Abstract:
An electrical component employing a metalized composite film with a heat-seal layer is disclosed. The use of the heat-seal layer in the disclosed configuration allows a wound film capacitor integral with a primary coil to be made without there being a need to impregnate the wound film unit with an epoxy or other resin, thus saving substantial manufacturing time.

Description:
FIELD OF THE INVENTION 
     This invention relates to metalized elements. More particularly, it relates to metalized elements made from polyester film composites, and more particularly to electrical components comprising wound film capacitors for use in flyback transformer circuits. 
     BACKGROUND OF THE INVENTION 
     Flyback transformers are used in the horizontal deflection circuit for cathode ray tubes (CRT), critical components of computer monitors, and are expected to find significant usage in digital television applications. Commonly used such transformers include elements manufactured by interleaving metalized polyester film with a coated heat sealable polyester film to form coils with an integral resonant capacitor. The coating side is wound against the metalized film and heat-sealed for protection against the corrosion of a final epoxy impregnation. Flyback transformers incorporating wound film capacitors acting simultaneously as resonance capacitors and primary coils are described for example by Okamura in U.S. Pat. No. 5,241,293, incorporated herein by reference. Such flyback transformers have a number of advantages compared to traditional designs involving wire-wound primary coils attached to separate resonance capacitors, including lower space requirements, lower magnetic flux leakage from the outside portion of the secondary coils, and reduction in ringing that otherwise occurs due to undesired resonance between the stray capacitance between the primary coil, and the secondary coil and the secondary leakage inductance. 
     Film capacitors wound by traditional methods result in an air layer between the film windings. Air layers in these types of high voltage capacitors cause corona discharge resulting in deleterious performance of the dielectric. Traditional wound film capacitors used in flyback transformers thus require impregnation of a resin such as epoxy to eliminate the air layer that exists between the individual film windings. The impregnation step requires a significant amount of manufacturing time to complete which limits the overall production yield. Therefore, elimination of his impregnation step would be advantageous to the high voltage capacitor manufacturer. 
     Thus there continues to be a need for improved wound film capacitors and methods for their manufacture. 
     SUMMARY OF THE INVENTION 
     In one aspect of the invention, there is provided an electrical component comprising: 
     a first elongated dielectric layer having superposed over a first surface thereof a metal layer substantially co-extensive with said first surface and over a second surface thereof opposite said first surface a heat-seal layer coextensive with said second surface; 
     a second elongated dielectric layer having superposed over a first surface thereof a metal layer substantially co-extensive with said first surface and over a second surface thereof opposite said first surface a heat-seal layer coextensive with said second surface; and 
     at least one terminal connected to each of said metal layers; 
     wherein said first and said second elongated dielectric layers are superposed with the heat-seal layer of the second dielectric layer in contact with the metal layer of the first dielectric layer and are wound together to form a substantially cylindrical structure, and wherein each of said dielectric layers and said heat-seal layers superposed thereon has a combined thickness of between about 0.5 μm and about 500 μm and wherein said heat-seal layer constitutes between about 0.5% and about 40% of said combined thickness. 
     In another aspect of the invention, there is provided a method for making an electrical component comprising: 
     (a) forming a first metalized element comprising a dielectric layer having superposed over a first surface thereof a metal layer substantially co-extensive with said first surface and over a second surface thereof opposite said first surface a heat-seal layer coextensive with said second surface; 
     (b) forming a second metalized element comprising a dielectric layer having superposed over a first surface thereof a metal layer substantially co-extensive with said first surface and over a second surface thereof opposite said first surface a heat-seal layer coextensive with said second surface; 
     (c) positioning said first metalized element over said second metalized element so that said metal layer of said first metalized element is in contact with said heat-seal layer of said second metalized element, and winding both metalized elements to form a generally cylindrical shape; and 
     (d) applying sufficient heat to said metalized elements to adhere said first and said second metalized elements together; and 
     (e) providing at least one electrical connection to each of said metal layers. 
    
    
     BRIEF DESCRIPTION OF DRAWINGS 
     FIG. 1 is an end view of a metalized element for making an electrical component according to one aspect of the invention. 
     FIG. 2 is an end view of a metalized element similar to that of FIG. 1, showing in addition a slip layer according to another aspect of the invention. 
     FIG. 3 is a top view of the metalized element of FIG.  2 . 
