GB2301230A - Toroidal core with a heat sink - Google Patents

Abstract

A toroidal magnetic core 40 includes a flat surface 42 which is arranged to be in contact with a non-ferrous metallic heat sink 50. The flat surface 42 may be formed on the outer surface of the core 40 and the toroidal magnetic core 40 may be D-shaped. An array of such toroidal cores may be arranged on a single heat sink 50 and can be mounted on a printed circuit board 51, such that each of the toroid cores 40 extends through a hole in the printed circuit board 51. Each row of toroid cores in the array may form a filter with non-adjacent rows operating at adjacent frequency levels. The above toroidal core arrangement may be used in a radio.

Description

RADIO FREQUENCY TOROID ASSEMBLY AND A RADIO Field of the Invention This invention relates to a radio frequency toroid assembly suitable for, but not exclusively limited to, use in an SSB radio harmonic filter. It also relates to a radio which includes such an assembly.

Background of the Invention SSB (Single Side Band) radios operating in high frequency ranges transfer transmitted power to an antenna through RF (Radio Frequency) coils of a harmonic filter. The harmonic filter filters out unwanted harmonics of the wanted signal. Each RF coil has a finite Q factor and therefore dissipates RF power as heat.

There is a need to provide high inductance coils in a small space and a suitable component for a high inductance coil is a toroid. A toroid is a coil wound around a core which extends in a circuit. A toroid conducts magnetic flux around a closed circuit (through the core). To provide a compact coil, high frequency range SSB radios use cores made of iron powder material. The maximum heating limit of the core is 125 degrees C.

Overheating above this temperature can cause damage to the core. SSB base stations can be required to operate in continuous wave transmission, resulting in continuous generation of heat.

There is a need for a toroid assembly having improved thermal characteristics.

Summarv of the Invention According to the present invention, a radio frequency toroid assembly is provided comprising a toroid having a magnetic core with a flat surface and a non-ferrous metallic heat spreader in contact with the flat surface of the magnetic core.

By coupling the core directly to a metallic non-ferrous body, heat is dissipated away from the toroid core. The body can additionally be used as a mount which can be coupled to a chassis. The electrical and electromagnetic properties of the coil are preserved.

By mounting several cores on the same mount, mutual heat exchange between cores can take place.

In a second aspect of the invention, a radio is provided comprising an assembly as defined above.

Brief Description of the Drawings FIG. 1 is a stereoscopic view of a prior art toroid core.

FIG. 2 illustrates the core of FIG. 1 mounted in an assembly known in the art.

FIG. 3 illustrates a known HF SSB harmonic filter incorporating seven of the assemblies shown in FIG. 2.

FIG. 4 is a side view of a toroid in accordance with the preferred embodiment of the invention.

FIG. 5 illustrates the toroid of FIG. 4 mounted in an assembly.

FIG. 6 is a schematic diagram of an HF SSB harmonic filter assembly incorporating a number toroids according to FIG. 4.

FIG. 7 is a stereoscopic drawing of the assembly of FIG. 6 with one of the toroids in position and the assembly mounted on a printed circuit board.

Detailed Description of the Drawings Referring to FIG. 1, an existing toroid core is shown. The core 10 comprises iron powder material formed in a cylindrical shape with a central hole. The entire surface of the core is coated in electrically insulating material. A toroid is formed around the core by winding insulated copper wire in a number of turns, each turn passing through the central hole.

FIG. 2 illustrates the toroid core of FIG. 1 mounted in a nylon plastic chassis 11. The chassis 11 merely provides mechanical support. The arrangement illustrated in FIG. 2 is known for use in a mobile SSB radio, and is therefore typically not used with a duty cycle of more than 1:4. In such a radio, heat dissipation is not a critical factor.

FIG. 3 illustrates a complete harmonic filter comprising seven individual chassis 11, 12 to 17, each having three toroids. In the case of chassis 11, the toroids are numbered 20, 21 and 22. Chassis 12 has toroids 23, 24 and 25 and chassis 17 has toroids 26, 27 and 28. RF switching circuitry 30 and 31 selectively switches one of the filters (depending on the frequency range) mounted within chassis 11, 12 to 17 between an RF input 32 and RF output 33.

Referring to FIG. 4, an improved toroid 39 is illustrated having a core 40 around and through which is wound a wire 41. The core 40 is Dshaped, with a flat surface 42. All the surfaces of the core 40, except the flat surface 42 are coated with an electrically isolating coating.

Whereas alternative arrangements are possible, an example being an arrangement in which the flat surface lies in a plane parallel to the circuit of magnetic flux 43, is preferred that the flat surface lies in a plain which is generally tangential to the flat surface, as shown in FIG. 4.

