Transformer With Magnetic Screening Foils

The invention relates to a transformer comprising a ferromagnetic core having one leg which supports a primary winding and at least one secondary winding with said windings being coaxial and insulated from each other, and between the windings and said leg and also between the primary and the secondary windings there are provided conductive screening foils which comprise electrical connections.

Transformers of this kind are used, for example, in switched mode power supplies with mains isolation. The screening foils serve for the capacitive screening of the primary and secondary windings so that the high-frequency voltages occurring therein cannot cause external interference. To this end, part of the screening foils is primarily grounded via the connections and another part is secondarily grounded. When a large magnetic flux is present at the area of the screening foils, for example, because the leg on which the windings are accommodated comprises an air gap, substantial eddy current losses occur in the screening foils.

An object of the invention is to provide a transformer of the described kind in which such eddy current losses in the screening foils are substantially smaller. To this end, the transformer in accordance with the invention is characterized in that at least the screening foil which is situated between the windings and the core leg is made of a soft-magnetic material.

The invention is based on the intuitive perception that the flux which crosses from the air gap to the other legs of the core can be substantially reduced by making at least the first screening foil thicker than the electromagnetic penetration depth. For non-ferromagnetic materials such as copper, the penetration depth is generally so large that very thick and hence expensive foils which are difficult to wind are required. Screening foils made of soft-magnetic materials, however, offer excellent results, even in the case of a small thickness.

It is to be noted that magnetic screens are known per se, for example, from German Auslegeschrift No. 1,237,677 and French patent specification No. 894,466. However, these references are not concerned with screening foils comprising electrical connections for capacitive screening.

A preferred embodiment of the transformer in accordance with the invention is characterized in that the screening foils are made of a soft-magnetic material and are covered with a layer of a material which is highly electrically conductive.

A step of this kind is known for the formation of short-circuited turns on a magnetic screen, see IBM Technical Disclosure Bulletin, Vol 12, Nr. 10 (March 1970), pp 1623-1624.

The invention will be described in detail hreinafter with reference to the accompanying diagrammatic drawing. The drawing is a side elevation and a partial sectional view of an embodiment of a transformer in accordance with the invention.

The transformer comprises a ferromagnetic core 1 which consists of two halves and which is made, for example, of ferrite, said core comprising two outer legs 3 and a central leg 5 which comprises an air gap 7 in order to prevent excessively fast saturation of the core. Around the central leg 5 there is arranged an insulating coil former 9 on which connection blocks 11 are formed, metal connection pins 13 being secured in said connection blocks 11.

Around the coil former 9 there are successively coaxially wound: a first conductive screening foil 15 made of a soft-magnetic material, a primary winding 17, a second conductive screening foil 19 also made of a soft-magnetic material, a first insulating foil 21 made of a synthetic material, a third conductive screening foil 23 made of a soft-magnetic material, a secondary winding 25, and a second insulating foil 27 made of a synthetic material.

The first and second screening foils 15 and 19 are connected, via connection wires 29 and 31, to a common connection pin 13 which, when the transformer is built into a circuit, is grounded on the primary side. The third screening foil 23 is connected, via a connection wire 33, to another connection pin 13 which is grounded on the secondary side. As a result, the primary and secondary windings 17 and 25 are capacitively screened so that no disturbing electromagnetic radiation can escape into the environment.

Because the screening foils 15, 19, 23 are made of a soft-magnetic material, their thickness, which itself is comparatively small (for example, approximately 50 .mu.m), is larger than the electromagnetic penetration depth for the frequency used (for example, approximately 300 kHz), so that magnetic flux cannot cross from the air gap 7, via the screening foils, to the outer legs 3. Eddy current losses are thus prevented in the screening foils. The screening foils may be made of, for example, mumetal or amorphous iron. In order to improve the conductivity, they may be covered, if desired, with a layer of a highly electrically conductive material, for example, tin.

Even though it is generally sufficient for the prevention of eddy current losses that only the first screening foil 15, which is situated between the leg 5 and the windings 17, 25, be made of a soft-magnetic material, it may be advantageous for the simplification of the manufacturing process to make all of the screening foils 15, 19, 23 of this material, as in the described embodiment.