Patent Application
20100259865
The invention relates generally to capacitors, and more specifically to film capacitors with improved dielectric properties.
Over the last decade, significant increases in capacitor reliability have been achieved through a combination of advanced manufacturing techniques and new materials. Film capacitors can be classified into three types based on the manufacturing technology, namely, film and foil capacitors, metallized film capacitors and mixed technology film capacitors.
Generally, film and foil capacitors consist of two metal foil electrodes separated by a layer of plastic film. One of the commonly used plastic films is polypropylene. Typically, a film and foil capacitor is made by alternating two pieces of aluminum foil with two layers of plastic film. These interleaved layers are wound around a spindle in a manner that prevents the metal layers from touching each other. The film and foil capacitors have been widely used in the power electronics industry. Metallized film capacitors differ from the film and foil capacitors in the sense that the aluminum foils are replaced by a layer of metal film that is vacuum deposited onto the layer of plastic film. The metal film layer is typically extremely thin, in the order of about 200-500 angstroms and is typically aluminum or zinc. The metallized capacitor has the advantages of size, simplicity, and cost of manufacturing. Mixed technology capacitors are a combination of both film and foil capacitors, and metallized film capacitors. Typically, these are high voltage capacitors.
While significant improvements have been made in capacitors in the past few decades, certain issues continue to exist, for example surface defects and electrical stress may lead to a reduction in life of a film capacitor. Surface defects may cause a scattering of breakdown voltages in a dielectric, resulting in varying breakdown voltages at various locations in a capacitor, leading to a lowering of the overall breakdown voltage of the capacitor. Film capacitors are also limited in life due to electrical stress at an anode of the capacitor. This leads to an associated degradation that occurs over a period of time in the capacitor.
Therefore, it would be desirable to design a film capacitor that would address the aforementioned problems and meet the current demands of industry applications.
In accordance with one aspect of the invention, a laminated film capacitor is provided. The film capacitor includes a first electrode and a first dielectric layer disposed upon the first electrode. It also includes a second dielectric layer disposed upon the first dielectric layer wherein the dielectric constant of the second dielectric layer is at least fifty percent greater than the dielectric constant of the first dielectric layer, and a metallized film disposed upon the second dielectric layer. The film capacitor further includes a second electrode disposed upon the metallized film.
These and other features, aspects, and advantages of the present invention will become better understood when the following detailed description is read with reference to the accompanying drawings in which like characters represent like parts throughout the drawings, wherein:
As discussed in detail below, embodiments of the present invention provide a film capacitor with improved dielectric properties. Some of the dielectric properties considered herein are dielectric constant, breakdown voltage, and loss factor. “Dielectric constant” of a dielectric is a ratio of capacitance of a capacitor, in which the space between and around the electrodes is filled with the dielectric, to the capacitance of the same configuration of electrodes in vacuum. As used herein, “Dielectric breakdown voltage” refers to a measure of dielectric breakdown resistance of a material under an applied AC or DC voltage. The applied voltage just before breakdown is divided by thickness of the dielectric to give the breakdown voltage. It is measured in kV/mm. In an ideal dielectric, the voltage wave and the current wave are 90° out of phase. In a real dielectric as the dielectric becomes less than 100% efficient; the current wave begins to lag the voltage in direct proportion. This results in a proportional power loss in the dielectric. The extent to which the current wave deviates from being 90° out of phase with the voltage is defined as the dielectric loss angle. The tangent of this loss angle is known as loss factor or dissipation factor.
A typical film capacitor comprises a polymer film interposed between two electrodes on either side. The polymer film acts as a dielectric in the film capacitor. In one embodiment of the present invention, a film capacitor disclosed herein comprises a first electrode upon which a first dielectric layer is disposed. The film capacitor also includes a second dielectric layer deposited on the first dielectric layer opposite the first electrode having a dielectric constant at least fifty percent greater than the dielectric constant of the first dielectric layer. Further, the film capacitor comprises a second electrode disposed upon the second dielectric layer. In another embodiment of the invention, a film capacitor includes a metallized film interposed between the second dielectric layer and the second electrode.
In an illustrated embodiment of the invention as shown in
Degradation in a film capacitor due to electrical stress is known to occur quite often at the anode of a film capacitor. The lifetime of a respective film capacitor can be increased through the addition of the second dielectric layer 16 with a much higher dielectric constant than the first dielectric layer 14 disposed beneath the second electrode 18. In a non-limiting example, the second dielectric constant of the second dielectric layer 16 is about 50%-75% greater than the first dielectric constant of the first dielectric layer 14. In another non-limiting example, the second dielectric constant of the second dielectric layer 16 is about 75%-100% greater than the first dielectric constant of the first dielectric layer 14. The second dielectric layer 16 causes a greater voltage gradient to occur at the first electrode 12 than at the second electrode 18. This improves overall voltage endurance of the film capacitor as voltage load on the second electrode 18 decreases. This further results in an increase in breakdown voltage. Consequently, this provides a means for reducing electrical stress at the second electrode 18 (anode) thereby reducing the degradation of the film capacitor. As a result, the lifetime of the film capacitor is enhanced.
