ELECTRICAL FUSE

Abstract: The present invention relates to an electrical fuse having a series circuit formed by a PTC element and a fusible conductor. To create an electrical fuse of above-mentioned type having an automatic trigger adjustment to ageing phenomena or increase of nominal resistance of the PTC element and having a simple and compact structure of the whole circuit it is suggested that the PTC element (2) and the fusible conductor (3) have close thermal coupling.

Description: Electrical fuse Description The present invention relates to an electrical fuse having a aeries circuit formed by a PTC element and a fusible conductor. The patent specification DE 41 43 095 discloses an electrical module of this type providing protection against instances of overloading caused by excessive currents. A PTC element operates in the switching mode, in which a temperature that corresponds to the current flow is established, which temperature in turn determines the resistance of the PTC element. In order to protect each PTC element against thermal overloading, a series circuit of the PTC element with a fusible conductor is provided. A conductive connecting element is connected between the PTC element and the fusible conductor. The said connecting element serves as a conductive connection between the fusible conductor and the PTC element, as a heat sink for the PTC element and, at the same time, as a protective housing for the fusible conductor. The fusible conductor is designed in such a way that it electrically isolates the PTC element from the external contacts in the event of major overloading that may lead to the destruction of the PTC element or to the burning of the circuit. At the instant when the fusible wire melts or vaporizes, the connecting element, which has a complicated structure, additionally serves as a plasma trap. The international patent application WO 95/35577 discloses an electrical fuse which comprises a series circuit of a disc-shaped PTC element with a fusible element of a printed circuit. In this case, the ceramic PTC element is conductively connected on one side of a substrate, which carries the printed circuit, by means of clamping contacts, the fusible conductor in the form of the printed fusible element being arranged on the rear side of the substrate. The PTC element is intended to be able to protect a downstream electrical circuit by means of fast and large resistance changes, within its reversible switching range. The overload range of the PTC element is defined by a current flow of 10 - 40 A at a voltage of 600 V. In this case, the extremely quick- acting fusible link is intended to effect complete electrical isolation of the circuit in order to protect the PTC element against burning and/or explosion. The arrangement of the individual elements and the structure of the circuit with the contact-making of the PTC element do not permit efficient manufacture of the circuit, tight limits being imposed on miniaturization due to the size that the ceramic PTC element is required to have for a low standard resistance. According to the prior art, the critical range with probable destruction of the PTC element is covered by a suitably rated fusible conductor. Both of the documents cited assume that below a critical range which is defined by overvoltages and instances of thermal overloading or high currents, the PTC element operates as a temperature-controlled resistor continuously and without any change in the predetermined properties in the switching mode. The fusible conductors are rated in accordance with these PTC properties. If the standard resistance of a PTC element increases during operation, for example due to ageing phenomena, the point of electrical isolation required for safety reasons is shifted without it being possible to adapt the triggering behaviour of the fusible conductor. Furthermore, in terms of its switching behaviour, each PTC element has a significantly greater sluggishness than a fusible link with a slow-acting characteristic. The object of the present invention, therefore, is to provide an electrical fuse of the abovementioned type with a simple and compact structure whilst avoiding the disadvantages mentioned above. This object is achieved according to the invention by virtue of the fact that the PTC element and the fusible conductor have close thermal coupling. An electrical fuse according to the invention comprises a series circuit formed by a PTC element and a fusible conductor, an ageing-dictated change in the resistance of the PTC element being taken into account. Thus, a PTC element already has a rising standard resistance after running through the reversible switching interval several times. With the same current flow, heat is thus produced to an increasing extent. Close thermal coupling, according to the invention, of the fusible conductor to the PTC element sets the thermal operating point of the fusible conductor in accordance with the specifications by the PTC element. As a result, the switching behaviour of the fuse element is influenced to an entirely essential degree. As a function of the temperature of the PTC element, the fusible conductor thus triggers, given a high temperature of the PTC element, as early as when there are slight additional, even extremely brief instances of overloading or overload spikes. In the event of high overcurrents , the PTC element reacts too sluggishly for most applications and therefore does not afford adequate protection. Even at a time way before a temperature which is critical for the PTC element has been reached, this very sluggish behaviour can lead to overloading of the