GB2218266A

GB2218266A - Electrical power resistor

Info

Publication number: GB2218266A
Application number: GB8907857A
Authority: GB
Inventors: Alfred Hodek; Ernst Karich; Wilhelm Rummel
Original assignee: Draloric Electronic GmbH
Current assignee: Draloric Electronic GmbH
Priority date: 1988-05-03
Filing date: 1989-04-07
Publication date: 1989-11-08
Anticipated expiration: 2009-04-07
Also published as: GB2218266B; DE3814987A1; FR2631154A1; GB8907857D0

Description

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22182266 1 Electrical Power Resistor The invention relates to. an electrical power resistor having an electrically insulating supporting member on which a resistance element is disposed.

Power resistors are known which comprise a resistance element in the form of a resistance wire or in the form of a resistance strip on a ylindrical supporting member of electrically insulating material. Such power resistors may be coated with a glaze or with a cement. They have a power density which is of the maximum order of magnitude of 2 watts per cm2. Power resistors in which a cylindrical supporting member provided with a resistance element in wire or strip form is disposed in a.housing of electrically insulating material have a greater power density of the order of magnitude of about 6 watts per cm2. A further increase in the power density is not possible in the known electrical power resistors because such a further increase in the power density would lead to local overheating of the supporting member or to inadmissibly great differences in temperatOre along the supporting member. Such differences in temperature result in inadmissible heat expansion which, in the extreme case, can lead to cracks or to fracture of the supporting member.

The present invention seeks to provide an electrical power resistor of the abbve-mentioned type with which power densities up to 25 wat ts per cm 2 and greater are possible.

According to a first aspect of the present invention there is provided an electrical power resistor having an J 2 electrically insulating supporting member on which a resistance element is disposed, wherein the supporting member is of plate-shaped construction and comprises at 1.east two insulating layers disposed one above the other, a planar temperature-equalisation element of a heat- conducting material being provided between adjacent insulating layers and the temperature-equalisation element and the associated insulating layers adjoining one another.

The individual insulating layers are preferably relatively thin, plateshaped ceramic members of aluminium oxide or of a similar electrically insulating material. The temperature-equalisation element may be a foil or a sheet of metal. For example, copper or aluminium foils or a sheets may be used. As a result of the construction of the supporting member from insulating layers and temperature-equalising elements arranged alternately one over the other and adjoining one another, there is the advantage that there are extremely short distances between the place where the heat is generated and the cooling medium, as a result of which the energy density can be comparatively very great without damage occurring tQ the power resistor.

As a result of the temperature-equalisation element provided between adjacent insulating layers there is a satisfactory heat conduction through the temperatureequalisation element so that possible differences in temperature between different parts of the power resistor, that is to say of the resistance element, are avoided, as a result of which, the risk of fracture is considerably reduced or eliminated up to comparatively 2 high power densities of 25 watts per cm and more.

i k_ 1 3 The plate-shaped supporting member may, of course, also comprise more than two insulating layers arranged one above the other a temperature- equalisation element being p-rovided between adjacent insulating layers in each case. The mechanic-ally firm connection between the insulating layers and the temperature-equalisation elements between adjacent insulating layers may be effected. for example. by external securing elements such as screw elements, clamping elements or the like. It is also possible, however, to connect the insulating layers to the temperature-equalisation elements provided in between, for example by conductive adhesives or by adhesives filled with ceramic or by any other adhesive materials.

It has proved advantageous for the temperatureequalisation element to have smaller dimensions in area than the insulating layers contiguous at the two opposite sides of the temperature-equalisation element. In this case, there is preferably a surrounding free area on each insulating layer between the peripheral edge of the /or each temperature- equalisation element and the peripheral edges of the two insulating layers adjacent to the temperature-equalisation element. In this way, depending on the number of insulating layers, a long creepage path, determined in particular by the free areas, results between the resistance elements and an external part of the power resistor or an earth connection, which may be a cooling element for example, so that, apart from the advantage of a very high power density, the further advantage results that such an electrical power resistor has a high dielectric strength or insulating strength. In this case, insulating strengths of the order of magnitude up to 6 kV or more are possible.

