Patent Publication Number: US-2007114010-A1

Title: Liquid cooling for backlit displays

Description:
RELATED APPLICATIONS  
      This application claims priority under 35 U.S.C. section 119(e) of co-pending U.S. Provisional Patent Application No. 60/735,757, filed Nov. 9, 2005, and entitled “Liquid Cooling Systems for Backlit LED Display Products,” which is hereby incorporated by reference. 
    
    
     FIELD OF THE INVENTION  
      The present invention is related to liquid cooling. Specifically, the present invention is related to providing liquid cooling for backlit displays.  
     BACKGROUND OF THE INVENTION  
      Light emitting diode (LED) technology has been making significant advancement in recent years. The advancement of LED technology has produced numerous applications such as interior and exterior (outdoor) lighting, compact or portable lamps, automotive lights, and also light sources for backlit display systems. In the near future, LED lamps are expected to replace traditional incandescent, halogen, and/or fluorescent lamps (particularly mercury and/or cold cathode flourescent lamps) due to cost and energy savings. Additional advantages of modern LED technology include, for example, brighter colors, more compact lighting solutions, independent color control, and higher reliability. LED based backlit displays, in particular, have advantages in terms of brightness, white balance, and color control. More specifically, LED backlit displays are typically comprised of tri-chromatic LED arrays that are finely tunable for optimum white and color balance.  
      However, these LED applications generally suffer from high cost and high heat issues. In particular, the color performance of an LED display is a closely related function of junction temperature of the LED arrays. Higher power displays with high brightness capability necessitate the use of higher power LED sources. High power LEDs in turn present significant thermal challenges for traditional methods of cooling. For instance, traditional methods of cooling have difficulty coping with the high heat flux of modern LEDs. Traditional heat pipe designs in particular are bulky, which defeats the small and/or thin form factor advantages of many LED applications. Further, heat pipes are limited in the amount of heat they can move, and also in the distance they can move the heat from the heat source, which negatively impacts the display screen size. Thus, improved thermal design for LED cooling is critical to support the expansion of LED applications.  
     SUMMARY OF THE INVENTION  
      A cooling system for a backlit device includes a first heat collector, a first radiator, a first pump, an interconnect tubing, and a fluid. The first heat collector preferably has a micro structure such as micro channels or micro tubes, and is maintained in contact with the backlit device. The first radiator is for distributing heat and the first pump is for driving a fluid flow. The interconnect tubing is interposed between the first heat collector, the first radiator, and the first pump, to form a closed cooling loop. The fluid is for conducting heat and is sealed within the closed cooling loop.  
      In some embodiments, a method of cooling a backlit device disposes a heat collector in intimate contact with the backlit device. The heat collector has a fluid. The heat collector is used to collect heat from the backlit device and transfer the heat to a radiator using the fluid. The method rejects the heat from the radiator and recirculates the cooled fluid through the heat collector. The heat collector of some embodiments has a micro structure, and the fluid is pumped through the micro structure.  
      Preferably, the backlit device comprises an LED backlit flat panel display, and the flat panel display is typically an edge type LED backlit display that generates a high amount of heat per edge. Each edge includes one or more arrays of LEDs. The LEDs typically generate heat in the range of approximately 100 Watts to 1000 Watts. In a particular embodiment, the flat panel display has a thin form factor in the range of approximately 0.5 inches to approximately 4.0 inches in depth.  
      The first heat collector of some embodiments comprises an extruded multiport tubing in intimate contact with the backlit device. The micro tube of some of these embodiments has internal channels of approximately 0.5 to 5.0 millimeters in width by 0.5 to 5.0 millimeters in height and a wall thickness of approximately 0.5 to 1.0 millimeters. Preferably, the tubes are formed of extruded aluminum, or an alloy of aluminum. Other materials can be used. The first heat collector, in some embodiments, is a manifold that has a plurality of parallel flow vanes. The flow vanes are for directing fluid flow in parallel such that the temperature of the first heat collector is substantially distributed throughout the heat collector and transferred to the fluid in an approximately homogenous manner. In some exemplary implementations, the maximum pitch between the flow vanes is approximately 1.0 to 5.5 millimeters.  
