Abstract:
The present disclosure includes an apparatus and method relating to LED fixtures having an LED module capable of operating at a temperature. The LED module is also capable of being operated within a desired temperature range having an upper limit and a lower limit. A fan is operable for cooling the LED module when the temperature of the LED module approaches the upper limit. A heater is operable for heating the LED module when the temperature of the LED module approaches the lower limit. A heat sink may be in thermal communication with the LED module such that the fan may cool and the heater may heat the heat sink. The controller may also be configured to selectively activate the heater and the fan. The controller may also be in electrical communication with a temperature sensor which is in thermal communication with the LED module such that the controller may read the temperature of the LED module. The controller may activate the fan and/or the heater depending on the temperature of the LED module.

Description:
CROSS REFERENCE TO RELATED APPLICATION 
     This application claims the benefit of U.S. Provisional Application No. 61/924,934 filed Jan. 8, 2014, herein incorporated by reference in its entirety for all purposes. 
    
    
     FIELD OF THE INVENTION 
     The present invention relates to LED based light fixtures. More particularly, but not by way of limitation, the present invention relates to a heater system for a high-power light emitting diode based light fixture. 
     BACKGROUND OF THE INVENTION 
     Generally speaking, light emitting diodes (LEDs) are finding their way into an ever increasing variety of light fixtures for an ever increasing number of applications. The popularity of LEDs has been driven by a number of factors, such as: a heightened awareness of the ecosystem spurred by the so called “climate change” debate; increased efficiency which can realize a rapid financial payback typically measured in months; exceptionally long bulb life compared to other lighting options; visually pleasing light quality; and an ever decreasing price in dollars per lumen of output. This list is not exhaustive and virtually every application for LED lighting will find advantages specific to the application. 
     LEDs offer significant advantages over every other known type of bulb. For example, LEDs produce at least four times the light produced by an incandescent bulb of the same wattage. However, if one measures harnessed light, the light actually striking an illuminated object, the LED typically delivers close to eight, sometimes as much as ten, times the light. This is due to the fact that the light from an LED is usually delivered conically rather than spherically, eliminating the need to reflect light headed toward the back of the fixture, an inherently inefficient process. 
     LEDs are also environmentally attractive compared to fluorescent lighting, or other gas discharge-types of lighting. While efficiencies of LEDs and gas discharge bulbs are similar, when measured in lumens per watt, harnessed light from LEDs is typically close to twice that of gas discharge-type bulbs. Further, LEDs have not reached their theoretical limit of efficiency, and indeed newer models continue to improve output, e.g., lumens per watt. In contrast, gas discharge bulbs are more fully evolved. In addition, gas discharge bulbs, including fluorescent tubes, use small amounts of mercury. There is growing concern over filling landfills with mercury. Admittedly, chemicals are used in the making of LEDs that may not be any less dangerous than mercury but: 1) LEDs are hermetically encapsulated so the chemicals will not find their way into the environment, unlike the glass envelope of gas discharge devices, which are easily broken during disposal; and 2) the bulb life of LEDs is significantly longer so radically fewer LED devices are finding their way into landfills in the first place. 
     In particular, LEDs are proving beneficial for all types of high-power lighting, such as street lights, parking lot lights, movie and television production lighting, theatre lighting, indoor high-bay lighting, projector lighting, and the like. There are many reasons for the switch to LEDs but in many of these applications fixtures are inaccessible and bulb life is of primary concern. 
     In the case of entertainment lighting, LEDs have proven themselves to render skin tones with exceptional accuracy, making the LED particularly useful for motion picture and television production. In addition, unlike gas discharge lamps, LEDs can easily be dimmed over their entire range of operation and, unlike incandescent bulbs, LEDs have only a small color temperature shift over the dimming range. Further, the absence of any significant amount of infrared radiation from LEDs means there is virtually no heat felt by the subject of the illumination. 