The electrodes 12, 18 typically comprise metal foils. In one embodiment, the electrodes 12, 18 comprise at least one of aluminum, copper, or zinc foil. The first dielectric layer 14 typically comprises an organic polymer, an inorganic material or a polymer film. A non-limiting example of an inorganic material is boron nitride (BN), mica, paper, silicon nitride (Si3N4) or aluminum nitride (AlN). Some non-limiting examples of a polymer film include polypropylene (PP), polyester (PET), polyphenylene sulfide (PPS), polycarbonate (PC), fluorenyl polester (FPE), polyetheretherketon (PEEK), polyethersulfone (PES), polyvinylidene fluoride (PVDF), polyimide, polyamide-imide, teflon (polytetrafluoroethelyne) (PTFE), polyvinylidene fluoride (PVDF-TrFE), Ultem® (polyetherimide) and combinations thereof. The second dielectric layer 16 may comprise an organic polymer, an inorganic material or polymer composite film. A non-limiting example of an inorganic material is boron nitride (BN), alumina (Al2O3), silica (SiO2), titania (TiO2), aluminium nitride (AlN) and silicon nitride (Si3N4). Some non-limiting examples of the polymer film used as second dielectric layer include polyethylene phthalate (PET), polyphenylene sulfide (PPS), polyvinylidene fluoride (PVDF). Some non-limiting examples of polymer composites comprising inorganic material or polymer can be Ultem® polyetherimide, PP, PET, PVDF, FPE, PEEK, PES, PC or PPS containing BN, Al2O3, SiO2, TiO2, Niobium pentoxide (Nb2O5), Tantalum pentoxide (Ta2O5) and combinations thereof. In a non-limiting example, the thickness of the second dielectric layer is in the range between about 0.3 microns to about 5 microns. A non-limiting example of a first dielectric constant is at least about 2. In a non-limiting example, the second dielectric constant is in the range between 3 and 100.
In an exemplary embodiment of the invention, the first dielectric layer 14 has a breakdown voltage in the range of 200 volts (V) and 800 V, whereas the second dielectric layer 16 has a breakdown voltage in the range of 50 V and 700 V. In a non-limiting example, the first dielectric layer 14 has a loss factor in the range of 0.0002 and 0.02, whereas the second dielectric layer 16 has a loss factor in the range of 0.003 and 0.03.
In another illustrated embodiment of the invention as shown in
In yet another embodiment of the invention as shown in
The aforementioned embodiments present some non-limiting advantages such as increase in dielectric constant, increase in dielectric breakdown voltage, reduction in surface defects and extended lifetime for film capacitors. Depending on the coating or laminating materials, their higher corona resistance and charge trapping capability could allow the base polymer to work under higher voltage and pulsed power. Polymers containing certain nanoparticles such as Al2O3 or BN have been found to show higher breakdown strength and dielectric constant. Particle filled polymers also could offer increased thermal conductivity. The higher glass transition temperature of Ultem® also allows a higher operation temperature of the capacitors. Surface defects, as previously described, cause a scattering of breakdown voltages in a dielectric resulting in varying breakdown voltages at various locations in a capacitor. Consequently, when such surface defects are reduced as in the case of the aforementioned embodiments, a narrower breakdown voltage range results.
The dielectric material of the present invention may be coated in several ways. These include spin coating, dip coating, brush painting, solvent casting, chemical vapor deposition and lamination of a thinner layer of the second polymers or polymer composites. In a non-limiting example, a surface of the first dielectric layer is cleaned to remove dust and contamination to enable a coated dielectric film to adhere to the base dielectric film. Non-limiting examples of cleaning include wet or chemical cleaning, plasma cleaning, or any combination thereof. Adherence of the coated dielectric material to the base dielectric film is desirable since voids or defects at an interface between the coated dielectric film and the base dielectric film can reduce the breakdown voltage. In one embodiment of the present invention, a coating material used has a dielectric constant at least 50% greater than the dielectric constant of the base dielectric film. In some embodiments, the coating material includes an organic polymer or an inorganic material or polymer. Non-limiting examples of a method of coating include coating from a solution, coating using chemical vapor deposition, plasma polymer polymerization, or a physical sputtering method.
The examples that follow are merely illustrative, and should not be construed to be any sort of limitation on the scope of the claimed invention.
While only certain features of the invention have been illustrated and described herein, many modifications and changes will occur to those skilled in the art. It is, therefore, to be understood that the appended claims are intended to cover all such modifications and changes as fall within the true spirit of the invention.
| Number | Date | Country | |
|---|---|---|---|
| Parent | 11273208 | Nov 2005 | US |
| Child | 12614892 | US |
