Transformer

Abstract

 The transformer comprises a core (1a, 1b) which is made of a soft-magnetic material and which comprises at least one limb (3a, 3b) on which a first and a second transformer winding (9a, 9b) are arranged one behind the other in the axial direction. The core comprises two portions (1a, 1b), each of which comprises at least a portion (3a, 3b) of the limb and is arranged so that the limb portions face one another and are situated one in the prolongation of the other. The first and second core portions (1a, 1b) and the transformer windings (9a, 9b) provided thereon are isolated from one another by an electrically insulating member which comprises a flat plate (11) which extends transversely of the longitudinal direction of the limb (3a, 3b) and which constitutes a partition between the core portions and projects a predetermined distance beyond the circumference of the core (1a, 1b) and the transformer windings (9a, 9b) in all directions. The transformer thus consists of two portions which are electrically fully isolated from one another. The air and creepage paths between these portions may be long, the dimensions of the transformer nevertheless being small.

Description

[0001] The invention relates to a transformer, comprising a core which is made of a soil-magnetic material and which comprises at least one limb on which a first and a second transformer winding are provided one behind the other in the axial direction, said first and second transformer windings being isolated from one another by an electrically insulating member, said core comprising two portions, each of which comprises at least a portion of the limb and is arranged so that the limb portions face one another and are situated one in the prolongation of the other.

[0002] A transformer of this kind is known from DE-A-1 932 174. The first transformer winding constitutes, for example the primary winding and is then generally connected directly or indirectly to the power supply mains. For reasons of safety the second winding (constituting the secondary winding) should then be adequately electrically isolated from the first winding. Generally speaking, the requirement is that the distance between the first and the second winding should at least have a given value through the air (air path) as well as via solid surfaces (creepage path). A customary requirement as regards the creepage path is, for example at least 6 mm. The electrically insulating member arranged between the first and the second coil former serves to increase the air and creepage paths without substantially increasing the dimensions of the transformer. In the known transformer, the ends of the first and the second coil former which are remote from one another must be provided with comparatively thick flanges because otherwise the distance between the two windings and the core would become inadmissibly small. This is because the core, to be considered to be electrically conductive, would then constitute a conductive connection between the two windings. The thick flanges, however, have the drawback that they increase the dimensions of the transformer.

[0003] It is an object of the invention to provide a transformer of the kind set forth whose dimensions may be smaller than those of the known transformer, all safety requirements nevertheless being satisfied.

[0004] To achieve this, the transformer in accordance with the invention is characterized in that the insulating member comprises a flat plate which extends transversely of the longitudinal direction of the limb and which constitutes a partition between the core portions and projects a predetermined distance beyond the circumference of the core and the transformer windings in all directions. The plate constitutes a complete electrical insulation not only between the first and the second winding but also between the core portions on which the first and the second coil former are provided. As a result, the transformer in accordance with the invention essentially consists of two electrically fully isolated portions.

[0005] Because a creepage path of at least 6 mm is required in many cases, a preferred embodiment of the transformer in accordance with the invention is characterized in that the plate extends at least 3 mm beyond the circumference of the core and the transformer windings in all directions.

[0006] An embodiment of the transformer in accordance with the invention which can be particularly simply and inexpensively manufactured is characterized in that the first and the second transformer winding are provided on a first and a second coil former, respectively, which coil formers constitute, in conjunction with the plate, a coil support in the form of an integral unit made of a suitable, electrically insulating plastic.

[0007] The invention will be described in detail hereinafter with reference to the drawing.

Fig. 1 is a perspective view of an embodiment of a transformer in accordance with the invention, without windings, and

Fig. 2 is a plan view of the transformer of Fig. 1, provided with the windings.

[0008] The transformer shown in the Figs. 1 and 2 comprises a core which is made of a soft-magnetic material, for example ferrite, and which consists of a first portion 1a and a second portion 1b. Each core portion 1a, 1b in the embodiment shown is E-shaped. It comprises a portion 3a, 3b of a limb (denoted by a dashed line in Fig. 2) and a portion 5a, 5b of a yoke. The core portions 1a and 1b are arranged so that the limb portions 3a and 3b face one another and are arranged one in the prolongation of the other in order to form a straight, rod-shaped limb, the yoke portions 5a and 5b forming an annular yoke surrounding the limb. On the first limb portion 3a there is provided a first coil former 7a on which a first transformer winding 9a is provided, the second limb portion 3b supporting a second coil former 7b on which a second transformer winding 9b is provided. Between the coil formers 7a and 7b there is provided a flat plate 11 which extends transversely of the longitudinal direction of the limb 3a, 3b and which constitutes, in conjunction with the coil formers, a coil support which is made as an integral unit of a suitable, electrically insulating plastic, for example by injection moulding.

