The present invention relates to a safety function, i.e. a fuse function, on a film for the manufacturing of capacitors, a metallized film intended for the manufacturing of capacitors ofthe type described in the preamble of claims 1 and 2. The invention relates also to a coiled or stacked capacitor manufactured therefrom.

Prior art

Coiled self-healing capacitors produced from whole or partially metallized paper or plastic films are known in the prior art. The electrodes ofthe capacitors are formed from the metal, and the film forms the dielectric. The plastic films which are used can be e.g. polyester, polypropene, polycarbonate, polystyrene, polyphenyl sulphide or the like. Suitable materials as a base for the metal are all those which can be metal- lized, coiled and which have suitable dielectric constants in order to be used as a di¬ electric in the capacitor. Mixed films of plastic and paper also exist.

It is known to produce patterns of metal-free areas in the surface ofthe metallized film which form the dielectric and the electrodes in the capacitor. In this manner the capa- citors can be divided into partial capacitors which can be arranged in series or in paral¬ lel. Normally the capacitors are ofthe so-called self-healing type. By self-healing it is meant that in the event of a possible flash-over in the capacitor the metal coating around the flash-over place vaporizes and a possible short circuit is prevented.

A known problem of the above mentioned type of capacitor is that despite the above mentioned self-healing property, damage can occur in which the capacitor is to a large extent or even completely destroyed, even if the cause ofthe damage is a local flash- over. Even if during production ofthe capacitor, the metallized films are designed so that their electrical characteristics are such that with a flash-over in the dielectric, the

metallization vaporizes to such an extent that a controlled burnt-out region occurs around the disruptive discharge channel, large damage can occur on both the metal¬ lization and the film itself. The damage produced depends naturally to a large extent on how high the temperature locally generated by the damage is. In many cases the vaporized area can be so large that the capacitor is destroyed.

It is known from the German Laid-Open Application 1 150 762 to produce patterns on metallized dielectric films intended for the production of capacitors. According to this document a metal layer is laid down on the film and thereafter the pattern is produced by grinding out a pattern made of a metal-free surface. In this process there is, how¬ ever, an obvious risk of producing unevenness in the metal coating which in itself is a potential risk that damage ofthe flash-over type shall occur.

It is also known to arrange coiled partial capacitors in series from amongst others EP-A 1-0 225 822, in which can be seen melting zones, fuse areas, intended in the case of flash-over or the like in the capacitor to melt so that the damaged area is isolated, EP-A 1-0 228 924, in which the electrode regions have a much higher resistivity than the regions which merely form the electrically conductive connections, EP-A 1-0 611 179, in which metallized plastic films have locally demetallized zones which separate paralleliogram-shaped surfaces with a surface area of approximately

T 1 cm 2.

US-A-5 136 462 (=EP-A2-0 450 455) describes in addition how attempts have been made in a number of patents to solve the problems which occur when flash-over and short circuits occur in capacitors ofthe above type. In amongst others EP-A 1 -0 243 288 it has been suggested that the metal coating should be divided in the longitudinal direction ofthe film by openings which extend essentially over the whole ofthe width ofthe film and in this way form metallized tongue-shaped pieces, wherein the coating has a greater thickness at the connection end in order to facilitate connection and

whereby at least one ofthe two capacitor layers in the region ofthe ends ofthe dividing openings has a reduced thickness. In the event of an unfavourable electrical load one or more of these regions, fuse regions, with reduced thicknesses is burnt away and the damage parts isolated.

It has, however, proven to be difficult to apply these areas with varying thicknesses and the necessary and desirable tolerances are difficult to achieve.

It has also been suggested that with the above described tongues which are formed through transverse openings, the base ofthe tongues should be tapered so that only a narrow connection remains which can easily be burnt away. These narrow connections can be considered as fuses. The possible loss of electrode surface will be dependent on how big these tongue-shaped areas are, and if the number of transverse openings is increased in order to minimize possible losses then this means that the capacitor becomes more bulky, i.e. in order to produce the same electrode surface, a longer metallized film is required.