     These markets have created a demand for higher and higher wattage LEDs. For many years, the limiting factor for the power of an LED fixture was the ability to dissipate heat from the LEDs into the environment. Other types of lighting radiate most of the heat produced by the bulb as infrared energy. In contrast, LEDs produce virtually no infrared. While in many ways this is an advantage, i.e. people under the light do not get hot, in some ways it is a disadvantage: all of the heat from LEDs must be conducted away. Fixture designers must strive to keep the LED die at less than 85° C., excessive temperature results in significant reduction in the life of the LED and, in the case of white LEDs, premature failure of the phosphors. Many LEDs exhibit a permanent shift in color temperature toward warmer hues when the LED is operated above its rated temperature. 
     For smaller LEDs the limiting factor is the amount of heat that can be conducted away in the electrical leads. In higher power devices, a cooper or aluminum slug is placed behind the semiconductor die to help carry heat away from the device. This technology seems to reach a practical limit at about ten watts per device. 
     In response, a number of manufacturers have begun using known chip-on-board, or COB, technology, to place a large number of LED dies in a very small area. The LEDs are mounted on an aluminum or copper core board so that the heat can be effectively transferred to a heat sink. While modules in the thirty to 300 watt range are now fairly common, several manufacturers claim modules in excess of 1000 watts are possible with current technology. High power modules, particularly modules having an input power of 30 watts, or more, create new challenges for fixture designers as significant amounts of heat must be dissipated into the environment. While theoretically traditional aluminum heat sinks could be constructed to take advantage of convection cooling for any foreseeable power level, at some point the amount of aluminum required would be prohibitive from both a cost standpoint and a fixture-handling standpoint. 
     Already, LED fixtures incorporating high-power LED modules have turned to forced air cooling, typically using a conventional rotating fan. With increased air flow, smaller and lighter heat sinks can be used, far offsetting the additional cost of the fan. In applications where noise is an issue, fixture designers strive for laminar air flow and thermostatically controlled fans to minimize noise. Heat pipes, water cooling, as well as other exotic techniques have been used to extract the heat from the relative small area around the LED module and dissipate it in a much larger volume. 
     Unfortunately, problems still exist in the use of such high power modules, such as: white LEDs exhibit a slight color shift as the operating temperature changes; and COB modules survive only a limited number of thermal cycles. Color shift over changes in operating temperature is of particular concern in motion picture and television production. While the human eye will adjust to such changes, film and video are unforgiving. The process for fixing a color shift in post-production is costly and time consuming. Adding to the problems is, the larger the heat sink, the longer it will take the fixture to achieve a steady-state temperature. In an LED fixture, it is not uncommon to see a 15 to 45 minute delay in achieving steady-state. 
     With regard to temperature cycling, it has been observed that, where LED fixtures are used only a few hours a day, modules will predominantly fail from temperature cycling as opposed to failure of the die or the phosphor. 
     Thus it is an object of the present invention to provide a system and method to reduce the effect of temperature cycling on the color temperature of the emitted light and on the life of a high power module. 
     SUMMARY OF THE INVENTION 
     The present invention provides an LED based light fixture where the effects of temperature cycling are significantly diminished. 
     In one preferred embodiment an LED fixture is provided which includes a high power LED module, a heat sink in thermal communication with the module for dissipating the heat produced by the module into the environment, and a heater for warming the module when the LED is very dim or off. 
     In another preferred embodiment the inventive LED fixture further includes a fan for moving air over the heat sink to increase the rate at which heat is dissipated from the heatsink and where the fan can be turned off while the heater is operable to reduce the amount of heat needed to maintain the temperature of the LED module. 
     In still another preferred embodiment, a semiconductor, such as a MOSFET, is used to produce heat when the LED module is turned off. 
     In yet still another preferred embodiment a Peltier device is employed so that, when the LED is operable, current is driven through the Peltier device in a first direction to cool the module and deliver heat to the heatsink. When the LED is not operated, the current through the Peltier device is reversed so the LED is warmed and heat is brought into the fixture from the heatsink. 
     In a further embodiment, the present disclosure includes an LED fixture having a housing; a heat sink supported by the housing; an LED module in thermal communication with the heat sink; and a heater in thermal communication with the LED module wherein when the LED module is inactive, the heater can be driven to add heat to the LED module. 
