Abstract:
A temperature control system for a light emitting diode assembly and associated lamp comprises a circuit board for supporting one or more light emitting diodes and a housing supporting the circuit board. A coolant circulation path is routed through the housing. The coolant circulation path is connected to a source of a substantially constant temperature circulating coolant. The temperature of the coolant is maintained around the constant by an internal combustion engine and a radiator.

Description:
BACKGROUND 
       [0001]    1. Technical Field 
         [0002]    The technical field relates generally vehicle lighting temperature control and more particularly to temperature control of light emitting diode based lamps. 
         [0003]    2. Description of the Technical Field 
         [0004]    Light emitting diodes (LED) are a type of semi-conductor device which emits light when electricity flows through them. LED efficiency in terms of conversion of power to light in the visible spectrum decreases with increasing temperature, with the decrease in efficiency accelerating dramatically when temperature increases above a particular threshold temperature. At the same time heat generation tends to increase. Removing heat from LEDs increasingly important at high power level operation of LEDs to keep operating temperature below the threshold. 
         [0005]    LED temperature has been controlled using many of the same techniques used to control semi-conductor device temperature. These techniques generally rely on increasingly heavy heat sinks and more complex heat transfer systems as the amount of heat to be removed increases. A basic heat sink for an LED may provide a heat conducting metal substrate on which the LED is mounted. The metal substrate provides an increased area from heat is radiated to the environment. The metal substrate may be cast with heat radiating fins to increase its radiating area. If fins alone are insufficient to reject the heat produced by an LED a temperature actuated fan may be employed to force air across the fins. Improved transmission of heat from an LED to the heat sink may be achieved by circulation of a liquid between the LED and the heat sink. Such an LED package includes a built in finned radiator, circulation paths for the liquid coolant and electronics to detect temperature and to drive circulation of air and possibly coolant. Each gradation of heat removal adds to the expense and to the power consumption of the device. Such heat sinks tend to become increasingly heavy which is a consideration in motor vehicle applications. 
       SUMMARY 
       [0006]    A temperature control system for a light emitting diode assembly and associated comprises a circuit board for supporting one or more light emitting diodes and a housing supporting the circuit board. A coolant circulation path is routed through the housing. The coolant circulation path is provided with connectors for connection to an external source of a substantially constant temperature, circulating coolant. The coolant is maintained at a constant temperature by an internal combustion engine and an associated radiator. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0007]      FIG. 1  is a schematic view of a motor vehicle coolant circulation system. 
           [0008]      FIG. 2  is a perspective view of a light emitting diode assembly for a vehicle lamp. 
           [0009]      FIG. 3  is an exploded view of the light emitting diode assembly of  FIG. 2  illustrating a possible coolant circulation path through the assembly. 
       
    
    
     DETAILED DESCRIPTION 
       [0010]    In the following detailed description, like reference numerals and characters may be used to designate identical, corresponding, or similar components in differing drawing figures. 
         [0011]    Referring now to the figures and in particular to  FIG. 1 , a motor vehicle coolant circulation system  11  is shown. Coolant circulation system  11  is connected to circulate engine coolant to an exterior lamp  10 . The coolant circulation system  11  includes a radiator  12  which is intended at temperatures not to exceed a limit temperature T. Engine coolant flows out of radiator  12  into coolant circulation lines  14 ,  18  which are connected to the exterior lamp  10  and to an internal combustion (IC) engine  20 , respectively. A coolant circulation line  16  carries coolant from lamp  10  to coolant circulation line  18  and through IC engine  20  to a pump  21  which forces the coolant into a coolant return line  22  for return to radiator  12 . 
         [0012]    Control over the coolant circulation system  11  resides in an engine control unit (ECU)  24  which, responsive to sensed coolant temperature from temperature sensor  15  can activate or deactivate a fan  28  positioned to force air through radiator  12 . Variance of coolant temperature above the limit T results in fan  28  being turned on. Coolant temperature below T less an offset results in the fan being turned off if it is already on. After a warm up period radiator  12  usually operates in a constant temperature range. 
         [0013]    ECU  24  communicates with an electrical system controller (ESC)  26  which in turn provides control signals for turning exterior lamp  10  on and off and which is connected to receive temperature measurements from lamp  10  from a temperature sensor  13 . In some applications ESC  26  can control the amount of power delivered to lamp  10 . ESC  26  can reduce the amount of power delivered to lamp  10 , or turn lamp  10  off, in response to measured temperature from temperature sensor  13  exceeding a maximum limit. 
         [0014]    Lamp  10  includes a light emitting diode (LED) assembly  30  which is shown in  FIG. 2 . Lamp  10  may also include appropriate lenses (indicated generally as exterior surface  17 ), reflectors and shields (not shown) to shape and direct light beams allowing the lamp to be adapted for use as a headlamp, a tail light, a marker light, a turn signal light or any other (usually) exterior application. Radiator  12  serves as a source of coolant/liquid which is introduced to the LED assembly through inlet  40  at a substantially constant temperature and which maintains the temperature of lamp  10  within a range well above freezing but below the threshold temperature where LED efficiency drops substantially. 
         [0015]    Reference to the liquid circulated through lamp  10  as “coolant” is conventional terminology for the water/alcohol mix used in engine cooling systems, however, the coolant can serve to transport heat either to or from LED assembly  30 . For example, coolant serves to transport heat from radiator  12  to lamp  10  when the LED&#39;s  36  are off. This allows for conduction of heat to the exterior surface  17  which can keep the surface warmed and prevent ice and snow buildup on the exterior surface under cold weather conditions. 
         [0016]    Referring to  FIG. 2 , an LED assembly  30  such as may be located in lamp  10  is shown in perspective view. LED assembly  30  comprises a housing  32 , a circuit board  34  applied to a major face of the housing, and a plurality of light emitting diodes (LED&#39;s)  36  surface mounted on the circuit board  34 . LED&#39;s  36  are encased in transparent epoxy lenses  38 . A coolant inlet  40  and a coolant outlet  42  are connected to one end of the LED assembly  30  for in and out flow of engine coolant from the coolant circulation system  11  through the LED assembly  30 . 
         [0017]    As illustrated in the partially exploded view of  FIG. 3 , circuit board  34  is applied to housing  32  directly over and covering a circulation cavity  44  within housing  32 . Circulation cavity  44  is illustrated as open thereby exposing engine coolant to direct contact with the major surface of the circuit board opposite from the side of the circuit board to which LED&#39;s  36  are mounted. The circulation path of engine coolant in circulation cavity  44  within housing  32  is generally indicated by path “A.” Alternatively, the circulation cavity  44  may be enclosed within housing  32  excluding coolant inlet  40  and coolant outlet  42 . Housing  32  may be made from a number of different materials, but is conveniently fabricated from aluminum. Circuit board  34  is usually made from a heat conducting metal. 
         [0018]    When IC engine  20  is on and engine coolant has reached in normal operating range, circulation of the coolant through lamp  10  keeps the lamp  10  and LED assembly  30  in a temperature range with an upper limit, but warm enough to prevent ice and snow accumulation even when the LED&#39;s  36  are not energized. No convection heating of the exterior surface  17  need be provided and the LED&#39;s  36  do not need to be turned on to keep ice and snow off the lamp  10 . The close proximity of the LED&#39;s  36  should keep the temperature of LED&#39;s  36  close to the coolant temperature under most conditions.