Because ofthe above reasons, in the above mentioned US-A-5 136 462 it has been suggested to divide the film into surface elements with small surfaces by arranging narrow openings with rounded ends, whereby at each end bridges with a narrower width than the openings can be found which conductively connect the surface elements. This is a solution which is different from those shown earlier because the electrode surface ofthe capacitor essentially forms a single surface, i.e. the capacitor is not divided up into series or parallel coupled partial capacitors. With a too high local current density, one or more of these bridges around one or more ofthe surface elements is burnt away and these surfaces are isolated from the capacitor with only a smaller loss of capacitor electrode surface as a consequence. The bridges constitute here a sort of fuse which insures that the damaged region is isolated and the rest of the capacitor can continue to function.

It is now apparent that, despite the arrangement ofthe above described fuses, there is a risk of damage occurring because of an unfavourable current density distribution in the fuse region.

Purpose ofthe present invention

The main purpose ofthe present invention is to produce a capacitor which takes up a small volume, i.e. where the surface ofthe metallized film is used to a maximum, and to produce a safe, self-healing capacitor which has a good safety function.

Yet a further object ofthe present invention is to produce a capacitor in which damage, which possibly occurs due to an overload, is isolated in a controlled manner, and in this way the risk of possible explosion and/or fire is minimized.

These and other objects, which will be obvious from the continued description, are achieved according to the invention with the use of a film ofthe type which is men¬ tioned in the characterizing part of claim 1. Further characteristics and developments of the arrangement according to the invention are mentioned in the other patent claims.

Short description ofthe drawings

In order for the present invention to be easily understood and carried out, it will be described with reference to the following drawings, where:

Figs la and lb show a schematic view of metallized films according to the invention, which films are intended for a series construction.

Figs 2a and 2b show a section through the film at line I - 1 in Figs la and lb.Fig 2c show the corresponding circuit diagram symbols.

Figs 3a and 3b show a schematic view ofthe metallized metal film intended for a single construction.

Figs 4a and 4b show a section through the films at line II - II in Figs 3a and 3b.

Fig 4c shows the corresponding circuit diagram symbols.

Figs 5a and 5b show a schematic view of metallized metal films intended for a series construction with three partial capacitors.

Figs 6a and 6b show a section through the films at line III - III in Figs 5a and 5b.

Fig 6c shows the corresponding circuit diagram symbols.

Figs 7a and 7b show a schematic view of metallized metal films intended for a series construction with four partial capacitors.

Figs 8a and 8b show a section through the films at line IV - IV in Figs 7a and 7b.

Fig 8c shows the corresponding circuit diagram symbols.

Fig 9 shows a simulated current density at a fuse function according to the state ofthe art.

Fig 10 shows a simulated current density at a fuse function according to the invention.

Fig 1 1 shows an enlargement ofthe fuse function shown in Fig 9.

Fig 12 shows an enlargement ofthe fuse function shown in Fig 10.

Description of a first embodiment ofthe invention

A first embodiment ofthe invention is shown in Figs 1 - 2. On the films 100 and 110, which form the dielectric ofthe capacitor, metallized surfaces are arranged which are divided into segments through certain areas being left metal-free during the metal¬ lization. These metal-free areas can naturally be produced by alternative means. The film 100 shown in Fig la has two metallized segments 102,103, separated by a metal- free region 101 extending along the film, and the film 110 shown in Fig lb has a metallized surface divided by means of narrow elongated metal-free regions, essenti- ally formed from the three parts 117,118,119;117',118',119', which extend from one edge ofthe film to the other, into essentially similarly shaped segments following one another 114,115,116. Along the edges ofthe film 110 there also extend two continuous metal-free edge regions 120,121. The parts 117,118,119;117\118\1 19' of the elonga¬ ted metal-free continuous regions are placed so that the first 117; 1 17' and the third 119; 119' parts are essentially displaced in parallel with respect to each other and form an angle β with the longitudinal direction ofthe band. The second part 1 18; 118' extends essentially ^' in the longitudinal direction of the film. In the transition between the different parts, the elongated metal-free regions 117,118,119; 1 17', 118', 1 19' extend in a gentle curve. The longitudinal extensions of parts 117 and 1 18 resp. 118 and 119 form in consequence an angle α with each other which is greater or equal to 90°, whereby the sum ofthe angles α + β = 180°.

