Field of the Invention

The present invention relates to the field of film power capacitors. More particularly, the invention relates to a wound film capacitor, with an improved heat evacuation and higher power ratings.

Background of the Invention

Film ca acitors

Film capacitors are made of a variety of polymers, such as polyester, polycarbonate, Teflon, polypropylene, and polystyrene.

There are three types of film capacitors:

- Film-foil capacitors: Capacitors of this type are made of alternating layers of plastic film and metal foil

- Metalized film capacitors: Capacitors of this type have the metal vacuum deposited directly on the film

- Hybrid series structure capacitors: Capacitors of this type include two metal foils and a metalized film, to form 2 capacitors in series.

In general, film-foil type is better for handling high current, while metalized film capacitors provide higher capacitance per volume. The hybrid type enables self healing capabilities, as will be explained later. The electrodes in a film-foil design are separate sheets of metal foil, wound with interlayer sheets of dielectric film material. These foil electrodes extend alternately out of each end of the capacitor roll beyond the dielectric surface. This provides a mass of metallic material, to which leads are attached by welding or soldering.

In the metalized type of design, the foil electrodes are replaced with a thin layer of 99% pure aluminum, vapor-deposited directly onto the dielectric film. The metalized design greatly reduces the physical size of the capacitor.

The hybrid capacitor type offers a distinct advantage over the film-foil type^ self-healing capability, resulting from the extreme thinness of the metalized electrode material. Whenever a flaw or weak spot in the dielectric results in a short-circuit condition (a point-defect short-circuit between the metallized electrodes), the dissipated heat vaporizes away a small pattern of the electrode film around the point of the short-circuit. As a result of the vaporization, the short condition is removed and the capacitor is again operational. This phenomenon is known as a "clearing" or "self-healing" process.

All the methods described above have not yet provided satisfactory solutions to the problem of manufacturing a wound film power capacitor with improved heat evacuation. Specifically, the existing standard hybrid foil/vacuum-deposited layout has a weak-point in the centre of the capacitor as will be explained later on.

It is therefore an object of the present invention to provide a method for manufacturing a wound film power capacitor with improved heat evacuation and a higher power rating.

Other objects and advantages of the invention will become apparent as the description proceeds.

Summary of the Invention

The present invention is directed to a wound film capacitor with improved heat evacuation, that comprises:

a) wound electric contacts consisting of two conductive metallic foils (e.g., aluminum or copper foils), each of which is connected to a different electrode of the capacitor;

b) a wound capacitive layer consisting of two metalized dielectric film material layers, facing each other and overlapping, each of which being metalized with vaporized metal (e.g., aluminum or copper), the wound capacitive layer forming two planar and essentially parallel walls; c) a pair of planar electrodes pressed against the edge of each wound foil, such that the edge of each foils that extends outwardly from the walls is bent and attached to an electrode; and

d) a mechanism for mechanically holding the assembly of the wound electric contacts, the wound capacitive layer and the pair of planar electrodes, together.

Alternatively, the wound film capacitor may consist of

a) wound electric contacts consisting of two conductive metallic foils, each of which is connected to a different electrode of the capacitor;

b) a wound capacitive layer consisting of a metalized dielectric film material layer being metalized with vaporized metal, and an overlapping dielectric film material layer, the wound capacitive layer forming two planar parallel walls;

c) a pair of planar electrodes pressed against the edge of each wound foil, such that the edge of each foils that extends outwardly from the walls is bent and attached to an electrode; and

d) a mechanism for mechanically holding the assembly of the wound electric contacts, the wound capacitive layer and the pair of planar electrodes, together. The wound film capacitor may further comprise a sprayed-on metallic agent (e.g., aluminum or copper) to form an electrode, whenever the foils are made of insolderable material and encapsulation material for forming the capacitor body.

The wound electric contacts may comprise two metallic foils, where each foil is connected to an electrode of the capacitor by first spraying a metal agent on to the outside of the foil, and then soldering the sprayed area to the capacitor electrode.

