Abstract:
A lighting system includes a base that is made of a thermally conductive material. Mounted within a cavity in the base and thermally interfaced to the base is a device that produces light (LED or LED array). Heat produced by the device that produces light conducts from the device to the base. A removable bezel is connected to the base along a conical frustum interface. The interface is formed at an angle with respect to a lengthwise axis of the bezel such that heat from the base conducts through the conical frustum interface and to the bezel from which the heat is radiated into the room ambient environment. An extension of the bezel includes an optional trim preferably made of the same or a similar material.

Description:
FIELD 
       [0001]    This invention relates to the field of lighting and more particularly to a system for dissipating heat from LED lighting systems. 
       BACKGROUND 
       [0002]    Since the days of Edison, the incandescent light has filled many a homes and businesses with safe, convenient, and affordable illumination. Incandescent light bulbs produce light by a flow of an electric current through a filament and thereby heating the filament to a very high temperature. The filament is prevented from oxidizing or burning by encapsulating the filament within a vacuum or within an inert gas formed within a glass enclosure that allows the light to exit while preventing introduction of air/oxygen around the filament. Since the filament normally operates at extremely high temperatures, there was little need in the past to cool filament-based lighting systems. 
         [0003]    The advances in high powered light emitting diode (LED) efficacies have exceeded incandescent and halogen light sources resulting in rapidly increasing adoption for general illumination applications. LEDs are semiconductor devices, in which, the forward biased flow of electrons across a P-N semiconductor junction produces light. LEDs are much more efficient than incandescent bulbs because more of the energy consumed by the LED is converted into light as opposed to heat (as is the case with incandescent lighting). An added benefit of LED lighting is that LEDs last much longer than incandescent lights, requiring less frequent replacement. The long life offsets an initially higher cost to produce LEDs. Typically, LEDs have lifetimes of 50,000 hours or more when operated at around 25° C. 
         [0004]    LED light output (or flux) is measured in lumens. Led light output and reliability are dependent upon temperature, a common characteristic for all LEDs. As LED case temperatures and corresponding junction temperatures increase, light output decreases and reliability typically decreases. Therefore, proper thermal management of the LEDs is critical to minimize the reduction in light output and maintain the expected reliability of the LEDs. Furthermore, because LEDs are semiconductors, they have a limited operating temperature range and will fail or have limited life if operated above that temperature. 
         [0005]    For many applications, LEDs fit in well, replacing incandescent equivalents without significant problems. Applications where there is sufficient air flow often provide sufficient cooling to properly operate LED based incandescent replacement bulbs because the ambient room air temperature is typically what is comfortable to people, between 60° F. to 80° F. Applications such as in a table lamp provide a reasonable ambient room air temperature for operation of an LED-replacement bulb. 
         [0006]    There are many applications where the ambient temperature is much higher than 60° F. to 80° F., creating problems with cooling the LEDs. One such example is in overhead recessed lighting (e.g. “Can Lights” or “Top Hats”). Such lighting is often recessed above a ceiling with little or no air circulation from the room below. In such cases, the heat sinks used to cool the LEDs are often located within the un-cooled space between the ceilings and next floor of a building or directly below an attic and often covered with insulation. In such cases, the air space around the heat sink is dead air space with typical ambient temperatures that often exceed 60° C. (140 F). These heat issues were less of a problem with incandescent light bulbs that are designed to operate in such high temperatures. However, these heat issues are critical issues for LED lighting. 
         [0007]    To permit operation of LED lighting in recessed lighting, manufacturers have resorted to including extra-large heat sinks to channel heat away from the LEDs. Such heat sinks help, but due to the typical dead air space temperatures, these heat sinks are not sufficient solutions for many applications. Furthermore, there are limitations on the size of such heat sinks due to the typical space above the ceiling and installation spaces such as the hole size through which the recessed lighting must pass during installation. 