      Preferably, the first heat collector is bonded to the backlit device by using a thermal interface material (TIM) layer. The TIM layer typically comprise at least one or more of Indium, a metallic coat, a thermal grease, a thermal pad, and/or a phase change material. In some embodiments, the first heat collector is also fastened to the backlit device by using a mechanical means. The mechanical means of these embodiments typically includes one or more of a screw, a bracket, and/or a clamp.  
      The cooling system of some embodiments further includes a reservoir and/or a fan. When implemented, the reservoir is for storing the fluid within the closed cooling loop, and further, preferably compensates for fluid loss over time. The fan is typically disposed in close proximity to the first radiator and is for rejecting heat from the first radiator.  
      The radiator of some embodiments has a thin form factor of approximately 15-50 millimeters. The fluid is typically selected from a set of cooling fluids comprising a glycol, an alcohol, a water based solution, and a dielectric solution. The cooling system of some embodiments includes a second heat collector, a plurality of radiators, and/or a plurality of pumps.  
    
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
      The novel features of the invention are set forth in the appended claims. However, for purpose of explanation, several embodiments of the invention are set forth in the following figures.  
       FIG. 1  illustrates a backlit display.  
       FIG. 2  illustrates a cooling system in accordance with some embodiments of the invention.  
       FIG. 3  illustrates the cooling system mounted to the backlit display according to some embodiments.  
       FIG. 4  illustrates a front view of a display with a cooling system according to some embodiments.  
       FIG. 5  illustrates a side view of the display and the cooling system of  FIG. 4 .  
       FIG. 6  illustrates a front view of a display and a cooling system of some embodiments.  
       FIG. 7  illustrates a side view of the display and cooling system of  FIG. 6 .  
       FIG. 8  illustrates a configuration for the cooling system of some embodiments.  
       FIG. 9  illustrates a heat collector manifold having parallel flow according to some embodiments.  
       FIG. 10  illustrates the heat collector incorporated into the cooling system of some embodiments.  
       FIG. 11  illustrates a side view of the heat collector bonded and/or coupled to the layers of an LED array for a backlit display.  
       FIG. 12  illustrates the flow of fluid across an LED array according to some embodiments of the invention.  
       FIG. 13  illustrates the steps of a method of cooling a backlit display in accordance with some embodiments of the invention.  
    
    
     DETAILED DESCRIPTION OF THE INVENTION  
      In the following description, numerous details are set forth for purpose of explanation. However, one of ordinary skill in the art will realize that the invention can be practiced without the use of these specific details. In other instances, well-known structures and devices are shown in block diagram form in order not to obscure the description of the invention with unnecessary detail.  
      Overview  
      Some embodiments of the invention provide a liquid cooling system for an LED backlit device, such as, for example, a flat panel display. These embodiments provide cooling to the LEDs of the display without significantly affecting the thin form factor of the device. The cooling system includes: one or more heat collectors; one or more radiator(s), fan(s), and/or fan radiators that have a small form factor; one or more mechanical pumps; tubing and interconnects to couple the elements of the cooling system together, and complete a closed cooling loop.  
      The heat collectors are typically made of an extruded multi port tubing which is in intimate contact with the device to collect heat from the device. For an LED backlit device, the LED arrays are traditionally a source of high heat. Accordingly, the heat collector is preferably disposed adjacent the LED arrays. The fluid, by the action of the pump(s), carries heated fluid from the heat collectors to the radiators, where the heat is rejected from the system. The cooled fluid is then (re)circulated by the pump(s) through the heat collectors to continuously draw more heat away from the hot LED array(s). Some embodiments further include a reservoir and/or a volume compensator to adjust the system for fluid loss over time.  
      Display  
      LED backlit displays are typically of the “direct” or the “edge” varieties. These categories generally refer to the location of the LEDs with respect to the view screen of the display. In typical LED backlit displays, the LEDs are organized into trichromatic (red, green, blue) arrays. With direct type LED displays, the LED arrays are generally uniformly distributed over the area of the display, such that the heat from the LED arrays is also generally distributed across the surface area of the display. For direct displays, macro or gross cooling solutions that blow cool air over the entire surface area of the distributed LED arrays is often sufficient.  