The wound electric contacts may comprise two copper foils, each of which is connected to an electrode of the capacitor by soldering or by mechanical pressure alone.

The dielectric film material may be selected from the group of- Polypropylene;

Polyethylene Terephthalate (PETP);

Polyethylene Naphtalate (PEN);

Polyphenylene Sulfide (PPS);

Polypropylene (PP);

paper. Brief Description of the Drawings

The above and other characteristics and advantages of the invention will be better understood through the following illustrative and non -limitative detailed description of preferred embodiments thereof, with reference to the appended drawings, wherein:

Fig. 1 (prior art) illustrates a configuration of the layers of a hybrid film/foil/vacuum-deposit wound film capacitor, with self- healing capability;

Fig. 2A illustrates a configuration of the layers of an improved wound hybrid film capacitor, according to a preferred embodiment of the invention; and

Fig. 2B illustrates a configuration of the layers of an improved wound hybrid film capacitor, according to another preferred embodiment of the invention.

Detailed Description of Preferred Embodiments

The present invention discloses a wound film power capacitor with improved heat evacuation. Accordingly, the foil used may optionally be made of copper, so as to gain improved thermal conductivity and lower losses. Typically, an electrode is formed by spraying a metallic agent on the planar edges of the wound foil, in order to connect the edges of the wound foil of all layers. Such spraying is used when the foils are made of aluminum, since soldering processes of aluminum are problematic. On the other hand, even though spraying is capable of providing the desired electric conductivity, the thermal conductivity is insufficient, due to the porosity of the sprayed layer. The porosity reduces the thermal conductivity of the electrode and generates a thermal junction that limits the power rating.

Using copper foils , as proposed by the present invention, allows construction of the capacitor body without the need for spraying copper on both planar sides of the wound films, in order to form an electrode on each plane. This configuration, allows assembling the capacitor by simply pressing planar electrodes against the edge of each wound foil (which always slightly extends outwardly) and then encapsulating the resulting construction.

Fig. 1 illustrates a conventional configuration of the layers of a wound film capacitor, with self-healing capability. The capacitor consists of two opposing aluminum foils 10, each of which is connected to a different electrode 15 of the capacitor. The capacitive layer consists of two Metalized Polypropylene (MPP) layers or a combination of an MPP layer 11 and a PP layer 12, facing each other. This arrangement forms a connection of two wound film capacitors arranged above each other and connected in series 13, where windings are equivalent to a plurality of small capacitors connected to each other in a parallel connection 14. Each equivalent capacitor is formed by one metal foil 10 and the metalized film 17. In case of failure due to point- defect short-circuit between an adjacent metalized film and a foil electrode 10, a self-healing mechanism will be activated, such that one of the series capacitors containing the point-defect will be shorted-circuit. At this point, since both capacitors are connected in series, the entire current will flow through the point-defect short-circuit and will vaporize the metallization around the point-defect short-circuit, thereby removing the metallization around the defect, and eliminating the short circuit condition.

Heat is generated in the capacitor by the current flowing in the Aluminum foil and in the metalized film as well as by dielectric losses in the dielectric film. Dissipation of the heat to the electrodes 15 is done through the aluminum foils 10.

The two capacitors are electrically connected in series by the metalized film in the center of the MPP layer. In the MPP layers, the current flows vertically between the two electrodes 15.

The current density in the metalized film reaches its maximum value in the center of the capacitor, since the center overlaps with the gap between the two foils 10. This means that the maximum heat is generated in this area. However, heat evacuation from this area is limited as no metal foil 10 is presented in the center of the capacitor. This deficiency limits the power that can be driven through the capacitor, since a hot area ("hot-spot)

16 develops in the center of the capacitor. In the configuration of Fig. 1, the only way to evacuate heat from the hot-spot is through the metalized film 17. However, metalized film 17 (which is evaporated) is very thin and has low thermal conductivity. Therefore, heat evacuation (which should be via foils 10) is problematic and practically, the generated heat creates a hot spot in the center of the capacitor.

