GB2158642A

GB2158642A - Hall plate encapsulations

Info

Publication number: GB2158642A
Application number: GB08511532A
Authority: GB
Inventors: Leif Erik Edlund; Lars-Anders Olofsson
Original assignee: Telefonaktiebolaget LM Ericsson AB
Current assignee: Telefonaktiebolaget LM Ericsson AB
Priority date: 1984-05-11
Filing date: 1985-05-07
Publication date: 1985-11-13
Also published as: SE8402562D0; DE3514059A1; SE8402562L; FR2564246A1; SE444623B; GB8511532D0

Abstract

The encapsulation comprises a dielectric circuit plate 10a having an aperture which receives a projection 14 on a lower pole piece 12. The Hall plate 15, which is of semiconductor material with a doped sensitive region, is mounted on the projection 14 and its electrodes are connected to conductive tracks 19 on the circuit plate. An upper pole piece 11 is mounted on the circuit plate via a spacer 10b and has a projection 13 which overlies the Hall plate and defines a small active air gap. The assembly may be placed in an air gap in a magnetic circuit carrying a coil the current flowing in which can thereby be measured. The circuit plate can be extended and may carry other circuit elements connected to the Hall plate by conductive tracks (Figure 5). <IMAGE>

Description

SPECIFICATION Fastening device for retaining a hall plate in a magnetic field The present invention relates to a fastening device for retaining a Hall plate in a magnetic field, the plate comprising, for example, a semiconductor plate with a doped surface layer.

Hall effect devices or so-called Hall plates of semiconductor material are fragile and must be retained by a fastening means in order to use them in practical applications. British Patent Specification No. 2 106 710 teaches fastening means for Hall plates comprising a body, e.g. of plastics, into which the plate is moulded, and into which pole plates of ferromatic material can also be moulded. The semiconductor plate is enveloped on both sides by dielectric material and has a large air gap between the pole plates. When measuring the magnetic field in the air gap of an iron core, this has a severe weakening effect on the magnetic field in the core.

Fastening devices for Hall plates are also known from US Patent Specification 2 725 504, where they are disclosed as consisting of insulating layers on the top and bottom sides of the semiconductor, with the pole plates of ferromagnetic material on the outsides of these layers. In this case as well, the semiconductor plate of dielectric material is enveloped on both sides so that the air gap will be large. Hall plates with this type of fastening means furthermore give a weak output signal due to the plate being thick. Hall plates with a strong output signal, which are thin, have their electrical connections on the top surface of the semiconductor plate and are difficult to insulate with this type of fastening.

According to the present invention, there is pro vided a fastening device which retains a Hall plate on a circuit plate for disposition in a magnetic field, wherein the circuit plate is provided with a hole, a pole plate of ferromagnetic material being fastened on one side of the circuit plate, said pole plate having a a projection inserted in said hole and on the end surface of which the Hall plate is arranged, and wherein a second pole plate of ferromagnetic mate rial is fastened by means of a spacing portion at the other side of the circuit plate in front of the hole in order to have a defined air gap between the second pole plate and the projection on the first pole plate.

The present invention will now be described, by way of example, with reference to the accompanying drawings, in which Figure 1 illustrates measuring a magnetic field in an iron core with the aid of a Hall plate; Figure2 illustrates in perspective a known Hall plate; Figure 3 is a cross-section of a fastening device in accordance with an example of the invention, together with a Hall plate; Figure 4 schematically illustrates a magnetic field for the fastening device, when measuring in an iron core; and Figure 5 illustrates an alternative embodiment of the invention with a fastening device built up on a plate.

Figure 1 illustrates a known use of a Hall plate for measuring electric current strength. An iron core 1 is provided with a coil 2 through which flows a current Im, which it is desired to measure. A gap 3 in the iron core accommodates a Hall plate 4 through which there passes a magnetic field B, the strength of which depends on the current Imb A known current 1h is supplied to the Hall plate, the output signal Vh of which responds to this current and to the magnetic field B. The output signal Vh thus gives a measure of the current m It is often a desire that the output signal Vh is large in relation to the current 1m which is to be measured.

The output signal Vh is in inverse proportion to the thickness of the Hall plate and an essential condition is therefore that the Hall plate is thin. Figure 2 illustrates a known such Hall plate 15. Aplate 5 of semiconductor material, e.g. silicon or gallium arsenide, has the active layer 7 of the plate on its upper surface 6. This layer has connection surfaces 8 for the current 1h and connection surfaces 9 for measuring the output signal Vh. The active layer 7 and the connection surfaces 8 and 9 have been achieved by a doping substance having been diffused into the base material at the surface of the plate. The layer 7 and the connection surfaces, the thickness of which is only about 1 Clam, have low resistivity compared with the base material in the plate 5.The connection surfaces are on the upper surface 6 of the plate 5, so that connection wires may be attached to the surfaces with conventional production techniques.

