Patent Publication Number: US-2004052077-A1

Title: Light emitting diode with integrated heat dissipater

Description:
TECHNICAL FIELD  
       [0001] This invention relates to light emitting diodes, and more particularly to a light emitting diode having a thermally conductive structure for dissipating heat.  
       BACKGROUND OF THE INVENTION  
       [0002] Light emitting diodes (LEDs) have been available since the early 1960&#39;s. Because of the relatively high efficiency of LEDs, LEDs are increasingly popular in a wider variety of applications, such as interior and exterior automobile lighting, traffic lights, outdoor signs, and other applications not considered practical in the past.  
       [0003] Even with new high-temperature LED technology, however, LEDs still exhibit a substantial decrease in light output when the temperature of the LED junction increases due to high current conditions. For commonly-used LEDs having a high thermal resistance, the relative flux decreases if the forward current increases beyond a certain point. For example, an increase of 75 degrees Celsius in the LED junction temperature may cause the luminous flux level to be reduced to one-half of its room temperature value. This phenomenon limits the amount of output from conventional LEDs.  
       [0004] There have attempts to reduce the thermal resistance of the LEDs in order to effectively conduct the heat to an external heat sink, allowing heat to dissipate through the heat sink into the ambient air. For example, U.S. Pat. No. 5,857,767 to Hochstein teaches mounting LEDs to a heat sink with electrically and thermally conductive epoxy. This structure does allow LEDs to be driven with higher currents than conventional printed circuit board assemblies while still maintaining a relatively low LED junction temperatures, thereby allowing increased light output. However, few LEDs are compatible with the Hochstein structure because most LEDs use a lead frame, which has a small surface area, to support the LED chip as well as to make electrical connections. The lead frame structure requires any heat in the cathode of the LED to conduct through long, narrow legs, making it difficult to remove any significant heat from the LED junction. This lack of surface area makes efficient heat dissipation to the ambient air difficult, if not impossible.  
       [0005] There is a need for a LED structure that can quickly remove heat from the LED junction as well as dissipate heat quickly to the ambient air.  
       SUMMARY OF THE INVENTION  
       [0006] Accordingly, the present invention is directed to a light emitting diode, comprising an anode, a thermally conductive cathode that is electrically isolated from the anode, a light-emitting diode chip disposed on the cathode and electrically coupled to the anode, and a heat sink individually associated with the light emitting diode and integrally coupled to at least one of the anode and the cathode.  
       [0007] The invention is also directed to a printed circuit board having a top surface and a bottom surface, comprising at least one light emitting diode having an anode, a thermally conductive cathode that is electrically isolated from the anode, a light-emitting diode chip disposed on the cathode and electrically coupled to the anode, a heat sink individually associated with the light emitting diode and integrally coupled to at least one of the anode and the cathode, a lens covering the light-emitting diode chip, and an electrical connection between said at least one light emitting diode and the printed circuit board. 
     
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
     [0008]FIG. 1A is a top view of a first embodiment of the present invention;  
     [0009]FIG. 1B is a front sectional view of the embodiment shown in FIG. 1A;  
     [0010]FIG. 2A is a top view of a second embodiment of the present invention;  
     [0011]FIG. 2B is a front sectional view of the embodiment shown in FIG. 2A;  
     [0012]FIG. 3A is a top view of a third embodiment of the present invention before being connected to a system heat sink;  
     [0013]FIG. 3B is a front sectional view of the embodiment shown in FIG. 3A after being connected to a system heat sink;  
     [0014]FIG. 3C is a side sectional view of the embodiment shown in FIG. 3A after being connected to a printed circuit board;  
     [0015]FIG. 3D is a side sectional view of the embodiment shown in FIG. 3A after being connected to a printed circuit board in an alternative manner;  
     [0016]FIG. 4A is a top view of a fourth embodiment of the present invention;  
     [0017]FIG. 4B is a front sectional view of the embodiment shown in FIG. 4A;  
     [0018]FIG. 4C is a front sectional view of the embodiment shown in FIG. 4A after being connected to a printed circuit board.  