         [0008]    Typical LED recessed lighting applications resort to large and heavy heat sinks to transfer heat from the LED junctions. Numerous light fixture applications exist where the heat sink is designed to be located in relatively high ambient temperatures of the dead space which are greater than room temperatures. Room temperatures are typically in the range of 18° C. to 26° C., but dead air spaces typically reach temperatures of anywhere from 40° C. to 60° C. Flow of heat from the LED junctions, through the heat sinks, and out to the surrounding air depends upon the temperature differential between the junction temperature and the temperature of the surrounding air. For example, if the junction temperature is 60° C. and the surrounding air temperature is also 60° C., no heat will flow and no heat will be dissipated. 
         [0009]    LED recessed down lights are often enclosed within a can enclosure which is in turn installed in a ceiling in commercial buildings. In such cases, dead air space exists between the next floor and a dropped ceiling constraining the heat flow from heat sinks. Similarly, in residential applications, LED recessed down lights are often installed in ceilings below an attic. In these attic locations, an insulation layer often surrounds the recessed down lights, further reducing the heat flow from LED loads to the air above the insulation layer. 
         [0010]    What is needed is a LED heat sink system that will dissipate sufficient heat such that the LEDs will operate within their specified temperature ranges in ceiling lighting systems. 
       SUMMARY 
       [0011]    A lighting system includes a base that is manufactured of a thermally conductive material. Mounted within a cavity of the base and thermally interfaced to the base is a device that produces light (LED or LED array). Heat produced by the device that produces light conducts from the device to the base. A removable bezel is connected to the base along a conical frustum interface. The interface is formed at an angle with respect to a lengthwise axis of the bezel such that heat from the base conducts through the conical frustum interface and to the bezel from which the heat is radiated into the room ambient environment. An extension of the bezel includes an optional trim preferably made of a similar material. The trim and bezel are fabricated as a single part or separate parts that are bonded or fastened together. 
         [0012]    In one embodiment, a lighting system is disclosed including a base that is formed of a material that conducts heat and having a device for producing light. The device for producing light is mounted to the base and is thermally interfaced to the base allowing heat to be conducted from the device to the base. The lighting system has a bezel connected to the base along a conical frustum interface. The conical frustum interface is formed at an angle with respect to a lengthwise axis of the bezel such that heat from the base efficiently conducts through the conical frustum interface and into the bezel and the heat is radiated from the bezel into room ambient air. 
         [0013]    In another embodiment, a lighting system is disclosed including a base that is formed of a material that conducts heat and having one or more light emitting diodes (LEDs) mounted to the base. The light emitting diode(s) are thermally interfaced to the base allowing heat to be conducted from the light emitting diode(s) to the base. A bezel is connected to the base along a conical frustum interface. The conical frustum interface is formed at an angle with respect to a lengthwise axis of the bezel such that heat from the base conducts through the conical frustum interface and into the bezel and the heat is then radiated from the bezel into room ambient air surrounding the bezel. 
         [0014]    In another embodiment, method of dissipating heat from the prior lighting system is disclosed including, the method including providing the lighting system as described prior and providing power to the light emitting diode(s), thereby the light emitting diode(s)produce both light and heat from the power. At least some of the heat from the light emitting diodes is conducted to the base, and consequently, at least some heat from the base is conducted to the bezel through the conical frustum interface. The heat is then conducted and/or radiated from the bezel into the room ambient air surrounding the bezel. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0015]    The invention can be best understood by those having ordinary skill in the art by reference to the following detailed description when considered in conjunction with the accompanying drawings in which: 
           [0016]      FIG. 1  illustrates a simplified thermal schematic of a typical LED lighting system. 
           [0017]      FIG. 2  illustrates a cross-sectional view of a LED lighting system of the prior art. 
           [0018]      FIG. 3  illustrates a cross-sectional view of a new LED lighting system. 
           [0019]      FIG. 4  illustrates a cross-sectional view of the new LED lighting system. 
           [0020]      FIG. 5  illustrates a cross-sectional, exploded view of the new LED lighting system. 
