Abstract:
An integrated liquid cooling device for electronic components addresses the need for efficient cooling created by ever increasing power densities of electronic components. The integrated liquid cooling device has a housing enclosing the electronic component, cooling liquid contained in the housing, a motor immersed in the cooling liquid and mounted to the housing, an impeller driven by the motor, and cooling surfaces on the exterior of the housing. The motor driven impeller creates a turbulent flow in the cooling liquid and a high velocity liquid flow over the electronic component, which rapidly transfers heat from the electronic component and distributes it throughout the interior of the housing. The cooling surfaces on the exterior of the housing dissipate this heat, either by free or forced convection, into the surrounding environment. Alternately, the integrated liquid cooling device may distribute this heat energy over an equipment case by circulating cooling liquid through a baffled enclosure that provides high velocity cooling liquid flow near the heat generating electronic component. Additional cooling capacity can be gained with the described integrated liquid cooling device by selecting a cooling liquid whose boiling point is near the operating temperature of the electronic component.

Description:
RELATED APPLICATION INFORMATION  
       [0001]     This application claims priority under 35 U.S.C. § 120 as a continuation-in-part of U.S. patent application Ser. No. 10/888,101, filed Jul. 9, 2004. 
     
    
     BACKGROUND OF THE INVENTION  
       [0002]     1. Field of the Invention  
         [0003]     The present invention relates to an apparatus and method for cooling electronic components, and more particularly, an integrated cooling device for use with electronic components.  
         [0004]     2. Description of Related Art  
         [0005]     Machines and devices that change energy from one form (chemical, mechanical, thermal, or electrical) to another are rarely 100% efficient. Likewise, devices that change or modulate electrical energy in one form to electrical energy in another form are rarely 100% efficient. For example, a transistor, integrated circuit or a microprocessor may change direct current into alternating current or into pulses on many outputs that signify numerical data. Where direct current is changed into alternating current, only part of the direct current input power is changed into alternating current. The remainder of the input power appears as heat. Where direct current is changed into pulses signifying numerical data, all of the direct current input power becomes heat, and the output is information, not energy.  
         [0006]     In many cases, the waste heat generated in a small machine or device is quite large, and the machine can neither radiate nor dissipate the heat to its surroundings, at a reasonable temperature, without providing an additional cooling mechanism. Semiconductors used in computers or as radio-frequency amplifiers are usually not very powerful, nor large, and do not generate a large amount of heat, but they may operate at a very high power density. The trend toward miniaturization and higher frequency operation of electronic devices, achieved through increased packing densities of gates in semiconductor devices, has resulted in ever increasing power densities. Predictions of powers of as much as 200 Watts over an area of two square centimeters (100 Watts per square centimeter) are being made. Such power densities are much too high for direct cooling with air.  
         [0007]     There have been various attempts to address cooling issues associated with the increasing power densities of modern electronics. However, each of these attempted solutions has undesirable characteristics. A fairly basic approach to cooling electronic components is by free or forced convection of air over a “heat sink.” Typically a heat sink will include an array of cooling fins that collectively have a larger surface area than the electronic component to be cooled. These fins are attached to a thermally conductive base (or heat spreader). Heat from the electronic component is conducted through the conductive base to the roots of the cooling fins and along the fins toward their surfaces. The heat is then transferred from the cooling fins to the surrounding atmosphere via either free or forced convection.  
         [0008]     The rate of heat transfer that can be accomplished by a heat sink is quite limited, however. The amount of power (the rate of heat flow) that can be transferred from the fins to the ambient air is a function of the average temperature difference between the fins and the air, the air velocity, and is proportional to the total fin area. For heat to spread radially outward to a large region, to which fins are attached, there must be a long heat path and a large temperature difference between the heated surface and the cool surface. (That is, in the case of a cooled electrical component, between the component itself at the contact point with the heat sink and the surfaces of the cooling fins). The total amount of heat transfer is proportional to the temperature difference between these two surfaces. At some power level, the device will become too hot to operate properly or perhaps even survive. This difficulty in effectively cooling electronic components is aggravated by the increased heat created by the increasing power densities encountered in modern electronic components.  
         [0009]     In prior art, a solution to the problem of the large temperature difference in the “spreader” portion of a heat sink and the “fin” portion when used to cool high power-density components has been the replacement of the “heat spreader” with a “heat pipe.” In a “heat pipe,” the heat generated by a device evaporates a liquid. The vapor rises (perhaps through the central one of two coaxial pipes) and condenses on a cool area (perhaps the outer one of the coaxial pipes which passes through attached air-cooled fins). The liquid then returns to the heat-generating device so the process can repeat. Here, the heat transferred depends upon the mass flow rate of the liquid multiplied by the heat of vaporization of the liquid. Heat pipes with large ratios of cooled fin area to heated area can be made without using a large difference in temperature between the evaporation temperature (the device temperature) and the condensation (fin) temperature. Limitations on heat pipes include the onset of film boiling on the heated surface (which insulates the surface, often followed by rapid temperature rise and failure), pressure build-up which inhibits boiling within the heat pipe, and the requirement that the heat pipe be properly oriented with respect to gravity.  