downstream electrical circuit and to damage thereof. The thermal coupling, according to the invention, of the fusible conductor to the PTC element makes it possible to replace the characteristic of the PTC element which is too sluggish in the high current range by the characteristic of the fusible conductor. This results in a novel fuse characteristic whose properties can be set both by the PTC element and by the fusible conductor. This novel behaviour of a fuse according to the invention will be explained in detail below with reference to a sketched family of characteristic curves. In a development, the close thermal coupling between the PTC element and the fusible conductor is brought about by virtue of the fact that the fusible conductor forms a lead to a contact of the PTC element. This provides not only direct electrical contact but also very good thermal contact. During operation, a thermal equilibrium will be established in the region of the contact point between fusible conductor and PTC element, the said thermal equilibrium being critically determined by the heat loss of the PTC element. Consequently, the thermal operating point of the fusible conductor is set by the PTC element. The fusible conductor may advantageously be designed as a wire or flat wire. In this case, it is preferable to use a fusible wire of the kind already used in tried and tested fusible links. Particularly when the dimensions are very small, it is also possible to use a bonding wire as the fusible conductor, by means of which an external contact is electrically conductively connected to a contact of the PTC element. In every case, the tasks of making contact with the PTC element and providing a fusible conductor are both fulfilled in this way, the proposed type of contact-making being additionally distinguished by the capability of expansion compensation between its contact points. The significance of this property becomes clear particularly in connection with Claim 9. The use of a wire as the fusible conductor considerably simplifies the manufacture of an electrical fuse according to the invention and also minimizes the overall circuitry outlay. The close thermal contact occurs at the base point of the wire on the surface of the PTC element and guarantees a reliable turn-off given electrical isolation in the event of persistently excessive heating of the PTC element. In particular, in addition to the two aforementioned elements of the fuse, only two external contacts and a substrate or fuse housing carrying the entire arrangement are required, as is described below using an embodiment. According to Claim 4, an electrically conductive layer can also be used as the fusible conductor, in order to form a lead to a contact of the PTC element. So that they are adapted to the task of a fusible conductor, such electrically conductive layers have a constriction at at least one point, wtiich constriction is interrupted when the fusible link is triggered. When used in an electrical fuse according to the invention, the constriction can be thermally closely coupled by an appropriate proximity to the PTC element, in contrast to the prior art, in this embodiment the fuse element and the PTC element are arranged on the same side of a substrate carrying the entire arrangement. This simplifies the insulation and/or partitioning of the entire fuse arrangement from its surroundings, as well as production. According to Claim 6, the fusible conductor can also be formed as a thick- film conductor. The advantages of such an embodiment become evident particularly when a pasty PTC compound is used, since in this case it is possible to manufacture the entire electrical fuse with its combined elements uniformly as a thick- film circuit. In a preferred embodiment, the thermal coupling between PTC element and fusible conductor is especially intensified by virtue of the fact that the fusible conductor is arranged on the PTC element. In this case, the fusible conductor is situated on a connection electrode of the PTC element, in particular. In an advantageous manner, the fusible conductor is borne directly by a PTC element metallization layer serving as contact area, the fusible conductor, with the exception of a connection point, being electrically isolated from the PTC element and/or the metallization layer by an electrically insulating layer. With a suitable material selection, the electrically insulating layer can be very thin, with the result that the thermal resistance between the PTC element and the fusible conductor is extremely low. In this way, the PTC element with good thermal coupling itself serves as a substrate for the fusible conductor. Without a separate substrate, the fuse according to the invention can be produced with a very compact structure and a minimum number of individual parts and process steps, in this case, the fusible conductor can be embodied as a wire or as a layer- type fusible conductor, depending on the desired triggering characteristic. If the fusible conductor is covered by a covering compound or curable, thermostable paste, in some cases it is even possible to dispense with an exterior housing, as will be shown using an exemplary embodiment. In a development of the electrical fuse described above, the contact located opposite the connection area of the fuse element is designed as a termination area on the electrically insulating layer. At least one external connection element, in particular a connection leg in wire form, is fastened to the termination area. Consequently, a fuse according to the invention is suitable directly for fitting in customary circuit boards of electrical