      In edge type displays, the LED arrays are grouped and concentrated at the top and bottom edges and/or the right and left edges (the rails) of the display. Optics direct the light from the rails through the remainder of the display. Due to the arrangement of the LED arrays, edge type displays often have cost savings and are advantageously very thin, in comparison to their direct type display counterparts. Some edge type displays, for example, are as thin as 0.50 inches deep, and use many fewer LEDs that are packaged in cost effective groups or arrays (rather than packaged discretely and more expensively as in direct displays). Costlier discretely packaged LEDs allow larger screen sizes for some direct displays, but further add to the thickness of a direct display&#39;s form factor. However, the main tradeoff for edge type displays is that the LED arrays must typically be very bright, and further, the heat from the LED arrays is concentrated within a smaller area of the display. Hence, the power consumed and also the heat generated by each concentrated rail of an edge type display is typically on the order of hundreds of Watts.  
       FIG. 1  illustrates an LED backlit display  100  in accordance with some embodiments of the invention. As shown in this figure, the display  100  has a number of LEDs. The LEDs of some embodiments are divided into a top LED array  102  along the top edge of the display  100 , and a bottom LED array  104  along the bottom edge of the display  100 . One of ordinary skill will recognize that the display  100 , the rails, and the LED arrays  102  and  104  of different embodiments has a number of shapes and sizes. For instance, some embodiments have side edge LED array rails, rather than top and bottom edge rails, while some embodiments have multiple arrays per edge. Regardless of the orientation of the rails, and/or the number of LED arrays, the LEDs of the display  100  typically generate a high amount of heat.  
      Cooling System and Fluid  
       FIG. 2  illustrates a cooling system  200  in accordance with some embodiments of the invention. As shown in this figure, the cooling system  200  has a pump  205  coupled to interconnect tubing  210  that runs from the pump  205  to a first micro tube heat collector  220 A.  
      From the first heat collector  220 A, the tubing  210  then directs fluid to a first radiator  215 A. In this embodiment, heat is collected by the first micro tube heat collector  220 A from a first rail of hot LED array(s), and rejected at the first radiator  215 A.  
      The tubing  210  then couples fluid from the first fan radiator  215 A to a second micro tube heat collector  220 B, which collects heat from a second rail of hot LEDs. The tubing  210  then couples fluid from the second heat collector  220 B to a second radiator  215 B, such that heated fluid is transported, by the action of the pump  205 , from the second heat collector  220 B to the second radiator  215 B, where the heat is rejected from the system.  
      The tubing  210  then returns the fluid from the second radiator  215 B to the pump  205 . Also shown in  FIG. 2 , the interconnect tubing  210  of some embodiments forms a closed path from the pump  205 , through the heat collectors  220  and radiators  215 , and back to the pump  205 . In these embodiments, the cooling system  200  typically contains a fluid that is sealed within the closed loop of the system  200 . Preferably, the fluid is suitable for cooling, such as, for example, water, a water based solution, a glycol type fluid, an alcohol type fluid, a dielectric solution, and/or another suitable cooling fluid.  
      Regardless of the type of fluid employed, the cooling system  200  illustrated in  FIG. 2  has certain advantages. For instance, the system  200  optimally cools the different edges of a display separately, but still implements a single closed cooling loop, which saves cost and maintains a small form factor for the system  200 .  
      One of ordinary skill will recognize that  FIG. 2  is representative, and thus the cooling system  200  of some embodiments includes more that one pump, various numbers of fan radiators, and alternate configurations of interconnect tubing.  
      Pump and Radiator  
      The pump  205  typically delivers fluid pressure of approximately two to seven pounds per square inch (PSI), while moving a volume of approximately one to two liters per minute. Some embodiments employ a pump that has a low cost and small dimensions to maintain the small factor of the entire system  200 . Quiet operation is an additional design consideration for the pump  205 , and hence, the pressure and flow rate of the pump  205  are constrained by these considerations. As described below, the pump  205  is further constrained by the implementation details of the other components, and particularly the heat collector  220 , of the system  200 . These components of the system  200  of some embodiments are provided by Cooligy, Inc. of Mountain View, Calif. For instance, the pump  205  of some embodiments includes mechanical, electro-kinetic, and/or electro-osmotic pumps. U.S. Pat. No. 6,881,039 B2 entitled “Micro-Fabricated Electrokinetic Pump” and issued Apr. 19, 2005, which is hereby incorporated by reference, describes certain types of pumps in greater detail.  