     FIG. 4 is an end view of a pre-winding assembly for making an electrical component according to the invention. 
     FIG. 5 is an exploded view of a pre-winding assembly for making an electrical component suitable for use in a flyback transformer, according to the invention. 
     FIG. 6 is a perspective view of the pre-winding assembly of FIG. 5, partially wound up, with layers partially cut away. 
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     The invention will next be illustrated with reference to the figures, wherein the same numbers indicate the same elements in all figures. Such figures are intended to be illustrative rather than limiting and are included herewith to facilitate the explanation of the invention. The figures are not to scale, and not intended as engineering drawings. Also, it will be appreciated that the devices of the invention may be used for a wide variety of applications, and accordingly the dimensions and materials useful for making them also cover a very wide range, and are sometimes interdependent on each other. Therefore, the invention should not be construed as limited by the materials and dimensions explicitly noted in the Figures and associated text. 
     Referring now to FIG. 1, there is shown an end view of a metalized element indicated generally at  10 , useful for making an electrical component according to the invention. Such a device may be a wound film capacitor, which may optionally be configured for use in a flyback transformer, as will be illustrated later. Metalized element  10  comprises a dielectric layer  12  with a heat-seal layer  18  on one surface, and a metal layer  14  on the opposite surface. The heat-seal layer covers essentially the entire surface of layer  12 , while the metal layer is shown with borders  16  on either side of it. Although two borders are shown in FIG. 1, neither need be present, or there may be only one. The borders, if present, may be from about 0.25 mm to about 5 mm in width. A 1-mm border is typically present on each side. 
     Dielectric layer  12  typically has a width from about 10 mm to about 500 mm, more typically about 40 mm. The combined thickness of dielectric layer  12  and heat-seal layer  18  is typically from about 0.5 μm to about 500 μm, preferably from about 2.5 μm to about 60 μm. Heat-seal layer  18  preferably constitutes from 0.5% to 40% of the combined thickness of layers  12  and  18 . It is preferably from about 0.1 μm to about 50 μm in thickness, and more preferably from about 1 μm to about 5 μm. 
     Layer  12  may comprise any electrically insulating material known in the art, including but not limited to polyethylene, polypropylene, polycarbonate, polyphenylene sulfide, polytetrafluoroethylene, polyethylene oxide, polypropylene oxide, polyetherether ketone and polyvinylidine fluoride, or copolymers of said homopolymers. It may be a polyester film, as is well known to those skilled in the art. In general, dielectric layer  12  may for example be produced from any suitable synthetic linear polyester that may be obtained in highly crystalline form after stretching and heat setting. Preferred polyester films are polyethylene terephthalate (PET) film and polyethylene naphthalate film. The most preferred polyester film is polyethylene terephthalate. 
     The polymer preparation and film manufacturing processes are well known to those skilled in the art and are disclosed in many texts, such as Encyclopedia of Polymer Science and Engineering, 2nd. Ed., Vol. 12, Wiley, N.Y., pp. 1-313, as well as in numerous patents, such as U.S. Pat. No. 3,107,139. The polymer is typically obtained by condensing the appropriate dicarboxylic acid or its lower alkyl diester with ethylene glycol. Polyethylene terephthalate is formed from terephthalic acid or an ester thereof, and polyethylene naphthalate is formed from 2,7-naphthalenedicarboxylic acid or an ester thereof. 
     Especially useful is polyethylene terephthalate that has been biaxially oriented and heatset. Such a material is well known in the art, and is described for example in U.S. Pat. No. 4,375,494 to Stokes, incorporated herein by reference. Dielectric layer  12  may also comprise a particulate additive to promote improved handling and winding properties by increasing surface roughness. The additive is typically present in an amount of from about 0.002% to about 0.5%, preferably from about 0.005% to about 0.02%, based on the weight of the polymer. Useful additives, which, may be for example particulate materials such as inorganic aluminosilicate clays, silica or alkali or alkaline earth salts. 
