Patent Application
20140254179
This invention relates to lighting fixtures and, more particularly, to thermal management of LED light fixtures.
In recent years, the use of light-emitting diodes (LEDs) in development of light fixtures for various common lighting purposes has increased, and this trend has accelerated as advances have been made in the field. Indeed, lighting applications which previously had typically been served by fixtures using what are known as high-intensity discharge (HID) lamps are now being served by LED light fixtures. Such lighting applications include, among a good many others, so-called canopy light for gasoline stations and the like, soffit-mounted light fixture, surface-mounted light fixtures, and a variety of factory lighting and commercial building lighting.
LED light fixtures present particularly challenging problems which relate to heat dissipation. Improvement in dissipating heat from fixture components is one significant objective in the field of LED light fixtures. It is of importance for various reasons, one of which relates to extending the useful life of the lighting products. Achieving improvements in thermal management without expensive additional structure and apparatus is much desired. It is also desired to achieve compactness in LED light fixtures while still al lowing excellent light output. Another major consideration in the development of LED light fixtures for various high-volume applications is controlling product cost even while delivering improved light-fixture performance.
In summary, finding ways to significantly improve the dissipation of heat from LED light fixtures and otherwise improve their performance without increase in cost of manufacturing would be much desired.
The present invention relates to improved LED light fixtures addressing the above concerns.
In certain embodiments, the inventive LED light fixture includes an enclosure formed by a base and a cover movably secured with respect to the base. At least one power-circuitry unit is secured with respect to the base such that, when the cover is closed, the power-circuitry unit is in thermal communication with the cover. The LED light source of the fixture is also secured with respect to the enclosure, and may include at least one LED emitter on LED-circuit board secured with respect to the base. In some embodiments, the light source is in thermal communication with the base such that during operation primary heat transfer from the power-circuitry unit and primary heat transfer from the LED emitter(s) are to separate enclosure members. More specifically, heat transfer from the LED emitter(s) is to the base, while heat transfer from power-circuitry unit is to the cover.
The base may be a single-piece metal casting. In certain embodiments, the cover is fully removable for complete access to within the enclosure. In some embodiments, the cover is a metal casting supporting a light-transmitting optical member over the LED emitter.
Some embodiments of the fixture include a housing which has at least first and second housing members. The second housing member is movably secured with respect to the first housing member and movable with respect thereto between use and non-use positions. The at least one power-circuitry unit is secured with respect to the first housing member, and in the use position is in thermal communication with the second housing member. In some of such embodiments, the power-circuitry unit is constrained with respect to the first housing member such that it has no more than one degree-of-freedom of movement and such that, when the second housing member is in its use position, the power-circuitry unit is in thermal communication primarily with the second housing member. Or, in some cases, the power-circuitry unit may be fixed in a position for primary thermal communication with the second housing member.
In certain embodiments, the power-circuitry unit has only one degree-of-freedom of movement. In some of such embodiments, such single degree-of-freedom of movement is back-and-forth movement along an axis, while in others the single degree-of-freedom of movement is rotational—e.g., about an axis fixed with respect to the first housing member.
In some embodiments, the power-circuitry unit is directionally biased toward the cover to facilitate thermal contact between the power-circuitry unit and the cover. In some of such embodiments, the fixtures include at least one resilient member between the power-circuitry unit and the base. The resilient member(s) is configured and positioned such that, when the cover is closed, the resilient member(s) push(es) the power-circuitry unit against the cover.
The resilient member(s) may include a compressible pad or pads. Certain versions of the compressible pad are sized to approximate the footprint of the power-circuitry unit on the base, thereby to facilitate thermal isolation between the power-circuitry unit and the base.
Some embodiments of the light fixture include first and second locators inter-engaged to constrain the power-circuitry unit in directions parallel to a constraint plane. The term “constraint plane,” as used herein, means a plane the coordinates of which remain substantially constant for the power-circuitry unit positioned with respect thereto.
In some of such embodiments, the first locator is secured with respect to the base and the second locator is on the power-circuitry unit. The first and second locators may allow back-and-forth movement of the power-circuitry unit along a direction substantially orthogonal to the constraint plane.
