Abstract:
One embodiment of a system for cooling a heat-generating device includes a base adapted to be coupled to the heat-generating device, a housing coupled to the base, a liquid channel formed between the base and the housing, where a heat transfer liquid may be circulated through the liquid channel to remove heat generated by the heat-generated device, and a heat pipe disposed within the liquid channel, where the heat pipe increases the heat transfer surface area to which the heat transfer liquid is exposed. Among other things, the heat pipe advantageously increases the heat transfer surface area to which the heat transfer liquid is exposed and efficiently spreads the heat generated by the heat-generating device over that heat transfer surface area. The result is enhanced heat transfer through the liquid channel relative to prior art cooling systems.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application is a continuation of U.S. patent application Ser. No. 11/294,825 entitled, “EMBEDDED HEAT PIPE IN A HYBRID COOLING SYSTEM,” filed Dec. 5, 2005 and having Attorney Docket No. NVDA/P002015. The subject matter of this related application is hereby incorporated herein by reference. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    1. Field of the Invention 
         [0003]    This invention relates generally to systems used to cool computer hardware and more particularly to an embedded heat pipe in a hybrid cooling system. 
         [0004]    2. Description of the Background Art 
         [0005]      FIG. 1  is an isometric view illustrating a prior art cooling system  100  used to cool heat-generating electronic devices in a computer system, such as a graphics processing unit (GPU). As shown, cooling system  100  characteristically includes a blower/fan  106 , fins  109  and a bottom plate  111 . Typically, cooling system  100  is thermally coupled to the GPU, for example using thermal adhesive or grease having thermal properties that facilitate transferring heat generated by the GPU to the bottom plate  111 . Cooling system  100  may also include a heat sink lid (not shown), which, among other things, prevents particles and other contaminants from entering blower/fan  106  and air blown from blower/fan  106  from escaping cooling system  100 . The heat sink lid, together with the fins  109  and the bottom plate  111 , define a plurality of air channels  108 . 
         [0006]    Blower/fan  106  is configured to force air through air channels  108  over bottom plate  111  such that the heat generated by the GPU transfers to the air. The heated air then exits cooling system  100 , as depicted by flow lines  114 , thereby dissipating the heat generated by the GPU into the external environment. This process cools the GPU, preventing the device from overheating during operation. Persons skilled in the art will understand that air channels  108  typically are configured to direct air blown from blower/fan  106  over bottom plate  111  and into the external environment in a manner that most efficiently removes heat from the GPU. 
         [0007]      FIG. 2  is a schematic diagram illustrating a computer system  200 , such as a desktop, laptop, server, mainframe, set-top box, and the like within which a conventional cooling system  100  for cooling the GPU  216  is incorporated. As shown, computing device  200  includes a housing  201 , within which a motherboard  204  resides. Mounted on motherboard  204  are a central processing unit (CPU)  206 , a processor cooler  208  for cooling CPU  206 , a system fan  210  for removing heat from computing device  200  and one or more peripheral component interface (PCI) cards  212 , each interfaced with a slot located in the back part of housing  201 . Motherboard  204  further incorporates a graphics card  202  that enables computing device  200  to rapidly process graphics related data for graphics intensive applications such as gaming applications. Graphics card  202  comprises a printed circuit board (PCB) upon which a plurality of circuit components (not shown), such as memory chips and the like, are mounted. In addition, graphics card  200  includes GPU  216 , mounted to one face of graphics card  202 , for processing graphics related data. 
         [0008]    Because the computational requirements of GPU  216  are typically quite substantial, GPU  216  tends to generate a large amount of heat during operation. If the generated heat is not properly dissipated, the performance of GPU  216  degrades. For this reason, cooling system  100 , which is configured to remove heat from GPU  216 , is coupled to GPU  216 . 
         [0009]    One drawback of these conventional blower/fan cooling systems is that, as processors become more powerful and generate more heat, the fan has to be operated at very high speeds to generate the airflow through the air channels and over the heat sink necessary to cool the processor. High speed operation tends to produce a substantial amount of unwanted acoustic noise, which is annoying to users of a computer. Also, in some instances, these types of conventional cooling systems may not even be able to meet the heat dissipation requirements of certain high-performance processors. Further compounding these issues is the fact that, while processors are becoming more powerful, the available space for cooling systems within computing devices is generally not increasing. Thus, substantial improvements in the efficiency of cooling systems are required to maintain pace with the evolution of processors. 