      The radiator(s) of some embodiments are actually comprised of two or more radiator elements disposed in certain configurations, such as, a parallel configuration for example, within a single housing. The multiple radiator elements of these embodiments are advantageously implemented with separate fins and fluid pathways for receiving one or more fluid inputs and/or outputs. As shown in  FIG. 2 , the radiators  215  are disposed side-by-side, and yet separately cool the LED arrays of each distinct edge of the display. This particular configuration allows the relocation of heat to a common locus for rejection from the system  200 . Regardless of their particular configuration, the radiators typically operate efficiently while still having a small form factor.  
      In some embodiments, the radiators are fan radiators that advantageously combine a radiator with a fan in a single unit. To reject sufficient heat for a typical LED backlit display, the fans should move in the range of 5 to 30 cubic feet per minute (cfm). Where a single fan is used, the air flow may cause undesirable noise. Where multiple fans are used such as shown in  FIGS. 4, 6 , and  8 , the air flow from each fan can be less and result in quieter operation of the system.  
      Typically, heated fluid flows along the fins of the radiator portion. Then, the heat is rejected from the fluid by the air flow generated around the fins by the fan. For instance, the radiators of some embodiments have a thickness of no more than about 15-50 millimeters. Radiators and heat rejection are described in further detail in United States Patent Application Serial No. [not yet assigned—COOL-01304] entitled “Cooling Systems Incorporating Microstructured Heat Exchangers,” filed Oct. 17, 2006, which is incorporated herein by reference.  
      As used herein, similar numerical identifiers represent similar features between figures. For instance,  FIG. 3  illustrates the cooling system  200  illustrated in  FIG. 2 , mounted to the backlit display  100  illustrated in  FIG. 1 , according to some embodiments  300  of the invention. As shown in this figure, the interconnect tubing  310  and the first heat collector  320 A of the cooling system  300  runs along the top LED array  302  and the second heat collector  320 B runs along the bottom LED array  304  of the display  100 . Preferably, the heat collectors  320  are in close, intimate contact with the LED arrays  302  and  304 , such that the heat generated by the LED arrays  302  and  304  is efficiently transferred to the fluid within the heat collectors  320  and the interconnect tubing  310 . As discussed in further detail below, some embodiments maximize thermal transfer between the LED arrays  302  and  304  and the heat collectors  320  by using a combination of mechanical coupling and thermal bonding. In these embodiments, the pump  305  moves the fluid along the interconnect tubing  310  through the fan radiators  315 , where heat transfers from the heated fluid to, and is dispersed by, the fan radiators  315 , before the cooled fluid returns back to the pump  305  for another pass through the closed cooling system  300 .  
       FIG. 4  illustrates a front view of a display with a cooling system according to certain embodiments of the present invention. As shown in  FIG. 4 , the cooling system  400  has a pump  405  and a set of fan radiators  415 , mounted on top of the display  100 . Also shown in  FIG. 4 , heat collectors  420 A and  420 B are mounted at the top and bottom of the display  100 , respectively. This configuration for the cooling system  400  has minimal impact upon the dimensions of the display  100 .  
       FIG. 5  illustrates a side view of the display  100  with the cooling system  400  of  FIG. 4 . As shown in  FIG. 5  several components of the cooling system  400 , such as the pump  505  and the fan radiators  515 , fit compactly above the display  100 , without significantly affecting its form factor. Also shown in  FIG. 5 , the heat collectors  520 A and  520 B have micro tubes that are disposed in close proximity to the edges of the display  100 , such that they do not add significantly to its form factor.  
       FIG. 6  illustrates a front view of a display and cooling system of alternate embodiments of the present invention. As shown in  FIG. 6 , the pump  605  and fan radiators  615  are placed on either side of the display  100 , while the heat collectors  620  are placed at the top and bottom of the display  100 . This alternative configuration also has a minimal impact on the dimensions of the display  100 .  FIG. 7  illustrates a side view of the display and the cooling system  600  of  FIG. 6 .  