In some embodiments, the first locator includes at least one post extending from the base to a distal post-end, and the second locator is, for each post, a hollow defined by the power-circuitry unit. Each post extends into the hollow such that the power-circuitry unit is slidable on the post(s) to facilitate thermal contact between the power-circuitry unit and the cover. There may be two pairs of posts and corresponding hollows, each such combination being spaced from other such combination(s).
The power-circuitry unit may include a heat-conductive casing which is in thermal contact with the cover. In some embodiments, the casing has a flange portion which defines the post-receiving hollow(s). The casing of the power-circuitry unit may be directionally biased toward the cover to facilitate thermal contact between the casing and the cover. At least one resilient member may be positioned between the casing and the base, the resilient member(s) being configured and positioned such that, when the cover is closed, the resilient member(s) push(es) the casing against the cover. As already stated, the resilient member may be in the form of a compressible pad; such compressible pad may be sized to approximate the footprint of the casing on the base, thereby to facilitate thermal insulation between the casing of the power-circuitry unit and the base.
Certain embodiments of the light fixture includes at least one bracket secured with respect to the base and holding the power-circuitry unit with respect to the base when the enclosure is open. In some of such embodiments, each bracket has an affixed end secured with respect to the base and a free end positioned to engage the power-circuitry unit. The free end of the bracket may define an aperture receiving the distal post-end.
In the embodiments where the power-circuitry unit includes a heat-conductive casing, the free end of the bracket may be positioned to engage the flange portion of the casing. In some of such embodiments with the free end of the bracket defining an aperture receiving the distal post-end, the flange portion of the casing may be positioned between the base and the free end of the bracket.
The term “non-linear array” as used herein with respect to LED light sources means a planar array of LED light sources which do not all lie along the same straight line. In other words, the array is at least two-dimensional, not linear. Furthermore, the two-dimensional array, which may be square or otherwise, includes a multiplicity of LED light sources, and can include as many as 70-240 or more LED light sources. Each LED light source may be an LED package which includes a single LED (or a closely-spaced group of LEDs) mounted either directly on the circuit board or on a submount on the circuit board, with what is commonly referred to as a primary lens over such LED(s).
The term “closed boundary” as used herein with respect to an array of LED light sources refers to the perimeter-line that has straight segments and circumscribes the array.
As used herein, the term “LED-populated area” means the circuit-board region within the closed boundary minimally circumscribing the LED light sources, provided that the circuit board has a non-linear array of LED light sources thereon with the spacing between adjacent LED light sources being no more than about three times the cross-dimension of each of the LED light sources. The term “non-LED-populated area” means the circuit-board region outside the LED-populated area. In some embodiments, the non-LED populated area can include other circuit elements, but in other embodiments it does not include any circuitry.
The term “optical aperture” as used herein means the light-fixture opening of smallest cross-sectional area through which aperture the light from the LED-populated area passes.
The term “substantially isothermal” as used herein in reference to the circuit board means that temperature variation across the circuit board is no more than 5° C.
As used herein in referring to portions of the devices of this invention, the terms “upward,” “upwardly,” “upper,” “downward,” “downwardly,” “lower,” “upper,” “top,” “bottom” and other like terms assume that the light fixture is in its usual position of use and do not limit the invention to any particular orientation.
In descriptions of this invention, including in the claims below, the terms “comprising,” “including” and “having” (each in their various forms) and the term “with” are each to be understood as being open-ended, rather than limiting, terms.
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Such arrangements, in which the light sources are in thermal communication with base 20 while power-circuitry unit 40 is in thermal communication with cover 30, is very advantageous. In other words, during operation of the light fixtures this arrangement provides primary heat transfer from the power-circuitry unit and primary heat transfer from the LED emitter(s) to separate major enclosure members, each of which serve as a heat sink
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The large non-LED-populated area surrounding the LED-populated area provides valuable advantages of anisotropic heat conduction during operation. In particular, heat generated by the LED light sources on the LED-populated area preferentially spreads in lateral directions across the entire circuit board more than in directions orthogonal to the circuit board into the heat-sink body. That is, the circuit board, which comprises a good thermally-conductive material, such as copper or aluminum, spreads the heat laterally away from the LED-populated area and allows rapid heat transfer to the heat-sink body from across the entire circuit board—even in such “hidden” positions as are beyond the boundary of the optical aperture.