         [0010]    Liquid cooling systems are beginning to emerge as a viable alternative to conventional blower/fan cooling systems. A liquid cooling system dissipates heat at a much greater rate than a comparable air cooling system. However, typical liquid cooling systems are driven by large pumps, which are prone to frequent failure and tend to consume a great deal of power. Moreover, such systems tend to use large quantities of liquid, circulating at a high flow rate, and therefore must be frequently replenished or replaced. 
         [0011]    To overcome some of these challenges, a hybrid cooling system is disclosed in U.S. patent application Ser. No. 10/822,958, filed on Apr. 12, 2004 and titled, “System for Efficiently Cooling a Processor,” which is herein incorporated by reference.  FIG. 3  is an isometric view of such a hybrid cooling system  300 . Similar to the system  100 , the hybrid cooling system  300  may be adapted for use in any type of appropriate computing device. As shown, hybrid cooling system  300  may include, without limitation, a fansink  302  and a hybrid cooling module  304 . As described in further detail below, fansink  302  and hybrid cooling module  304  may operate independently or in combination to dissipate heat from a processor or other heat-generating device within the computer system. 
         [0012]    Fansink  302  is configured in a manner similar to cooling system  100  of  FIG. 1  and includes, without limitation, a fan  308 , walls  306  and a bottom plate  318 . Cooling system  300  also includes a heat sink lid  320 , which, among other things, prevents particles and other contaminants from entering fan  308  and air blown from fan  308  from escaping system  300 . Heat sink lid  320 , together with walls  306  and bottom plate  318  of fansink  302 , define a plurality of air channels  322 . 
         [0013]    Hybrid cooling module  304  is adapted to be integrated with fansink  302 . Hybrid cooling module  304  is thermally coupled to a portion of bottom plate  318  and includes, without limitation, a liquid channel  312 , an inlet  314 , an outlet  316  and a plurality of air channels  310 . Hybrid cooling module  304  is coupled to a pump, which is adapted for circulating a heat transfer liquid (e.g., water or any other liquid suitable for transferring heat) through a closed loop that includes liquid channel  312 . As shown in  FIG. 3 , the pump circulates liquid from hybrid cooling module  304  through a heat exchanger prior to supplying the liquid back to hybrid cooling module  304 . Inlet  314  and outlet  316  are configured for respectively supplying and removing the heat transfer liquid to liquid channel  312 . Air channels  310  are adapted for coupling to air channels  322  and for transporting forced air from fan  308  to the local environment. Air channels  310  are positioned over and around liquid channel  312  such that liquid channel  312  is substantially enclosed within air channels  310 . 
         [0014]    When cooling a processor or other heat-generating device, fan  308  forces air through air channels  322  of the fansink  302  and air channels  310  of the hybrid cooling module  304  such that the heat generated by the processor transfers to the air as the air passes over bottom plate  318 . The heated air then exits system  300 , as depicted by flow lines  324 , thereby dissipating the heat generated by the processor into the local environment. In addition, as previously described, the pump circulates the heat transfer liquid through liquid channel  312  of hybrid cooling module  304 , and heat generated by the processor transfers to the circulating heat transfer liquid as well as to air in air channels  310 . Liquid channel  312  is adapted for transporting heat transfer liquid through a downstream heat exchanger, which dissipates heat from the heat transfer liquid into the local environment. 
         [0015]    Fansink  302  and hybrid cooling module  304  may be implemented independently or in combination to dissipate heat from a processor, in order to dissipate heat from the processor in the most efficient manner. For example, fansink  302  may be implemented to dissipate a majority of the generated heat, hybrid liquid cooling module  304  may be implemented to dissipate a smaller quantity of heat, and the proportions of heat dissipated by fansink  302  and hybrid cooling module  304  may be dynamically adjusted. Alternatively, one of fansink  302  and hybrid cooling module  304  may be implemented as a primary means for heat dissipation, while the other mechanism is implemented on an as-needed basis to dissipate excess heat. 