         [0010]     In order to make still larger reductions in the temperature of a cooled device with respect to ambient temperature, it is necessary to use forced convection of heat in solid or liquid material between the device and its surroundings. Some approaches to cooling heat-generating devices make use of moving solid structures to transport heat from one location to another. Such an approach can be more easily visualized as the heat transfer provided by a reciprocating piston in an internal combustion engine. A piston in an internal combustion engine is heated by the combustion of fuel during the combustion stroke of an Otto cycle engine and the piston then distributes this heat along the stroke depth of the cylinder walls as it travels.  
         [0011]     The concept of using a moving part to transfer heat has been applied to the cooling of electronic components. Some devices using this transfer mechanism have used thermally conductive reciprocating sheets or rotating disks in thermal contact with a heat-generating component of a relatively smaller surface area. In these devices, heat is transferred from the heat-generating component to the moving part. Typically the movement of the part distributes the heat relatively evenly over the larger surface area of the moving part. Heat is then dissipated from the moving part through convective cooling (typically, with cooling fins).  
         [0012]     Nevertheless, the use of a moving part to remove heat from an electronic component, as provided by the prior art, has several shortcomings. First, in some of the prior-art moving-part cooling devices, the moving part is in direct contact with the heat-generating device. This direct contact leads to friction heating and wear of both the moving part and the heat-generating component. If, as in other prior-art devices, a fluid is used to provide a lubricating interface between the heat-generating component and the moving part, heat must be transferred between the heat-generating component, the relatively poorly conductive lubricating fluid, and into the moving part. In some prior art moving-part-based cooling devices, the heat would then be transferred through another poorly-conductive-fluid interface to cooling fins. Therefore, the heat transfer path provided by a moving-part-based cooling device is impeded as it passes through several interfaces with low thermal conductivity.  
         [0013]     There is another reason that the use of a moving solid part to transfer heat is inferior as compared to simple liquid cooling. The solid moving part used to transfer heat is often a thermally conductive material such as a metal. The thermal capacity of a metal (i.e., the specific heat multiplied by density and temperature rise, or the amount of energy that may be carried by a given volume of the material) is often less than that of a liquid such as water or many other liquids with similar properties. Therefore, the use of a liquid as a heat transfer medium is preferable to a solid moving part because additional heat may be carried by an equal volume of liquid. While still liquids and liquids in laminar flow tend to have lower thermal conductivities (which limit the liquid&#39;s ability to rapidly distribute heat energy) than thermally conductive materials such as metals, a liquid in the turbulent flow regime will have similar heat distributing abilities.  
         [0014]     An additional advantage of liquid cooling over solid moving-part-based cooling is that the cooling effect provided by a liquid cooling medium can be significantly enhanced through the use of nucleate boiling. Nucleate boiling occurs in liquid cooling devices with high-velocity cooling streams when the temperature of the heat-generating component is slightly higher than the boiling temperature of the cooling liquid. In nucleate boiling, very small bubbles of cooling liquid vapor are swept off of the interface between the heat generating component and the cooling liquid by the flowing liquid. These bubbles then condense within the cooling liquid stream. Through the nucleate boiling mechanism, the heat transfer coefficient (a measure of the cooling ability of the system) may be increased by a factor of as much as ten over normal liquid cooling. The enhanced cooling ability provided by nucleate boiling could not occur in a cooling system relying on a solid moving part for heat transfer. High-velocity fluid flow inhibits the onset of film boiling and consequent “burn out” that can occur in low-velocity boiling-fluid systems (as mentioned above in connection with heat pipes). High-velocity water-cooling systems utilizing nucleate boiling have reliably transferred as much as several kilowatts of power per square centimeter of cooled area.  
