Patent Application
20080285266
The exemplary embodiment relates to the illumination arts. It finds particular application in connection with thermal management in a ballast for a lighting fixture and will be described with particular reference thereto.
Lighting fixtures, such as those which support fluorescent lamps, typically employ a ballast to control the current supplied to the lamps. Typically, the ballast used for a fluorescent lighting fixture is an electronic ballast. The ballast includes a circuit board which carries components such as filters, rectifiers, and capacitors, for rectifying the AC current input and providing a high voltage during initial start-up of the lamp. These components can be damaged if subjected to high temperatures. Under most operating conditions, however, any heat generated by the ballast is dissipated to the atmosphere via the ballast housing or through the relatively cool fixture and the ballast does not suffer overheating problems.
Fluorescent lamps are now being used in a variety of applications, such as industrial applications and in facilities with high ceilings where the ambient temperatures are relatively high. It is not uncommon for ambient temperatures to reach 50-60° C., or more, in some environments. Such temperatures can compromise the operation of the ballast and result in shorter operational lifetimes. Where the fixture is positioned in a relatively inaccessible location, it is inconvenient to access the fixture for ballast replacement.
In accordance with one aspect of the exemplary embodiment, a fluorescent lamp assembly includes a fixture which supports at least one fluorescent lamp. A ballast is mounted to the fixture for controlling a supply of electrical current to the fluorescent lamp. A thermal transfer material is interposed between the ballast and a portion of the fixture whereby heat is transferred between the ballast and the fixture.
In accordance with another aspect, a method of operating a fluorescent lamp assembly in an environment which reaches temperatures in excess of about 50° C. during operation of the lamp, is provided. The method includes supporting at least one fluorescent lamp on a fixture, mounting a ballast to the fixture, and interposing a thermal transfer material between the ballast and a portion of the fixture whereby heat is transferred between the ballast and the fixture during operation of the lamp.
In accordance with another aspect, a fluorescent lamp assembly includes a ballast and a fixture which supports a fluorescent lamp, the ballast being adhesively attached to the fixture with a thermal transfer material comprising a room temperature vulcanizing silicone and optionally a particulate material. The thermal transfer material aids in conducting heat from the ballast to the fixture during operation of the fluorescent lamp at an ambient temperature of at least 50° C.
With reference to
The chassis 12 defines a cavity for receiving the fluorescent lamp. The chassis may be formed of a reflective material or interiorly coated with a reflective material to reflect light from the lamp 14 which is incident thereon.
A ballast 22, such as an electronic ballast, supplies the electrodes 18 with current. In the illustrated embodiment, electrical wiring 24 connects with the pins 20 through the chassis. The ballast includes a ballast housing 26, which houses electronic circuit board 28 which supports the electronic components of the ballast thereon. The ballast is mounted to a part of the fixture by fixing members, such as bolts 32, screws, or the like. In the illustrated embodiment, the ballast is mounted to a ballast box 30 at one or both ends of the ballast housing. In other embodiments, the center of the ballast hosing may be mounted to the ballast box. The ballast box 30 mounts the ballast to the rest of the fixture. In particular, the ballast box is removably mounted at either end to the chassis. The ballast box is formed from metal, such as a thin gauge sheet metal, such as steel or aluminum, and serves as a heat sink for the ballast. In particular the ballast box 30 may have a large surface area from which heat is transferred to the atmosphere by conduction and radiation. The ballast box may be coated with an emissive material, such as a black coating, which aids thermal transfer from the ballast to the surrounding atmosphere.
