Patent Publication Number: US-8994273-B2

Title: Light-emitting diode fixture with an improved thermal control system

Description:
BACKGROUND OF THE INVENTION 
     1. Field of the Invention 
     The present invention relates to a light-emitting diode fixture and, in particular, to a light-emitting diode fixture with an improved thermal control system. 
     2. Description of the Related Art 
     Light-emitting diodes, like any semiconductor, emit heat during their operation. This is because not all of the electrical energy provided to a light-emitting diode is converted to luminous energy. A significant portion of the electrical energy is converted to thermal energy which results in an increase in the temperature of the light-emitting diode. In resistor driven circuits, as the temperature of the light-emitting diode increases, the forward voltage drops and the current passing through the PN junction of the light-emitting diode increases. The increased current causes additional heating of the PN junction and may thermally stress the light-emitting diode. 
     Thermally stressed light-emitting diodes lose efficiency and their output is diminished. In certain situations, optical wavelengths may even shift causing white light to appear with a blue tinge. Thermally stressed light-emitting diodes may also impose an increased load on related driver components causing their temperature to increase as well. This may result in broken wire bonds, delaminating, internal solder joint detachment, damage to die-bond epoxy, and lens yellowing. If nothing is done to control the increasing temperature of the light emitting diode, the PN junction may fail, possibly resulting in thermal runaway and catastrophic failure. 
     Thermal control of light-emitting diodes involves the transfer of thermal energy from the light-emitting diode. Accordingly, one aspect of light-emitting diode fixture design involves efficiently transferring as much thermal energy as possible away from the PN junction of the light-emitting diode. This can generally be accomplished, at least in part, through the use of a heat sink. However, for more powerful light-emitting diode fixtures in the 20 to 60 watt range or in applications where numerous light-emitting diodes are disposed within a confined space, an additional cooling means may be required to maintain performance. This is because the thermal energy generated by the light-emitting diodes may at times exceed the thermal energy absorbed and dissipated by the heat sink. In these situations a cooling fan is typically used in combination with the heat sink. 
     In a conventional thermal control system for light-emitting diode fixtures, a heat sink and a cooling fan are thermally coupled to a light source comprised of a plurality of light-emitting diodes. A thermal sensor senses the temperature of the light source and signals a controller to operate a variable speed cooling fan, based on the temperature of the light source, to maintain the fixture within a desired temperature range. However, the need for a controller, typically in the form of a microprocessor, increases the number of components in the thermal control system and thereby increases manufacturing costs. 
     SUMMARY OF THE INVENTION 
     It is an object of the present invention to provide an improved light-emitting diode fixture. 
     There is accordingly provided a light-emitting diode fixture comprising a first housing portion and a second housing portion spaced-apart from the first housing portion. A cooling device is disposed within the first housing portion and is in fluid communication with the second housing portion. First and second printed circuit boards are disposed within the second housing portion. A light-emitting diode and a negative coefficient thermistor array are mounted on the first printed circuit board. A heat sink is thermally coupled to both the light-emitting diode and the negative coefficient thermistor array. A rectifier is mounted on the second printed circuit board. The rectifier is electrically connected in series with the cooling device and the negative coefficient thermistor array. Current used to power the cooling device flows from the power supply to the cooling device and through the negative coefficient thermistor array. The negative coefficient thermistor array controls said current flow from the power supply to the cooling device based on a temperature of the heat sink which is thermally coupled to the thermistor array, thereby controlling the output of the cooling device based on the temperature of the heat sink. The current used to power the cooling device may also flow through an LED array. The negative coefficient thermistor array may be connected in series in the powering line (wire) of the cooling device. There may also be a positive coefficient thermistor mounted on the first printed circuit board. The positive coefficient thermistor may be thermally coupled to the heat sink. 
     The first housing portion may be vented and the second housing portion may be vented. The fixture may further include a collar disposed about the light-emitting diode. The heat sink and the collar may be on opposite sides of the first printed circuit board. There may be an aperture in the first printed circuit board and at least two radially extending fins on the collar. The aperture in the first printed circuit board may be disposed between said at least two radially extending fins on the collar. There may be a reflector which is thermally coupled to the collar. There may be a passageway extending through the heat sink. The aperture in the first printed circuit board and the passageway in the heat sink may be aligned. 
     The light-emitting diode fixture may also include a positive coefficient thermistor, a switching diode, a resistor array, a setting resistor and an indicator. The light-emitting diode may be connected with the rectifier and the positive coefficient thermistor. The light-emitting diode may be electrically connected with the positive coefficient thermistor and the switching diode. The switching diode may be electrically connected with the resistor array and the setting resistor. The setting resistor may be connected with the switching diode and the indicator. The positive coefficient thermistor, resistor array and light-emitting diode indicator may all be connected to a negative bus of the rectifier. The positive coefficient thermistor is mounted on the first printed circuit board. The indicator may be a light emitting diode indicator. 
    