         [0015]     There have been many prior art approaches to cooling heat-generating components using liquid cooling. In many common applications of liquid cooling, the heat generated by a device is transferred to a high-velocity liquid (typically flowing through the tiny channels of a heat sink in contact with the device). The liquid is conveyed (typically in another hose, pipe, or other conduit) to a small auto-radiator-like heat exchanger, and finally, the liquid is returned (typically via hose, pipe, or other conduit) to the pump and the heat-generating device so the process can repeat. In some cases, by allowing direct contact between the cooling fluid and the semiconductor package, the transfer of heat is enhanced. This results from the elimination of temperature differences between the heat sink and fluid and in the heat-sink compound used in the joint between the component and the heat sink. Here, the heat transferred again depends upon the mass flow rate of the liquid multiplied, in this case, by the specific heat of the liquid and the temperature change. For maximum heat transfer, the recirculating cooling liquid must be pumped, at high velocity, through small channels in the heat exchanger, and connecting pipes. Especially when operating at the high mass-flow rates required for maximum heat transfer, such systems may suffer from large pressure drops, or “head losses.” 
         [0016]     Another approach to using liquid cooling to cool heat generating components attempts to address the shortcomings of cooling systems using recirculating liquid cooling. In this alternate approach, the cooling components are more integrated. A cooling device housing is thermally connected to a heat generating component, and a cooling liquid is contained in the housing and is circulated inside the housing by an impeller. Completely integrated cooling devices are more easily usable in high power density electronic devices such as computers. The application is simpler because the cooling device is essentially a single component that thermally contacts the heat generating electronic components (rather than using separate liquid conduits, pumps and heat exchangers that would be required for a recirculating liquid cooling system). Additionally, by integrating the cooling components, the pump energy required can be used directly to transport the cooling liquid at high velocity across the heated surface rather than to overcome the head losses that are encountered in a less integrated liquid cooling system.  
         [0017]     But, prior art attempts to integrate cooling device components have been problematic. Since prior-art integrated cooling devices have thermally or electrically conductive impellers driven by externally-created electromagnetic fields, heat transfer must occur between the impeller and the cooling fluid with its relatively poor thermal conductivity. A more direct heat transfer mechanism could provide enhanced cooling. Further, the thermally and electrically conductive impeller of the prior art requires compromises in the housing of the cooling device. The prior art integrated cooling devices have a composite housing, including layers of metal and plastic, to allow external electromagnetic fields to pass through the surface of the housing and motivate the impeller. The composite structure may not be able to withstand the amounts of heat generated by ever increasing power densities of modern electronic components. This indirectly motivated impeller of the prior art may also revolve at a lower speed than the speed of the motivating electromagnetic field. Therefore, another drawback of the prior art is poor liquid circulation and possible difficulties in achieving a turbulent flow in the cooling liquid. Alternatively, the impeller of prior-art integrated cooling devices is motivated by a shaft that, in turn, is driven by an external motor. Such an external motor shaft drive arrangement requires a seal between the motor and the housing and impeller. As the seal begins to wear, leaking cooling liquid can lead to problems such as leakage, reduced cooling effectiveness and ultimately damage the heat-generating components or adjacent electronic components Additionally, the prior-art attempts to provide an integrated liquid cooling mechanism have failed to take advantage of the additional cooling ability provided by nucleate boiling.  
         [0018]     Therefore, in light of the prior art, there is a need for an integrated cooling device that has a direct, high thermal capacity, low-temperature-difference heat-transfer path, that circulates the cooling liquid in turbulent flow without requiring sealing between rotating components and the housing, and that can advantageously use nucleate boiling to cool the heat-generating components.  
       SUMMARY OF THE INVENTION  
       [0019]     The present invention provides a method and apparatus for cooling electronic components using an integrated circulating liquid cooling device.  
         [0020]     The integrated cooling device of the present invention comprises a housing, cooling liquid, a motor, an impeller, and cooling surfaces. It also allows for expansion of the cooling liquid as the temperature changes. Contained within the housing is the cooling liquid. Immersed in the cooling liquid is a motor driving an impeller. In certain embodiments of the present invention, the heat producing electronic component is also immersed in the cooling liquid such that the cooling liquid flows over the heat producing electronic component when the motor drives the impeller. The surface of the housing is thermally connected to cooling surfaces such as cooling fins, a thermally conducting computer or other-electronic-equipment case, or a low-velocity fluid path carrying a second cooling fluid other than air.  
         [0021]     The housing may have any shape, but if a close fitting impeller is desired, the shape of the interior surface of the housing can be generated by rotating about an axis, a line generatrix following any arbitrary path between any two points on that axis. For example, the shape of the housing would be a cylinder if the generatrix rotated about the axis were formed of three straight-line segments that form a rectangle with the axis as the fourth side. The generatrix of a hat shaped enclosure would have five joined straight-line segments between the points on the axis. A curved line-segment would generate a spherical or domed surface. Additionally, the line generatrix may include at least one feature that generates at least one ridge on the resulting interior surface of the housing. Such a feature yields increased surface area as compared with a non-ridged housing. The ridge may be oriented with its highest portion extending into the interior of the housing, or, it may be oriented with its highest portion extending into the wall of the housing (such that a groove is created in the wall of the housing) This increased surface area results in increased cooling capacity of the integrated cooling device. In one preferred embodiment in which high-velocity coolant flow would occur over the largest possible interior surface, the motor and impeller are concentric with and closely fit the housing. Alternatively, the impeller and motor could be located so they provide the highest velocity coolant liquid flow to only that area of the housing where the greatest heat transfer is needed (e.g., near the component to be cooled). Thus, a small motor could provide local high-velocity flow.  
