GB2261868A - Transformer bobbin - Google Patents

Abstract

A bobbin, for a switched mode power supply transformer, comprises nesting, interlocking skeletal bobbin elements 10, 18, 20 each bearing windings and each incorporating creepage barriers 14, 16 and 22, 24 respectively providing 3 mm and 6 mm creepage paths. The windings extend the full width W between the barriers. The elements nest 18 over 10, 20 over 18, over interwinding tape which is applied over the full width T of the elements including the creepage barriers. <IMAGE>

Description

Title Transformer Bobbin Field of the invention This invention relates to a bobbin for carrying the windings of a transformer, more especially a switched mode power supply (smps) transformer.

Background to the invention Present practice in relation to smps transformers is to employ either vertically segmented or horizontally segmented bobbins for carrying the primary and secondary windings.

Figure 1 of the accompanying drawings shows the principle of a vertically segmented bobbin, wherein the layers of the primary and secondary coils are marked "P" and "S" respectively. The disadvantage with this type of arrangement is that the leakage inductance values are too high to enable the transformer to be employed for higher frequency operation.

Figure 2 shows the principle of a horizontally segmented bobbin, suitable for an smps transformer required to operate at high frequencies. The layers of the primary (P) and secondary (S) coils are separated by insulating tape, and inserted barriers, providing extended creepage paths are also incorporated into the bobbin.

The disadvantage of this type of arrangement is that, if the creepage barriers are formed of layers of tape wound on to the bobbin production is time consuming and expensive and difficult to automate. Alternatively, if the barriers are formed as elements of moulded insulating material these are difficult to hold in place and thus give rise to serious handling problems. Moreover, the creepage barriers are additional to the thickness of the end wall of the bobbin, and are thus wasteful of the theoretically available winding area.

The invention has for its object to provide a transformer bobbin which retains the low leakage of horizontally segmented windings whilst overcoming the above-described problems of production.

The invention According to the invention, there is provided a transformer bobbin comprised by a plurality of nesting skeletal bobbin elements each incorporating spaced first and second creepage barriers.

In a preferred arrangement, the windings are extended over the full width of the spacing between the first and second barriers, interwinding tape is applied over the full width of the elements including the barriers, and the skeletal bobbin elements nest over the applied interwinding tape.

The bobbin elements preferably comprise an inner bobbin element with integral terminals and at least one outer skeletal bobbin element.

The at least one outer skeletal bobbin element may conveniently incorporate wire guidance pins, allowing the wire lead-out to be taken back across the winding and terminated. Slots in the at least one skeletal bobbin element allow the wire lead-out to exit the bobbin at different angles, reducing wire lead-out crossover.

The bobbin elements are preferably provided with ratchet like formations which fix the nested bobbin elements together in a secure assembly.

Desirably, the assembled bobbin as described complies with IEC 65, which specifies a 6 mm creepage path plus 0.4 mm distance through insulation (or three layers of insulating material) between the input and output windings but the principle may be applied to any dimension of extended creepage path.

However, in accordance with a further feature of the invention, this does not mean that a 6 mm barrier must be provided all round the bobbin. Where no lead-outs cross the barrier, a barrier 3 mm wide can provide a 6 mm creepage path. In accordance with this further feature of the invention, therefore, a 3 mm barrier is provided all round the bobbin except at the lead-out position, where the barrier is increased to 6 mm width. In this way, an additional 3 mm of winding width is created in a bobbin of the same overall dimensions.

The above-described bobbin construction has the advantages firstly that, as the two creepage barriers are moulded as parts of a common element, they are easier to handle and more securely held in the correct position; secondly that the barrier forms the end wall of the bobbin, thus making more effective use of the available winding space; thirdly that, not only is the low leakage of horizontally segmented windings maintained, but the increased winding width further reduces leakage; and fourthly that, as hitherto explained, wide guidance slots and projections can readily be incorporated.