     In yet a further embodiment, the present disclosure includes an LED fixture having an LED module capable of operating at a temperature; the LED module capable of being operated at least within a desired temperature range having an upper limit and a lower limit; a fan operable to cool the LED module when the temperature of the LED module approaches the upper limit; and, a heater operable to heat the LED module when the temperature of the LED module approaches the lower limit. A heat sink may be in thermal communication with the LED module. A controller may be configured to selectively activate the heater and the fan. A controller may be in electrical communication with a temperature sensor which is in thermal communication with the LED module such that the controller may read the temperature of the LED module. The controller may activate the fan and/or the heater depending on the temperature of the LED module. 
     The present disclosure further includes a method for maintaining the temperature of an LED module within prescribed limits including the steps of: 
     a. providing a fan for cooling the LED module; 
     b. providing a heater for heating the LED module; 
     c. providing a controller configured to selectively activate the fan and the heater; 
     d. providing a temperature sensor in thermal communication with the LED module and in electrical communication with the controller; 
     e. in the controller, reading the temperature of the LED module; 
     f. activating the fan when the temperature of the LED module approaches an upper limit; and 
     g activating the heater when the temperature of the LED module approaches a lower limit. 
     Further objects, features, and advantages of the present invention will be apparent to those skilled in the art upon examining the accompanying drawings and upon reading the following description of the preferred embodiments. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         FIG. 1  depicts a preferred embodiment of the inventive system for heating an LED device in its general environment. 
         FIG. 2  provides a cutaway view from the right side to show the interior of the fixture of  FIG. 1 . 
         FIG. 3  provides a rear view of a preferred embodiment of a heat sink as used in the fixture of  FIG. 1 . 
         FIG. 4  depicts an LED fixture similar to that of  FIG. 1  using a Peltier device to pump heat between the LED and the heat sink. 
         FIG. 5  provides a block diagram of a preferred embodiment which employs a MOSFET as a heater element. 
         FIG. 6  provides a block diagram of a preferred embodiment which employs a resistive load as a heat element. 
     
    
    
     DESCRIPTION OF THE PREFERRED EMBODIMENTS 
     Before explaining the present invention in detail, it is important to understand that the invention is not limited in its application to the details of the construction illustrated and the steps described herein. The invention is capable of other embodiments and of being practiced or carried out in a variety of ways. It is to be understood that the phraseology and terminology employed herein is for the purpose of description and not of limitation. 
     Referring now to the drawings, wherein like reference numerals indicate the same parts throughout the several views, one embodiment of a high power LED video production light employing the present invention  10  is shown in its general environment in  FIG. 1 . Typically fixture  10  is pivotally mounted to a stand  12 , or perhaps a truss  14 , by a bail, or yoke,  16 . Mount  16  allows fixture  10  to be tilted up or down and locked in place with knob  18  (a second knob is typically provided on the opposite side of the lighting instrument, but not shown). Between the stand mounting and yoke  16 , the light can generally be directed as desired. 
     In one preferred embodiment, LED fixture  10  includes a Fresnel lens  20  located towards the front of the instrument to allow focusing of the beam. As will be discussed in more detail below, the LED may be moved relative to lens allowing adjustment of the width of the beam. 
     While many of the features of a particular LED lighting instrument are not important to practicing the present invention, for the sake of clarity, a brief discussion of the construction of a typical LED fixture suitable for use with the present invention is provided. Turning to  FIG. 2 , LED fixture  10  includes: a housing  200 ; Fresnel lens  202  retained in front cover  204  at a forward end of housing  200 ; a rear cover  206  covering the back of housing  200 ; a stepper motor  208  mounted in rear cover  206  for driving lead screw  210 ; a heat sink  212  includes a threaded nut  214  driven by lead screw  210  to move the heat sink forward and backward within the housing as screw  210  is driven by motor  208 , thereby focusing the light exiting the fixture. As will be apparent to one of ordinary skill in the art, while the focusing mechanism is described as employing a stepper motor  208  and lead screw  210 , virtually any type of mechanism for moving heat sink  212  would suffice as long as heat sink  212  can be accurately positioned. Such mechanisms include purely manual systems and/or automated systems (i.e., servo driven systems). 