An advantage of the arrangement shown is that it is found that the re-design of the toroid from the ideal cylindrical shape to the D-shape does not substantially degrade its magnetic and electrical characteristics.

Referring to FIG. 5, it is illustrated that the core 40 is mounted on a core base, or heat spreader 50. The heat spreader 50 is a planar sheet of copper coated with solder. The heat spreader 50 is mounted underneath a printed circuit board (PCB) 51 and the core 40 protrudes through an aperture 52 in the PCB 51. The core 40 is fixed to the heat spreader 50 by reflow soldering.

It is found that coupling of the iron powder metallic core 40 to a nonferrous heat-conducting heat spreader 50 does not adversely affect the magnetic flux flow in the iron powder core, but effectively absorbs the heat from the core.

In the preferred embodiment, a number of cores equivalent to core 40 are mounted together on a single heat spreader 50 to form a filter module.

The preferred filter module comprises seven filters operating over seven contiguous frequency ranges from 1.6 MHz to 30 MHz. Each filter comprises three toroids mounted side-by-side. The number of windings of copper wire 41 around each toroid core is selected according to the desired frequency range of the filter. The ranges of operation of the seven filters are set out in table 1.

TABLE 1 Range Frequencies 1 1.6 to 2.43 MHz 2 2.43 to 3.7 MHz 3 3.7to5.6MHz 4 5.6to8.5MHz 5 8.5 to 12 MHz 6 12 to 19.8 MHz 7 19.8 to 30 MHz Thus, 21 toroids are mounted on a single heat spreader 50 in seven rows of three toroids per row. Such a module is labelled module 60 in FIG.

6. Module 60 comprises seven filters 61 to 67 operating over frequency ranges 1 to 7 respectively. It can be noted from the figure that, with the exception of filters 64 and 65, filters operating over adjacent frequency ranges are positioned in non-adjacent positions in the module 60. This is found to be advantageous in reducing the influence of one filter over an adjacent filter. Each of the filters 61 to 67 is connected to an input switch of an input switch bank 70 and an output switch of an output switch bank 71.

The input switch bank connects an input 72 to the various filters and the output switch bank 71 connects the various filters to an RF output 73. The switch banks 70 and 71 are under the control of RF switching circuitry 74.

The arrangement of FIG. 6 has the advantage that the module 60 can be formed as a discrete module manufacture using surface mount technology and soldered to the other components by means of leads 75.

FIG. 7 shows that the PCB 51 with it heat spreader 50 and various toroids 40 (arranged in seven rows of three toroids per row or such other configuration as is necessary to span the desired frequency range) is, when in use, mounted as a module in a cut-out hole 78 of a larger PCB 79 of an SSB radio. The leads 75 support the PCB 51 of the module 60. The leads 75 are connected to copper tracks on the surface of PCB 79 by a surface mount technique, that is to say by first gluing the leads 75 to the tracks with solder paste and then reflow soldering.

The filter module 60 can be manufactured with ease using surface mount technology. If a filter module 60 is faulty, it can readily be replaced.

The preferred embodiment of the invention as described is considerably more compact than prior art arrangements. Thus, the surface area of the PCB 78 of the filter of FIG. 6 can be 116to or 1/8th of the surface area of the PCB of the filter shown in FIG. 3. This represents a very substantial reduction in size, weight and cost.

Claims (8)

Claims

1. A radio frequency toroid assembly comprising: a toroid having a magnetic core with a flat surface and a non-ferrous metallic heat spreader in contact with the flat surface of the magnetic core.

2. An assembly according to claim 1, wherein the toroid conducts magnetic flux around a circuit through the core and the flat surface lies in a plane which is generally tangential to the flux circuit.

3. An assembly according to claim 2, wherein the core is D-shaped in cross-section.

4. An assembly according to any one of claims 1 to 3, wherein the heat spreader is generally planar and a plurality of similar toroids are disposed with the flat surfaces of their respective cores being in contact with the planar heat spreader.

5. An assembly according to claim 4 further comprising a printed circuit board extending across the planar heat spreader, with the toroids protruding through apertures in the printed circuit board.

6. An assembly according to claim 5, comprising a plurality of rows of toroids, together forming a radio frequency filter operating over a plurality of contiguous frequency ranges, each row being a part of a radio frequency filter operating over a separate one of said plurality of frequency ranges.

7. An assembly according to claim 6, wherein filters operating over adjacent frequency ranges are dispersed across non-adjacent rows.

8. A radio comprising a printed circuit board with a hole formed in the printed circuit board and an assembly according to any one of claims 5, 6 and 7 mounted in the hole.