           [0021]      FIG. 6  illustrates a perspective view of the shape of the heat sink to bezel interface of the new LED lighting system. 
       
    
    
     DETAILED DESCRIPTION 
       [0022]    Reference will now be made in detail to the presently preferred embodiments of the invention, examples of which are illustrated in the accompanying drawings. Throughout the following detailed description, the same reference numerals refer to the same elements in all figures. 
         [0023]    Referring to  FIG. 1 , a simplified thermal schematic of a typical LED lighting system is shown. This schematic is for a typical thermal circuit showing the heat flow from a LED or LED array to the air space above and below a ceiling. Note, for simplicity, this schematic does not include heat conducted into the ceiling tiles and/or wiring system, etc. 
         [0024]    An LED array is defined as a group of series and/or parallel electrically connected LEDs mounted on a single platform such as but not limited to a metal core circuit board. 
         [0025]    This thermal schematic of  FIG. 1  shows the heat flow Q T  which is driven by temperature gradients where heat flows from higher temperatures to cooler temperatures. Q T  is related to the power dissipation of the LED (or LEDs)  30  (see  FIGS. 3-5 ) which has a junction temperature T j , typically required to be in the range of less than 85° C. Assuming the heat sink has a lower temperature T h  than the junction temperature T j , heat flow Q T  is from the junction to the heat sink through a thermal resistance Re jh . The thermal resistance Re jh  is due to the interface between each individual LED  30  semiconductor junction and the heat sink, including typically plastic packaging and electrical leads from the semiconductor device. 
         [0026]    Now, assuming that the heat sink has a higher temperature T h  than the dead air space, Tdas, a portion of the total heat flow, Q T , will flow from the heat sink to the dead air space Q das , limited by the efficiency (or thermal resistance) of the heat sink denoted by Re hdas . Such efficiencies are factors of the surface area of the heat sink and the temperature differential between the heat sink T h  and the dead air space Tdas. 
         [0027]    The remaining portion of the heat flow Q T  flows from the heat sink through the bezel to the room ambient air T ra . It is assumed that most of the heat transfer to ambient air is accomplished by natural convection cooling. The ability to transfer heat from the heat sink to the ambient air is affected by temperature differences between the heat sink temperature, T h , and the temperature of the room ambient air, T ra . Typically, ceiling lighting systems have bezels  20 / 120  (see  FIGS. 2 and 3 ) that are exposed to room ambient air and, therefore, will transfer heat from the bezels  20 / 120  to the room ambient air, assuming the temperature of the bezel, T b , is greater than the temperature of the room ambient air, T ra . 
         [0028]    The flow of heat from the heat sink to the bezel is not absolute and is limited by the interface/connection between the heat sink and the bezel, denoted Re hb , Likewise, the ability for heat to flow from the bezel to the room ambient air is also limited by the design of the bezel, taking into account the material of the bezel, color, surface area, etc. This is denoted by Re bra . 
         [0029]    Therefore, the total heat dissipation (or flow) is limited by the amount of heat that flows from the heat sink into the dead space (above the ceiling) plus the amount of heat that flows from the heat sink into the ambient air, represented by the formulas: 
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         [0030]    The thermal resistance from heat sink T h  to room ambient air T ra  includes the interface resistance between the heat sink and bezel Re hb  and the interface resistance between the bezel and the room ambient air, Re bra . The division of heat flow QT between Q das  and Q ra  is dependent on the temperature gradients to each air location T das  for the dead air and T ra  for the room air, as well as thermal resistances Re hdas  and Re hb +Re bra . 
         [0031]    Note that the thermal resistance Re m  includes the thermal resistance from the LED junction to the case of the LED array  30  plus the thermal resistance from the LED case to the heat sink. 