circuits with holes. However, fitting in known housing designs matched to the EC standard contact grid is also possible, with contact-making of the housing connections on the termination areas. In a particularly advantageous development, the PTC element is composed of a polymer material. This material is distinguished by an extreme change in resistance with temperature. The change in resistance is caused by temperature-dict ted thermal expansion. In order to compensate for this expansion movement, the use of a fusible conductor in wire form is preferred with contact-making in the main expansion direction. However, permanent contact-making perpendicular to the main expansion direction is also possible. This feature is explained below using an exemplary embodiment. The integration of a PTC element in iniaturizable electrical fuses can advantageously be carried out using polymer bodies in sheet form. It is possible to make contact with such polymer bodies reliably due to their flat shape, and they have only a very small structural height in conjunction with a low dead weight and good PTC properties. According to Claim 11, preference is given to PTC elements of the type described whose temperature -dictated expansion essentially follows one main direction, with the result that the expansion direction is advantageously perpendicular to a given bearing face of the PTC element. An electrical fuse according to the invention is also advantageously possible by the contact-making of a ceramic PTC element, or of a PTC element formed by a sintered body, in the manner described. For example, it is possible to use barium titanate (BaTi0 3 ) as the PTC element in a fuse according to the invention, as a result of which the properties of such a fuse are distinctly improved in relation to comparable fuses according to the prior art. In a development, the arrangement of an electrical fuse according to the invention is chosen such that it can also be inserted into known housings, thereby providing already proven and reliable protection from the external surroundings. Moreover, such housings are manufactured in large numbers with customary standard contact grids and afford sufficient room for the very compact arrangement of an electrical fuse according to the invention. According to Claim 14, the dimensioning of an electrical fuse according to the invention can be reduced to such an extent that it can be embodied as an SMD- mountable element. As a result, a fuse according to the invention has been created which can be integrated without any problems in modern production and mounting processes in a manner conforming to automatic machines. The switching characteristic of the fusible conductor is controlled thermally by the heat loss of the PTC element. At the turn-off instant, however, it can also advantageously be assisted by arc-inhibiting or arc- extinguishing materials. Such materials are preferably arranged in the region of the blowing zone of the fusible conductor. According to Claim 16, the fusible conductor is constructed in such a way that a diffusion process can proceed during operation. That leads to a turn-off behaviour of the fusible conductor which is more slow- acting than quick-acting fuses. In particular, a layer of tin is electrodeposited for this purpose on the fusible conductor, which layer diffuses from the surface to a point within the fusible conductor, as a function of the temperature. In contrast to the above-described fuses according to the prior art, a simple and readily iniaturizable structure with greatly improved switching properties is always produced even when all of the abovexnentioned features and developments of the invention are taken into account and/or realized. Exemplary embodiments of the invention are explained in more detail below with reference to the drawing. The figures show: Fig. 1 a sketch of a family of characteristic curves; Fig. 2 a perspective illustration of one embodiment of a fuse without a surrounding housing; Fig. 3 a perspective illustration of a further embodiment of a fuse without a housing, and Figs 4a, 4b two sketched views of a further embodiment of a fuse. Fig. 1 shows, as a sketched example, a characteristic curve of an electrical fuse comprising a series circuit formed by a PTC element and a thermally coupled fusible conductor. The characteristic curves of a PTC element and of a fusible conductor of a fuse are plotted as switching time t against current intensity I in this illustration, with a logarithmic scale on both axes. In this case, the characteristic curve of the PTC element is situated before the characteristic curve of the fusible conductor of a fuse in the region of low currents. In the region of higher currents, the characteristic curves finally converge at a point of intersection. The characteristic curves then diverge further as the currents continue to rise, the fusible conductor of a fuse always lying with its characteristic curve distinctly before the characteristic curve of the PTC element. Therefore, in this region the fusible conductor reacts significantly earlier than the PTC element. The point of intersection of the two characteristic curves thus defines the boundary between two regions in which a thermally coupled series circuit formed by the two elements behaves in a greatly different manner. In the region to the left of the point of intersection, the PTC element effects overload regulation with reversible isolation by regulating its inherent resistance, in other words in the form of a repeatable switching operation. Situated to the right of the point of intersection is the region of permanent and complete