     [0019]FIG. 5A is a top view of a fifth embodiment of the present invention;  
     [0020]FIG. 5B is a front sectional view of the embodiment shown in FIG. 5A;  
     [0021]FIG. 5C is a front sectional view of the embodiment shown in FIG. 5A after being coupled to an external heat sink;  
     [0022]FIG. 5D is a front sectional view of the embodiment shown in FIG. 5A after being coupled with a printed circuit board;  
     [0023]FIG. 5E is a front sectional view of the embodiment shown in FIG. 5A when used when coupled with a printed circuit board in an alternative manner. 
    
    
     DETAILED DESCRIPTION OF THE DRAWINGS  
     [0024]FIG. 1A and 1B are top and front sectional views, respectively, of one embodiment of an LED structure  100  according to the present invention, A cathode  150  and anode  160  in the LED structure are made from strips of thermally conductive material, such as copper, aluminum or another similar material. The anode  160  and cathode  150  strips are disposed next to each other and are held together in any known manner that allows the anode  150  and cathode  160  to be electrically isolated from each other, such as non-conductive adhesive or optical epoxy used to form the LED body.  
     [0025] In this embodiment, a reflector cup  120  is machined in the cathode  150  to hold an LED chip  110 . A bound wire  130  electrically couples the LED chip  110  to the anode  160 . A lens  140  covers the LED chip  110  and the bound wire  130  for protection and for directing light output from the LED chip  110  to the outside environment.  
     [0026] The anode  160  and cathode  150  each have a heat sink portion  170  that can be bent and inserted through openings in a printed circuit board  190  to extend below the bottom surface of the board  190 . Conductive adhesive  190  electrically connects the LED structure to the printed circuit board  190 .  
     [0027] In the specific embodiment shown in FIGS. 1A and 1B, the anode  150  and cathode  160  are also held together by an optional heat equalizer  180 . The heat equalizer  180  can be made from any thermally conductive material and can be the same material as the anode  160  and cathode  150 . The heat equalizer  180  is connected to the anode  160  and cathode  150  with electrically non-conductive adhesive  185 . Because much of the LED&#39;s heat is generated at the cathode  150 , the heat equalizer  180  absorbs the heat from the cathode  160  and transfers it to the anode  160  heat sink to distribute heat evenly between the two heat sink portions  170 . Note that by allowing the heat sink  170  to extend below the bottom surface of the printed circuit board  170  rather than simply pressing the heat sink  170  flat against the printed circuit board  170  surface, both surfaces of the heat sink  170  are exposed to the ambient air, increasing the surface area through which heat can dissipate.  
     [0028]FIG. 2A and 2B are top and front sectional views, respectively, of an alternative LED structure  200  according to the present invention. In this embodiment, the heat equalizer  185  is a thermally conductive strip having portions, much like the heat sink portions  170  described above, that extend through an opening in the printed circuit board  190 . The anode  160  and cathode  150  are formed as planar members connected to and supported by the top surface of the printed circuit board  190 . Conductive adhesive  195  provides the electrical connection between the LED  200  and the printed circuit board  190 .  
     [0029] In this embodiment, the heat equalizer  185  acts as the primary heat dissipater and is not electrically connected either to the anode  160  or the cathode  150 . Similar to the embodiment in FIGS. 1A and 1B, the bent portions of the heat equalizer  185  in FIGS. 2A and 2B allow air to circulate around both surfaces of the heat equalizer  185 , improving heat dissipation.  
     [0030]FIG. 3A and 3B are top and front sectional views, respectively, of yet another alternative LED structure  300  according to the present invention. In this embodiment, the anode  160  and cathode  150  have narrow electrically conductive leads  301   a,    301   b.  The cathode  150  also includes a comparatively large extension portion  302  that acts as a heat sink. The extension portion  302  is formed as part of the cathode  150  because the cathode generates most of the LED&#39;s heat, as noted above.  
     [0031] Providing narrow leads  301   a,    301   b  along with an extension portion  302  having a large surface area combines the convenience of soldering high thermal resistance leads  301   a,    301   b  with high heat dissipation through the extension  302 . More particularly, the high thermal resistance of the leads  301   a,    301   b,  because of their small cross-sectional areas, prevent the LED chip  110  from thermal damage during the soldering process. This high thermal resistance, however, also prevents effective heat dissipation. The extension  302  solves this problem by providing a large surface area through which heat can dissipate. Thus, this embodiment provides separate structures for heat dissipation and for electrical connection.  