         [0022]     The housing may be completely filled or almost completely filled with a cooling liquid. If the housing is completely filled with liquid, a bellows, flexible diaphragm, or other flexible element must be provided in the housing to allow for thermal expansion of the cooling liquid. On the other hand, if a small volume filled with a compressible gas is provided within the housing, it will have no effect on the satisfactory operation of the cooling device. Any void in the cooling liquid will be forced into the center of the housing by centrifugal forces when the motor spins the liquid. In some cases, the compressible gas may be comprised only of the vapor phase of the cooling liquid.  
         [0023]     The placement of the motor and impeller inside the housing and immersed in the cooling liquid provides several advantages. The motor&#39;s placement within the housing creates a more fully integrated cooling device than that of the prior art, which had an external motor. The level of integration afforded by the cooling device of the present invention is especially desirable given the ever-shrinking size constraints imposed by modern electronic components. Further, the motor&#39;s placement within the housing significantly reduces the potential for leakage and damage to the components to be cooled since, unlike the prior art, there is no need to seal the housing around a rotating impeller drive shaft. Also, since the impeller is directly driven, rather than motivated indirectly by external electromagnetic fields, the impeller need not be composed of an electrically conductive material. Unlike the prior art, heat transfer from the heat-generating component to the cooling surface will be more direct because the housing of the present invention need not be composed of a composite material to allow electromagnetic fields (used to motivate the impeller of prior-art integrated cooling devices) to pass through. Rather, since the cooling device of the present invention features an enclosed motor that directly drives an impeller, the housing material may be any material with the desired durability and thermal properties (including metals that would interfere with the indirect drive system of the prior art integrated device). It will also be obvious that the integrated cooling device can be further integrated with the semiconductor package thus eliminating the temperature differences in a joint filled with heat-sink compound.  
         [0024]     In choosing the cooling liquid, one must make a satisfactory compromise between thermal properties such as thermal capacity, mechanical properties such as viscosity, and other properties that make the liquid compatible with the immersed brushless motor and the immersed heat producing electronic component in embodiments where the electronic component is immersed. These last properties include lubrication and corrosion protection. A number of hydrocarbons, synthetic oils, chlorinated hydrocarbons, or fluorocarbons have such properties. One also might consider alcohols, glycols and mixtures of these with water. Mixtures of oil and water with emulsifying agents might also be considered, particularly if some attention is given to the surface treatment of the motor parts.  
         [0025]     Additionally, the cooling liquid could be chosen to gain enhanced heat transfer characteristics through nucleate boiling. To take advantage of nucleate boiling, the cooling liquid chosen should have a boiling temperature approximately equal to the operating temperature of the heat-generating component.  
         [0026]     Motors capable of operating in a liquid immersed environment are known in the art. For example, any shaded-pole or poly-phase alternating-current motor can operate in a liquid. A direct-current brushless motor (a polyphase motor with a solid-state inverter or commutation circuit, quite similar to those that propel the small fans used to move air in computers) could be mounted in the housing immersed in the cooling liquid. Such a motor will work well when immersed in a variety of liquids. Electrical connections to the motor and, in embodiments where it is immersed in the liquid, the heat producing electronic component, are made through hermetic seals. The small fan motor mentioned have their stationary pole pieces and coils inside the rotor, which may be a permanently magnetized cylinder with a central shaft rotating in a bearing in the center of the pole pieces. The magnetized rotor may be surrounded by magnetic shielding, such as a soft iron cylinder, to strengthen the fields inside the rotor and reduce those outside the rotor, which might interfere with electronic circuits. The impeller is attached to the rotor of the motor. The impeller is comprised of a plurality of fins or vanes that are approximately normal to the outside circumference of the rotor and the inside circumference of the housing in order to form a propeller or paddlewheel-like structure inside the housing. A small clearance exists between the edges of the vanes and at least part of the walls of the housing. Unlike the prior art integrated devices, the impeller vanes do not have to be either a good thermal or electrical conductor.  
