GB2263362A

GB2263362A - Radio interference suppression device

Info

Publication number: GB2263362A
Application number: GB9200153A
Authority: GB
Inventors: Sidney E Francis
Original assignee: Pag Ltd
Current assignee: Pag Ltd
Priority date: 1992-01-06
Filing date: 1992-01-06
Publication date: 1993-07-21
Anticipated expiration: 2012-01-06
Also published as: GB9200153D0; GB2263362B

Abstract

A capacitor for suppressing interference in a switched mode circuit is connected to a transformer linking the input and output sections of the circuit. The capacitor is placed outside the core (10) wound assembly of the transformer and comprises parallel conductive plates 21, 22 separated by a dielectric layer 23. The dielectric layer (23) projects beyond the edges of the plate to define a path around the dielectric layer greater in length than the plate separation. The capacitor may be separate from the transformer or, as shown, comprise concentric tubular members surrounding the core (10) on which is wound the primary and secondary windings. <IMAGE>

Description

Radio Interference Suppression Device The present invention relates to devices for suppressing radio interference from electrical equipment, and in particular from equipment comprising primary and secondary sections.

Electrical equipment containing switch mode circuits having primary and secondary sections is liable to create radio interference due to capacitative energy transfer between the sections. This is a problem if the equipment is used near radio devices. The problem is particularly acute in Safety Class II equipment as defined by British Standard 3456, which is not earthed.

The radio interference can be reduced by connecting a capacitor between the primary and secondary sections.

However, known capacitors are not suitable for this duty.

Accordingly, a first aspect of the invention provides a capacitor for suppressing radio interference in a switch mode circuit having input and output sections linked by a transformer, the transformer comprising a primary and secondary coil wound around a core to form a wound assembly, wherein the capacitor is positioned outside the wound assembly and comprises a pair of parallel conductive plates separated by a gap that is filled by a dielectric layer, and means for connecting the capacitor between the parts of the transformer connected to the input and output sections of the circuit respectively, the dielectric layer projecting beyond the edges of the plates to define a path around the dielectric layer, between the edges of the plates, that is greater in length than the gap between the plates.

In a preferred arrangement, said path is at least 8 mm long.

Preferably, the plates are coterminous and the dielectric layer projects at least 4 mm from the edges of the plates.

Preferably, the capacitor surrounds the wound assembly and comprises a tubular section of dielectric material sandwiched between inner and outer conductive plates, each plate comprising a concentric tubular section of conductive material. The plates may have a gap extending therealong. The gaps may be radially aligned.

Alternatively, the capacitor may be a separate unit from the transformer.

Preferably, the dielectric layer comprises one or more of layers of isolation tape. The tape may be of a type used in said transformer construction.

Preferably, the dielectric layer comprises three layers of type fifty six transformer isolation tape.

According to a second aspect of the invention there is provided a transformer comprising a core around which primary and secondary coils are wound, wherein the coils and core are housed in a capacitor which is connected between the primary and secondary coils and which comprises a tubular section of dielectric material sandwiched between concentric tubular conductive layers.

The invention provides a capacitor which is known to meet the required safety requirements in its construction. The capacitor may be created in two different ways.

The first approach to the capacitor construction is to create it from two concentric strips of metal foil wound as the final winding on a power transformer. The foils are separated by an insulating dielectric of a quality that meets the requirements of the relevant international safety standards (normally three layers of insulating tape of the required dielectric strength).

In order to meet the required creepage and clearance distance (normally 8 mm) the width of the insulating dielectric is the full width of the winding window, and the width of each of the foils is 8 mm less than the window. The distance from one of the folds to the other round the edge of the insulation is the required 8 mm.

The lead-out connections from the capacitor are normally two suitable wires, one connected to each of the foils. The lead-out wires connect to suitable points in the primary and secondary circuits.

