Electricity: electrothermally or thermally actuated switches – Electrothermally actuated switches – Fusible element actuated
Reexamination Certificate
2000-03-03
2002-05-07
Picard, Leo P. (Department: 2835)
Electricity: electrothermally or thermally actuated switches
Electrothermally actuated switches
Fusible element actuated
C337S290000, C337S295000, C439S890000
Reexamination Certificate
active
06384708
ABSTRACT:
FIELD OF THE INVENTION
The present invention relates to an electrical fuse element which comprises
a substrate with two contacts arranged on opposite end sides,
terminal areas connected to the contacts which run in the same plane and
a fusible conductor electrically connected in a conducting manner
to the contacts via the terminal areas.
BACKGROUND OF THE INVENTION
Fuse elements of the above-mentioned type are today preferably produced as surface-mounted devices (SKD) using fusible conductors in the form of conducting layers or pieces of wire. Owing to the small dimensions, it is attempted by the use of special materials and/or by a complex inner structure to extend the fullest voltage range in which such components can be used.
An example for a SMD-type fuse of the above mentioned type is disclosed in WO-A-96/08832. This suminiature circuit protector consists of several layers of ceramic material, where on each layer terminal areas connected to a fuse element are arranged. Terminal areas of different layers are interconnected in parallel or in series by lead-throughs extending from one layer to another through the ceramic material.
It is the object of the present invention to develop fuse elements of the aforementioned type at low production costs for use in higher voltage ranges with an improved breaking capacity.
The object is achieved according to the invention by
the terminal areas and the fusible conductor
being arranged separated from one another by an insulator and
electrically connected to one another via leadthroughs,
where the terminal areas and the fusible conductor run in different planes
In known SMD fuse elements, the fusible conductor, as the actual functional element of a fuse, goes over directly into the other electrically conducting components of the fuse, in particular into the terminal areas. For this purpose, usually all the components are arranged on the surface of a substrate. At the moment of breaking the current, the fusible conductor melts through in the region of the hottest area, the “hot spot”. The current flow is not instantaneously interrupted, however, but is maintained by an arc. According to the prior art, it is attempted by particular material selection and/or design measures to quench this breaking arc as quickly as possible and to suppress the subsequent striking of a secondary arc. While the breaking arc or primary arc is produced whenever breaking occurs and is fed by the melting material of the fusible conductor itself, in the case of arcing back, that is when a secondary arc is produced, the metal adjoining the fusible conductor—usually in the form of conducting tracks—is also involved in the arcing process. Consequently, the secondary arc spreads beyond the region of the actual fusible conductor and may even reach the external terminals of the SMD fuse element. In this case, the fuse can no longer perform a protective function and even additionally damages surrounding components by the arc.
SUMMARY OF THE INVENTION
A different geometry is described in AU-B-40791/78. Here, a houshold type fuse is disclosed where on opposite sides of an insalator terminal areas are arranged. Two fusible links are also arranged on opposite sides of the insalator, electrically connected to the terminal areas and to each other by leadthroughs. In this arrangement, a distance is kept between the terminal areas and the fusible links. However, arc flashover is not effectively prevented by this arrangement.
With otherwise the same switching geometry, by contrast with such fuses according to the prior art, a fuse element according to the invention suppresses the effect described by the fusible conductor being arranged separated from all the other parts of the fuse by an insulator. Lead-throughs provide the electrically conducting connection of the fusible conductor through the insulator to the external contacts. When breaking the current, after consuming or vaporizing the conductive material of the fusible conductor, the arc burns up to the lead-throughs in the insulator. From this moment on, there is no more material to he vaporized available, since the material of the lead-throughs lying in the insulator cannot be melted and vaporized by the arc. Even a possibly struck secondary arc must consequently extinguish quickly, since it can no longer be maintained. The fuse element consequently breaks the current reliably and, on account of the minimizing of conductive material available for the arc, quickly after it blows. Accordingly, with the same dimensioning and overall size, a fuse according to the invention has a considerably greater breaking capacity than known fuses, since it always keeps the arc confined to the region of the fusible conductor, with the result that, after consuming or vaporizing the small amount of conductive material of the fusible conductor between the lead-throughs on the insulator, the arc can no longer find any further “food”.
According to the invention, the terminal areas and the fusible conductor are arranged separated from one another by the insulator in such a way that they run in different planes. An arc flashover is consequently prevented particularly effectively.
The insulator is advantageously made up by one or more layers of dielectric pastes. The insulator may thus be arranged as an insulating layer on the substrate, preferably by screen printing. Many inexpensive processes of adequate accuracy, in particular using pastes capable of cofiring, are known from the field of thick-film and thin-film circuitry. In very cost-effective processes, insulators can consequently be produced in multiple repeats as dielectric layers which also have a surface quality which allows the use of known processes for applying or attaching and contacting a fusible conductor on the respective insulator with great reliability.
In a preferred embodiment, the insulator is formed by the substrate itself, with the result that no additional material has to be used for the separation of terminal areas and fusible conductor. This feature also allows at least one process step to be saved in comparison with customary production processes. In a preferred embodiment of the invention, the two planes on which the fusible conductor on the one hand and the terminal areas on the other hand are arranged spatially separated from one another and connected via lead-throughs represent the upper side and the underside of the substrate.
The short burning duration and the strict spatial confinement of the arc described above also make it possible to use customary fusible conductor coverings in the “hot spot” towards the outside, preferably a glass covering. In mass production, this additionally has the effect of lowering the unit price of fuse elements according to the invention.
A fuse element according to the invention is advantageously not restricted to the use of a particular substrate material. For example, a composite plastic, such as for example FR4, or other customary circuit board materials may be used as the substrate material. Preferably, however, a ceramic material and, in particular, a glass ceramic is used as the substrate in a fuse according to the invention.
In a particularly advantageous development, the leadthroughs are designed as plated-through holes and, according to Claim 11, consist of a conductive sintered material, which is preferably filled into holes of a refractory substrate, such as for example a ceramic, and subsequently solidified in a thermal process. With these comparatively narrow lead-throughs, when an arc occurs there may also be a phenomenon referred to as the channelling effect with a positive influence on the extinction of the arc, by which effect an arc passing through a narrow channel “blows itself out”.
However, ceramic manufacturers also offer ready-made and ready-sintered substrate materials, which can be provided with plated-through holes by drilling and heating once the drilled holes have been filled with sinterable material. Applying the terminal areas and any leads to the plated-through holes on the one hand and a fusible condu
Baus Andreas
Jöllenbeck André
Rupalla Manfred
Banner & Witcoff , Ltd.
Vortman Anatoly
Wickmann-Werke GmbH
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