         [0027]     Cooling surfaces such as a group of fins or vanes, a thermally conductive computer case, a channel carrying a second low-velocity liquid coolant other than air, or any other thermally conductive and dissipative elements are thermally connected to the outer surface of the housing. When the integrated cooling device is operating, heat is transferred from the heat generating component to the cooling liquid. The turbulent flow of cooling liquid motivated by the impeller then distributes the heat energy substantially evenly throughout the liquid and over the inside surfaces of the housing. This distribution may be aided by nucleate boiling and vapor condensation in the liquid. The heat energy is then conducted through the walls of the housing to the cooling surfaces thermally attached to the large outside surfaces of the housing. Finally, the heat energy is dissipated from the cooling surface to the atmosphere or other environment by free or forced convection that may be produced for example by a blower external to the integrated cooling apparatus. It will be obvious that heat from the outside surface of the enclosure or fins could just as well be carried away by low velocity liquid. A more complete understanding of the integrated cooling device will be afforded to those skilled in the art, as well as a realization of additional advantages and objects thereof, by a consideration of the following detailed description of the preferred embodiment. Reference will be made to the appended sheets of drawings, which will first be described briefly. 
     
    
     BRIEF DESCRIPTION OF THE DRAWINGS  
       [0028]      FIG. 1  is a cross-sectional side view diagram showing an exemplary integrated cooling device according to the invention.  
         [0029]      FIG. 2  is a top view diagram showing the integrated cooling device of  FIG. 1 .  
         [0030]      FIG. 3  is a cross-sectional top view of an alternative embodiment of the invention in which heat is transferred to the environment through the intermediary of an electronic computer case rather than fins.  
         [0031]      FIG. 4  is a cross-sectional side view diagram showing a third embodiment of the present invention with an immersed heat producing electronic component.  
         [0032]      FIG. 5  is a cross-sectional top view of an alternative arrangement according to the third embodiment of the present invention in which heat is transferred from an immersed heat producing electronic component to the environment through the intermediary of an electronic computer case rather than fins.  
         [0033]      FIG. 6  is a cross-sectional side view of an integrated cooling device according to the third embodiment of the present invention in which high power density electronic components and support electronic components are mounted immersed in cooling liquid inside the housing. 
     
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT  
       [0034]     The present invention provides an apparatus for cooling an electronic component with an integrated cooling device that overcomes the limitations of the prior art. In the detailed description that follows, like numerals are used to denote like elements appearing in one or more of the figures.  
         [0035]     Referring to  FIG. 1 , an embodiment of the integrated cooling device  10  of the present invention is depicted in a cross-sectional side view. The integrated cooling device  10  is comprised of a housing  12 , a motor stator further comprising pole-pieces and windings  14 , a cylindrical permanent magnet rotor  15 , an impeller  16 , cooling liquid  18 , and cooling surfaces  22 . The housing  12  of the integrated cooling device  10  has inside surfaces  24  and outside surfaces  26 .  FIG. 1  depicts the housing  12  as a right circular cylinder, although it should be appreciated that multiple geometries of housings are contemplated within the scope of the present invention. For example, hat-shaped, spherical, or dome-shaped geometries of the housing may be used to allow a close-fitting impeller. The housing  12  is sealed to prevent the cooling liquid  18  from escaping. The housing  12  and any electrical connections (not depicted) for the motor  14  may be hermetically sealed. The housing  12  is thermally connected to a heat generating component  50 . The housing  12  may be comprised of a material with a relatively high thermal conductivity such as a metal. Optionally, the package of the heat-generating component could be integrated with the housing to further reduce temperature differences. Advantageously, a metal housing allows for effective transfer of heat from the heat generating component  50  to the integrated cooling device  10 .  
         [0036]     The motor stator  14  is mounted to the inside of the housing  12 . In the depicted embodiment, the motor stator  14  is mounted concentrically with respect to the housing  12 . This concentric mounting causes high-velocity coolant flow to occur over the largest possible interior surface of the housing  12 . Alternatively, as described in connection with  FIG. 3  below, the impeller and motor could be located so they provide the highest velocity coolant liquid flow to only that area of the housing where the greatest heat transfer is needed. This alternate embodiment could allow the use of a smaller motor and impeller than is required to generate high-velocity liquid flow across the largest possible interior surface of the housing  12 .  
         [0037]     The impeller  16  is affixed to and is rotated by the rotor  15  of the motor. The impeller  16  is comprised of a plurality of vanes  30  approximately normal to the outside circumference of the rotor  15  and, for the case of concentric mounting, the inside surface  24  of the housing  12  in order to form a paddlewheel-like structure inside the housing  12 . The vanes  30  of the impeller  16  extend from the rotor  15  to a location sufficiently close to the inside surface  24  of the housing  12  to provide reasonably high-velocity coolant flow at the housing inner surface. Advantageously, this enclosed motor configuration allows for an integrated cooling device that does not need to be sealed around an external impeller drive shaft. Electrical connection to the motor may be made through hermetically sealed insulators (not shown).  