     [0032]FIGS. 3B through 3D illustrate various ways in which the LED structure  300  of FIG. 3A can be coupled to the printed circuit board  190 . FIG. 3B shows a structure where the extension  302  is bent to form foot portions  302   a  that can be coupled to a system heat sink  304 . The system heat sink  304  can be designed for coupling to another board or can even have an insulating coating and an electrical circuit printed directly on the heat sink  304 .  
     [0033]FIG. 3C shows an alternative connection structure where the extension  302  is bent and then inserted through openings in the printed circuit board  190  so that they extend below the bottom surface of the board  190 . The connection shown in FIG. 3D also allows portions of the extension  302  to extend below the board  190 , but in this embodiment the LED structure is inserted from underneath the board  190  so that a portion  306  of the extension mates with the bottom surface of the printed circuit board  190  while the lens  140  extends through an opening in the board  190 . This embodiment also allows the extension  302  to extend below the board  190  and expose a large surface area to the ambient air.  
     [0034]FIG. 4A and 4B are top and front sectional views, respectively, of another LED structure  400  according to the present invention. In this structure, the anode  160  is ring-shaped and connected to the cathode  150  with a non-conductive adhesive layer  185 . The cathode  150  in this embodiment is a flat conductive plate. The anode  160  has an opening  402  that surrounds the LED chip  110 . Similar to other embodiments, the cathode  150  in this embodiment also acts as a heat sink.  
     [0035]FIG. 4C illustrates one way in which the embodiment shown in FIGS. 4A and 4B can be connected to a printed circuit board  190 . In this embodiment, the anode  160  is coupled to the bottom surface of the printed circuit board  190  with a conductive adhesive  195  to form the electrical connection. The lens  140  extends through an opening in the printed circuit board  190 .  
     [0036]FIG. 5A and 5B are top and front sectional views, respectively, of yet another alternative LED structure  500  according to the present invention. In this embodiment, the anode  150  is a planar conductive plate having an opening  502  for accommodating the LED chip  110 . The cathode  160  is formed as a substantially flat, thermally conductive plate to provide additional surface area for heat dissipation, allowing the cathode  160  to be used as a heat sink. The high thermal conductivity of the structure shown in FIGS. 5A and 5B makes soldering less appropriate than electrically conductive adhesive for attaching the LED to the printed circuit board.  
     [0037]FIG. 5C shows the LED structure attached to the system heat sink  304  with an electrically and thermally conductive adhesive. As noted above, the system heat sink  304  may have an insulating coating and an electrical circuit printed on its surface.  
     [0038]FIGS. 5D and 5E show two ways in which the LED of FIGS. 5A and 5B can be connected directly to the printed circuit board  190 . In FIG. 5D, the top surface of the cathode  150  is coupled to the bottom surface of the printed circuit board  190  so that the lens  140  can extend upwardly through an opening in the printed circuit board  190 . In this embodiment, all electrical connections are preferably on the bottom surface of the board  190 . Heat then dissipates through the bottom surface of the cathode  150 . The relatively large surface area of the cathode  150  ensures that heat can be dissipated to the ambient air quickly.  
     [0039]FIG. 5E shows an alternative mounting structure where the cathode  150  is bent and inserted through openings in the printed circuit board  190 , allowing the ends of the cathode  150  to extend below the bottom board surface while arranging the anode  160  and LED  110  on the top board surface. The LED is connected to the board  190  with conductive adhesive  195 . In this configuration, air can circulate around both sides of the cathode  150 , increasing the heat dissipation surface area.  
     [0040] As a result, the invention integrates a heat sink into an LED structure to allow efficient heat dissipation from the LED into the ambient air. More particularly, the inventive structure creates an LED having a large cross-sectional area and a direct path between the LED chip and the heat sink, increasing the efficiency in which heat is removed from the LED chip. The efficient heat dissipating properties of the inventive LED structure allows the LED junction temperature to be kept low even as the forward current through the LED chip is increased to increase the light output. As a result, the inventive LED structure allows the LED to be driven with a much higher current than previously thought possible, allowing increased overall light output per LED. Further, the inventive structure preserves efficient heat dissipation even when the LED is mounted on a printed circuit board, eliminating the need for an external heat sink.  
     [0041] It should be understood that various alternatives to the embodiments of the invention described herein may be employed in practicing the invention. It is intended that the following claims define the scope of the invention and that the method and apparatus within the scope of these claims and their equivalents be covered thereby.