DE69702719T2 - ELECTRICAL FUSE - Google Patents

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

The present invention relates to an electrical fuse with a series circuit consisting of a PTC element and a fusible element.

Such an electrical component for protection against overloads caused by excessive currents is known from patent DE 41 43 095. A PTC element works in switching mode, in which a temperature corresponding to the current flow is set, which in turn determines the resistance of the PTC element. To protect each PTC element from thermal overload, a series connection of the PTC element with a fusible element is provided. A conductive connection element is connected between the PTC element and the fusible element. It serves as a conductive connection between the fusible element and the PTC element, as a heat sink for the PTC element, and at the same time as a protective housing for the fusible element. The fusible element is designed in such a way that it galvanically separates the PTC element from the external contacts in the event of a large overload, which can lead to the destruction of the PTC element or to the circuit burning down. At the moment of melting or evaporation of the melting wire, the complex connection element also serves as a plasma catcher.

The international patent application WO 95/35577 discloses an electrical fuse that includes a series connection of a disk-shaped PTC element with a fuse element of a printed circuit. The ceramic PTC element is conductively connected to one side of a substrate carrying the printed circuit by means of clamping contacts, with the fuse conductor in the form of the printed fuse element being arranged on the back of the substrate. The PTC element is intended to be able to protect a subsequent electrical circuit within its reversible switching range by means of rapid and large resistance changes. The overload range of the PTC element is defined by a current flow of 10-40 A at a voltage of 600 V. Here, the extremely fast fuse is intended to provide complete galvanic isolation of the circuit to protect the PTC element from burning or exploding.

The arrangement of the individual elements as well as the structure of the circuit with the contacting of the PTC element do not allow for an efficient production of the circuit, whereby miniaturization is strictly limited by the size of this component required for a low standard resistance of the ceramic PTC element.

According to the state of the art, the critical area with the probable destruction of the PTC element is covered by a suitably designed fuse element. Both documents mentioned assume that the PTC element works permanently and without changing the predetermined properties in switching mode as a temperature-controlled resistor below a critical area, which is defined by overvoltages and thermal overloads or high currents. The fuse elements are designed according to these PTC properties. If the normal resistance of a PTC element increases during operation, for example due to aging, the point of galvanic isolation required for safety reasons is shifted without the triggering behavior of the fuse element being able to be adjusted.

Furthermore, each PTC element has a significantly greater inertia in its switching behavior than a fuse with a slow characteristic.

It is therefore an object of the present invention to provide an electrical fuse of the type mentioned with a simple and compact structure while avoiding the disadvantages mentioned above.

This object is achieved according to the invention in that the PTC element and the fusible conductor have a close thermal coupling.

An electrical fuse according to the invention comprises a series circuit of a PTC element and a fuse element, whereby an age-related change in the resistance of the PTC element is taken into account. A PTC element therefore has an increasing normal resistance after several times running through the reversible switching interval. With the same current flow, heat is thus produced to an increasing extent. A close thermal coupling of the fuse element to the PTC element according to the invention sets the thermal operating point of the fuse element in accordance with the specifications of the PTC element. This has a very significant influence on the switching behavior of the fuse element. Depending on the temperature of the PTC element, the fuse element is thus triggered at a high temperature of the PTC element even with small additional, even extremely short-term overloads or overload peaks.

In the case of high overcurrents, the PTC element reacts too slowly for most applications and therefore does not offer sufficient protection. This very slow behavior can lead to an overload of the connected electrical circuit and damage to it long before a critical temperature for the PTC element is reached. The thermal coupling of the fusible element to the PTC element according to the invention makes it possible to replace the characteristics of the PTC element, which are too slow in the high current range, with the characteristics of the fusible element. This results in a new type of fuse characteristic, the properties of which can be adjusted by both the PTC element and the fusible element. This new type of behavior of a fuse according to the invention will be explained in more detail below using a sketched field of characteristics.

In a further development, the close thermal coupling between the PTC element and the fusible element is brought about by the fusible element forming a supply line to a contact of the PTC element. This creates not only direct electrical contact but also very good thermal contact. During operation, a thermal equilibrium is established in the area of the contact point between the fusible element and the PTC element, which is largely determined by the heat loss of the PTC element. The thermal operating point of the fusible element is thus set by the PTC element.