1 4 The last insulating layer remote from the resistance element can be dispbsed on a plan supporting surface of a cooling member. This cooling member may be a plateshaped element of solid metal, a cooling member provided with cooling fins or a cooling member which has forced cooling. This forced cooling may involve a coolant which is brought into contact with the cooling member. Air or a cooling liquid may be considered as coolant. - According to how the resistance element of the electrical power resistor is constructed, electrical resistance values betw een less than /equal to 0.1 ohm and greater than/ equal to 100 k ohm can be realised. If the resistance element is an etched or stamped foil structured by a laser beam, resistance values of the order of magnitude between 0.1 and 100 ohms can be produced. If the resistance element consists of a wire or strip material, resistance values between 1 ohm and 1 k ohm in order of magnitude can be produced. It is also possible to apply the resistance element to the supporting member in the form of a thick film, for example of cermet, structured in path form, which is done by a screen printing process for example. In this case, resistance values between 1 ohm and 100 k ohm in order of magnitude can be realised. Naturally, larger or smaller resistance values of the electrical power resistor can also be produced with the said different resistance elements. Further advantages of the power resistor according to the invention consist in that its inductance is comparatively low regardless of whether the resistance element is provided in planar or meander form or in the form of a flat coil.

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Thus the inductance in a power resistor with a silkscreen printed thickfilm resistance element of the 9rder of magnitude of about 4 ohms is only 0.08 pH. If the res istance element is constructed in meander shape or in the form of a flat coil, its inductance is about 0.15)iH or less. A further advantage is afforded by the high impulse loading capacity of the power resistor. In addition, the power resistor according to the invention is very user-friendly because its external dimensions can easily 'be adapted as desired.to the particular application of each user. It is likewise possible that the user may mount the power resistor according to the invention on a cooling device provided for it.

In a power resistor according to the invention, wherein the resistance element i's connected to connected to connecting members, the resistance element and the insulating layers of the supporting member with the at least one temper atur e-equal isatIon element provided between the insulating layers can be covered with an electrically insulating covering out of which the connecting members project. Naturally, it is also possible to construct the power resistor without an electrically insula. ting covering and to provide only the connecting members with an insulation in order to ensure adequate insulation between the connecting members and the at least one possible cooling member.

In this manner, a quasi -:symmetrical construction of the power resistor results and hence a further increase in the possible powerdensity of the electrical power resistor. In this construction of the power resistor also, each temper ature-equalisat ion element preferably has smaller dimensions in area than the insulating layers which are contiguous on the two opposite sides of 6 k each temperature-equalisation element. In order to achieve a high dielectric strength in this case,it is an advantage for there to be a surrounding free area on ea-ch insulating layer between the peripheral edge of each temperature-equalisation element and the peripheral edge of the two insulaing layers adjacent to the temperature-equalisation element. The creepage path between the resistance element provided between the supporting members and and an outer part of the power resistor, which may be an earth connection for example, or a cooling member, or two cooling members, or two cooling members remote from one another, is determined by these surrounding free areas of each insulating layer and by the adjoining surrounding narrow face of each insulating layer.

At least one of the two last insulating layers remote from the resistance element may be disposed on a plane supporting surface of a cooling member. In order to achieve a symmetrical heat dissipation, it is an advantage if the last two insulating layers remote frorc. the resistance element are each disposed on plane supporting surface of a cooling m ember. In this case, it is, of course, also possible for the two opposite cooling membe.rs to be connected to one another laterally beside the power resistor.

In an electrical power resistor of the kind described above, wherein the resistance element is connected to connecting members. the resistance element and the insulating layers of the two supporting members with the temperature-equalisation elements provided between the insulating layers, may be covered with an electrically insulating coveking out of which the connecting members 1 I 4.

2 0 1 30 7 project. Since such an electrically insulating covering, although it serves to improve the protection against contact, nevertheless can, on. the other hand, impair the radiation of heat into the environment of the electrical power resistor, it is also possible to dispense with such an electrically insulating covering and to provide only the connecting members of the resistance element with an electrically insulating covering, in order to insulate the connecting members sufficiently frqm the at least one cooling member which can be combined with the power resistor.

Regardless of whether the electrical resistor comprises a multi-layer supporting member on which the resistance element is disposed or whether the power resistor comprises two multi-layer supporting members between which the resistance element is provided, the resistance element is provided, the resistance element can lie against a mal n surface of the or each.supporting member. In this case, it is a question of a resistance element substructed in path-form or of planar construction, which may be of a foil or sheet material, of a thickfilm material or the like. It is also possible, however, for the resistance-element-to be wound round a plate-shaped substrate of. electrically insulating material. This substrate can be of the same material as the insulating,layers of the each supporting member. Since the plate-shaped substrate usually has only a small wall thickness or the order of magnitude of about 1 mm, even a resistance element thus constructed has only a relatively small inductance.