         [0032]    In recessed down light applications, heat sink size is often limited not only to weight but also to size. Height is typically limited by the space above the ceiling and diameter is limited by existing lighting standard sizes, where typical recessed down light diameters are limited to 4 inch, 5 inch, and 6 inch diameter sizes, etc. The size and weight for the heat sink limits the efficiency of heat transfer from the heat sink to the dead air space, Re hdas , and due to the often low expected temperature differentials between the heat sink and the dead air space, the heat sink alone is often not sufficient to properly cool the LED or LED array  30 . 
         [0033]    Most recessed down lights include a bezel  20 / 120 . Bezels not only provided a decorative look but also covered the interface between the ceiling material  40  (see  FIGS. 2 and 3 ) and the ceiling fixture. Since an outer surface area of the bezel  20 / 120  is surrounded by room ambient air, which is typically cooler than dead air space, it is advantageous to use the bezel to transfer at least some of the heat from the LED or LED array  30  into the room ambient air. 
         [0034]    Referring to  FIG. 2 , a cross-sectional view of a LED lighting system of the prior art is shown. Given the recessed down light size constraints, the heat sink  112  typically must fit within a specific size hole in the ceiling  40 . The bezel  120  has an opening for allowing light to pass and results in a horizontal annular ring surface area interface  116  where the bezel  120  contacts the heat sink  112 . Although this approach provides for transfer of some heat from heat sink  112  to the bezel  120  and, therefore, to room ambient air, the limited surface area of the interface  116  where the bezel  120  contacts the heat sink  112  results in a high thermal resistance Re hb - 2 , limiting heat flow Q hb - 2  and Q ba - 2  from the heat sink  112  to room air ambient T ra . In this example, the contact area  116  between the bezel  120  and the heat sink  112  does not provide optimal heat transfer. Assuming flow of heat to the dead air space is constant, Q das - 2 , total heat flow Q t - 2  from the LED or LED array is reduced due to this higher thermal resistance through this low-efficiency interface  116 , Re hb - 2 . 
         [0035]    Referring to  FIG. 3 , a cross-sectional view of a new LED lighting system is shown. In this, the surface area  16  where the bezel  20  contacts the heat sink  12  is in the form of a conical frustum (see  FIG. 6 ). This approach provides for a greater transfer of heat from heat sink  12  to the bezel  20  and, therefore, to room ambient air, by providing an increased surface area of the interface  16  where the bezel  20  contacts the heat sink  12 , resulting in a lower thermal resistance Re hb - 3  as compared to Re hb - 2  of the prior art. This results in a greater heat flow Q hb - 3  and Q ba - 3  from the heat sink  12  to room air ambient T ra  compared to that of the prior art shown in FIG.  2 . The conical frustum interface  16  between the bezel  20  and the heat sink  12  provides greater heat transfer given the size constraints of the bezel  20  and the heat sink  12 . Since Re hb - 2  is greater than Re hb - 3  (greater thermal resistance in the prior art) and assuming flow of heat to the dead air space in both scenarios is constant (Q das - 2 =Q das - 3 ), total heat flow from the LED or LED array Q t - 3  in the system of  FIG. 3  is greater than total heat flow Q t - 2  in the system of  FIG. 2 . The improved total heat flow provides for lower LED and LED array junction temperatures over a wider range of ambient room air temperatures and dead air space temperatures; resulting in improved operation and life of the LED or LED array. 
         [0036]    Referring to  FIGS. 4 ,  5  and  6 , an improved LED lighting system  10  is shown. In this exemplary LED lighting system  10 , the LED or LED array  30  (or any known or future light source) is mounted within a cavity  34  of a heat sink  12  that also serves as a base, frame, or enclosure. 
         [0037]    The typically cylindrically shaped cavity  34  is of appropriate diameter to fit the LED or LED array  30 . In one embodiment, the LED or LED array  30  is an array or cluster of LEDs mounted on a metal core board. Several other components of a typical lighting system are shown but are not required in this system, such as, mounting clips  14  to secure the LED lighting system  10  against the ceiling surface  40  (e.g. ceiling tile or drywall). Other optional components include a reflector  32  and a diffuser  36 . In such, the reflector  32  redirects light to a desired location and the diffuser typically comprises an acrylic material with a translucent finish to produce a softer lighting effect. 