     Especially useful are particulate materials obtained by calcining precursor silicone resin particles by the method disclosed by Siddiqui in International Application No. PCT/US98/26429, filed Dec. 11, 1998 and published Jul. 8, 1999 under International Publication No. WO 99/33910, incorporated herein by reference. Particularly suitable are such particles where about 100 percent have a particle sized below about 6 μm, and at least about 95 percent have a particle size below about 5 μm, and 50 percent have a particle size below about 2.2 μm. The particles typically have an average size of about 2 to about 3 μm, preferably about 2 to about 2.4 μm, more preferably about 2.2 μm. Such materials, due to their narrow particle size distribution and relatively spherical shape, are especially useful when the metalized element is used to make a capacitor for use in a flyback transformer application, where they cause less dielectric loss and pitting at the particle-polymer interface than do most particulate additives. 
     Heat-seal layer  18  should be capable of forming a bond to itself or to metal layer  14  and/or to dielectric layer  12 , or preferably all of these, by heating to soften the polymeric material of the heat-seal layer and applying pressure, at a temperature that does not soften or melt the polymeric material of the dielectric layer. In one embodiment, heat-seal layer  18  has a glass transition temperature that is lower than the glass transition temperature of dielectric layer  12 . 
     Heat-seal layer  18  typically comprises a polyester resin, particularly a copolyester resin derived from one or more dibasic aromatic carboxylic acids, such as terephthalic acid, isophthalic acid and hexahydroterephthalic acid, and one or more glycols, such as ethylene glycol, diethylene glycol, triethylene glycol and neopentyl glycol. Layer  18  may comprise terephthalate-containing polyester. A preferred copolyester is derived from one or more of terephthalic acid, isophthalic acid and hexahydroterephthalic acid and one or more glycols, preferably ethylene glycol. The preferred copolyesters which provide satisfactory heat-sealing properties in the amorphous state are those of ethylene terephthalate and ethylene isophthalate, especially in the molar ratios 60 to 90 mol % ethylene terephthalate and correspondingly 40 to 10 mol % ethylene isophthalate. Particularly preferred copolyesters comprise 70 to 85 mol % ethylene terephthalate and 30 to 15 mol % ethylene isophthalate, for example a copolyester of approximately 80 mol % ethylene terephthalate and approximately 20 mol % ethylene isophthalate. 
     Dielectric layer  12  in combination with heat-seal layer  18  should have a dielectric strength of at least about 136 volts/μm, an insulation resistance of at least about 5000 mohm-μf. More preferably, the combination of dielectric layer  12  and heat-seal layer  18  should have a dielectric strength of at least about 227 volts 1 μm, an insulation resistance of at least about 10,000 mohm-μf. One way to achieve these electrical values is to use virgin polyesters for making dielectric layer  12  and heat-seal layer  18 , i.e. without any recycled polyester content. 
     Metal layer  14  typically has a thickness in the range of 10 to 5,000 angstroms, most preferably from about 80 to about 300 angstroms. The layer may comprise any metal. Nonlimiting examples of useful metals include aluminum, palladium, zinc, nickel, gold, silver, copper, indium, tin, chromium, titanium, zinc/aluminum alloy, copper/aluminum alloy or copper/zinc/aluminum alloy. Aluminum is most preferred. 
     FIG. 2 shows, in another embodiment of this invention, a metalized element  110  comprising a dielectric layer  112 , a metal layer  114  having borders  116  on one surface of  112 , and a heat-seal layer  118  on the opposite surface of  112 . Metalized element  110  additionally comprises, on the surface of heat-seal layer  118  not in contact with dielectric layer  112 , a slip layer  120  comprising a particulate slip additive  121 . Such a layer is useful for improving the winding properties of the metalized composite when it is formed into a capacitor. Nonlimiting examples of suitable formulations and methods of applying slip layer  120  are described by Berry et al in U.S. Pat. No. 6,656,222, incorporated herein by reference. Such a layer may have a thickness from about 0.05 μm to about 0.4 μm, preferably from about 0.1 μm to about 0.2 μm, and may comprise a water-soluble or water-dispersible acrylic resin containing as a slip additive submicron alumina, zirconia, silica, or a combination of these. Particularly useful is fumed silica having an average primary particle size from about 10 nm to about 500 nm, preferably from about 40 nm to about 50 nm. 
     The surface provided by such a layer affords excellent handling properties and acceptable heat-sealing properties in film composites comprising a biaxially oriented and heat-set polyethylene terephthalate dielectric layer and a 70 to 85 mol % ethylene terephthalatel30 to 15 mol % ethylene isophthalate copolyester heat-seal layer. It may typically provide a static coefficient of friction for the heat-seal layer tested against itself in the range 0.40 to 0.50, and a heat-seal strength measured by sealing the heat-seal layer to itself in the range 40 to 120 N/mm 2 . The method for determining heat-seal strength is given in the Test Methods section. 