As. has already been stated, the resistance element may simply be planar in construction if the required X 8 resistance value of the power resistor is to be low, or the resistance element may be structured in path-form if higher resistance values of the power resistor are desired.

Preferred embodiments of the present invention will now be described, by way of example only, with reference to the accompanying drawings, of which:

Figure 1 shows a longitudinal section through a first embodiment of a power resistor in' accordance with the present invention; Figure 2 shows a longitudinal section through a second embodiment of the present invention; Figure 3 shows a longitudinal section through a third embodiment of the present invention; Figure 4 shows a section through a fourth embodiment of the present invention; and Figure 5 shows"a section through a fifth embodiment of the present invention which differs from the first four embodiments particularly in the construction of its resistance element.

Figure 1 shows a first embodiment of the electrical power resistor 10 which comprises a supporting member 12 and a resistance element 14 disposed on the supporting member 12." The resistance element 14 is a foil or a sheet of an electrical resistance material, which may be made with 1 large area or be structured in path-form. The resistance element 14 comprises two A 1 k.w 9 connecting members 16 which project away from the resistance element 14. in the same direction. The supporting member 12 comprises insulating layers 18 and a temper ature-equal isat ion element 20 provided between the two insulating layers 18. The insulating layers 18 are of plate- shaped construction. They consist, for example,, of an aluminium oxide c e r a m i c. The ternperature-equalisation element 20 is, for example a sheet or a foil of copper or aluminium. The resistance element 14 is provided, on the top surface remote from the supporting member 12, with an electrically insulating covering element 22 which has two recesses 24 through which the connecting members 16 of the--resistance element 14 extend. The mechanical connection of the covering element 22, of the resistance element 14, of the two insulating layers 18 and of the temperature-equal isat ion element 20 of the supporting member 12 is effected, for example, by means of connecting el,ements (not illustrated) such as screws, clamps or the like. it is also possible to bond the individual parts of the power resistor 10 to one another.

As can easily be seen from the drawing, the temperatureequalisation element 20 has smaller dimensions in area than each insulating layer 18 of the multi-layer supporting member 12. At the same time, the temperature-equalisation element 20 is preferably provided between the associated insulating layers 18 in such a manner that the temper atur eelement 20 forms a surrounding free area 26 on each insulating layer 18. The dielectric strength of the electrical power resistor 10 is determined by these surrounding free areas of the J Q insulating layers 18,and by the narrow face 28 extending round each insulating layer 18. The covering element 22 of-electrically.insulating material serves the same purpose.

is The electrical power resistor 10 illustrated in Figure 2 differs from the power resistor 10 shown in Figure 1 only in that it is provided with a plate-shaped cooling member 30. The same compon.ents are designated by the same reference numerals in Figures 1 and 2 so that it is superfluous to describe all these- components again in detail in connection with Figure 2.. The cooling member 30 can be assembled with the covering element 22, the resistance element 14, the insulating layers 18 and the temper atur e-equalisat ion element 20 by means of an adhesive or by means of another securing element (not illustrated), known per se, such as screws or clamps, to form a unit.

In Figure 3, an electrical power resistor 10 is illustrated wherein the resistance element 14, which has a plane surface and may be structured in path form, is disposed on a supporting member 12-which comprises three insulating layers 18 and two temperature-equalisation elements 20 provided between the insulating layers 18. The insulating layers 18 may all have the same wall thickness or may have different wall thicknesses from one another. The same applies to the wall thicknesses of the temperature-equalisation e lements 20. The insulating layer 18 furthest away from the resistance element 14 is in direct contact with a cooling member 30 over a large area. The resistance element 14 is provided with connecting members 16 which project out of an elect-ricaliy ins.ulating covering 32. The electrically insulating covering 32 covers the top surface 34 of the cooling member 30, at least for the most part. In this construction of the electrical power resistor 10 also, the temperature-equalisation elements 20 have extents in area which are smaller than the dimensions of the insulating layers 18 so that free areas 26 result which extend round the edge of each insulating layer 18.

Figure 4 shows a electrical power resistor 10 having a resistance element 14 with a plane surface which is arranged between two supporting members 12. Each of the supporting members 12 comprises three insulating layers 18 with temperature-equalisation elements 20 provided between adjacent insulating layers 18. The resistance element 14 is connected to connecting members 16 which project away from the power the power resistor 10 in opposite directions. The two supporting members 12 gripping the resistance element 14 are each in contact, over a large area, with a cooling member 30 and the two cooling members 30 lie against the insulating layers furthest away form the resistance element 14. Each connecting member 16 has an insulating element 36 in order to ensure adequate electrical insulation between each connecting member 16 and the two cooling members 30. In this construction of the electrical power resistor also, the temper atur e-equalisati on elements 20 have smaIler dimensions in area than the insulating layers 18 so that surrounding free areas 26 result round the insulating layers 18, at their edge.