By forming and placing parts 1 17,118,119 resp. 1 17',118', 1 19' after each other in the longitudinal direction ofthe film, the metallized segments formed in this way will be divided into two parts diagonally displaced with respect to each other separated by an elongated constricted region, whereby the diagonally displaced parts will form essenti¬ ally mirror images of each other with reference to a line through the narrowest part 150 ofthe elongated region, peφendicular to the longitudinal direction ofthe region. This part 150 forms the fuse function according to the invention. It is evident from the

Figures that the angle α influences the length ofthe elongated region. Through varying the angle α the fuse region can be designed so that a desired current distribution and fuse function can be achieved.

The above mentioned metal-free surfaces 120,121 along the edges ofthe film 110 are intended to ensure that during forming ofthe contacts the metallized surfaces ofthe film 110 do not come into contact with the electrical contacts ofthe capacitor.

Figs 2a resp. 2b show the films 100 and 110 according to Figs la resp. lb placed above one another, as they are placed before coiling of the capacitors, in a section along a line I - 1 in Figs la and lb.

By comparison ofthe distribution ofthe metal coating on the films 100 resp. 1 10, it is evident that the fuse function 150 on film 110 is so placed that it always corresponds to a capacitively inactive surface, i.e. a metal-free surface, on film 100. This positioning ofthe fuse function forms part ofthe invention and corresponding positioning can be found in all the examples below, even if it is not mentioned.

During the production ofa capacitor with the use ofthe above mentioned films, a film 1 10 is placed on a a film 100 and thereafter these films are coiled into a roll. Film 1 10 is preferably narrower than film 100 because ofthe above given reason. In this way coiled capacitors are formed. During the manufacturing of stacked capacitors, films 100 and 1 10 are coiled in the same way but around an object with a large radius. Sub¬ sequently, a contact layer of metal is formed using known techniques, e.g. flame-spray- ing, on the part ofthe coiled capacitor which is formed from both ofthe long sides of film 100. Finally, these capacitors are cut into appropriate sizes, and connection con¬ tacts are arranged. As mentioned above, film 1 10 is narrower than film 100, whereby the metallized segments 1 14, 1 15 resp. 1 16 will not be affected by the sprayed metal, i.e. film 1 10 will not have contacts formed on it during the shaping of the capacitor.

Description of a second embodiment ofthe invention

A second embodiment ofthe invention is shown in Figs 3 - 4. There are similarly arranged metallized surfaces 214, 215 resp. 216 on the films 200 and 210 which form the dielectric ofthe capacitor.

The two films 200 and 210 shown are essentially similar, and as is shown in Figs 3a and 3b they are placed in opposing directions in relation to each other. The directions ofthe films are shown in the Figures by arrows.

Only the film shown in Fig 3a is described below since the film in Fig 3b is an identical film. The film 200 has a metallized surface divided by means of narrow elongated metal-free regions, essentially formed of three parts 217, 218, 219; 217', 218', 219', which extend from one edge ofthe film to the other edge into essentially similarly shaped segments 214, 215 resp. 216 following each other. The first 217 and the third 219 parts of these parts are essentially displaced in parallel with respect to one another, and their extensions form an angle β with the longitudinal direction of the band, and wherein the second part 218 extends in the longitudinal direction ofthe film. This part 218 is, however, placed nearer one edge ofthe film, i.e. non-symmetrically in relation to the two edges ofthe film. In the transition between the different parts the longitudinal metal-free region extends in a gentle curve. The longitudinal directions of parts 217 and 218 resp. 218 and 219 form consequently an angle α with each other which is equal to or greater than 90°, whereby the sum ofthe angles α + β = 180°.