     FIG. 3 is a top view of the metalized element of FIG. 2, showing optional margins  116  and  117  along the sides and at the ends of metal layer  114 , respectively. 
     Referring to FIG. 2, fabrication of the metalized element  110  may be performed by first forming a film composite comprising dielectric layer  112  and heat-seal layer  118 , biaxially orienting the film composite, optionally applying to the surface of heat-seal layer  118  a layer  120  comprising a slip additive  121 , heatsetting the film composite, and applying to the dielectric layer  112  a metal layer  114 . 
     The polyester film composite may be formed by solvent casting or extrusion of the heat-seal layer onto the surface of a self-supporting film of the dielectric layer material, which is preferably a biaxially oriented and heat-set film of polyethylene terephthalate. A convenient and alternative process for the manufacture of such a film composite includes multiple extrusion through a multiple orifice die or coextrusion of the composite layers, e.g. broadly as described in U.S. Pat. No. 3,871,947, followed by molecular orientation by stretching in one or more directions and heat setting. A convenient process and apparatus for coextrusion, known as single channel coextrusion, which is described in U.S. Pat. No. 4,165,210 and GB patent specification No. 1,115,007, comprises simultaneously extruding streams of the first and second polyesters from two different extruders, uniting the two streams in a tube leading to a manifold of an extrusion die, and extruding the two polyesters together through the die under conditions of streamline flow so that the two polyesters occupy distinct regions of the flow without intermixing whereby a composite is produced. 
     Biaxial orientation of the dielectric layer such as a polyethylene terephthalate layer may be accomplished by stretching the film composite in sequence in two mutually perpendicular directions typically at temperatures in the range of about 78 to 125° C. 
     Generally, the conditions applied for stretching the composite may function to partially crystallize the heat-seal layer and in such cases it is preferred to heat set under dimensional restraint at a temperature greater than the crystalline melting temperature of the heat-seal layer and permit or cause the composite to cool in order to render the heat-seal layer essentially amorphous. Therefore, the stretching operation is preferably followed by heat setting under dimensional restraint typically at a temperature in the range 150 to 250° C. Convenient processes for stretching and heat setting are described in U.S. Pat. No. 3,107,139. 
     Slip layer  120  may be formed by applying to heat-seal layer  118  a formulation containing a slip additive  121 , preferably during an interdraw stage of the biaxial orientation process, that is, after the film has been stretched in one direction but prior to stretching in the orthogonal direction. Techniques for such coating are well known in the art, and are described for example by Chappell, Jr. et al in U.S. Pat. No. 5,985,437, incorporated herein by reference. For example, the coating composition may be applied as an aqueous dispersion at a concentration and amount sufficient to produce a coating with the desired adhesion properties. The coating composition typically comprises about 3% to 18% total solids, preferably about 5% to about 10% total solids. As is well known to those skilled in the art, total solids refers to the total amount of non-volatile material in the coating composition, even though some of these materials may be non-volatile liquids at room temperature. 
     Any conventional coating method, such as spray coating, roll coating, slot coating, meniscus coating, immersion coating, wire-bar coating, air knife coating, curtain coating, doctor knife coating, direct and reverse gravure coating, and the like, can be used to apply the coating composition. The coating is typically applied as a continuous coating. The wet coating preferably has a thickness of about 7.5 to about 8.5 μm, as measured by wet infrared gauge. After drying, the coating typically has a thickness of about 0.05 μm to about 0.4 μm, preferably, about 0.1 μm to about 0.2 μm. Although coating from a solution or dispersion as described here may be a convenient way of applying slip layer  120 , other means such as for example lamination may also be used. 
     Deposition of metal layer  114  is usually performed by vacuum deposition, by methods and using equipment known in the art. For example, a vacuum deposition apparatus available from Balzers GmbH (Bingen, Germany) may be used. The surface resistance of the resulting layer may be from about 0.9 to about 8 ohm/sq., preferably from about 1.0 to about 3.0 ohm/sq. A typical resistance is 1.25 ohm/sq. Other methods such as electroplating and sputtering may be used, and are well known in the art. The film, either before or after metal deposition, may optionally be subjected to a surface treatment or other coating treatment with a resin which may further improve the metal adhesion, electrical performance or other characteristics as desired. 