         [0038]    The LED(s)  30  (or other light emitting devices) is/are mechanically mounted to the heat sink  12  providing a thermal resistive path represented by Re jh  as shown in  FIG. 1 . To minimize the thermal resistance and therefore maximize thermal conductivity and thermal transfer between the LED junction and heat sink, heat conductive paste (e.g. heat sink grease) or heat conductive pad material is often placed between the LED(s)  30  and the heat sink  12 . In some embodiments, the heat sink  12  includes fins  13  to increase the overall surface area of the heat sink  12 , thereby increasing conduction of heat into the dead air space above the ceiling surface  40 . The conduction (or radiation) of heat from the heat sink  12  to the dead air space above the ceiling surface  40  is represented by Re hdas  as shown in  FIG. 1 . 
         [0039]    The heat sink  12  is made of any suitable material such as aluminum or copper and, optionally, has one or more fins  13  that provide increased surface area for radiation of heat into the area above the ceiling  40 . 
         [0040]    Heat will only radiate from the base heat sink  12  and optional fins  13  if the temperature of the dead air space, T das , is lower than the temperature of the base heat sink  12  (and optional fins  13 ), T h . For most installations of such lighting systems  10 , the ambient temperatures of the dead air space is often too high to provide sufficient heat removal by radiation from the base heat sink  12 . Therefore, for many installations, especially during warm seasons, the heat sink  12  and optional fins  13  will not radiate sufficient heat to properly cool the LED(s)  30 , resulting in decreased life of the LED(s), improper lighting brightness, undesired color shift, LED failure, etc. 
         [0041]    Therefore, it is desirable to remove more heat than is possible with only the base heat sink  12  through either radiation or conduction, especially when the temperature of the dead air space is high. 
         [0042]    The lighting system  10  includes a bezel  22  and optional trim  20 . The exemplary bezel  22  includes an opening with optional multiple concentric circular groves  24  as a typical example, though any shape and form of bezel  22  and optional trim  20  is anticipated. As an example, the concentric circular grooves  24  provide a certain aesthetic look but also increase thermal radiation by increasing the exposed surface area of the bezel  22 , thereby improving heat conduction to the ambient air. Likewise, the trim  20  provides a decorative feature as well as covering the often rough cut opening in the ceiling material  40  and providing an additional sink for heat produced by the LEDs  30 . 
         [0043]    With incandescent lighting, the bezel  22  and trim  20  was basically decorative, in that, it provides a certain aesthetic look while covering the often rough-cut opening in the ceiling material  40 . In the disclosed lighting system, the bezel  22  and optional trim  20  not only provides this same decorative feature, but it also provides an additional sink for heat produced by the LEDs  30 , thereby reducing the overall heat of the base heat sink  12  and, consequently, the heat of the LEDs  30 . 
         [0044]    The bezel  22  has in interface surface  16 B. The shape of the interface surface  16 B is in the form of a truncated cone or frustum (see  FIG. 6 ). This geometrical shape is like slicing the top of a cone leaving a circular top. In this embodiment, the circular top is an opening to permit the propagation of light from the LED or LED array. The base  12  has a similar interface surface  16 A in the form of a similar frustum. Hence, the interface  16  between the interface surface  16 B of the bezel  22  and the interface surface  16 A the base  12  is a conical frustum. The angle, α, is any angle between 1 and 89 degrees, though a 45 degree angle is shown. 
         [0045]    In one embodiment of the lighting system  10 , the bezel  22  is removable from the base  12 . The base  12  has snaps or threads  17  and the bezel has mating snaps or threads  27 , or any other removable mating system as known in the industry. Many methods exist to secure the bezel  22  to the heat sink base  30 . The threaded fitting  17 / 27 , as shown, is one example in which the bezel  22  tightens against the base heat sink  12  through the rotation of the bezel  22 . Any system for attaching the bezel  22  to the base heat sink  12  is anticipated including, but not limited to, a press fit or friction fit. 