    
     
       BRIEF DESCRIPTIONS OF DRAWINGS 
       The invention will be more readily understood from the following description of the embodiments thereof given, by way of example only, with reference to the accompanying drawings, in which: 
         FIG. 1  is a perspective, partially broken away view of an improved light-emitting diode fixture; 
         FIGS. 2A and 2B  are section views of the light-emitting diode fixture of  FIG. 1 ; 
         FIG. 3  is an elevation view of a printed circuit of the light-emitting diode fixture of  FIG. 1 ; and 
         FIG. 4  is a circuit diagram of the light-emitting diode fixture of  FIG. 1 . 
     
    
    
     DESCRIPTIONS OF THE PREFERRED EMBODIMENTS 
     Referring to the drawings and first to  FIG. 1  an improved light-emitting diode fixture  10  is shown. The light-emitting diode fixture  10  includes a first housing portion  12  and a second housing portion  14 . The first housing portion  12  and second housing portion  14  are spaced apart and coupled by a connector  16 . The connector is hollow and permits fluid communication between the first housing portion  12  and the second housing portion  14 . In this example the connector  16  is integral with the first housing portion  12 . In other examples the connector may be integral with the second housing portion or the connector may be a separate component. The first housing portion  12  is vented and has a plurality of openings, for example openings  18   a  and  18   b , which extend through an end  20  of the first housing portion which is opposite of connector  16 . The second housing portion  14  is also vented and has a plurality of openings, for example openings  22   a  and  22   b , which extend through an end  24  of the second housing portion which is opposite of the connector  16 . There are also openings, for example openings  26   a  and  26   b , in a side wall  28  of the second housing portion  14 . Wiring  30  extends from the first housing portion  12  and there is a reflector  32  disposed within the second housing portion  14 . 
     The first housing portion  12  and the second housing portion  14  of the fixture  10  are shown in greater detail in  FIGS. 2A and 2B . In this example, and as shown in  FIG. 2A , the first housing portion  12  has a perforated lid  34  which forms the end  20  of the first housing portion with the plurality of openings  18   a  and  18   b . The perforated lid  34  is releasably secured to the first housing portion  12  with stand-off connectors  36   a  and  36   b . Release and removal of the perforated lid  34  allows access to an interior of the first housing portion  12 . There is a cooling device in the form of a fan  40  in this example disposed within the first housing portion  12  together with positive fan wire  41  and the negative fan wire  42 . The positive fan wire  41  and a negative fan wire  42  run from the first housing portion  12 , through respective isolator switches  44  and  46 , to the connector  16 . The positive fan wire  40  and the negative fan wire  42  may run through a side wall  48  of the connector  16  as shown in  FIG. 2A . 
     In this example the second housing portion  14  also has a perforated lid  50 . The perforated lid  50  of the second housing portion  14  forms the end  24  of the second housing portion  14  with the plurality of openings  22   a  and  22   b . The perforated lid  50  is threadedly secured to the second housing portion  14 , but is releasable to allow access to an interior of the second housing portion  14 . Referring now to  FIG. 2B , there is a first printed circuit board  52  disposed within the second housing portion  14 . In this example, the first printed circuit board  52  is a first printed circuit board which is shown in greater detail in  FIG. 3 . The first printed circuit board  52  has a metal core  54  surrounded by a non-conductive substrate  56 . There is an LED array  58  mounted on the metal core  54  together with a positive coefficient thermistor  60  and a negative coefficient thermistor array  62 . The LED array  58  is surrounded by thermal collar  64  which is thermally coupled to the first printed circuit board  52 . There are also apertures, for example apertures  66   a  and  66   b , in the metal core  54  of the first printed circuit board  52  as well as apertures, for example apertures  68   a  and  68   b , in the surrounding non-conductive substrate  56  of the first printed circuit board  52 . The thermal collar  64  has a plurality of radial fins, for example radial fins  70   a  and  70   b , that are positioned such that the apertures  66   a  and  66   b  in the metal core are between the radial fins in this example. The thermal collar may be made of aluminium. The shape of the apertures  66   a  and  66   b  may affect airflow. 
     Referring back to  FIG. 2B , the thermal collar  64  couples the first printed circuit board  52  to the reflector  32 . In this example the reflector  32  is metallic and may function to increase the surface area upon which convective heat exchange may occur. There is also a heat sink  72  thermally coupled to the first circuit board  52 . The thermal collar  64  and heat sink  72  are on opposite sides of the first printed circuit board  52 . This allows for heat exchange to occur on both sides of the first printed circuit board  52 . In this example the heat sink has a base  74  and a plurality of fins, for example fins  76   a  and  76   b , extending from the base. There are a plurality of passageways, for example passageways  78   a  and  78   b , extending through the base  74  of heat sink  70 . The apertures in the base of the heat sink are aligned with the apertures in the metal core  54  of the first printed circuit board  52  as shown in  FIG. 2B  for apertures  66   a  and  78   a . This allows air to flow from the fan through the second housing portion  14  of the fixture  10 . Air may also flow through the apertures  68   a  and  66   b  in the substrate  56  of the first printed circuit board  52 . 