The fuse element can advantageously be designed as a wire or flat wire. Preferably, a fuse wire is used here, as is already used in tried and tested fuses. Particularly with very small dimensions, a bonding wire can also be used as a fuse element, through which an external contact is electrically connected to a contact of the PTC element. In any case, the tasks of contacting the PTC element and creating a fuse element are simultaneously fulfilled, with the proposed type of contact also being characterized by the ability to compensate for expansion between its contact points. The importance of this property becomes particularly clear in connection with claim 9.

The use of a wire as a fusible conductor significantly simplifies the manufacture of an electrical fuse according to the invention and also minimizes the overall circuitry effort. The close thermal contact occurs at the base of the wire on the surface of the PTC element and guarantees reliable shutdown with galvanic isolation in the event of persistent excessive heating of the PTC element. In particular, in addition to the two elements of the fuse mentioned, only two external contacts and a substrate or fuse housing supporting the entire arrangement are required, as described below using an embodiment.

According to claim 4, an electrically conductive layer can also be used as a fusible conductor in order to form a supply line to a contact of the PTC element. In order to adapt to the task of a fusible conductor, such electrically conductive layers have a constriction at at least one point, which is interrupted when the fuse is triggered. When used in an electrical fuse according to the invention, the constriction can be thermally closely coupled by a corresponding 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 or sealing off of the entire fuse arrangement from the environment and also simplifies production.

According to claim 6, the fuse element can also be formed as a thick-film conductor. The advantages of such an embodiment become particularly obvious when using a pasty PTC mass, since in this case the entire electrical fuse with its combined elements can be manufactured in a uniform manner as a thick-film circuit.

In a preferred embodiment, the thermal coupling between the PTC element and the fusible conductor is particularly intensified by arranging the fusible conductor on the PTC element. The fusible conductor is located in particular on a connection electrode of the PTC element. The fusible conductor is advantageously carried directly by a metallization of the PTC element that serves as a contact surface, with the fusible conductor being electrically separated from the PTC element and/or the metallization by an electrically insulating layer, with the exception of one connection point. The electrically insulating layer can be very thin if the material is selected appropriately, so that the thermal resistance between the PTC element and the fusible conductor is extremely low. The PTC element with good thermal coupling thus serves as a substrate itself. for the fuse element. Without its own substrate, the fuse according to the invention can be manufactured with a very compact structure and a minimal number of individual parts and process steps. Depending on the desired triggering characteristics, the fuse element can be designed as a wire or as a layered fuse element. If the fuse element is covered by a covering compound or hardenable, thermally stable paste, in some cases an outer housing can even be dispensed with, as will be shown using an example.

In a further development of the electrical fuse described above, the contact that is opposite the connection surface of the fuse element is designed as a termination surface on the electrically insulating layer. At least one external connection element is attached to the termination surface, in particular a wire-shaped connection leg. A fuse according to the invention is therefore directly suitable for installation in conventional circuit boards of electrical circuits with holes. However, installation in known housing designs with adaptation to the IEC standard grid is also possible by contacting the housing connections on the termination surfaces.

In a particularly advantageous development, the PTC element is made of a polymer material. This material is characterized by an extreme change in resistance with temperature. The change in resistance is caused by temperature-related thermal expansion. To compensate for this expansion movement, the use of a wire-shaped fusible conductor is preferred when contacting in the main expansion direction. However, permanent contact perpendicular to the main expansion direction is also possible. This feature is explained below using an example.

The integration of a PTC element into miniaturizable electrical fuses can advantageously be achieved using film-shaped polymer bodies. Such polymer bodies can be reliably contacted due to their flat shape and have a very low overall height with a low weight and good PTC properties. According to claim 11, PTC elements of the type described are preferred whose temperature-related expansions essentially follow a main direction, so that the expansion direction is advantageously perpendicular to a given contact surface of the PTC element.

An electrical fuse according to the invention is also advantageously possible by contacting a ceramic PTC element or a PTC element formed by a sintered body in the manner described. For example, barium titanate (BaTiO₃) can be used as a PTC element in a fuse according to the invention, whereby the properties of such a fuse are significantly improved compared to comparable fuses according to the prior art.