Description of embodiment A transformer bobbin in accordance with the invention is now described by way of example with reference to the accompanying drawings, in which: Figures 1 and 2 show the prior art; Figure 3 shows a bobbin construction (for an smps transformer) in accordance with the invention; Figures 4A and 4B are explanatory diagrams; and Figure 5 shows a modification of a skeletal bobbin element.

Referring to Figure 3, an smps transformer bobbin comprises an inner bobbin element 10 having integral terminals 12. The bobbin element 10 has a 6 mm creepage barrier 14 at the terminal end and a 3 mm creepage barrier 16 at the other end.

The bobbin also comprises nesting skeletal bobbin elements 18 and 20, the element 18 for nesting over the inner bobbin element 10 and the element 20 for nesting over the element 18, as indicated by the dotted lines in the drawing. The skeletal bobbin elements 18 and 20 are generally similar, and each incorporate a 6 mm creepage barrier 22 at one end and a 3 mm creepage barrier 24 at the other end. The barriers 22, 24 are linked on opposite sides by partial side walls 26, 28, at least one of which is equipped with integral wire guidance pins or projections 30. The wide slots 32 between the partial side walls 26, 28 enable the lead-outs to exit the bobbin at different angles.

Ratchet-type formations 34, 36 integrally formed with the elements enable the elements to be securedly fixed in the assembled condition.

In the assembled bobbin, the winding (not shown) on each bobbin element extends over the entire width W between the first and second barriers 22, 24, while interwinding tape is applied over the full width T of the bobbin element, including the creepage barriers. The bobbin elements 10, 18, 20 are thus designed to nest over the applied interwinding tape.

As indicated in Figure 4A, a creepage path of 6 mm is obtainable with a 3 mm wide barrier provided a lead-out is not crossing the barrier at that point. However, where a lead-out does cross the barrier, as shown in Figure 4B, a 6 mm wide barrier is essential.

In accordance with a modification of the invention, shown in Figure 5, the above-described nesting bobbin elements are provided with a 3 mm barrier 38, instead of a 6 mm barrier, except at the position 40 of a lead-out, where the barrier has a portion 42 of width increased to 6 mm.

This creates an extra 3 mm of winding width.

Various modifications of the above-described and illustrated arrangements are possible within the scope of the invention defined by the appended claims.

Claims (11)

Claims

1. A bobbin for a transformer, especially an smps transformer, comprised by a plurality of nesting skeletal bobbin elements each incorporating spaced first and second leakage barriers.

2. A bobbin according to claim 1, wherein the required leakage path is provided directly by the first barrier and indirectly by the second.

3. A bobbin according to claim 1 or claim 2, wherein the windings are extended over the full width of the spacing between the first and second barriers, interwinding tape is applied over the full width of the elements including the barriers, and the skeletal bobbin elements nest over the applied interwinding tape.

4. A bobbin according to claim 1 or claim 2 or claim 3, wherein the bobbin elements comprise an inner bobbin element with integral terminals and at least one outer bobbin element.

5. A bobbin according to claim 4, wherein the at least one outer bobbin element is formed with integral projections or pins for wire lead-out.

6. A bobbin according to claim 4 or claim 5, wherein the at least one outer bobbin element is formed with wide slots allowing wire lead-out at different angles.

7. A bobbin according to any of claims 1 to 6, wherein the bobbin elements are provided with integral ratchetlike formations for securing said elements in the nested condition.

8. A bobbin according to claim 2 or any claim appendant thereto, wherein the bobbin elements each have creepage barriers all round the element.

9. A bobbin according to claim 8 wherein the width of each creepage barrier is equal to one half the creepage path or the full creepage path.

10. A bobbin according to claim 2 or any claim appendant thereto, wherein the bobbin elements each have first and second equal width creepage barriers except at the position of a lead-out, where the width is increased to double that of the first and second creepage barriers.

11. A transformer bobbin substantially as hereinbefore described with reference to the accompanying drawings.