The ballast housing 26 includes a closure in the form of a planar member 40 which defines a planar exterior surface 42. The planar surface 42 contacts an adjacent planar surface of the ballast box 30 when affixed thereto. In the illustrated embodiment, the ballast box 30 supports the ballast 22 so that the ballast is spaced from the chassis by an air gap 43, and thereby to some degree is spaced from the heat generated by the fluorescent lamps. A major portion of the excess heat generated by the ballast 22 is transferred to the planar member 40 by virtue of the location of the circuit board. Under normal ambient conditions, a sufficient contact area exists between the planar surface 42 and the ballast box 30 to ensure that the excess heat is dissipated. However, in high-ceiling locations and facilities where high temperatures are experienced, the heat may not be dissipated rapidly enough to maintain the circuitry of the ballast at a suitable operating temperature. Accordingly, a thermal transfer material 44 is interposed between the ballast box 30 and the ballast housing 26. For example, the thermal transfer material 44 may be applied to the ballast box 30 or planar surface 42 prior to fixing the ballast to the ballast box with bolts, screws or other fixing members. The thermal transfer material 44 may be applied in the form of a gel, paste, pad, or the like. The material 44 reduces the air gaps between the ballast box 30 and the ballast housing 26, thereby increasing the area of thermal contact and decreasing the thermal resistance.
The thermal transfer material 44 may be an adhesive composition which retains its adhesive character, i.e., does not flow appreciably, at the operating temperature of the ballast 22 or temperature of surfaces of the fixture which the material 44 contacts. In one embodiment, the thermal transfer material 44 is applied as a flowable liquid or viscous material or paste and cures to a substantially non-flowable material, e.g., solid, gel, or highly viscous material, at room temperature. Exemplary curing agents include platinum catalysts and organic peroxide compounds. The thermal transfer material transfers heat more effectively between the ballast and the fixture as compared to when it is absent.
Exemplary thermal transfer materials include polymers, such as silicones and acrylic-based materials. Exemplary silicones are room temperature vulcanizing (RTV) silicones which cure to a relatively firm material in the atmosphere. Such materials are obtainable from General Electric Co., Silicone Products Division, e.g., RTV108 silicone. This material has adhesive properties which are retained under lamp operating conditions. Other suitable polymers include viscous waxes, asphaltic pitches, and the like.
U.S. Pat. No. 5,493,041, which is incorporated herein in its entirety by reference, discloses crosslinked organofunctional siloxane waxes and methods for their preparation, which may be utilized herein. For example, the crosslinked organofunctional silicone wax can be prepared by reacting a commercially available siloxane having silicon-bonded hydrogen atoms with a slight stoichiometric excess of an olefin and an unconjugated alpha, omega-diene crosslinker in the presence of a transition metal (e.g., platinum) catalyst. Olefins are known in the art and commercially available. Mixtures of olefins having different numbers of carbon atoms can be used, for example mixtures of olefins having 30 carbon atoms and olefins having greater than 30 carbon atoms can be used to prepare the crosslinked organofunctional silicone wax. The crosslinker can be an organic group, an organosilicone having an average of at least two alkenyl groups bonded to silicon atoms per molecule, a combination thereof, and others.
Other suitable curable silicone materials are described in U.S. Pat. No. 6,639,008 to Lewis, et al., the disclosure of which is incorporated herein by reference.
Optionally, a thermally conductive particulate filler material is dispersed in the polymer matrix. Exemplary particulate materials include aluminum oxide, silicon nitride, aluminum nitride, boron nitride, silver, gold, copper, platinum, nickel, diamond, magnesium oxide, zinc oxide, zirconium oxide, silicon carbide, titanium dioxide, other high performance thermal transfer materials, and combinations thereof. The particles may have a size of less than about 500 microns. When present, the filler is typically present in a range between about 10 weight % and about 95 weight %, based on the weight of the thermal transfer material. More typically, the filler is present in a range between about 20 weight % and about 90 weight %.
The lamp assembly may be mounted to a support surface, such as a ceiling (not shown), for example, through the chassis or ballast box. In the exemplary embodiment, the lamp is situated in an environment where temperatures can be relatively high, such as above 50° C. or above 60° C., or more, during operation of the lamp, such as in a high-ceilinged facility or in a factory where considerable heat is generated. As a result, the air surrounding the ballast does not conduct heat away from the ballast 22 to the same extent that it would do in normal environments. The RTV silicone, or other thermal transfer material, allows heat to be conducted away from the ballast to the lamp fixture.
With reference to
The invention has been described with reference to the preferred embodiments. Obviously, modifications and alterations will occur to others upon reading and understanding the preceding detailed description. It is intended that the invention be construed as including all such modifications and alterations.