In a further development, the arrangement of an electrical fuse according to the invention is chosen so that it can also be used in known housings, thus providing tried and tested and reliable protection against the external environment. In addition, such housings are manufactured in large numbers in common standard grids and offer sufficient space for the very compact arrangement of an electrical fuse according to the invention.

According to claim 14, the dimensions of an electrical fuse according to the invention can be reduced to such an extent that it can be designed as an SMD-mountable element. This creates a fuse according to the invention that can be easily integrated into modern production and assembly processes in an automated manner.

The switching characteristic of the fuse element is thermally controlled by the heat loss of the PTC element. It can However, at the moment of shutdown, arc-inhibiting or arc-extinguishing materials can still be used to support this. Such materials are preferably placed in the area of the melting zone of the fusible conductor.

According to claim 16, the fuse element is constructed in such a way that a diffusion process can take place during operation. This leads to a more sluggish shutdown behavior of the fuse element compared to fast-acting fuses. In particular, a layer of tin is galvanically applied to the fuse element, which diffuses from the surface into the interior of the fuse element depending on the temperature.

In contrast to the fuses according to the state of the art described above, even when all of the above-mentioned features and further developments of the invention are taken into account or implemented, a simple and easily miniaturizable structure with greatly improved switching properties always results.

In the following, embodiments of the invention are explained in more detail using the drawing. The figures show:

Fig. 1 is a sketch of a characteristic curve field;

Fig. 2 is a perspective view of an embodiment of a fuse without a surrounding housing;

Fig. 3 a perspective view of another embodiment of a fuse without housing and

Fig. 4a, 4b two sketched views of another embodiment of a fuse.

Fig. 1 shows a sketched example of a characteristic curve of an electrical fuse consisting of a series connection of a PTC element and a thermally coupled fuse element. The characteristic curves of a PTC element and a fuse element are plotted as switching time t against the current I in a double logarithmic representation. The characteristic curve of the PTC element lies ahead of the characteristic curve of the fuse element in the low current range. In the higher current range, the characteristic curves finally converge at an intersection point. The characteristic curves then diverge further as the current continues to rise, with the fuse element with its characteristic curve always lying significantly ahead of the characteristic curve of the PTC element. The fuse element therefore reacts much earlier in this range than the PTC element.

The intersection of both characteristic curves defines the boundary between two areas in which a thermally coupled series circuit made up of both elements behaves very differently. In the area to the left of the intersection, the PTC element provides overload control with reversible separation by regulating its own resistance, i.e. in the form of a repeatable switching process. To the right of the intersection is the area of permanent and complete galvanic separation, in which, when the temperature of the PTC element has already risen significantly, the fuse element provides galvanic separation of the circuit to protect against damage that can occur either due to temperature on the PTC element or due to the PTC element's too slow switching behavior on the subsequent circuit alone.

The intersection point as the boundary between the areas of reversible and galvanic or irreversible separation only represents a fixed point when the temperature of the PTC element and thus the working temperature of the fuse element are constant. Here it is shown as an example for a temperature of 125ºC. This point shifts with the entire fuse characteristic curve due to the close thermal coupling of the temperature of the PTC element. accordingly, as indicated by the arrows in Fig. 1. This also prevents the PTC element from operating in the critical current range.

Fig. 2 shows a perspective view of an embodiment of a fuse 1, which is formed from a series connection of a PTC element 2 and a fusible conductor 3. The fusible conductor 3 serves in the form of a wire 4 as a supply line from an outward-leading contact 6 to a connection point 7 of the PTC element 2. The PTC element 2 is in the present case preferably designed as a polymer 8 with PTC properties. The polymer 8 is characterized by a preferred stretch direction 9. It is manufactured by various manufacturers as a film-shaped polymer body 10, of which only a section adapted to the application is installed in the fuse 1. Here, metallizations 12 are applied in planes perpendicular to the preferred stretch direction 9 on the section of the film-shaped polymer body 10. The upper metallization 12 forms the connection point 7 for the wire 4; the lower metallization 12 is conductively connected to a second external contact 6, for example by an electrically conductive connecting layer 13, which consists of a conductive adhesive or a solder paste.

The wire 4 runs freely between the connecting layer 13 on the left contact 6 and the metallization 12 of the polymer 8. This area 14 can be filled with an arc-extinguishing or arc-inhibiting material within a fuse housing (not shown here). This not only provides guidance but also mechanical stabilization of the wire 4.