         [0038]     In the depicted embodiment, the housing  12  is almost completely filled with a cooling liquid  18 , with the remainder filled by a compressible gas  20 . The compressible gas  20  allows the housing  12  and cooling liquid  18  to expand without distorting the housing  12  during operation of the integrated cooling device  10 . In some cases the compressible gas may be comprised only of the vapor phase of the cooling liquid  18 . Regardless of the orientation of the integrated cooling device  10  relative to gravitational forces, centrifugal forces generated by the rotating impeller  16  will force the compressible gas  20  into the center of the housing  12  during operation of the integrated cooling device  10 . In alternate embodiments of the present invention, the housing  12  may be completely filled with a cooling liquid  18 . In these alternate embodiments where the housing  12  is completely filled with cooling liquid, an expansion bellows, flexible diaphragm, or other flexible element must be provided in the housing to allow for thermal expansion of the cooling liquid.  
         [0039]     The cooling liquid  18  could be chosen from a number of liquids based on their thermal properties and compatibility with the motor stator  14  and rotor  15 . For enhanced cooling through nucleate boiling, the cooling liquid  18  should have a boiling temperature approximately equal to the operating temperature of the heat generating component  50 . Advantageously, nucleate boiling increases the heat transfer coefficient of the liquid by a factor of up to approximately ten. A turbulent flow condition is created in the cooling liquid  18  through the rotation of the impeller  16 .  
         [0040]     Cooling surfaces  22  are thermally connected to the outside surfaces  26  of the housing  12 . The cooling surfaces  22  may be an array of fins or vanes that allow heat transfer to the surrounding atmosphere via free or forced convection. Alternatively, the fins could be replaced by any other heat conductive and dissipative device such as a computer case for example. When the integrated cooling device  10  of the present invention is operated, heat energy is transferred from the heat generating source  50  through the housing  12  where it is distributed substantially evenly by the turbulent cooling liquid  18  throughout the inside surfaces  24  of the housing  12 . The heat energy is then conducted through the housing  12  to the outside surfaces  26  of the housing  12  and dissipated from the cooling surfaces  22  by free or forced convection. Advantageously, the heat transfer pathway provided by the present invention efficiently transfers heat from a high-power-density small area device such as a semiconductor device or other electronic component to cooling surfaces  22  having a relatively large total surface area.  
         [0041]      FIG. 2  is a cross-sectional top view diagram showing the integrated cooling device of  FIG. 1 .  
         [0042]      FIG. 3  shows an exemplary arrangement of many of the components of the integrated cooling device shown in  FIGS. 1 and 2 .  FIG. 3  shows a motor stator  14 , permanent magnet rotor  15 , and impeller vanes  30  essentially identical to those shown in  FIGS. 1 and 2 . The integrated cooling device of  FIG. 3 , however, is optimized to transfer heat from the heat generating component  50  to an electronic equipment or computer case surface  40 , represented by a large rectangle. In thermal contact with the electronic equipment or computer case surface  40 , is an enclosure  38  with a lower surface joining an outer cylindrical wall  34  and a concentric inner cylindrical wall  36 . At their upper edges, the cylindrical walls  34  and  36  are joined by a surface, not shown, but having the same shape as the lower surface, and forming a closed toroidal enclosure with a rectangular cross section. The enclosure contains a cooling liquid  18 , and may be hermetically sealed. It should be recognized that while the enclosure  38  is depicted here as a toroidal structure with a rectangular cross section, other geometries of enclosure may be employed within the scope of the present invention. The motor and impeller assembly,  14 ,  15 , and,  16 , fits within the enclosure  38 , surfaces with small clearances between the impeller at top, bottom, and the side walls  34  and  36 . A primary baffle  42  divides the enclosure into two channels that can carry cooling liquid  18  in opposite directions, clockwise and anticlockwise. Two secondary baffles,  44  and  46 , join with the primary baffle  42 , and the small openings between these secondary baffles  44  and  46  and the cylindrical walls,  34  and  36 , insure that there will be high-velocity cooling liquid  18  flow adjacent to the heat-generating component  50 , and lower-velocity flows elsewhere around the toroidal enclosure  38 . Unlike the situation in the embodiment of  FIGS. 1 and 2 , in which there was a fairly high power density at the roots of the fins  22 , in this embodiment, the coolant spreads the heat over, more nearly, the entire surface area of the equipment case  40 . The heat-generating component  50  is mounted on one of the surfaces of the toroidal enclosure  38  close to the impeller vanes  30  where the rate of heat transfer will be very high.  