         [0046]    The bezel  22  thermally interfaces to the base  12  in a conical frustum  16  (see  FIG. 6 ). The bezel  22  has an interface surface  16 B in the form of a conical frustum that interfaces with an interface surface  16 A of the heat sink base  12 . The interface surface  16 A of the heat sink base is also in the form of a conical frustum of substantially the same size and angle as the interface surface  16 B of the bezel  22 . The interfaces ( 16 A/ 16 B) are at an angle with respect to the plane of the ceiling  40  and although any angle is anticipated, an angle of approximately 45 degrees is shown. This conical frustum interface method provides an increased surface area for contact between the interface surface  16 A of heat sink base  12  and the interface surface  16 B of the bezel  22 . The increased surface area results in a decrease in the thermal resistance, Re hb , and therefore greater heat flow, Q hb , from the heat sink  12  to the bezel  22  and optional trim  22 . Since more heat Q hb  now flows to the bezel, assuming a constant thermal resistance, Re bra , for a given material and surface area of the bezel  22  and optional trim  20 , more heat flows into the ambient, Q ba , thereby providing for improved cooling of the heat sink  12 , Q t  and, consequently, the LED(s)  30 . 
         [0047]    Additionally, because the cavity  34  is exposed to ambient air, further radiation of heat is made possible because the surface area within the cavity  34  also radiates some heat to the ambient air. 
         [0048]    Irregularities between the interface surfaces  16 A/ 16 B are anticipated as a result of production tolerances. When such irregularities are present, slight air gaps at the interface  16  have the potential of reducing heat flow from the base heat sink  12  to the bezel  22  due to the increased thermal resistance due to gaps within the interface  16  as opposed to direct contact between metals such as aluminum. To mitigate this effect, it is anticipated to include any known thermal interface material such as heat sink grease within the thermal interface  16 , thereby further improving the heat conduction characteristics of the thermal interface  16  between the base heat sink  12  and the bezel  22 . 
         [0049]    The trim  20  is optional, though preferred, providing improved cooling. The trim  20  provides additional surface area that radiates heat into the room ambient air. In some embodiments, trim  20  and bezel  22  are a single piece. In other embodiments, trim  20  and bezel  22  are separate pieces, bonded together or removably bonded together by any means known including, but not limited to, welding, press fit, adhesive, glue, fasteners, etc. When the trim  20  and bezel  22  are separate pieces bonded by a material, it is preferred that the bonding material has a low thermal resistance for a higher thermal conductivity. 
         [0050]    The heat sink base  12 , bezel  22 , and the trim  20  are made of the same or different materials. It is preferred that the materials are thermally conductive materials such as, but not limited to, aluminum or copper. Similar materials will have the same expansion ratios due to heating and help to preserve a tight interface  16  with minimal air gaps. 
         [0051]    The exemplary LED lighting system  10  is shown as an example of one possible construction of the disclosed inventions. Any suitable materials are anticipated, beyond that which are disclosed, including aluminum alloys, tin, copper, steel, etc., though aluminum is known to be a cost-effective material with good thermal conduction. Although exemplary LED light sources  30  are used as examples in this disclosure, the lighting system  10  is not limited to only LED light sources and are anticipated for use with any thermally sensitive lighting source either known or a future thermally sensitive light source. 
         [0052]    Equivalent elements can be substituted for the ones set forth above such that they perform in substantially the same manner in substantially the same way for achieving substantially the same result. 
         [0053]    It is believed that the system and method as described and many of its attendant advantages will be understood by the foregoing description. It is also believed that it will be apparent that various changes may be made in the form, construction and arrangement of the components thereof without departing from the scope and spirit of the invention or without sacrificing all of its material advantages. The form herein before described being merely exemplary and explanatory embodiment thereof. It is the intention of the following claims to encompass and include such changes.