         [0016]    One drawback to using the hybrid cooling system  300  is that, when the pump is inoperative and no heat transfer liquid is circulated through the liquid channel  312 , a substantial amount of cooling capacity is lost because air cooling provided by the fansink  302  is limited to the air channels  310 ,  322  that are not obstructed by the liquid channel  312 . In other hybrid cooling system configurations, the fansink and the liquid cooling module may be “stacked” such that the fansink is disposed on top of the hybrid cooling module. In such configurations, when the pump is inoperative and no heat transfer liquid is circulated through the liquid channel, the standing liquid in the liquid channel acts like an insulator and retards the heat transfer between the processor or other heat-generating device and the walls of the fansink air channels, substantially decreasing the cooling efficiency of the hybrid cooling system. In addition, when such a “stacked” hybrid cooling system is installed in a peripheral component interconnect (PCI) slot, height restrictions become a concern. Consequently, the height of the fansink air channels may be reduced to allow the system to fit within the allocated space. Reducing the height of the air channels reduces the effective heat transfer area of the air channels, further decreasing the cooling efficiency of the hybrid cooling system. 
         [0017]    As the foregoing illustrates, what is needed in the art is a way to increase the efficiency of hybrid cooling systems, especially when the liquid cooling portion of the system is not being used. 
       SUMMARY OF THE INVENTION 
       [0018]    One embodiment of a system for cooling a heat-generating device includes a base adapted to be coupled to the heat-generating device; a housing coupled to the base; a liquid channel formed between the base and the housing, wherein a heat transfer liquid may be circulated through the liquid channel to remove heat generated by the heat-generated device; and a heat pipe disposed within the liquid channel, wherein the heat pipe increases the heat transfer surface area to which the heat transfer liquid is exposed. Among other things, the heat pipe advantageously increases the heat transfer surface area to which the heat transfer liquid is exposed and efficiently spreads the heat generated by the heat-generating device over that heat transfer surface area. The result is enhanced heat transfer through the liquid channel relative to prior art cooling systems. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0019]      FIG. 1  is an isometric view illustrating a prior art system used to cool a processor. 
           [0020]      FIG. 2  is schematic diagram illustrating a computing device adapted for use with a system for cooling a processor, according to one embodiment of the present invention. 
           [0021]      FIG. 3  is an isometric view illustrating a prior art hybrid cooling system for cooling a heat-generating electronic device. 
           [0022]      FIGS. 4A-C  are various views/schematics of a hybrid cooling system with an embedded heat pipe, according to one embodiment of the present invention. 
           [0023]      FIG. 5  is an alternative embodiment of the hybrid cooling system of  FIGS. 4A-C , according to one embodiment of the present invention. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0024]      FIG. 4A  is an exploded isometric view of a hybrid cooling system  400 , according to one embodiment of the present invention. Hybrid cooling system  400  is configured to be thermally and structurally coupled to a printed circuit board (PCB), such as the graphics card  402  or the graphics card  202  of  FIG. 2 , and implemented with a computer system, such as the computer system  200  of  FIG. 2 . Mounted on a top side, the graphics card  402  includes GPU  416  (more clearly depicted in  FIG. 4C ) and other components, such as memory units (not shown) and a power supply (not shown). Preferably, the graphics card  402  is configured to connect to a computer system via a standard peripheral component interconnect PCI slot. Further, the hybrid cooling system  400  is configured so that when it is mounted to the graphics card  402 , the cooling system  400  and the graphics card  402  will fit substantially within one standard PCI slot of a computer system. In alternate embodiments, the hybrid cooling system  400  may be configured to be coupled to any type of PCB for use in cooling a heat-generating electronic device mounted on that circuit board, such as an accelerated graphics port (AGP) card. 
         [0025]    The hybrid cooling system  400  includes, without limitation, a base  405 , a lid  410 , a fan  415 , a hybrid cooling module  420 , a heat pipe  425 , a heat sink  430 , a heat exchanger (as shown in  FIG. 3 ) and a pump (as shown in  FIG. 3 ). The base  405 , the hybrid cooling module  420 , the heat pipe  425 , and the heat sink  430  are made from a thermally conductive material, such as aluminum or copper. The lid  410  and the fan  415  may be made from plastic or any other appropriate material. 