For obtaining a large output signal Vh, it is also an essential condition that the magnetic field strength at the Hall plate is large. As mentioned, a condition here is that the air gap is small, although the field strength can be affected by the magnetic field being concentrated to the Hall plate. In Figure 2 there is illustrated an embodiment of a fastening device in accordance with the invention in which these desires are realized. Two poles plates 11 and 12 of ferromagnetic material, each have projections 13 and 14, respectively, which are mutually opposite and also have their opposite surfaces parallel. An air gap is formed between the projections, the surfaces of which are considerably less than those of the pole plates and the Hall plate 15, described above, is placed in the air gap.A frame 10 of dielectric material between the pole plates, which surrounds the projections, determines the distance between the pole plates and thereby the width of the air gap.

The pole plates are attached to the frame with the aid of glue, the frame being combined from a lower part 10a and an upper part 1 Ob. At its under surface, the Hall plate is attached to the projection 14, e.g. with the aid of glue, and has its upper surface 6 at a small distance from the projection 13. The base material in the plate 5 constitutes an insulating layer between the active layer 7 and the projection 14. Electrical connection wires 16 connect the connection surfaces 8 and 9 with the connection surfaces 17 on the lower part 10a of the frame. In turn, these are electrically connected to outer connection surfaces 18 on the outside of the frame with the aid of conductors 19.

The connection surfaces 17 and 18 together with the conductors 19 are applied to the lower part 1 Oa of the frame by a method such as that used for printed circuits before the upper part 1 Ob of the frame is attached to the lower part.

The air gap of the described fastening device may be small, since there is no insulation material around the Hall plate, and the distance between the projections 13 and 14 only needs to be large enough so that the plate 5 is accommodated therebetween with room for the connection wires 16. The magnetic field at the Hall plate is schematically illustrated in Figure 4. The pole plates 11 and 12 abut with their outwardly facing surfaces against the poles of the iron core 1, and by the pole plates having their least distance from each other at the projections 13 and 14 the magnetic field B is concentrated thereto.

The frame in the embodiment comprises two parts 1 Oa and 1 Ob, but may naturally be formed integrally, with openings for electrical conductors to the Hall plate. The fastening device described in connection with the embodiment is, as will be seen, a capsule which can be sealed, and very high demands on impermeability can be met. The frame 10 is of impervious material in such a case, e.g. ceramics, and the pole plates are attached to the frame with such as glass material or solder. In the latter case the frame has a metal coating at the pole plates so that the solder can adhere to it.

An alternative embodiment of the invention is illustrated in Figure 5, where a fastening device 20 is built up on a circuit plate 21 carrying electric circuits 22, with the circuit plate as a part of the fastening device. The pole plate 12 is attached to the underside of the circuit plate with its projection 14 in a hole in the circuit plate. On the upper side of the circuit plate there is a frame part 24 attached so that it surrounds this hole, and the pole plate 11 is attached to the frame with its projection 13 directly opposite the projection 14. The Hall plate 15 on the projection 14 is connected to the electrical circuits 22 on the circuit plate via the connection wires 16 and conductors 23 printed on the surface of the circuit plate.

Claims

1. A fastening device which retains a Hall plate on a circuit plate for disposition in a magnetic field, wherein the circuit plate is provided with a hole, a pole plate of ferromagnetic material being fastened on one side of the circuit plate, said pole plate having a projection inserted in said hole and on the end surface of which the Hall plate is arranged, and wherein a second pole plate of ferromagnetic mate rial is fastened by means of a spacing portion at the other side of the circuit plate in front of the hole in order to have a defined air gap between the second pole plate and the projection on the first pole plate.

2. Afastening device according to claim 1, wherein the spacing portion is provided by a closed frame.

3. A fastening device according to claim 1 or 2, wherein the circuit plate and the spacing portion are of ceramic material.

4. A fastening device according to claim 2 or 3, wherein the pole plates are fastened to the spacing portion and the circuit plate respectively with the aid of a diffusion impervious material.

5. Afastening device which retains a Hall plate, substantially as herein described with reference to Figure 3 or Figure 5 of the accompanying drawings.

6. Afastening device which retains a Hall plate for disposition in a magnetic field, comprising a frame defining a hole, a pole plate of ferromagnetic material fastened on one side of the frame, said pole plate having a projection inserted in said hole and on the end surface of which the Hall plate is arranged, and a second pole plate of ferromagnetic material fastened by means of a spacing portion at the other side of the frame in front of the hole in order to have a defined air gap between the second pole plate and the projection on the first pole plate.