      As shown in  FIG. 7 , several components of the cooling system  600 , such as the pump  705  and the fan radiators  715 , fit compactly on the side of the display  100 . Accordingly, the cooling system  600  of these embodiments does not significantly affect the form factor of the display  100 .  
       FIG. 8  illustrates another alternative configuration  800  for the cooling system of some embodiments. Specifically,  FIG. 8  illustrates that the pump of some embodiments, such as the pump  605  and  705  illustrated respectively in  FIGS. 6 and 7 , is formed by a coupling of multiple pump devices  805 , to optionally provide alternative fluid dynamics to the cooling system  800 .  
      Heat Collector  
      As mentioned above, some embodiments employ a micro tube heat collector in close contact with the LED array of a display to collect and disperse the heat from the LED array.  FIG. 9A  illustrates a micro tube heat collector  920 A in accordance with some embodiments. As shown in this figure, the heat collector  920  has an inlet  925  and an outlet  930 , for the flow of fluid. The heat collector  920 A relies upon micro scale heat conduction principles for its operation. An exemplary description of such small scale heat collection principles is described in relation to a heat “exchanger,” in U.S. patent application Ser. No. 10/882,132, entitled “Method and Apparatus for Efficient Vertical Fluid Delivery for Cooling a Heat Producing Device” filed Jun. 29, 2004, which is hereby incorporated by reference.  
      Owing to the length of the arrays of LEDs, the heat collector of some embodiments could suffer from temperature gradients within the heat collector and undesirable fluid pressure drop. For example, the temperature and pressure in the region most adjacent to the inlet of the heat collector is different than the temperature and pressure of the region that is near the outlet of the heat collector. This has particularly undesirable effects for image display applications because the quality of the displayed image depends in some measure on temperature homogeneity of the LED arrays. Moreover, the temperature at each LED affects its individual performance and useful life.  
      Some embodiments of the present invention mitigate the temperature difference, from the region adjacent to the inlet to the region near the outlet of the heat collector, by increasing the pressure and/or flow rate at the pump. At sufficiently high flow rates and/or pressures, such as 2.0 liters per minute and/or about 7.0 psi of pressure, for example, the fluid moves quickly enough through the heat collector  920 A such that a minimal temperature gradient occurs and any pressure drop does not affect the cooling efficacy of the system. However, as mentioned above, increasing the properties of the pump, such as flow rate and/or pressure, typically has undesirable tradeoffs such as an increase in the cost, noise, and/or the dimensions of the pump, or the other elements of the system, or constrains the type of pumping mechanism for the system. An alternative embodiment contemplates increasing a cross sectional volume of the micro tube to allow more fluid to flow at lower pressures.  
      Still other embodiments mitigate the temperature difference and pressure drop within the heat collector by using a parallel flow manifold.  FIG. 9B  illustrates one example of a heat collector  920 B that employs a manifold structure having parallel flow according to the invention. As shown in this figure, the heat collector  920 B of these embodiments has an inlet  925 , an outlet  930 , and a series of parallel flow fins  935 . Typically, cooled fluid enters through the inlet  925  and flows in parallel through each of flow fins  935 , in approximately simultaneous fashion. The flow fins  935  of some embodiments have dimensions in the range of 0.5 to 5.0 millimeters in width by 0.5 to 5.0 millimeters in height. The flow fins can be formed by extrusion of a thermally conductive material such as aluminum or an aluminum alloy. In this manner, fluid flow is more evenly distributed through the heat collector  920 B, such that temperature differences between portions of the heat collector  920 B are mitigated. Thus, for the exemplary heat collector  920 B illustrated in  FIG. 9B , the fluid flow is more evenly distributed and the temperature difference is reduced from the left side to the right side of the figure.  
       FIG. 10  illustrates the heat collector  920 B illustrated in  FIG. 9B  incorporated into the cooling system of one embodiment. As shown in  FIG. 10 , the heat collector  1020  is coupled to the LED array of a backlit display. Preferably, the heat collector  1020  is intimately coupled to the LED array to improve the thermal efficiency of the heat transfer from the LED array to the fluid. Most preferably a TIM or thermal grease is used. A fluid inlet  1025  and a fluid outlet  1030  of the heat collector  1020  are coupled to the interconnect tubing  1010  of a cooling system  1000  to form a closed loop. Also shown in  FIG. 10 , the cooling system  1000  includes a pump  1005  for providing fluid pressure and flow through the loop, and a fan radiator  1015  for dispersion of heat from the fluid.  
