GB2502315A - Transformer beads with flux-interrupting grooves - Google Patents

Abstract

A transformer bead or core 10' comprises a body formed with at least first and second channels 16, 18, which may be cylindrical, for receiving transformer windings wherein at least one groove or notch 30 is formed in the body extending between and parallel to the channels. The groove acts to provide a flux gap such that when the transformer bead 10' is used to carry two separate transformer windings 20, 22, the groove 30 interrupts the flux path occurring in the magnetic bead 10' substantially reducing cross-coupling. The groove 30 may have an outwardly curved, tapered cross-section with a flat base. There may be upper and lower grooves 30, directly opposing each other, and the depth of the groove or grooves may range from 20 to 80% of the body 10' height. The bead 10' may be incorporated into an electronic device or circuit (40, Figure 4). The transformer may be used in a cable television splitter circuit.

Description

Title: Transformer Beads

Field of the Invention

This invention relates to improvements to transformer beads and in particular transformer beads with multiple holes or channels extending through the bead.

Background to the Invention

Transformer beads with two holes or channels extending through the body of the bead are used in single coil and transformer applications, with the coil or transformer a windings arranged between the two holes, or with two separate coil or transformer assemblies wound onto the core through separate holes passing over the body of the core so permitting a very compact two-transformer assembly that can be used on a printed circuit board. However the close proximity of the two transformers due to their positioning on a common bead can affect circuit performance.

Summary of the Invention

In accordance with one aspect of the present invention, there is provided a transformer bead comprising a body formed with at least first and second cylindrical channels or holes for receiving transformer windings wherein at least one groove is formed in the body extending between and parallel to the cylindrical channels. The groove acts to provide a flux gap such that when the transformer bead is used to carry two separate transformer windings, the groove interrupts the flux path occurring in the magnetic bead, or core. Cross-coupling is substantially reduced in the core so improving the properties of transformers provided by the bead and windings, which in turn improves the performance of electronic circuits in which the transformers are used.

Preferably the groove has a tapered cross-section and more preferably outwardly curved side walls and a flat base. As will be appreciated, a groove must necessarily be formed in an outer surface of the body.

Where the groove comprises outwardly curved side walls with a flat base, preferably the curvature of the curved side walls is complementary to a radius extending from a centre of the respective cylindrical channel to a closest outer edge of the body. Thus

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the groove subdivides the body into two substantially cylindrical portions to reduce unwanted flux effects. The first and second cylindrical channels may be associated with different radii in which case the groove will have curved side walls of different curvature.

If desired two opposing grooves may be provided in upper and lower outer surfaces of the body. Typically the upper and lower grooves directly oppose each other, spaced apart by a portion of the body.

a The depth of the groove, or the combined depth of the upper and lower groove preferably ranges from 20 to 80% of the body height and more preferably between 40 to 60% of the body height, such that the groove depth is sufficient to introduce a flux gap between respective transformers formed by windings wound on the body.

The transformer bead may comprise additional cylindrical channels, otherwise known as holes, and additional grooves such that at least one groove is formed between channels that are intended to receive separate transformer windings. Thus for example, the body may include four channels, the first and fourth channels intended to receive two separate transformers and the second and third channels receiving windings for a common transformer such that the four channels carry three transformers or coils between them. In this situation, a fir st groove is provided between the first and second channels and a second groove between the third and fourth channels so as to interrupt flux paths between the three transformers.

In accordance with another aspect of the invention, there is also provided an electronic device or electronic circuit incorporating a transformer bead as aforesaid.

The invention will now be described, by way of example, and with reference to the accompanying drawings in which: Figure 1 shows a schematic diagram of a prior art transformer bead; Figure 2 shows a first embodiment of a transformer bead in accordance with the present invention; Figure 3 shows a second embodiment of the present invention; Figure 4 shows an exemplary circuit incorporating a transformer bead in accordance with the present invention; and Figure 5 shows a third embodiment of the present invention.

Description

Figure 1 shows a prior art transformer bead 10 made from a magnetic material such as ferrite. Typically the bead is made by adding appropriate binders and lubricants to a ferrite powder with the desired magnetic properties, pressing this mixture into a suitably shaped mould and sintering at a temperature of between 1100°C and 14000 C a in a controlled atmosphere. The bead can be subsequently machined. The bead is substantially cuboid with curved end walls 12, 14 with two cylindrical channels 16, 18 extending thercthrough such that each channel is positioncd to bc a constant distance from the curved wall edge. Curved walls 12, 14 have respective radii or curvature of R1 and R2, R being the distance from the centre axis of the channel to the outer surface of the curved wall. Typically the width W ranges from 10 to 2mm, the length L ranges from 10 to 2mm and the height H ranges from 5 to 1mm. In Figure 1, bead 10 carries windings 20, 22 associated with two separate transformers although windings for one transformer can be wound between the two channels 16, 18 if desired. An example of the common flux path 24 is marked and with this prior art bead shape, the flux of the separate transformers links and cross-couples.

Such a core is used for applications where two transformers are required, such as directional couplers and two-hole splitters, where the windings arc wound around the outside of bead 10, the bcad body and winding combining to provide a transformer.

Winding two separate sets of windings onto a single bead or core to produce two transformers allows the windings to be connected to form a very compact assembly prior to fitting to a printed circuit board.

Figure 2 shows a transformer bead 10' in accordance with the present invention formed with a groove 30 running parallel to and equi-spaced from cylindrical channels 16, 18. The groove depth d is sufficient to introduce a flux gap when bead 10' is wound with windings 20 and 22 associated with separate transformers.

Typically the groove depth will be around 50% of height H although the maximum depth is that at which the ferrite can still retain structural integrity for a given thickness and not break. Thus d can be in the range 20% to 80% of height Fl.

Groove 30 typically has a flat bottom 32 with outwardly tapered side wafls 34, 36 with the curvature of walls 34, 36 complementary to radii Ri and R2 respectively so that groove 30 effectively divides bead 10' into two cylindrical magnetic components with circular flux paths 24 with flux coupling between the two components substantially reduced even though the components are connected.

a Figure 3 shows an alternative embodiment where bead 10" has three separate cylindrical channels 16', 18' 18" or holes extending through the body of the bead and whcrc one set of transformer windings 20' is associated with channel 16' and transformer windings 22', 22" forming a second transformer are associated with channels 18' and 18". Groove 30 decouples the flux associated with windings 20' from windings 22", 22", giving exemplary flux paths 24".

As will be appreciated by those skilled in the art, a transformer bead can be provided with any number of channels with grooves provided between those channels intended to carry maetically separated transformer windings, see for example Figure 5 where a four hole bead provides three separate transformers, two grooves 30; 30" being provided to ensure a flux gap between each transformer. Such a transformer bead can be used in any appropriate circuit where at least two transformers are needed and where cross-coupling is undesirable, for example where cross-coupling is reducing circuit efficiency. An example of a circuit in which such a bead can be used is shown in Figure 4 which illustrates an exemplary cable television (CATV) splitter circuit 40 comprising input ports 42, 44 and output port 46 and having two transformers 48, 50.

Transformers 48, 50 are wound on bead 10' as shown in Figure 2 so as to avoid cross-coupling effects in the circuit.