         [0043]      FIGS. 4 and 5  show a third embodiment of the integrated cooling device of the present invention. In the third embodiment of this invention, at least one heat-generating component  60  is mounted inside the liquid-filled housing  12 .  FIG. 4  depicts this third embodiment as corresponding to the integrated cooling device of the first embodiment as shown in  FIG. 1 .  FIG. 5  depicts this third embodiment as corresponding to the integrated cooling device as shown in  FIG. 3 . In the third embodiment, the heat-generating component  60  is mounted closely spaced to the impeller  16  and the motor  14 ,  15  driving the impeller  16 . This mounting arrangement for the heat generating component may be applied to either the structure of the first embodiment of the present invention (depicted in  FIG. 4 ) or to the second embodiment (depicted in  FIG. 5 ). The third embodiment, as depicted in  FIG. 4 , comprises a housing  12 , at least one heat generating component  60  mounted within the housing  12 , a motor stator further comprising pole-pieces and windings  14 , a cylindrical permanent magnet rotor  15 , an impeller  16 , cooling liquid  18 , and cooling surfaces  22 . A more detailed description of these component elements is provided above in the description of  FIG. 1 . In the integrated cooling device depicted in  FIG. 4 , when the motor  14 ,  15  is running, the impeller will create high-velocity fluid flow over the heat-producing component  60  surface and provide a high heat transfer coefficient from the component to the liquid.  
         [0044]      FIG. 5  depicts the arrangement of the heat generating component of the third embodiment as corresponding to the integrated cooling device of the second embodiment of the present invention. The integrated cooling device depicted in  FIG. 5  comprises a motor stator  14 , a permanent magnet rotor  15 , an impeller  16  with impeller vanes  30 , an enclosure  38  containing the motor and impeller assembly  14 ,  15 , and  16 , a cooling liquid  18  contained in the enclosure  38 , a heat-generating component  60  mounted inside the enclosure  38  near the impeller  16 , a primary baffle  42 , two secondary baffles  44  and  46 , A more detailed description of the corresponding component elements is provided above in the description of  FIG. 3 . In the integrated cooling device of  FIG. 5 , cooling liquid with lower velocity near the interior surface of the enclosure will still be able to communicate the total amount of heat to the heat absorbing or dissipating surfaces attached to the exterior of the enclosure.  
         [0045]     When exploiting the third embodiment, it will often be convenient to mount additional support electronic components and circuitry inside the enclosure with the heat-generating component  60  or components. This joint mounting can substantially reduce the number of conductors that must be brought through hermetic seals in the enclosure wall. For example, if it were desired to cool the microprocessor and the graphics chip of a personal computer by placing them in a liquid filled enclosure, each near a common impeller or each near its own impeller and brushless motor, it might also be convenient to locate many of the memory chips and other support chips, or perhaps the whole motherboard, within the same enclosure.  
         [0046]      FIG. 6  depicts an integrated cooling device featuring the immersed chip location of the third embodiment with jointly mounted electronic components of an electronic system  100 . The integrated cooling device comprises: a first motor stator  114 , a first permanent magnet rotor  115 , a first impeller  116  with impeller vanes  130 , a second motor stator  164 , a second permanent magnet rotor  165 , a second impeller  166  with impeller vanes  130 , a first baffle  144 , a second baffle  146 , a housing  112  containing the first motor and impeller assembly  114 ,  115 , and  116 , the second motor and impeller assembly  164 ,  165 , and  166 , a cooling liquid  18  contained in the housing  112 , and cooling surfaces  122 . While two motor and impeller assemblies  114 ,  115 , and  116  and  164 ,  165 , and  166  are depicted, joint mounting can be applied to integrated cooling devices of the present invention having only a single motor and impeller assembly, or having more than two motor and impeller assemblies.  
         [0047]     The first stator  114  and the second stator  164  are each mounted to the inside of the housing  112 . In the depicted embodiment, the first stator  114  is mounted concentrically with respect to a first baffle  144  and the second stator  164  is mounted concentrically with respect to a second baffle  146 . The first baffle  144  and the second baffle  146  may be segmented to allow for increased cooling liquid flow. As depicted, the first and second baffles  144 ,  146  are segmented, each comprising two baffle segments. Each of the two baffle segments comprising the segmented baffle may be approximately a ninety degree arc of the wall of a right circular cylinder. As depicted, the two cylindrical baffle segments making up each of the first and second baffles  144 ,  146  are arranged to create partially enclosing right circular cylindrical chambers that channel high velocity fluid flow created by the first and second impellers  116 ,  166  on a plurality of high power density heat generating electronic components  160  mounted within the housing  112 . Preferably, the heat generating electronic components  160  are mounted within the partially enclosing right circular cylindrical chambers created by the segmented first and second baffles  144 ,  146 .  