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The present invention provides efficient ways for addressing thermal challenges and extracting increased amounts of light from the LEDs of LED light fixtures. One such way, as described above, is increasing the surface area of the printed circuit board without changing the configuration of the LED array thereon. This takes advantage of the extra circuit-board material for heat-transfer purposes.
Given the thermal purposes of this invention, the material used for the LED circuit board should be selected with particular regard to its thermal conductivity. Using a metal-core printed circuit board is particularly advantageous. A simple metal-core circuit board is comprised of a solder mask, a copper circuit layer, a thermally-conducting thin dielectric layer, and a much thicker metal-core base layer. Such layers are laminated and bonded together, providing a path for heat dissipation from the LEDs. The base layer is by far the thickest layer of the circuit board and may be aluminum, or in some cases copper, a copper alloy or another highly thermally-conductive alloy. Such highly-conductive base layer facilitates lateral conduction of heat in the board from beneath the LED-populated area to and across the non-LED-populated area. And since board temperatures remain high even across the non-LED-populated area, the total area of substantial thermal transfer from the circuit board to the heat sink is beneficially large—substantially larger than just the LED-populated area.
For example, if instead of sizing the circuit board to closely match the size of the LED array, the circuit board is enlarged to have a non-LED-populated area around an LED-populated area with such the non-LED-populated area extending beyond the optical aperture. In one example, such circuit-board enlargement decreases the temperature of the LEDs by 2° C. without adding manufacturing costs, and this allows an increase on total lumen output. Larger decrease in temperature and larger increase in total lumen output are possible depending on non-LED-populated area of such circuit board.
The present invention provides a further way for addressing thermal challenges in LED light fixtures. In particular, the thermal load of the driver (power-circuitry unit) is substantially removed from the fixture member (e.g., the base member) which is in primary thermal communication with the LED circuit board, and instead is transferred to a separate fixture member such as the light-fixture cover. In one example, such thermal “repositioning” of the driver provides a decrease in the LED temperature of about 2° C., and the thermal separation of the driver from the LED circuit board also lowers the driver temp by 2° C. This permits drive current to be increased while still maintaining a 100,000 hour driver life rating and allowing an increase on total lumen output.
In some examples of light fixtures of this invention, enlargement of the non-LED-populated area is combined with separation of the primary thermal paths of the LEDs and the LED driver. In one example, this combination of thermal advantages decreases the LED temperature by 4° C. and allows a 15% increase in the drive current which resulted in 13% increase in total lumen output.
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As also seen in FIGS. 1 and 15-17, a light-transmissive member 31 is positioned in cover opening 34. Light-transmissive member 31 may include a phosphorescent material such that at least some of the light emitted by the fixture has a different wavelength than light emitted from the LED-populated area. For example, the LED-populated area may include LED sources of the type emitting light with wavelength of a blue color, and in order to achieve a customary white-color light, a so-called “remote phosphor” technique is used. The remote-phosphor technique typically utilizes blue LED(s)—generally considered to be the color that delivers maximum efficacy. The phosphor that generates the white light is included on a lens or diffuser such as light-transmissive member 31 by coating or otherwise. Such “remote phosphor” technique delivers better efficacy than do phosphor-converted LEDs, since the phosphors are more efficient in conversion when operating at the lower phosphor temperatures made possible by such remote configurations. For example the LEDs can be blue LEDs where the blue light excites the phosphorescent material, such as yittrium aluminum garnwt or YAG, to produce a secondary emission of light where the blue light and the secondary emission produce white light. In other embodiments, different color LEDs can be used together with individual white LEDs (blue LEDs plus phosphor) or with blue LEDs in a remote phosphor configuration where the light-transmissive element is coated and/or impregnated with the phosphorescent material.
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Heat-dissipating surfaces 27 extend substantially orthogonally to front surface 26 of base plate 200. As seen in
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The low-profile configuration of the light fixture permits installation against the structure with a relatively small aperture formed in structure surface 1 for electrical connections. This is beneficial in installations for outdoor canopies such as those used at gasoline stations. In particular, the small connection aperture minimizes access of water to the fixture. Another benefit provided by the light fixture according to the present invention is that all major components are accessible for servicing from the light-emitting front of the fixture, under the canopy.
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While the principles of the invention have been shown and described in connection with specific embodiments, it is to be understood that such embodiments are by way of example and are not limiting.
