Patent Publication Number: US-2007120138-A1

Title: Multi-layer light emitting device with integrated thermoelectric chip

Description:
BACKGROUND  
      1. Field of the Invention  
      The present invention generally relates to a light emitting diode (LED). More specifically, the invention relates to a mechanism for cooling the LED.  
      2. Description of Related Art  
      A white LED assembly typically includes a chip, a phosphor conversion layer and a housing. The process of producing light from electrical power is somewhat inefficient at approximately 10% efficiency, the rest of the electrical power is converted to heat energy. Therefore, the temperature of the chip will rise as the LED chip produces light. With this temperature increase, the amount of light produced decreases, particularly in terms of lumens per watt of electrical drive power. If the temperature continues to rise, eventually, the LED chip will fail as the actual chip temperature will exceed the maximum junction temperature for the chip. Currently, the use of the heat sinks, spacing of heat generating devices, and convection cooling are all used to reduce the temperature of the LED chip in an LED assembly. These techniques however, restrict the size and power of the LED assembly, thereby limiting the application of such assemblies.  
      In view of the above, it is apparent that there exists a need for an improved LED assembly.  
     SUMMARY  
      In satisfying the above need, as well as overcoming the enumerated drawbacks and other limitations of the related art, the present invention provides such an improved LED assembly or package.  
      An LED package according to the principles of the present invention includes an LED chip and a thermoelectric device (TED) coupled thereto. The LED chip is covered with a conversion layer, typically a powder coating of phosphor, that is held in place by a transparent coating or a matrix material into which the phosphor is embedded. A thermoelectric device is integrated into the LED package in order to decrease the temperature of the chip and to increase the heat flux from the package into a heat sink. The heat is transferred by thermal conduction from the package slug to the heat sink through the circuit substrate. The thermoelectric device can be located under the silicon submount, under the chip, on top of the slug, and on top of the lead frame, depending on the LED package structure. The thermoelectric device can also be located on the bottom of the slug, in order to lower the temperature of the slug and nearly the entire package, while increasing the heat expelled from the package into the substrate and heat sink.  
      Further objects, features and advantages of this invention will become readily apparent to persons skilled in the art after a review of the following description, with reference to the drawings and claims that are appended to and form a part of this specification. 
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       FIG. 1  is a side sectional view of an LED package according to the present invention; and  
       FIG. 2  is a side sectional view of another embodiment of an LED package according to the present invention.  
    
    
     DETAILED DESCRIPTION  
      Referring now to  FIG. 1 , an LED chip package embodying the principles of the present invention is illustrated therein and designated at  10 . As its primary components, the LED chip package  10  includes an LED chip  12  and a thermoelectric device  18 .  
      The LED chip  12  is covered by a conversion coating layer  14  that is attached to a first side of the LED chip  12 . The conversion coating layer  14  may be made of phosphor or other commonly used conversion coating material, and the light generated by the LED chip  12  is transmitted through the conversion coating layer  14 . Attached to a second side of the LED chip  12  is a submount layer  16 . The submount layer  16  may be made of silicon or any other material suitable for that purpose. Attached to the submount layer  16 , opposite of the LED chip  12 , is a thermoelectric device (TED)  18 . The TED  18  is attached on a first side to the submount layer  16  and is configured to actively draw heat through the submount layer  16  and away from the LED chip  12 . The second side of the TED  18  is attached to a slug layer  20 . The slug layer  20  is made of a highly themoconductive material, so that the heat drawn from the LED chip  12  by the TED  18  can be effectively deposited into and dissipated through the slug layer  20 . One preferred material for the slug layer  20  is copper because of its high thermal conductivity.  
      The slug layer  20  is in turn mounted to an LED housing  22 . The LED housing  22  not only supports and protects the LED chip  12 , but it also serves to further dissipate heat from the slug layer  20 .  
      Attached to the LED housing  22  and surrounding the LED chip  12  is a lens  24 . The lens  24  focuses the light generated by the LED chip  12  and also serves to protect the LED chip  12  and conversion coating layer  14 .  
      The LED housing  22  may further be attached to a heat sink  26  through an attachment layer  28 . The attachment layer  28  can be solder, a thermoconductive adhesive or similar material. Preferably, the heat sink  26  has a large surface area and mass, relatively speaking, and is configured to further dissipate the heat received from the LED housing  22  through convection. Accordingly, it is preferred that the heat sink  26  has a high thermal conductivity and, as such, may be made from copper or similar material.  
      As the LED chip  12  heats up during illumination, heat is transferred from the LED chip  12  to the submount layer  16 , to the slug layer  20  and to the rest of the package. As the slug layer  20  increases in temperature, the heat from the slug layer  20  is transferred to the submount layer  16  onto which the LED chip  12  is mounted. The amount of heat dissipated from the LED package  10  and the resulting temperature of the LED chip  12  are contingent upon the package thermal resistance. The package thermal resistance is calculated as the difference in temperature of the LED chip  12  from the bottom of the slug layer  20  divided by the thermal power dissipation of the LED chip  12 . Hence, if the slug layer  20  is at 60° C. and the LED chip  12  is at 100° C. while driving the LED chip  12  at 1 W, the resulting thermal resistance is 40° C./1 W or 40° C. /W.  
      The TED  18  aids the flow of heat from the LED chip  12 . When mounted under the submount layer  16 , the TED  18  reduces the temperature of the LED chip  12 , which is attached to the “cold” side of the thermoelectric device  18 . The heat is drawn through the “cold” side of the TED  18  and across a temperature gradient thereby increasing the temperature of the “hot” side of the TED  18 . The “hot” side then conducts heat to the slug layer  20  due to the difference in temperature according to equation 1. 
 