[0009] The plate 11 constitutes a partition between the two core portions 1a, 1b and between the two transformer windings 9a, 9b, so that the transformer consists of two portions which are electrically fully isolated from one another. The length of the creepage paths for electric current, existing between these portions, is determined by the degree of projection of the plate 11 beyond the circumference of the core 1a, 1b and the transformer windings 9a, 9b. The plate 11 preferably projects a distance d of at least 3 mm beyond this circumference in all directions. In Fig. 1 the distance d is measured, in the horizontal as well as in the vertical direction, between the edge of the plate 11 and the nearest surface of the core 1a, 1b. However, it will be apparent that, should one of the windings 9a, 9b project above the yoke 5a, 5b, the distance d in the vertical direction should be measured between the upper side of the relevant winding and the edge of the plate 11. The length of the creepage path between the two transformer portions amounts to 2d plus the thickness of the plate 11 (approximately 0.4 mm). The air path between the transformer portions always equals at least the creepage path in the described construction. When d equals 3 mm, the creepage path thus amounts to at least 6 mm. The transverse dimensions of the transformer (width and height in Fig. 1) can be very small despite the comparatively long creepage path, for example no more than 25 mm.

[0010] At the ends of the coil formers 7a, 7b which are remote from the plate 11 there are provided flanges 13a, 13b which bound the transformer windings 9a, 9b in the axial direction in conjunction with the plate 11. Because the transformer consists of two electrically isolated portions due to the plate 11, it is not strictly necessary to insulate the transformer windings electrically from the core portions 1a, 1b. Therefore, if desirable, the coil formers 7a, 7b and the flanges 13a, 13b can be omitted.

[0011] At the lower side of each core portion (see Fig. 1) there is provided a contact strip 15a, 15b which is made of an electrically insulating plastic and in which there are secured connection pins 17a, 17b which are electrically connected to the transformer windings 9a, 9b and which can be soldered, for example into a printed circuit board. The contact strips 15a, 15b may form part of said coil support. However, they may alternatively be separately manufactured and bonded to the core 1a, 1b, for example by means of a suitable adhesive. In the absence of coil formers 7a, 7b, the plate 11 can also be bonded to the core portions 1a, 1b by means of a suitable adhesive.

[0012] The core of the described embodiment consists of two E-shaped portions 1a, 1b. The invention, however, can be used equally well for transformers comprising cores formed in a different manner and comprising more than one portion, for example cores consisting of I-shaped or C-shaped portions. In all cases the limb is formed by the portion of the core on which the windings are wound one behind the other, in the axial direction.

[0013] It is to be noted that the presence of the plate 11 between the core portions 1a, 1b means that an air gap is present in the magnetic circuit formed by these core portions. This may be advantageous because in some applications a transformer comprising an air gap may be necessary or desirable. The thickness of the plate 11 can be readily adapted to the desired size of the air gap. In the event of a very small air gap, the plate 11 may consist of one or more layers of a very thin, plastic foil.

Claims

1. A transformer, comprising a core (1a, 1b) which is made of a soft-magnetic material and which comprises at least one limb (3a, 3b) on which a first and a second transformer winding (9a, 9b) are provided one behind the other in the axial direction, said first and second transformer windings being isolated from one another by an electrically insulating member, said core comprising two portions (1a, 1b), each of which comprises at least a portion (3a, 3b) of the limb and is arranged so that the limb portions face one another and are situated one in the prolongation of the other, characterized in that the insulating member comprises a flat plate (11) which extends transversely of the longitudinal direction of the limb (3a, 3b) and which constitutes a partition between the core portions (1a, 1b) and projects a predetermined distance beyond the circumference of the core and the transformer windings (9a, 9b) in all directions.

2. A transformer as claimed in Claim 1, characterized in that the plate (11) projects at least 3 mm beyond the circumference of the core (1a, 1b) and the transformer windings (9a, 9b) in all directions.

3. A transformer as claimed in Claim 1 or 2, characterized in that the first (9a) and the second transformer winding (9b) are provided on a first (7a) and a second coil former (7b), respectively, which coil formers constitute, in conjunction with the plate (11), a coil support in the form of an integral unit of a suitable, electrically insulating plastic.

Drawing