     Referring now to FIG. 4, there is shown an end view of a pre-winding assembly, indicated generally at  222 , comprising two metalized elements  210  and  210 ′ in position for being wound to make an electrical component in accordance with this invention. Metalized elements  210  and  210 ′ are shown overlaid such that they are adjacent and substantially coextensive with each other, with heat-seal layer  218  of metalized element  210  shown adjacent metal layer  214 ′ of metalized element  210 ′. Preferably, metalized elements  210  and  210 ′ comprise slip layers such as shown at  120  in FIG.  2 . For simplicity, these are not shown in FIGS. 4,  5 , and  6 . 
     FIG. 5 shows an exploded view of a pre-winding assembly similar to that of FIG. 4, indicated generally at  322 . The assembly, which is suitable for making an electrical component for a flyback transformer, additionally shows terminals  330   a  and  330   b  attached via connectors  328  to the ends of metal layer  314 ′ of metalized element  310 ′, and a terminal  326  attached via a connector  324  to metal layer  314  of metalized element  310 . In the embodiment shown, metalized element  310  overlies  310 ′, but this may be reversed. 
     Also, in the embodiment shown, the terminals are sandwiched in place by the connectors, which are typically aluminum films spot-welded in place to the metal layers. Other means of connecting terminals are known in the art, and are contemplated by this invention. The number and positioning of terminals  326 ,  330   a  and  330   b  shown in FIG. 5 are especially suitable for use in preparing a capacitor capable of operating as both the primary coil of a flyback transformer and as a capacitor electrically connected thereto. However, the invention also contemplates other numbers and positioning of terminals, depending on the requirements of the finished device. As a nonlimiting example, elimination of either of terminals  330   a  and  330   b , optionally with repositioning of terminal  326  and/or the other of  330   a  and  330   b , may result in a pre-winding assembly suitable for making a simple capacitor. Other possible arrangements of terminals will be apparent to those of skill in the art. 
     FIG. 6 shows a partial section view of the pre-winding assembly of FIG. 5, partially wound up. The winding may be done such that the resulting wound assembly has heat-seal layer  318 ′ outermost, as shown in this embodiment, but it may instead be wound with metal layer  314  outermost. 
     To make the capacitor, the pre-winding assembly  322  is wound up to produce a wound assembly, which is then hot-pressed to cause the heat-seal layers on each of metalized elements  310  and  310 ′ to adhere to the other metalized element. Typically this may be done under a pressure of about 40 kg/cm 2  at a temperature of about 140° C. for about 5 minutes. Winding equipment such as is available from Metar (Metan, Switzedand) may for example be used for this operation, according to commonly used methods. Hot-pressing may for example be performed using apparatus available from Thermal Technology, Santa Rosa, Calif. This is typically followed by the application of a liquid epoxy resin, commonly one based on bisphenol A, to coat and seal the capacitor. A commonly used resin is EPALLOY 5000, available from CVC, Maple Shade, N.J. Other coating and sealing compositions might also be used, as are known in the art. 
     TEST METHODS 
     Heat-seal Strength Measurement 
     Heat-seal strength is measured by sealing the heat-seal layer to itself or to the dielectric layer at 140° C. for 0.5 second under a pressure of 103 kPa (15 psi), cooling to room temperature, and measuring the force required under linear tension per unit width of seal to peel the sealed films apart at a constant speed of 5.08 mm/second. Heat-seal values are quoted as the heat-seal strength for a film composite consisting of a dielectric layer and a heat-seal layer wherein the heat-seal layer comprises 20% of the total composite thickness. 
     Electrical Property Measurements 
     The dielectric strength of film capacitors is tested according to the JIS standard using the average dielectric breakdown voltage per micron of film thickness for 100 capacitors tested at a ramp rate of 100 DC volts/second to failure (ASTM method 2318). 
     The insulation resistance measurement is done by measuring the average resistance and capacitance of 100 wound film-foil capacitors at 360 volts and at 25° C. The property is then expressed as the product of resistivity and capacitance in megohms-microfarad (mohm-μf). 
     Having described the invention, we now claim the following and their equivalents.