         [0026]    A bottom side of the base  405  is thermally coupled to the GPU  416  so as to conduct heat generated by the GPU  416  during operation. The base  405  may also be thermally coupled to other heat generating electronic devices on the graphics card  402 , such as memory units and the power supply, to conduct heat generated by those electronic devices as well. The heat sink  430 , also shown in  FIG. 4B , is coupled to a top side of the base  405  over at least some of the memory units and at least a portion of the GPU  416  to enable heat generated by these devices and transferred through the base  405  to be transferred to air forced through air channels within the heat sink  430  by the fan  415 . A second heat pipe (not shown) may be disposed beneath the heat sink  430  to improve heat distribution throughout the heat sink  430 . As described in greater detail herein, the fan  415  forces air through the air channels  420   e  of the hybrid cooling module  420  to enable heat generated by the GPU  416  to be removed and transferred to the local environment. 
         [0027]      FIG. 4B  is a top view of the hybrid cooling system  400  without the lid  410  and having hidden lines to show the embedded heat pipe  425  and the GPU  416 .  FIG. 4C  is a sectional schematic of the hybrid cooling system  400 . As shown, the hybrid cooling module  420  is coupled to the top side of the base  405  and is disposed laterally on the base  405  above the GPU  416 . The hybrid cooling module  420  is mounted on to the base  405  so that a liquid channel  420   d  is formed between the base  405  and the hybrid cooling module  420 . A seal (not shown) is disposed therebetween to prevent leakage of a heat transfer liquid  440  (e.g., water) within the liquid channel  420   d . Alternatively, the hybrid cooling module  420  may have its own base, with the liquid channel formed between that base and a top  420   f  of the hybrid cooling module  420 , and be sealed prior to installation on the base  405 . The hybrid cooling module  420  includes a liquid inlet  420   a  and a liquid outlet  420   b . The inlet  420   a  is coupled to an outlet of the pump via tubing (not shown), and the outlet  420   b  is coupled to an inlet of the heat exchanger via tubing (not shown). The pump and the heat exchanger may be located distally from the graphics card  402  in the computer chassis  201  or outside of the computer chassis  201 . A plurality of fins  420   c  are formed in the top  420   f  of the hybrid cooling module  420 . The fins  420   c  and the top  420   f  form air channels  420   e  through the hybrid cooling module  420 , which may be covered by the lid  410 . In one embodiment, the hybrid cooling module  420  is an integrated part, but in alternative embodiments, the components of the hybrid cooling module  420 , such as the fins  420   c  and the top  420   f , may be separate elements coupled together in some technically feasible fashion. 
         [0028]    The heat pipe  425  is disposed in the liquid channel  420   d . Preferably, the heat pipe  425  is press fit into the liquid channel  420   d  to ensure good contact with the base  405  and the top  420   f  of the hybrid cooling module  420 . The heat pipe  425  may even be press fit to such an extent to deform the heat pipe  425  from an originally circular cross-section to a substantially oval-shape cross-section, as shown in  FIG. 4C , to better ensure coupling between the base  405  and the top  420   f  of the hybrid cooling module  420 . The heat pipe  425  may also be thermally coupled to the base  405  and the hybrid cooling module  420  with thermal adhesive or solder. The heat pipe  425  is formed in a substantially “U” shape so that a portion of the heat pipe  425  may substantially extend the length of each side of the liquid channel  420   d . Alternatively, the heat pipe  425  may be substantially “S” shaped along the longitudinal axis to increase the contact area with the heat transfer liquid  440 . As most clearly shown in  FIG. 4B , the hybrid cooling module  420  is preferably disposed relative to the GPU  416  so that the curved portion of the heat pipe  425  resides above the GPU  416 . The outside surface of the heat pipe  425  may be textured to increase the heat transfer rate from the heat pipe  425  to the heat transfer liquid  440 . The workings of the heat pipe  425  are conventional and well-known by those skilled in the art. 