     A second printed circuit board  80  is also disposed within the second housing portion  14 . The second printed circuit board  80  is spaced apart from the first printed circuit board  52  by a flange  82  which extends along an inner wall  84  of the second housing portion  14 . In this example there is a central opening  86  in the second printed circuit board  80  to allow the heat sink  72  to extend through the second printed circuit board as shown in  FIG. 2B . The central opening  86  in the second printed circuit board  80  also allows air to flow through second printed circuit board and into the second housing portion  14 . Employing both the first printed circuit board  52  and the second printed circuit board  80  may decrease the thermal load on each of the printed circuit boards. 
     Referring back to  FIG. 2A , there are stand-off connectors  88  and  90  disposed within the second housing portion  14 . The stand-off connectors  88  and  90  face an open end  92  of the second housing portion  14 . In this example the connector  16  is received by the open end  92  of the second housing portion  14  and the connector  16  engages the stand-off connectors  88  and  90 . This mechanically couples the first housing portion  12  to the second housing portion  14  and completes the electric circuitry of the fixture  10 . This is because the positive fan wire  41  and the negative fan wire  42  run from the first housing portion through connector  16  to the printed circuit boards  52  and  80  disposed in the second housing portion  14 . The stand-off connectors  88  and  90  also ensure an electrical connection between the printed circuit boards  52  and  80 . 
     Referring now to  FIG. 4  a circuit diagram of the fixture  10  of  FIG. 1  is shown. A plurality of light-emitting diodes, for example light-emitting diodes  58   a  and  58   b , form the LED array  58 . The light-emitting diodes may be electrically connected in both parallel and series. An AC power supply  92  provides current to the fixture. A rectifier  94  in the fixture rectifies the alternating current to direct current which powers the LED array  58 . The direct current also powers a motor  96  of the fan  38 . The positive terminal of the rectifier  94  is electrically connected in parallel to the positive terminal of the LED array  58  and the positive terminal of the fan motor  96 . 
     The negative coefficient thermistor array  62  is electrically connected in series between a negative terminal of the fan motor  96  and a negative terminal of the rectifier  94 . The negative coefficient thermistor array  62  includes a plurality of negative coefficient thermistors, for example negative coefficient thermistors  62   a  and  62   b , which are thermally coupled to the heat sink  72  by means of the first printed circuit board  52  as shown in  FIGS. 2A and 2B . The negative coefficient thermistors  62   a  and  62   b  may be electrically connected in both parallel and series. The negative coefficient thermistor array  62  is sensitive to the temperature of the heat sink  72 . As the temperature of the heat sink  72  increases, the resistance of the negative coefficient thermistor array  62  decreases. As the temperature of the heat sink  72  decreases, the resistance of the negative coefficient thermistor array  62  increases. Accordingly, the flow of direct current to the fan motor  96  is dependent on the temperature of the heat sink  72 . The negative coefficient thermistor array  62  generally functions in manner as described in U.S. Pat. No. 8,070,324 which issued on Dec. 6, 2011 to Kornitz et al., and the full disclosure of which is incorporated herein by reference. However, in this example as part of a negative feedback control loop. 
     There is a positive coefficient thermistor  60  electrically connected in series between the negative terminal of the rectifier  94  and the negative terminal of the LED array  58 . The positive coefficient thermistor  60  functions to protect the LED array  58  from overheating in combination with overcurrent. There is also a resistor array  98  electrically connected in series between the negative terminal of the rectifier  92  and the negative terminal of the LED array  58  through a switching diode  106 . The resistor array  98  functions to restrict the current flowing to the LED array  58  when the LED array  58  overheats and may make the fixture more energy efficient. The positive coefficient thermistor  60  and resistor array  98  are electrically connected in parallel along a common negative bus. There is also a resistor  100  and an indicator in the form of a light-emitting diode  104  electrically connected in series between the cathode of the switching diode  106  and the common negative bus  102 . The resistor  100  is electrically connected to an anode of the light-emitting diode  104  and a cathode of the light-emitting diode  104  is electrically connected to the negative bus  102 . The resistor  100  is a setting resistor and functions as a setting device of the light-emitting diode  104 . The light emitting diode  104  functions as an indicator of the regime of the fixture. The negative terminal of the LED array  58  is electrically connected with an anode of the switching power diode  106 . A cathode of the switching power diode  106  is electrically connected with resistor array  98  and resistor  100 . 
     Employing two printed circuit boards  52  and  80 , shown in  FIG. 2B , allows for separate arrangement of components of the electric circuitry. This decreases the thermal load on the individual printed circuit boards  52  and  80 . In this example the LED array  58 , negative coefficient thermistor array  62 , and positive coefficient thermistor  60  are disposed on the first printed circuit board  52 . The integrated microcircuits for the rectifier  94 , the resistor array  98  and the resistor  100 , and the diodes  104  and  106  are disposed on the second printed circuit board  80 . Separation of heat releasing elements in the electric circuitry assists in heat dissipation in the fixture. 
     It will be understood by a person skilled in the art that many of the details provided above are by way of example only, and are not intended to limit the scope of the invention which is to be determined with reference to the following claims.