         [0048]     Alternatively, if the first and second baffles  144 ,  146  are not segmented, spaces are left at their ends. In this alternate baffle arrangement, the vanes  130  of the impellers  116 ,  166  are angled with respect to their axis of rotation such that high-velocity coolant flow would occur in the axial direction in the cylindrical enclosures and in the radial direction at the ends of the cylindrical enclosures over the high-power-density electronic components  160 . Other arrangements of baffles may be made having different numbers, geometries, and positioning of baffles than those depicted. Those other arrangements of baffles are within the scope of the present invention.  
         [0049]     The first and second impellers  116 ,  166  are each affixed to and rotated by the corresponding rotor  115 ,  165 . The first and second impellers  116 ,  166  are each comprised of a plurality of vanes  130  that are approximately normal to the outside circumference of the corresponding first or second rotor  115 ,  165  such that each impeller  116 ,  166  is a paddlewheel-like structure within the partially enclosing right circular cylindrical chamber created by the corresponding first or second segmented baffles  144 ,  146 . Alternatively the vanes  130  may make an angle to the axis of rotation so the impellers are more propeller-like and the baffles  146 ,  166  are full cylinders.  
         [0050]     At least one high power density heat generating electronic component  160  and at least one support electronic component  170  are mounted inside the housing  112 . The electronic components  160 ,  170  are electrically connected through the housing  112  with hermetically-sealed connections  172 . Likewise, electrical connections are made through the housing  112  to the motor and impeller assemblies  114 ,  115 , and  116  and  164 ,  165 , and  166  with hermetically-sealed connections  172 . Advantageously, the arrangement of the motor and impeller assemblies  114 ,  115 , and  116  and  164 ,  165 , and  166  provide primary cooling to the high power density heat generating electronic components  160  such as microprocessors or graphics chips. Further advantages are achieved by jointly mounting the high power density components  160  and support electronic components  170  such as memory chips within the housing  112  to reduce the number of electrical connections that would otherwise be required to pass through the housing  112 .  
         [0051]     The housing  112  may be a two piece assembly such that it is further comprised of a first housing portion and a second housing portion. The first housing portion is preferably comprised of a metal. The cooling surfaces  122  are thermally connected to the first housing portion. The cooling surfaces  122  are preferably metal cooling fins. The second housing portion may be comprised of a material such as a partially metallized plastic, glass, or ceramic. The electronic components  160 ,  170  and motor-impeller assemblies  114 ,  115 , and  116  and  164 ,  165 , and  166  may be soldered or otherwise connected to the second housing portion providing hermetically-sealed electrical connections  172  through the second housing portion.  
         [0052]     The integrated cooling device of the present invention may be used in a novel method of the present invention to cool an electronic component. In this method, the integrated cooling device, comprising a housing, cooling liquid contained in the housing, a motor mounted inside the housing, an impeller driven by the motor, and cooling surfaces thermally connected to the housing, is thermally connected to the electronic component to be cooled, and the motor of the integrated cooling apparatus is activated to drive the impeller. The additional step of selecting the cooling liquid such that the cooling liquid has a boiling point that is approximately equal to an operating temperature of the electronic component will enhance the cooling provided by this method.  
         [0053]     Having thus described a preferred embodiment and alternate embodiments of an integrated cooling device, it should be apparent to those skilled in the art that certain advantages of the described invention have been achieved. It should also be appreciated that various modifications, adaptations, and alternative embodiments thereof may be made within the scope and spirit of the present invention. For example, a cylindrical housing with a centrally mounted motor arranged to cool one electronic component has been illustrated. However, it should be apparent that the inventive concepts described above would be equally applicable to an embodiment in which multiple electronic components are thermally connected to the housing. Likewise, using a hat shaped enclosure which would provide more room for the motor or a dome shaped housing to better resist internal pressure might be a beneficial modification. Alternatively, a motor and impeller could be mounted close to a heat-generating device mounted, for example, on one side of a rectangular box, or multiple motor and impeller assemblies could be mounted within, and closely spaced to portions of a single enclosure of a more complex shape. Baffles of various shapes could also be used within the enclosure to advantageously direct the coolant fluid flow. Also, appropriate magnetic shielding, such as a soft iron cylinder around a magnetic motor rotor, as mentioned above, or soft iron parts in the housing, could be incorporated in the integrated cooling device to reduce any external magnetic fields produced by the device that might interfere with the proper operation of nearby electronic circuits. A housing of high electrical conductivity and sufficient thickness will also provide substantial attenuation of time varying magnetic fields by virtue of eddy currents or the “skin effect.”