The films 200 resp. 210 have a metal-free region 220 resp. 221 extending along one edge ofthe film.

Figs 4a resp. 4b show films 200 and 210 according to Figs 3a resp. 3b placed above one another, as they are positioned before the coiling of capacitors, in a section along a line II - II in Figs 3a and 3b.

Coiled or stacked capacitors can subsequently be made from these films in the above described matter with the difference that as the films are laterally displaced in relation to one other, every second layer will be contacted on one side ofthe coiled or stacked capacitor and the other layers will be contacted on the opposing side.

Figs 5a and 5b and Figs 7a and 7b show schematically an extended series construction and Figs 6a and 6b, and 8a and 8b sections through the films on lines III - III in Figs 5a and 5b resp. lines IV - IV in Figs 7a and 7b. The pattern ofthe metallized segments and metallized surfaces are built up in the same way as in the earlier described pattern for series construction according to Figs la and lb, and Figs 2a and 2b.

The films according to the invention have preferably a width of between 5 to 100 mm, one or two metal-free edges 110, 121, 220, 221 along the longitudinal edges ofthe films with a width of approximately 0,5 - 5 mm, a shortest distance D (shown in Figs lb and 3a) between two following longitudinal regions 117, 118, 119; 217, 218, 219 at the connecting curved parts of approx. 0,1 - 10 mm, a bending radius ofthe curved parts of approx. 1 - 10 mm, the distance along the film edge between two consecutive longitudinal regions can preferably vary between 5 - 100 mm.

The angle α which is formed between two parts following each other ofthe longitudi¬ nal region is 90° or greater, preferably between 90° and 135°, most preferably 100° to 120°.

Fig 9 shows the simulated values ofthe relative current density in a fuse region according to the prior art. The metallized surface represented by 900, the metal-free region by 901 and the fuse region by 950. The current densities given in the table are given in relative terms. It can be seen that the current density is extremely high but unequally distributed in the fuse region 950. Along a central line IX - IX through the fuse region the relative current density varies between 4.1 and 1 1.0. With a reduction

ofthe width ofthe slits the above effect becomes even more pronounced. In order that a fuse with this shape should bring about the desired effect, care must be taken so that the minimum value does not become too low, because in such a case the fuse function cannot achieve its object. This means also that a possible maximum value can climb so high that the security and function ofthe fuse function is put into jeopardy and the desired security is not achieved. It is consequently difficult to produce a fuse with exactly specified characteristics in this embodiment.

It has now surprisingly been shown that in the embodiment according to the inven- tion, see Fig 10, a current density distribution in the fuse region which is much more advan-tageous than that shown earlier for an arrangement according to the prior art (Fig 9) is achieved. Fig 10 shows simulated values ofthe relative current density in a fuse region according to the invention. The metallized surface is represented by 900', metal-free regions by 901 ' and the fuse region by 950'. The current densities given in the table are given in relative terms. It can be seen that the current density is extremely high but equally distributed in the fuse region 950'. Along a central line X - X through the fuse region, the relative current density varies between 10.5 and 1 1.0. This means that with an embodiment ofthe fuse according to the invention it is easy to achieve a controlled burning-out and to design the fuse region so that with a high degree of certainty the current density is achieved at which a controlled burning- out of the fuse function occurs. Through changing the angle α the relative length ofthe fuse can be regulated and thereby indirectly the current density reached in the region 950'.

In this way consequently a current strength can be achieved which is sufficient to burn out the fuse but which does not locally go to such high values that damages occur to the surroundings. Through the elongated shape ofthe fuse region the fuse in the indi¬ vidual cases can be dimensioned with a greater precision than in the case illustrated by the prior art. Neither are the tolerances during manufacturing as small as they are with

the arrangements manufactured according to the prior art, because of the elongated shape ofthe fuse.

The fuse function, the metallized film having such a fuse, and capacitors manufactured from these films according to the invention, which are described above with reference to the described embodiments, can naturally be modified within the scope ofthe appended claims.