electrical isolation, in which, when the temperature of the PTC element has already undergone a severe increase, the fusible conductor performs electrical isolation of the circuit in order to afford protection against damage which may occur either on the PTC element, in a manner dictated by the temperature, or on the downstream circuit alone, as a result of the excessively sluggish switching behaviour of the PTC element. The point of intersection as a boundary between the regions of reversible and electrical or irreversible isolation constitutes a fixed point only when the temperature of the PTC element is constant and thus the operating temperature of the fusible conductor of a fuse ia constant. Here, it is illustrated for a temperature of 125°C, by way of example. This point shifts with the entire fusible link characteristic curve on account of the close thermal coupling of the temperature of the PTC element in a manner corresponding to that illustrated by the arrows in Fig. 1. This also avoids operation of the PTC element in the critical current range. Fig. 2 shows a perspective illustration of one embodiment of a fuse 1, which is formed from a series circuit comprising a PTC element 2 and a fusible conductor 3, The fusible conductor 3, in the form of a wire 4, serves in this case as a lead from a contact 6, which passes to the outside, to a connection point 7 of the PTC element 2. In the present case, the PTC element 2 is preferably embodied as a polymer 8 with PTC properties. The polymer 8 is distinguished by a preferred expansion direction 9. It is produced by various manufacturers as a polymer body 10 in sheet form, of which only a portion matched to the use is fitted in the fuse 1. In this case, metallization layers 12 are applied to the portion of the polymer body 10 in sheet form, in planes perpendicular to the preferred expansion direction 9. The upper metallization layer 12 forms the connection point 7 for the wire 4; the lower metallization layer 12 is conductively connected to a second external contact 6, for example by means of an electrically conductive bonding layer 13 composed of a conductive adhesive or a solder paste. The wire 4 runs free between the bonding layer 13 on the left-hand contact 6 and the metallization layer 12 of the polymer 8. This region 14 can be filled with an arc-extinguishing or arc-inhibiting material inside a fuse housing (not illustrated further here) . Not only guidance but also mechanical stabilization of the wire 4 can be formed as a result of this. There is also the option of covering the region 14, in which the blowing zone of the fusible conductor is located, by a potting compound in order to protect the PTC element 2 against an arc. The wire 4 may also be replaced by a bonding wire in the case of a very high degree of miniaturization or in order to form a fusible link. On account of the thermal coupling, an extremely quick- cting characteristic of the fusible conductor is generally unnecessary. On its surface, the wire 3 is provided with a coating which is made of tin and is preferably electro- deposited. The wire 3 itself is composed for example of silver, copper or else gold. The effect of the surface coating with tin is that during operation of the fusible conductor, a temperature-dependent diffusion process can proceed, as a result of which the turn-off behaviour of the wire becomes increasingly slowe -acting in comparison with quick-acting fusible links. A similar procedure presents itself with other fusible conductor designs as well . Figure 3 shows a perspective illustration of a further embodiment of a fuse according to the invention. In this case, an electrically conductive layer 16, which has a constriction 18 at a location 17, has been applied to a substrate 15. The conductive layer 16 formed in this way can be produced for example as a thick film using a screen printing method. Besides a printed circuit, a photoche ically produced circuit board is alternatively conceivable for this application. The external contacts 6 are eleσtrodeposited or applied as sintered pastes on the substrate 15. In the present case, the conductive layer 16 extends from the left-hand contact 6 to a point shortly before the right-hand contact 6. Between the right-hand contact 6 and the conductive layer 16, the direct electrical connection is interrupted in a region 19. The interruption region 19 is covered by a portion of the polymer body 10, in sheet form, of the PTC element 2, which, at one end, is conductively connected to the wide end of the layer 16 and, at the other end, is electrically conductively connected to the external contact 6. Consequently, a series circuit formed by a fusible conductor 3 and a PTC element 2 has once again been produced, in a further embodiment, between the two external contacts 6. It should be pointed out that the region 14 of the blowing zone of the fusible conductor 3 is arranged closely adjacent to the PTC element 2, thereby realizing close thermal coupling. In the present case, this blowing region 14 can be covered by a potting compound having suitable arc-extinguishing material properties. The fuse illustrated in Fig. 3 represents a prototype of an S D-mountable element 20. The process for producing such an element can be standardized by likewise applying the PTC element as a paste using a screen printing method or another thick-film method. However, it is also possible to use conventional standard PTC elements, for example as portions of a polymer sheet 10. Such polymer bodies 10 in sheet form are regularly provided with metallization layers 12 as contacts on both surfaces. For use in the