It is also possible for the area 14 in which the melting zone of the fuse element is located to be covered by a casting compound to protect the PTC element 2 from an arc. In the case of very high miniaturization or to form a fuse, the wire 4 can also be replaced by a bonding wire. Due to the thermal coupling, an extremely fast characteristic of the fusible element is generally not required.

The surface of the wire 3 is coated with tin, which is preferably applied electroplated. The wire 3 itself consists of silver, copper or gold, for example. The surface coating with tin means that a temperature-dependent diffusion process can take place during operation of the fuse element, which makes the switch-off behavior of the wire increasingly slower compared to fast-acting fuses. A similar procedure is also suitable for other fuse element designs.

Fig. 3 shows a perspective view of another embodiment of a fuse according to the invention. Here, an electrically conductive layer 16 has been applied to a substrate 15, which has a constriction 18 at a point 17. The conductive layer 16 formed in this way can, for example, be produced as a thick layer in a screen printing process. Alternatively, in addition to a printed circuit, a photochemically produced circuit board is also conceivable for this purpose. The external contacts 6 are attached to the substrate 15 galvanically or as sintered pastes. In the present case, the conductive layer 16 extends from the left-hand contact 6 to just before the right-hand contact 6. Between the right-hand contact 6 and the conductive layer 16, the direct electrical connection is interrupted in an area 19. The interruption area 19 is covered by a section of the film-shaped polymer body 10 of the PTC element 2, which is conductively connected on one side to the wide end of the layer 16 and on the other side to the external contact 6. Thus, a series circuit made up of a fusible conductor 3 and a PTC element 2 has been produced between the two external contacts 6 in a further embodiment. It should be noted that the area 14 of the melting zone of the fusible conductor 3 is arranged closely adjacent to the PTC element 2, whereby a close thermal coupling is achieved. In the present case, this melting area 14 can be covered by a casting compound with suitable arc-extinguishing material properties.

The fuse shown in Fig. 3 represents a prototype of an SMD-mountable element 20. The process for producing such an element can be standardized by applying the PTC element as a paste in a screen printing or other thick-film process. However, conventional standard PTC elements can also be used, for example as sections of a polymer film 10. Such film-shaped polymer bodies 10 are usually provided with metallizations 12 as contacts on both surfaces. For use in the present embodiment of a fuse 1, the contact surface that would be connected to the fusible conductor 3 and the external contact 6 is separated in the area 19 to avoid a short circuit. The current thus flows from the end of the fusible conductor 3 to the contact 6 through the PTC element 2.

Analogous to the integration of a film-shaped polymer body 10 described above, a similarly constructed ceramic body, for example made of barium titanate, can be used. The embodiments described for Figs. 2 and 3 can therefore be produced without any problems using all common PTC designs.

Due to the deliberately chosen thermal coupling between the fuse element and the PTC element as well as the performance of modern PTC materials, the dimensions of the fuses described can be so small that, for example, the fuse arrangements shown and described in Fig. 2 and 3 can be installed in known fuse housings, such as the TR5® or the SM3®. This gives the Fuses have an additional external protective cover. This means they are better shielded against external influences and, at the same time, the environment is effectively protected from escaping plasma by the fuse element if the fuse is switched off. In addition, such housings are adapted to the corresponding IEC grid spacing of the connections and are manufactured in large numbers as mass-produced items.

Two views of another embodiment of a fuse 1 are sketched in Fig. 4a and 4b. In this very compact design of a fuse 1, the fuse element 3 is arranged directly on the PTC element 2, which creates a very intensive thermal coupling between the two circuit elements for automatic adjustment of the fuse element operating point to signs of aging or increases in the normal resistance of the PTC element. The PTC element 2 can be formed by either a ceramic or a plastic body and also serves as a substrate for the fuse element 3. In the embodiment shown, the fuse is therefore constructed without a substrate 15 at all, in contrast to the design in Fig. 2, for example.