Fig. 2A illustrates a configuration of the layers of an improved wound film capacitor, according to a preferred embodiment of the invention. In this embodiment, the capacitor consists of two conductive metal foils 40, each of which is connected to a different electrode 21 . Sandwiched between the foils 40, there are two metalized PP films 22a and 22b,. None of these metalized layers is connected to the electrodes 21.

This structure forms two capacitors in series between the two metal foils 40. Each capacitor is made by the polypropylene (PP), sandwiched between the metal foil 40 on one side and the metalized (evaporated) layer

17 on the other side. The capacitors which are arranged side by side (one after the other), are described schematically in 23. As a result, the series connection (which is essential for self-healing capability) is enabled in the same way described above in Fig. 1.

The advantage of this arrangement (in comparison to Fig. l) is that all thermal conduction paths for heat evacuation are either along the length of the metal foil, which is a reasonable thermal conductor (that evacuates the heat generated due to the currents flowing through it), or passing through the metalized film 17 (in a direction perpendicular to the metalized film area, which is evaporated on the PP layer), which is so thin that its thermal resistance in this perpendicular direction is not significant. In addition, in this arrangement, no electrical current flows along the high resistive metalized layer (i.e., in the vertical direction in Fig. 2A or in any direction which is parallel to the plane of the metalized layer), and therefore, no heat is generated in the metalized layer. The fact that foils 40 are extending almost along the entire gap between electrodes 21, allows efficient heat evacuation from the wound layers. The gap 41 between the edge of each foil 40 and the close electrode 21 does not generate heat, since it does not have an opposing conductive foil and therefore, does not form a capacitive segment.

This arrangement, allows self-healing in case of failure due to point-defect short-circuit, while substantially improving the heat evacuation. Fig. 2B illustrates a configuration of the layers of an improved wound film capacitor, according to another preferred embodiment of the invention. In this embodiment, the capacitor consists of two conductive metal foils 40, each of which is connected to a different electrode 21 (a planar face of the wound capacitor). Sandwiched between the foils, there are two polypropylene films 22a and 22c (which are capacitive elements), such that film 22a is MPP metalized with vaporized aluminum, and film 22c is PP and overlapping 22a. None of these films is connected to the electrodes.

Here again, the advantage of this arrangement (in comparison to Fig. l) is that all thermal conduction paths for heat evacuation are either along the length of the metal foil, which is a reasonable thermal conductor (that evacuates the heat generated due to the currents flowing through it), or passing through the metalized film 17 (in a direction perpendicular to the metalized film area, which is evaporated on the PP layer), which is so thin that its thermal resistance in this perpendicular direction is not significant. In this arrangement as well, no electrical current flows along the high resistive metalized layer (i.e., in the vertical direction in Fig. 2B or in any direction which is parallel to the plane of the metalized layer), and therefore, no heat is generated in the metalized layer. The fact that foils 40 are extending almost along the entire gap between electrodes 21, allows efficient heat evacuation from the wound layers. The gap 41 between the edge of each foil 40 and the close electrode 21 does not generate heat, since it does not have an opposing conductive foil and therefore, does not form a capacitive segment.

This arrangement as well, allows self-healing in case of failure due to point-defect short-circuit, while substantially improving the heat evacuation. The advantages of this arrangement in comparison to Fig. 1 is essentially similar to the advantages of the embodiment shown in Fig. 2 A.

The above examples and description have of course been provided only for the purpose of illustration, and are not intended to limit the invention in any way. As will be appreciated by the skilled person, the invention can be carried out in a great variety of ways, employing more than one technique from those described above, and different metallic and dielectric materials (such as Polypropylene, Polyethylene Terephthalate (PETP), Polyethylene Naphtalate (PEN), Polyphenylene Sulfide (PPS), Polypropylene (PP) or paper) other than used in the description, all without exceeding the scope of the invention.