present embodiment of a fuse 1, the contact area which would be connected to the fusible conductor 3 and to the external contact 6 is interrupted in order to avoid a short circuit in the region 19. Thus, the current flows from the end of the fusible conductor 3 up to the contact 6 through the PTC element 2. In a manner analogous to the abov -described integration of a polymer body 10 in sheet form, it is possible to proceed using a similarly constructed ceramic body, for example made of barium titanate. Thus, the production of the embodiments described with reference to Pigs 2 and 3 is unproblematic using all customary PTC designs. On the basis of the deliberately chosen thermal coupling between fusible conductor and PTC element and also the performance of modern PTC materials, the dimensions of the fuses described may be small enough to enable the fuse arrangements described and illustrated in Figs 2 and 3, for example, to be fitted in known fuse housings, such as e.g. that of TR5 * or of SM3 * . As a result, the fuses obtain an additional exterior protective hood. They are thus better screened against external influences and, at the same time, their surroundings are also effectively protected against escaping plasma in the event of disconnection of the fuse by the fusible conductor. Moreover, housings of this type are matched to the corresponding IEC contact grid spacings of the connec- tions and are manufactured in large numbers as mass- produced articles. Two views of a further embodiment of a fuse 1 are sketched in Figures 4a and 4b. In this very compact design of a fuse 1, the fusible conductor 3 is arranged directly on the PTC elements 2, as a result of which a very intensive thermal coupling is established between the two circuit elements for the purpose of automatic adaptation of the fusible conductor operating point to ageing phenomena or standard resistance increases of the PTC element. In this case, the PTC element 2 may be formed either by a ceramic or by a plastic body and simultaneously serves as a substrate for the fusible conductor 3. Thus, in the exemplary embodiment illustrated, the fuse will be constructed entirely without a substrate 15, in contrast e.g. to the design of Fig. 2. The fusible conductor 3 is embodied as a thick- film fusible conductor in this exemplary embodiment. However, it may also be designed as a wire- type fusible conductor, since good thermal coupling with the PTC element 2 is ensured in both cases on account of the close proximity. In the present exemplary embodiment, the two connection points 7 of the fusible conductor 3 are in each case located on the PTC element 2, one connection point 7 being arranged directly on the metallization layer 12. In order to avoid a short circuit of the fuse element 3, the second connection point 7 is located on a metallic termination area 22 which, for its part, is fastened on an electrically insulating layer 23. Consequently, the termination area 22 is electrically isolated from the metallization layer 12 and the material of the PTC element 2 by the insulating layer 23. For this purpose, however, it is not necessary, in contrast to what is illustrated in Fig. 4a, to provide the metallization layer 12 with a cutout 24 for the insulating layer 23. The layers are each very thin and can thus easily be arranged lying one over the other. The termination area 22 also makes available, in addition to the second connection area 7 for the fusible conductor 3, an option for the electrically conductive fastening of an external connection element 25. In this case, connection legs 26 in wire form have been chosen as connection elements 25, with the result that the fuse 1 can be inserted into circuit boards with holes. Of course, the termination area 22 can also be designed in one piece with the fusible conductor 3, for example in the case of a flat-wire fusible conductor or a fusible conductor 3 in the form of a sheet-metal stamping. The fastening and contact-making on the metallization layer 12 of the PTC elements 2 then proceed in the usual way. The termination area 22 can be fastened for example by adhesive bonding on the electrically insulating layer 23, in which case the material of the insulating layer 23 itself may also serve as an adhesive. Fig. 4b shows the embodiment of Fig. 4a in a side view, which reveals the simple structure of the fuse element 1. The insertion of the insulating layer 23 produces a current path from a connection leg 26 through the PTC element 2 to the first connection point 7 of the fusible conductor 3, via the fusible conductor 3 to the second connection point 7 and to the termination area 22 and thus to the second connection leg 26. As a result of the cutout 24 in the metallization layer 12 of the PTC element 2, the fusible conductor 3 and the PTC element 2 are electrically isolated from one another only by the relatively low thickness of the insulating layer 23. However, this small separation ensures the desired, good thermal coupling of the fusible conductor 3 with the PTC element 2. If the fusible conductor 3 is covered by known coverings or potting compounds, the embodiment of Figs 4a, 4b can also be used without a separate housing, since the plasma liberated by the fusible conductor in the event of turn-off can be captured and thus cannot cause any damage to the surrounding circuit. By virtue of the compact structure sketched in a side view in Fig. 4b, it is also easily possible to insert this embodiment, without any further structural adaptation measures, into known housing designs from the field of equipment fuses, such as for example that of TR5 * mentioned above.