     Mounted to the forward surface of heat sink  212  is LED module  214 . LED module  214  is preferably a chip-on-board (“COB”) module. One such module is a VERO  29  LED module manufactured by Bridgelux, Inc. of Livermore, Calif. Chip-on-board technology involves attaching a semiconductor die directly to a circuit board. Very small bond wires are then attached directly to the die and to pads on the circuit board. Chip on board technology is a mature process relative to semiconductors in general, but is a relatively new process and still evolving as to LED modules. Heat sink  212  transfers heat from LED  214  to dissipate the heat into the environment. A fan  218  circulates air through fins (as best seen in  FIG. 3 ) of heat sink  212  and exhausts the heated air from housing  200 . Preferably fan  218  is thermostatically controlled so that the speed of the fan is no higher than it needs to be to achieve adequate cooling of LED  214  in light of the brightness of LED  214  and the ambient temperature, and thus produce no more noise than is absolutely necessary. 
     Circuit board  216  includes circuitry for driving LED  214 , for controlling the brightness of LED  214 , and for controlling the speed of fan  218 . As will be apparent to one skilled in the art, fan  218  may be driven at a speed relative to the brightness of LED  214 , or alternatively, a temperature sensor may be mounted near LED module  214  and the circuitry of board  216  may monitor the temperature and control fan  218  to maintain the temperature within a prescribed range, typically in neighborhood of 65 degrees Celsius. 
     When LED  214  is driven at less than full brightness, but with sufficient power to require some fan cooling, the temperature of LED  214  can be controlled within a fairly narrow range, typically within a 20 degree Celsius window. In a preferred embodiment, the desired temperature range is between approximately 55 degrees Celsius and approximately 75 degrees Celsius. However, it is understood that as LED technology provides LEDs that are designed to operate at higher temperatures, one skilled in the art would recognize that the desired range would increase and/or move upward. While the precise point at which the fan is stopped will vary based on the efficiency of the heat sink, particulars of the housing, the ambient temperature, etc., at some point, about 10% of full power in the preferred embodiment, the temperature of LED module  214  will fall outside the prescribed range even with fan  218  stopped. There are several reasons that it is desirous to maintain the LED temperature within a prescribed range such as, by way of example and not limitation: stable color temperature, stable forward voltage of the LED, reduced temperature cycling on the LED dies, improved life of the module, etc. To achieve temperature control when LED  214  is inactive or driven at a power level below that which allows temperature regulation via fan  218 , heater  220  may be driven to add additional heat to heat sink  212 . In a basic embodiment, heater  220  may be driven to add additional heat directly to the LED module  214 . Circuitry on board  216  provides on/off control of power for heater  220 . Proportional control of heater  220  may be provided, but simple on/off control can easily be used to control the temperature of LED  214  within a reasonable range. 
     Turning to  FIG. 3 , heat sink  212  preferably includes a body  306  for mounting LED  214  ( FIG. 2 ) and heaters  220 ; one or more heat tubes  304  for conducting heat away from body  306 ; and a plurality of fins  302  for dissipating the heat into the environment. In a preferred embodiment, heaters  220  are electrical resistors. While resistors are available in a variety of shapes, sizes, package types, etc., any package which allows good heat transfer to body  306  would be acceptable. Further, the power rating of the resistor is not critical so long as the total heat dissipation of heaters  220  is at least high enough to provide the necessary heat to maintain the LED near 65 degrees Celsius, at least in one preferred embodiment, with the LED turned off and the fan not running in a typical indoor environment. In one preferred embodiment the sum power dissipation of the heaters is approximately 10% of the rated power of the LED module. 
     In another preferred embodiment, a semiconductor, such as a MOSFET, can be used as the heater.  FIG. 5  provides a block diagram of one such embodiment comprising: MOSFET  506 ; current sense register  508  for measuring the electrical current flowing through MOSFET  506 ; controller  502  which includes input  514  o receive a signal  510  from current sense register  508  indicative of the current flowing through MOSFET  506 ; and a digital to analog converter (DAC)  504  in communication with controller  514  via data bus  512  such that controller  502  can adjust the voltage provided by DAC  504  to MOSFET  506  and thus control the current flowing through MOSFET  506 . When incorporated into an LED fixture, preferably a temperature sensor will be provided to measure the temperature of the LED module. Thus controller  502  can proportionately control the heat produced by MOSFET  506  to maintain the LED module temperature at a prescribed level. 