In this embodiment, the fuse element 3 is designed as a thick-film fuse element. However, it can also be designed as a wire fuse element, since in both cases a good thermal coupling with the PTC element 2 is ensured due to the close proximity. In any case, in the present embodiment, both connection points 7 of the fuse element 3 are located on the PTC element 2, with one connection point 7 being arranged directly on the metallization 12. To avoid a short circuit of the fuse element 3, the second connection point 7 is located on a metallic termination surface 22, which in turn is attached to an electrically insulating layer 23. The termination surface 22 is thus electrically separated from the metallization 12 and the material of the PTC element 2 by the insulating layer 23. For this purpose, however, unlike in Fig. 4a shown, it is not necessary to provide the metallization 12 with a recess 24 for the insulating layer 23. The layers are each very thin and can thus easily be arranged one above the other.

In addition to the second connection surface 7 for the fuse element 3, the termination surface 22 also provides an option for the electrically conductive attachment of an external connection element 25. In this case, wire-shaped connection legs 26 have been selected as the connection elements 25, so that the fuse 1 can be used in drilled circuit boards. Of course, the termination surface 22 can also be formed as a single piece with the fuse element 3, for example in the case of a flat wire fuse element or a fuse element 3 in the form of a stamped sheet metal part. The attachment and contacting to the metallization 12 of the PTC element 2 then takes place as usual. The termination surface 22 can be attached to the electrically insulating layer 23, for example by gluing, whereby the material of the insulating layer 23 itself can also serve as an adhesive.

Fig. 4b shows the embodiment of Fig. 4a in a side view, from which the simple structure of the fuse element 1 is evident. By inserting the insulating layer 23, a current path is created from a connecting 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 surface 22 and thus to the second connecting leg 26. Due to the recess 24 in the metallization 12 of the PTC element 2, the fusible conductor 3 and the PTC element 2 are only electrically separated from one another by the relatively small thickness of the insulating layer 23. However, this small distance ensures the desired good thermal coupling of the fusible conductor 3 with the PTC element 2.

When the fusible conductor 3 is covered by known covers or casting compounds, the embodiment of Fig. 4a, 4b can also be used without its own housing, since the plasma released by the fuse element in the event of shutdown can be collected and thus cannot cause any damage to the surrounding circuit.

Due to the compact design outlined in a side view in Fig. 4b, it is also easy to use this embodiment without any further structural adaptation measures in known housing designs from the device security sector, such as the TR5® mentioned above.

Claims (16)

1. Electrical fuse with a series circuit consisting of a PTC element and a fusible element, characterized, that the PTC element (2) and the fusible conductor (3) have a close thermal coupling. 2. Electrical fuse according to claim 1, characterized in that the fusible conductor (3) forms a supply line to a contact of the PTC element (2). 3. Electrical fuse according to claim 1 and/or claim 2, characterized in that 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 (3) 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-film circuit. 7. Electrical fuse according to one of the preceding claims, characterized in that - the fusible conductor (3) - is arranged on the PTC element (2), in particular on a metallization serving as a contact surface (12) of the PTC element (2), wherein - the fusible conductor (3), with the exception of the connection point (7), is electrically separated from the PTC element and/or the metallization (12) by an electrically insulating layer (23). 8. Electrical fuse according to claim 7, characterized in that - the contact opposite the connection surface (7) (6) - is formed on the electrically insulating layer (23) as a termination surface (22), - to which at least one external connection element (25) can be fastened, in particular a wire-shaped connection leg (26). 9. Electrical fuse according to one of the preceding claims, characterized in that the PTC element (2) consists of a polymer (8). 10. Electrical fuse according to claim 9, characterized in that the PTC element (2) is formed by a section of a film-shaped polymer body (10). 11. Electrical fuse according to claim 9 or claim 10, characterized in that temperature-related expansions (9) of the PTC element (2) take place essentially perpendicular to a support surface of the PTC element (2). 12. Electrical fuse according to one of claims 1-8, characterized in that the PTC element (2) consists of a ceramic, in particular of barium titanate. 13. Electrical fuse according to one of the preceding claims, characterized in that the fuse (1) in known housing can be used. 14. Electrical fuse according to one of the preceding claims, characterized in that the fuse (1) is designed as an SMD-equipped element (20). 15. Electrical fuse according to one 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 melting zone of the fusible conductor (3). 16. Electrical fuse according to one of the preceding claims, characterized in that the surface of the fusible conductor (3) is coated with tin and this coating was applied in particular by a galvanic process.