      As mentioned above, some embodiments optimize the heat transfer from the LEDs of a display to the heat collector of these embodiments.  FIG. 11  illustrates the means by which some embodiments maximize coupling, bonding, and thermal transfer. As shown in this figure, a typical LED display includes a set of LEDs  1195  positioned on an substrate layer  1190 , that is in turn disposed on a metal layer  1185 . The substrate layer  1190  is typically comprised of an electrical insulator such as a ceramic or FR4 material, for example, while the metal layer  1185  is typically comprised of an aluminum or copper type material. Hence, the metal layer  1185  is capable of heat conductance and/or spreading. Accordingly, some embodiments include a thermal interface material (TIM) layer  1180  between the heat collector  1120  and the metal layer  1185  of the display. The TIM layer  1180  typically comprises an inorganic and/or an organic substance that thermally bonds and transfers heat from the metal layer  1185  of the display&#39;s LED arrays to the heat collector  1120 . Examples of inorganic thermal interface materials include metallic coat and Indium, while examples of such organic substances include thermal grease, thermal pads, and/or phase change materials. The TIM layer  1180 , thus, often has thermally adhesive properties.  
      Alternatively, the TIM layer  1180  of some embodiments bonds the heat collector  1120  directly to the substrate layer  1190 , without the need for the metal layer  1185 . Also shown in  FIG. 11 , some embodiments include a physical coupling means  1175  to mechanically affix the heat collector  1120  to the TIM layer  1180 , the metal layer  1185 , and/or the substrate layer  1190 . The coupling means  1175  includes a variety of mechanical implementations such as, for example, screws, brackets and/or clamping means. By providing a mechanical force to affix the heat collector  1120 , to the layers of the backlit device, the coupling means  1175  adds to the structural integrity of the system, and further promotes heat transfer from the device to the heat collector  1120 .  
      As mentioned above, the heat collector  1120  is preferably coupled and/or bonded to the heat source in such a way as to optimize thermal transfer. Some embodiments also orient the flow of fluid through the heat collector  1120  in order to further maximize the conduction of heat via the travel of the fluid. For instance,  FIG. 12  illustrates a top view of the fluid flow across a set of hot LEDs  1295  in an array. As shown in this figure, cool fluid is directed through a manifold having a set of vanes that are approximately parallel. Also mentioned above, the maximum pitch between the vanes of some embodiments is in the range of 1.0 to 5.5 millimeters. The cool fluid travels in close proximity to the trichromatic (RGB) LEDs  1295  of the arrays, such that the fluid conducts heat, and carries the heat away from the LEDs  1295 .  
      Method  
       FIG. 13  is a process flow  1300  illustrating the steps of some of these embodiments. As shown in  FIG. 13 , the process  1300  begins at the step  1305 , where the heat from the backlit device is collected in a heat collector. As described above, the heat collector of some embodiments includes a micro tube that has a fluid. Preferably, the micro tube is disposed in intimate contact with the backlit device. In some embodiments, the heat collector comprises two or more parallel flow fins. The flow fins direct the fluid flow in parallel such that the temperature of the heat collector is substantially distributed. Once the heat is collected from the device at the step  1305 , the process  1300  transitions to the step  1310 , where the heat is transferred to a fluid. Then, the process  1300  transitions to the step  1315 .  
      At the step  1315 , the heat is transferred to a radiator by using the fluid, and the process  1300  transitions to the step  1320 . At the step  1320 , the heat is dispersed or rejected from the radiator and then, at the step  1325 , the cooled fluid is circulated and/or re-circulated through the system. After the step  1325 , the process  1300  concludes. The (re)circulation of the fluid is typically performed by using a pump. Optionally, excess fluid is stored in a reservoir, which also preferably compensates for any loss of fluid over time.  
      While the invention has been described with reference to numerous specific details, one of ordinary skill in the art will recognize that the invention can be embodied in other specific forms without departing from the spirit of the invention. Thus, one of ordinary skill in the art will understand that the invention is not to be limited by the foregoing illustrative details, but rather is to be defined by the appended claims.