     In the resistive heater embodiment as discussed with reference to  FIG. 3 , in one preferred embodiment, thermostatic control of the heater may be accomplished through the system depicted in  FIG. 6 . Preferably resistive heater  604  may be switched off and on via transistor  610  under the control of output  612  of controller  602 . Temperature sensor  606  provides temperature feedback to controller  602  via input  614 . As will be apparent to one skilled in the art, sensor  606  can be a thermistor, or an integrated temperature sensor having a voltage output, in which case input  614  would be an analog input. Alternatively, temperature sensor  606  may provide a digital output, in which case input  614  may actually comprise a serial data bus, such as, by way of example and not limitation, a SPI bus, and ITC bus, a one wire serial bus, or the like. 
     Preferably heater  604  and sensor  606  are mounted on heat sink  608  proximate the LCD module. In a preferred embodiment controller  602  is a microcontroller, FPGA, or similar programmable device configured to read the temperature from sensor  606  and provide a pulse width modulated signal at output  612  to maintain a prescribed temperature at the LED module. Providing proportional control of a heat to maintain a heat sink temperature within a prescribed range within the skill level of one of ordinary skill in the art. 
     As depicted in  FIG. 4 , in another preferred embodiment, LED fixture  400  includes: housing  402  having a Fresnel lens  404  retained in front cover  406 ; a rear cover  408 ; a servo motor  410 , such as a stepper motor, brushless DC motor, etc., mounted in rear cover  408  and configured to drive lead screw  412 ; a carriage assembly  414  driven by lead screw  412  so as to be positionable along a longitudinal axis extending between front cover  406  and rear cover  408 . 
     Carriage assembly  414  includes heat sink  420  having LED module  416  attached on a forward facing surface  418  of heat sink  414  and Peltier device  422  sandwiched between heat sink  420  and LED  416 . As will be apparent to one skilled in the art, the light emitted by LED  416  can selectively focused by adjusting the position of carriage  414  relative to lens  404 . 
     A fan  424  mounted in rear cover  408  draws air through heat sink  420  and exhausts the heated air from housing  402 . When electrical current is driven through a Peltier device in a first direction, one surface of the Peltier device will get cooler while the opposite surface will get hotter. Thus, a Peltier device acts as a solid state heat pump. As will be apparent to one skilled in the art, Peltier device  422  can be used to cool LED module  416  when the LED is operating and allow heat sink  420  to heat to a much higher temperature than would be possible without Peltier device  422  thus allowing heat sink  420  to dissipate heat more efficiently. When the LED  416  is driven at low power, or not at all, the current through Peltier device  422  can be reversed to heat the LED  416 . 
     As in the previously described embodiment, the fan speed can be varied to assist cooling the LED  416 , and can be stopped entirely when the Peltier device  422  is used to warm the LED  416 . Methods for reversing electrical current are well known in the art, such as with an H-bridge driver which may be located on circuit board  426 . 
     It should be noted that while preferred embodiments of the inventive LED heater have been discussed as employed in a Fresnel-type LED fixture, the invention is not so limited. The inventive techniques may be used in any type of LED fixture where stable color temperature is important over a range of operating conditions or where life of the LED may be adversely affected by temperature variations or temperature cycling. It should also be noted that while the preferred embodiments have discussed using either resistive heat or a Peltier device to produce heat, the invention is also not so limited. Any method of producing could be used including, but not limited to, combustion, friction, and the like. 
     Finally it should be noted that, while the preferred embodiment of the inventive LED heater system have been shown and described as incorporating a fan, the invention is not so limited. The inventive system would work equally well with convection cooled LEDs. 
     Thus, the present invention is well adapted to carry out the objects and attain the ends and advantages mentioned above as well as those inherent therein. While presently preferred embodiments have been described for purposes of this disclosure, numerous changes and modifications will be apparent to those skilled in the art. Such changes and modifications are encompassed within the spirit of this invention.