In Figures 1 to 4, electrical power resistors 10 are illustrated which comprise a resistance element 14 with a plane surface, possibly structured in path form. In 0 t 12 1 1 contrast, the electrical power resistor 10 shown in Figure 5 comprises a resistance element 14 which is wound round a plate shaped substrate 38. The plateshaped substrate 38 can be made with the same dimensions as the insulating layers 18 which,, together with the temperature-equalisation- elements 20 provided between adjacent insulating layers' 18, form the two supporting members 12 between which the resistance element 14 wound on the plate-shaped substrate 38 is gripped. In Figure 5, a resistance element 14 is shown which is structuted in path form with gaps 42 between the individual turns 40. Apart from this, the power resistor 10 according to Figure 5 corresponds essentially to the electrical power resistor 10 shown in Figure 4, that is to say it is likewise constructed with two cooling members 30 and with connecting members 16 projecting axially away from the power resistor 10 in o.pposite directions. Here, too, an electrically insulating covering is designated by the reference numeral 32 and completely covers the power resistor 10 between the two cooling members 30, only the two connecting members 16 projecting out of the electrically insulating Covering 32.

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Claims

1---An electrical power resistor having an electrically insulating supporting member on whi ch a resistance element is disposedr wherein the supporting member is of plate-shaped construction and comprises at least two insulating layers disposed one above the other, a planar temperature-equalisation element of a heat-conducting material being provided between adjacent insulating layers and the temper atur e-equal isation element and the associated insulating layers adjoining one another.

2. An electrical power resistor according to Claim 1, wherein the temperature-equalisation element has smaller dimensions in area than the insulating layers adjoining the two opposite sides of the temperatur e-equalisat ion element.

3. An electrical power resistor according to Claim 2, wherein there is a surrounding free area on each insulating layer between the peripheral edge of the or each of the two insulating layers adjacent to the temperature-equalisation element.

4. An electrical power resistor according to any one of Claims 1 to 3, wherein the insulating layer remote from the resistance element is disposed on a plane supporting surface of a cooling member

5. An electrical power resistor according to any one of Claims 1 to 4. wherein the resistance element is connected to connecting members and the resistance element of. the supporting member are covered by an electrically insulating covering out of which the connecting members project.

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6. An electrical power resistor having an electrically insulating supporting member on which a resistance element is disposed between two supporting members of pl.ate-shaped construction, each supporting member comprising at least two plate-shaped insulating layers disposed one over the other, a planar temperatureequalisation element of a heat-conducting material being provided between adjacent insulating layers, and the temper atur e-equal i sat ion element and the associated insulating layers adjoining one another.

7. - An _electrical power resistor according to Claim 6, wherein each temperature-equalisation element has smaller dimensions in area than the insulating layers adjoining the two opposite sides of the temperatureequalisation element.

8. An electrical power resistor according to Claim 7, wherein there is a surrounding free area on each insulating layer between the peripheral edge of each temperature-equalisation element and the peripheral edge to the two insulating layers adjacent thereto.

9. An electrical power resistor according to any one of Claims -6 to 8, wherein at least one of the two insulating layers remote from the resistance element is disposed on a plane supporting surface of a cooling member.

10. An electrical power resistor according to any one of Claims 6 to 9, wherein the resistance element is connected to connection members and the resistance elemen.t and the supporting members are covered with an electrically insulating covering out of which the connection members project.

1 1 11. An electrical power resistor according to any one of- Claims 1 to 10, wherein the resistance element lies against a main surface of the supporting member.

12. An electrical power resistor according to any one of Claims 1 to 10, wherein the resistance element is wound round a plate-shaped substrate of electrically insulating material.

13. An electrical power resistor substantially as herein described with reference to Fig.1, Fig.2, Fig.3, Fig.4, or Fig.5 of the accompanying drawings.

1 Publishedl.989at ie Patent Office, State House, 6671 High Hollorn, London WUR 4TP. Further copies maybe obtained from The Patent Office. Sales Branch, St Mary Oray, Orpington, Kent BR5 3RD. Printed by Multiplex techniques ltd. St Mary Cray, Kent, Con. 1/87