An alternative method is similar to the above method, but the capacitor is not part of the transformer construction. Two sheets of metal foil are placed one above the other with the required insulation to meet safety requirements sandwiched between them. The insulation material extends beyond the edges of the foil to provide the required creepage distance between the foils. A lead-out wire is connected to each foil and these terminate at suitable points in the primary and secondary circuits.

In order that the invention and its various other features may be understood more easily, an embodiment thereof will now be described by way of example only.

with reference to the drawings, wherein: Fig. 1 is a schematic diagram of a transformer incorporating a suppression device according to the present invention; Fig. 2 is a lateral cross-section through the transformer and suppression device; Fig. 3 is a section on 3-3 in Fig. 2; and Fig. 4 is a detail of region 4 in Fig. 3.

Fig. 1 illustrates the interference suppression device incorporated into a transformer of conventional construction. The function of the device is to reduce radio interference from equipment that incorporates rapid switching primary and secondary sections that are inductively linked by a transformer. The transformer comprises a core 10 around which are wound three secondary coils 12 and a primary coil 14. The transformer also comprises primary and secondary screens 15, 16 positioned between the primary and secondary coils which are layers of 0.05 mm copper sheet. The screens are electrostatic screens incorporated in the transformer to limit the transfer of capacitively coupled energy between the windings.

The primary and secondary screens are concentric and are separated by three layers of isolation tape. The coils and the screens are separated from each other also using isolation tape. The isolation tape is of a particular specification for transformer use. An example is type fifty six tape made by the 3M company.

The completed transformer is cylindrical in shape as shown in Fig. 2 and is wound on a former (18 in Figs. 3 and 4 only).

The suppression device comprises a hollow, cylindrical capacitor 20 that surrounds the transformer. The capacitor is insulated from the transformer and comprises two sheets of 0.05 mm copper 21, 22 separated by an insulating layer 23. A gap 24 runs along the full length of each copper sheet so that they do not form complete cylinders. The sheets are connected to the primary and secondary screens respectively. The voltage of the primary and secondary screens is the reference voltage for the input and output stages of the transformer respectively.

Figs. 3 and 4 illustrate the construction of the ends of the capacitor 20. The insulating layer 23 projects beyond the sheets 21, 22 by 4 mm at each end. The insulation layer 23 comprises three layers of said transformer isolation tape. The construction of the capacitor is identical to that of the structure formed by the primary and secondary screens 15, 16.

The transformer and suppression means are covered by an outer layer of said isolation tape (not shown in drawings).

In use, the capacitor captures capacitively coupled energy leaving the transformer and returns it to the primary coil 14.

The suppression device has a number of advantages for transformer applications.

(i) Current engineering practice requires a minimum air gap of at least 8 mm between primary and secondary stage components that are not insulated. In the present invention this is provided by the insulating layer 23 projecting 4 mm from the edges of the sheets 21, 22. Thus, the distance through the air from sheet 21 to sheet 22 is 4 mm + 4 mm + the thickness of the insulating layer, this total distance being greater than 8 mm. This is indicated as AfA+B in Fig. 4.

(ii) The suppression means uses standard isolation tape to form the insulating layer 23. This ensures that the suppress ion device is constructed to the same specification as the transformer, and makes for simplicity of construction.

The use of three layers of tape is preferred, because the likelihood of any flaws in the tape coinciding is very small.

(iii)The copper plates, 21, 22 separately act as shielding in addition to the primary and secondary screens 15, 16.

(iv) The suppression device provides effective suppression for equipment incorporating a transformer using a simple device that is easy to construct. The alternative, is to supply costly filtering equipment at the input stage of the transformer. The device is particularly suitable for use with non-earthed electrical apparatus, Safety Class II Apparatus.

It is not essential that the capacitor 20 be formed around the transformer. Instead, the capacitor can be formed as a separate unit. This can be added to existing transformers. The plates of the capacitor can be rolled together to form a compact structure.

The size of the air gap can be altered by altering the thickness of the insulating layer 23 and/or the distance A the insulating layer projects from the edges of the plates.