Claims: Claims 1. Electrical fuse having a series circuit formed by a PTC element and a fusible conductor, characterized in that the PTC element (2) and the fusible conductor (3) have close thermal coupling. 2. Electrical fuse according to Claim 1, characterized in that the fusible conductor (3) forms a lead to a contact of the PTC element (2) . 3. Electrical fuse according to Claim 1 and/or Claim 2, characterized in tha the fusible conductor (3) is formed by a wire (4) , in particular by a bonding wire. 4. Electrical fuse according to Claim 1 and/or Claim 2, characterized in that the fusible conductor (2) is formed by an electrically conductive layer (16) . 5. Electrical fuse according to Claim 4, characterized in that the electrically conductive layer (16) has a constriction (18) at at least one point (17) . 6 Electrical fuse according to Claim 4 and/or Claim 5, characterized in that the electrically conductive layer (16) of the fusible conductor (3) is formed by a thick- ilm circuit. 7. Electrical fuse according to one or more of the preceding claims, characterized in that - the fusible conductor (3) - is arranged on the PTC element (2) , in particular on a metallization layer (12) of the PTC element (2) , which metallization layer serves as a contact area, - the fusible conductor (3) , with the exception of the connection point (7), being electrically isolated from the PTC element and/or the metallization layer (12) by an electrically insulating layer (23) . 8. Electrical fuse according to Claim 7, characterized in that - the contact (6) located opposite the connection area (7) - is designed as a termination area (22) on the electri- cally insulating layer (23) , - to which termination area it is possible to fasten at least one external connection element (25) , in particular a connection leg (26) in wire form. 9. Electrical fuse according to one or more of the preceding claims, characterized in that the PTC element (2) is composed of a polymer (8) . 10. Electrical fuse according to Claim 9, characterized in that the PTC element (2) is formed by a portion of a polymer body (10) in sheet form. 11. Electrical fuse according to Claim 9 and/or Claim 10, characterized in that temperature-dictated expansions (9) of the PTC element (2) take place essentially perpendicularly to a bearing face of the PTC element (2) . 12. Electrical fuse according to one or more of Claims 1 - 8, characterized in that the PTC element (2) is composed of a ceramic, in particular of barium titanate. 13. Electrical fuse according to one or more of the preceding claims, characterized in that the fuse (1) can be inserted into known housings. 14. Electrical fuse according to one or more of the preceding claims, characterized in that the fuse (1) is embodied as an SMD-mountable element (20) . 15. Electrical fuse according to one or more of the preceding claims, characterized in that the fuse (1) contains arc- inhibiting or arc-extinguishing materials which are preferably arranged in a region (14) of the blowing zone of the fusible conductor (3) . 16. Electrical fuse according to one or more of the preceding claims, characterized in that the surface of the fusible conductor (3) is coated with tin and this coating has been applied by means of an electrodeposition process, in particular.

Inventors: FROECHTE BERND, JOELLENBECK ANDRE, POESSNICKER PETER, RUPALLA MANFRED, FROECHTE, BERND, JOELLENBECK, ANDRE, POESSNICKER, PETER, RUPALLA, MANFRED
Applicants: FROECHTE BERND, JOELLENBECK ANDRE, POESSNICKER PETER, RUPALLA MANFRED, WICKMANN WERKE GMBH, FROECHTE, BERND, JOELLENBECK, ANDRE, POESSNICKER, PETER, RUPALLA, MANFRED, WICKMANN-WERKE GMBH
Application year: 1997
Main classification: H01H85
IPC codes: H01C7, H01H37, H01H85