21 d until they are accommodated into the larger circular region of the slot, such as 22a to 22d. The wires are then twisted together to form an effective connection terminal that can be directly inserted into suitably sized and positioned holes on a printed circuit board. In Figure 6 these wound together wires form terminals shown as 23 on the upper part and 24 on the lower part of the transformer.Similarly on the other cheek not visible in Figure 6 there would be two more sets of connection terminals formed in exactly the same way, so that in all there would be fourteen connection terminals, exactly the same configuration as shown in Figures 1 and 2. This would mean thatforthe primary winding there would in fact be just a single wire forming a terminal and it is found that the gauge of the wire, i.e. 0.9 mm is sufficient to form a relatively sturdy connection terminal for direct insertion into the printed circuit board. But all the other terminals will be a plurality of wires wound together and subsequently soldered by dip soldering in the same way as the wires are soldered to the crimped portions of the terminals of the embodiment of Figures 1 to 4.

Yet another embodiment, Figure 7, shows a terminal strip 31, also made of resin bonded fibre which has five key-hole shaped slots 31 a to 31 e, each slot designed to accommodate a single wire 0.9 mm in diameter. The strip 31 would be fixed to the transformer winding bobbin in exactly the same way as the strip shown in Figure 1 or FigureS.

The slots 31 a to 31 e have a key-hole shape such that the narrowest portion of the slot is just smaller than the 0.9 mm diameter wire so that when this wire is pushed into the slot is snaps into the circular portion at the end of the slot and is retained and does not come back out. This embodiment is suitable for a transformer where only one wire per winding is used, although alternatively a number of wires for a single winding can still be used, each wire being taken to its own respective slot such as 31 a, and the printed circuit board is arranged to common up the wire of a particular winding.

Claims (8)

1. A small transformer for direct mounting on a printed circuit board comprises an injection moulded plastics winding spool, at least one spool cheek having a heat resistant strip of insulating material fixed to one edge and carrying several connection tags projecting in the same direction for direct insertion in the printed circuit board.

2. A transformer as claimed in claim 1, wherein the strip has at least one notch having several wires located therein and soldered together and forming one of the connection tags of the transformer for direct insertion in the printed circuit board.

3. Atransformeras claimed in claim 1 or claim 2, the winding spool having cheeks about 4 cm square and a plurality of windings on the spool of a wire having a diameter of the order of 0.9 mm, at least one of the windings having a tag terminating two or more wires for direct insertion in the printed circuit board.

4. A transformer according to claim 1, claim 2 or claim 3, wherein at least one of the tags is a metal part fitted in the heat resistant strip and a crimp portions between which the ends of a multi-wire winding are crimped.

5. A transformer as claimed in claim 4, wherein the crimped wires are located in a plane outboard of the pertaining winding spool cheek, and the crimped wires have been dip soldered.

6. A transformer as claimed in claim 2 or claim 3 appended thereto, wherein the notch has a first narrow portion extending from the edge of the strip inwardly, and a second wider portion, whereby each of the several wires of the winding has been inserted one by one through the narrow portion and is located in the wider portion, soldered together, so that they are trapped and cannot be removed sideways from the notch.

7. A transformer as claimed in any preceding claim, wherein the injection moulded winding spool has in each side cheek a number of slots through which the wires of the winding extend, the heat resistant strip extending across the ends of the slots to thereby close off the slots.

8. A transformer substantially as hereinbefore described with reference to any of the accompanying drawings.