The device can be used in other units where there is an isolation gap and fast switching gives radio interference.

The number of secondary coils linked to the primary coil in the transformer can be varied.

Claims

1. A capacitor for suppressing radio interference in a switch mode circuit having input and output sections linked by a transformer, the transformer comprising a primary and secondary coil wound around a core to form a wound assembly, wherein the capacitor is positioned outside the wound assembly and comprises a pair of parallel conductive plates separated by a gap that is filled by a dielectric layer, and means for connecting the capacitor between the parts of the transformer connected to the input and output sections of the circuit respectively, the dielectric layer projecting beyond the edges of the plates to define a path around the dielectric layer, between the edges of the plates, that is greater in length than the gap between the plates.

2. A capacitor according to claim 1, wherein said path is at least 8 mm long.

3. A capacitor according to claim 2, wherein the plates are coterminous and the dielectric layer projects at least 4 mm from the edges of the plates.

4. A capacitor according to any preceding claim, wherein the capacitor surrounds the wound assembly and comprises a tubular section of dielectric material sandwiched between inner and outer conductive plates, each plate comprising a concentric tubular section of conductive material.

5. A capacitor according to claim 4, wherein each plate has a gap extending therealong.

6. A capacitor according to claim 5, wherein the gaps are radially aligned.

7. A capacitor according to any of claims 1 to 3, wherein the capacitor is a separate unit from the transformer.

8. A capacitor according to any preceding claim, wherein the dielectric layer comprises one or more of layers of isolation tape.

9. A capacitor according to claim 8, wherein the tape is of a type used in said transformer construction.

10. A capacitor according to claim 9, wherein the dielectric layer comprises three layers of type fifty six transformer isolation tape.

11. A capacitor substantially as herein described, with reference to the accompanying drawings.

12. A transformer substantially as herein described.

12. A transformer comprising a core around which primary and secondary coils are wound, wherein the coils and core are housed in a capacitor which is connected between the primary and secondary coils and which comprises a tubular section of dielectric material sandwiched between concentric tubular conductive layers.

13. A transformer substantially as herein described.

Amendments to the claims have been filed as follows 1. A capacitor for suppressing radio interference in a switch mode circuit having input and output sections linked by a transformer, the transformer comprising a primary and secondary coil wound around a core to form a wound assembly, wherein the capacitor is positioned outside the wound assembly and comprises a pair of parallel conductive plates separated by a gap that is filled by a dielectric layer comprising one or more layers of isolation tape, and means for connecting the capacitor between the parts of the transformer connected to the input and output sections of the circuit respectively, the dielectric layer projecting beyond the edges of the plates to define a path around the dielectric layer, between the edges of the plates, that is greater in length than the gap between the plates.

2. A capacitor according to claim 1, wherein the tape is of a type used in said transformer construction.

3. A capacitor according to claim 2, wherein the dielectric layer comprises three layers of type fifty six transformer isolation tape.

4. A capacitor according to any preceding claim, wherein said path is at least 8 mm long.

5. A capacitor according to claim 4, wherein the plates are coterminous and the dielectric layer projects at least 4 mm from the edges of the plates.

6. A capacitor according to any preceding claim, wherein the capacitor is a separate unit from the transformer.

7. A capacitor according to any preceding claim, wherein the capacitor surrounds the wound assembly and comprises a tubular section of dielectric material sandwiched between inner and outer conductive plates, each plate comprising a concentric tubular section of conductive material.

8. A capacitor according to claim 7, wherein each plate has a gap extending therealong.

9. A capacitor according to claim 8, wherein the gaps are radially aligned.

10. A capacitor substantially as herein described, with reference to the accompanying drawings.

11. A transformer comprising a core around which primary and secondary coils are wound, wherein the coils and core are housed in a capacitor which is connected between the primary and secondary coils and which comprises a tubular section of dielectric material sandwiched between concentric tubular conductive layers.