         [0029]    In one embodiment, the heat pipe  425  is a passive heat transfer device, employing two-phase flow to achieve an extremely high thermal conductivity. The heat pipe  425  includes a vapor chamber  424  and a wick structure  425   w  which draws liquid  425   l  (e.g. water) to a heat source  499  (created by the heat generated by the GPU  416  and transferred through the base  405 ) by the use of capillary forces. The liquid  425   l  evaporates in the wick  425   w  when heated and the resulting vapor  425   v  escapes to the vapor chamber  424  of the heat pipe  425  where the vapor  425   v  is then forced by a resulting pressure gradient to cooler regions of the heat pipe  425  for condensation. The condensed liquid is then returned to the heat source  499  via the capillary action. Further detail on the design and implementation of heat pipes in electronics cooling applications may be found in an article by Scott D. Garner, P. E., entitled “Heat Pipes for Electronics Cooling Applications,” available at http://www.electronics-cooling.com/resources/EC_Articles/Sep96 — 02.htm, which is incorporated herein by reference. 
         [0030]    Operation of the hybrid cooling system  400  will now be described. Heat flow from the GPU  416  and through the hybrid cooling module  420  is denoted by heat paths  435   a  and  435   b . Heat is transferred from the GPU  416 , through the base  405 , and to the heat pipe  425 . The heat vaporizes the liquid  425   l  in the wick  425   w . The vapor  425   v  is forced away from the GPU  416  towards the cooler regions of the heat pipe  425 , which are shown in  FIG. 4B . As the vapor  425   v  travels through the heat pipe  425 , heat is transferred through the sides of the heat pipe  425  to the heat transfer liquid  440  circulating within the liquid channel  420   d  (when the pump is operated), as depicted by heat path  435   b . The heat transferred to the heat transfer liquid  440  is transported to the heat exchanger where it is dissipated into the local environment. Heat is also transferred through the top of the heat pipe  425  to the top  420   f  of the hybrid cooling module  420 , as depicted by heat path  435   a . The heat continues through the top  420   f  the fins  420   c , where the heat is transferred to the air being forced through the air channels  420   e  by the fan  415 . The heat is subsequently dissipated out into the local environment as well. When the pump is inoperative, and no heat transfer liquid  440  circulates through the liquid channel  420   d , heat only travels along heat path  435   a , as described above. 
         [0031]    Disposing the heat pipe  425  in the liquid channel  420   d  improves the heat transfer capability of the cooling system  400  relative to the cooling system  300  when the pump is both inactive and active. When the pump is inactive, the heat pipe  425  remains operational, since it is a passive device, and thus provides a direct heat path  435   a  between the GPU  416  and the fins  420   c . As such, the heat pipe  425  substantially improves heat transfer through the liquid channel  420   d  to the fins  420   c  versus the prior art hybrid cooling system  300  in which, as previously described, the non-circulating heat transfer liquid acts as an insulator and impedes the heat transfer between the GPU and the fansink portion of the system. When the pump is active, the sides of the heat pipe  425  increase the heat transfer surface area to which the circulating liquid  440  is exposed, thereby increasing the rate of heat transfer to the heat transfer liquid via heat path  435   b  relative to prior art systems. 
         [0032]    In an alternative embodiment, the heat pipe  425  may be added into a liquid channel of a liquid-only cooling system  500 , thereby realizing the benefit of increasing the heat transfer area to the circulating heat transfer liquid  440 , as described above. For example, as shown in  FIG. 5 , a housing  520  may be used instead of the hybrid cooling module  420 , with a liquid channel  420   d  defined between the top portion of the housing  520  and the base  405 . The heat pipe  425  is embedded in the liquid channel  420   d , as previously described herein. Again, in operation, heat is transferred from the GPU  416 , through the base  405 , and to the heat pipe  425 . The heat vaporizes the liquid  425   l  in the wick  425   w . The vapor  425   v  is forced away from the GPU  416  towards the cooler regions of the heat pipe  425 . As the vapor  425   v  travels through the heat pipe  425 , heat is transferred through the sides of the heat pipe  425  to the heat transfer liquid  440  circulating within the liquid channel  420   d , as depicted by heat path  435   b.    
         [0033]    In another alternative embodiment, the hybrid cooling system may be configured to be coupled to heat-generating electronic devices other than a GPU, such as a central processing unit (CPU), an application-specific integrated circuit (ASIC), another type of special purpose processing unit, memory elements and the like. 
         [0034]    Although the invention has been described above with reference to specific embodiments, persons skilled in the art will understand that various modifications and changes may be made thereto without departing from the broader spirit and scope of the invention as set forth in the appended claims. The foregoing description and drawings are, accordingly, to be regarded in an illustrative rather than a restrictive sense.