 Q=mc ( T   1   −T   a ) 
 
 Where Q is the heat energy, m is the mass of the body, c is the thermal capacitance, T 1  is the induced temperature and Ta is the ambient temperature. Further, the change in the temperature over time is defined according to equation 2. 
 
 dQ/dt =( T   1   −T   a )/ R   th    (2) 
 
 Where dQ/dt is the thermal power and R th  is the thermal resistance across the interface. 
 
      Also, the TED  18  creates a temperature gradient across it as a function of the input current. The voltage of the TED  18  is dependent on the existing temperature gradient: 
 
 dT   ted   =h   ted ( dT   induced ) K   o    P   ted    (3) 
 
 Where dT ted  is the reverse temperature gradient caused by the TED  18 , h ted  is the efficiency of the TED  18 , K o  is the power coefficient of temperature gradient, and dT induced  is the pre-existing temperature drop across the TED  18 . 
 
      Therefore, the efficiency of the TED  18  is dependent on the induced temperature gradient and the temperature gradient created is dependent on the input power. Therefore, the higher the temperature difference across the TED  18  the lower its effectiveness. However, if the temperature gradient is relatively small, the efficiency is high and the temperature drop on the LED chip  12  can be created more efficiently. In this embodiment, the efficiency of the TED  18  is high enough that the amount of power required to create a reverse temperature gradient contributes more to the heat extraction than to heat input of the LED chip  12 .  
      The TED  18  is powered in electrical series with the LED chip  12 , making the current through the TED  18  proportional to the drive current of the LED chip  12 . Accordingly, the current through the TED  18  is roughly proportional to the power dissipated in the LED chip  12 . This allows the TED  18  to work only as hard as it is required.  
      Referring now to  FIG. 2 , an alternative embodiment of an LED chip package according to the principles of the present invention is illustrated therein and designated at  110 . The LED chip package  110  includes an LED chip  112  that is covered on a first side by a conversion coating layer  114 . The conversion coating layer  114  may be made of phosphor or other material suitable for that purpose, and the light generated by the LED chip  112  is transmitted through this conversion coating layer  114 .  
      Attached to a second side of the LED chip  112  is a submount layer  116  which may be made of silicon or other well known submount materials. Attached to the submount layer  116 , opposite the LED chip  112 , is a slug layer  120  of a highly themoconductive material. Based on this construction, heat from the LED chip  112  is effectively deposited into and dissipated through the slug layer  120 . One material that is suitable for the slug layer  120  is copper because of its high thermal conductivity.  
      An LED housing  122  is attached to the slug layer  120  and protects the LED chip  112 . The LED housing  122  also serves to further dissipate heat from the slug layer  120 .  
      Attached to the LED housing  122  and surrounding the LED chip  112  is a lens  124 . The lens  124  focuses the light generated by the LED chip  112  and in addition to the LED housing  122 , also serves to protect the LED chip  112  and conversion coating layer  114 .  
      Attached to the LED housing  122 , opposite the LED chip  112 , is a TED  118 . The TED  118  is attached on a first side to the LED housing  122  and is configured to actively draw heat through the LED housing  122  and away from the LED chip  112 . The second side of the TED  118  is attached to a heat sink  126  through an attachment layer  128 .  
      As with the prior embodiment, the attachment layer  128  may be made of solder or a thermoconductive adhesive and the slug layer  120  is also a highly thermoconductive material. This enables heat drawn from the LED chip  112  by the TED  118  to be effectively deposited into and dissipated through the heat sink  126 . Accordingly, the heat sink  126  is preferably made of copper or similar material. Additionally, the heat sink  126  has a large surface area and is configured to further dissipate the heat from the LED housing  122  through convection.  
      As discussed above in connection with the prior embodiment, the TED  118  is powered in electrical series with the LED chip  112 , making the current through the TED  118  proportional to the drive current of the LED chip  112 . Accordingly, the current through the TED  118  is roughly proportional to the power dissipated in the LED chip  112 . This allows the TED  118  to work proportionally to the LED chip  112  and, therefore, only as hard as it is required.  
      As a person skilled in the art will readily appreciate, the above description is meant as an illustration of implementation of the principles this invention. This description is not intended to limit the scope or application of this invention in that the invention is susceptible to modification, variation and change, without departing from the spirit of this invention, as defined in the following claims.