Abstract:
An apparatus includes a circuit having a heat-generating circuit component, and structure for guiding a two-phase coolant along a path which brings the coolant into direct physical contact with either the circuit component or a highly thermally conductive part which is thermally coupled to the circuit component. The coolant absorbs heat generated by the circuit component, at least part of the coolant changing from a first phase to a second phase in response to the heat absorbed from the circuit component, where the second phase is different from the first phase.

Description:
BACKGROUND OF THE INVENTION 
   Some types of circuits produce very little heat, and can be operated with a passive cooling technique, such as convection cooling. On the other hand, there are a variety of circuits which produce extremely large amounts of heat, and require some sort of active cooling. One example is a high-power phased array antenna system, which consumes large amounts of electrical power, typically on the order of tens of thousands of watts. 
   These phased array antenna systems commonly include monolithic microwave integrated circuits (MMICs), which consume substantial amounts of power, and which generate substantial amounts of heat. Pre-existing systems of this type usually include MMICs that need cooling for a thermal density of no more than about 100 Watts/in 2 . However, the industry trend is toward the development and use of MMIC devices which consume substantially higher amounts of power and which dissipate substantially greater amounts of heat. One example is gallium nitride (GaN) MMIC technology, and another example is gallium arsenide (GaAs) MMIC technology. In a GaN MMIC, the typical thermal density can be on the order of at least 400 Watts/in 2 , when the radio frequency duty cycle is at 100%. Although pre-existing techniques for cooling phased array antenna systems have been generally adequate for their intended purposes, they use a single phase coolant, such as a refrigeration-cooled polyalphaolefin (PAO). Such a pre-existing cooling system with a single-phase coolant is only capable of handling thermal densities up to about 100 Watts/in 2 . Consequently, these pre-existing cooling systems and techniques are not entirely satisfactory for the levels of heat dissipated by next-generation technology in phased array antenna systems. 
   SUMMARY OF THE INVENTION 
   From the foregoing, it may be appreciated that a need has arisen for a method and apparatus for providing more effective cooling of heat-generating circuit components. According to one form of the present invention, a method and apparatus are provided to address this need, and involve: operating a circuit which includes a heat-generating circuit component; and guiding a two-phase coolant along a path which brings the coolant into direct physical contact with the circuit component or with a highly thermally conductive part which is thermally coupled to the circuit component. The coolant absorbs heat generated by the circuit component, at least part of the coolant changing from a first phase to a second phase in response to heat absorbed from the circuit component, where the second phase is different from the first phase. 

   
     BRIEF DESCRIPTION OF THE DRAWINGS 
     A better understanding of the present invention will be realized from the detailed description which follows, taken in conjunction with the accompanying drawings, in which: 
       FIG. 1  is a diagrammatic perspective view of part of a phased array antenna system which embodies the present invention; 
       FIG. 2  is a diagrammatic exploded fragmentary perspective view of a slat which is a component of the system of  FIG. 1 ; and 
       FIG. 3  is a diagrammatic fragmentary sectional top view of part of a slat which is an alternative embodiment of the slat shown in FIG.  2 . 
   

   DETAILED DESCRIPTION OF THE INVENTION 
     FIG. 1  is a diagrammatic perspective view of part of an apparatus which is a phased array antenna system  10 . The antenna system  10  includes a frame or chassis  12 . The chassis  12  supports a plurality of modules of a type commonly known as slats, three of which are shown in  FIG. 1  at  16 - 18 . In an operational configuration, a larger number of slats would be installed in the frame  12 , so that the frame  12  is effectively filled with slats. 
   In the embodiment of  FIG. 1 , the slats are all identical, including the three slats shown at  16 - 18 . Each slat has, along an edge opposite from the frame  12 , a row of outwardly projecting antenna elements  21 . The antenna elements  21  on all of the slats collectively define a two-dimensional array of antenna elements. Each slat includes circuitry with transmit and receive capabilities, and this circuitry is operationally coupled to the antenna elements  21  on that slat. The frame  12  carries electrical signals and coolant to and from each of the slats, in a manner which is known in the art and which is therefore not illustrated and described here in detail. In this regard,  FIG. 1  diagrammatically shows a cooling and condensation system  27  which is capable of cooling a coolant in its vapor phase, so as to return the coolant to its liquid phase. Coolant from the frame  12  is circulated through the system  27  along a path which is indicated diagrammatically in  FIG. 1  by reference numeral  26 . 
   In a pre-existing phased array antenna system, the transmit/receive circuitry includes monolithic microwave integrated circuits (MMICs), which need active cooling that can remove heat at a rate which is less than about 100 Watts/in 2 . Newer MMICs include gallium nitride (GaN) technology, and dissipate significantly greater amounts of heat, for example 500 Watts/in 2 . In the embodiment of  FIG. 1 , the antenna system  10  uses GaN MMIC technology, and includes a cooling configuration capable of efficiently removing heat at rate of 500 Watts/in 2 . In fact, the cooling arrangement provided for the antenna system  10  of  FIG. 1  is capable of removing heat at rates up to approximately 1,000 Watts/in 2.    
   As mentioned above, the slats in the antenna system  10  of  FIG. 1  are all effectively identical. Therefore, for purposes of explaining how they are cooled, the following discussion addresses only one of the slats in detail, in particular the slat  16 . In more detail,  FIG. 2  is a diagrammatic exploded fragmentary perspective view of the slat  16  of FIG.  1 . As shown in  FIG. 2 , the slat  16  includes a slat member  41 , a cooling plate  42 , and a cover plate  43 , the cooling plate  42  being sandwiched between the slat member  41  and the cover plate  43 . In the disclosed embodiment, the slat member  41 , cooling plate  42  and cover plate  43  are each made of aluminum silicon carbide (AlSiC), due in part to the fact that it has a coefficient of thermal expansion (CTE) comparable to that of other components within the slat, so as to avoid thermally-induced structural stress. However, one or more of the parts  41 - 43  could alternatively be made of some other suitable material. 
   The slat member  41  has a row of the antenna elements  21  supported thereon. A multi-layer circuit board  47  is supported on a side of the slat member  41  which faces the cooling plate  42 . The circuit board  47  is electrically coupled to the frame  12  ( FIG. 1 ) through connectors in a manner which is known in the art and therefore not illustrated and described here in detail. The circuit board  47  has, on the side thereof which is not visible in  FIG. 2 , a plurality of radio frequency (RF) probes that are disposed along the edge adjacent the antenna elements  21 . When the antenna system  12  is transmitting, the probes excite the antenna elements  21  in response to a suitable RF signal from the circuit board  47 . On the other hand, when the antenna system  12  is receiving, the antenna element excite the probes in response to an incoming RF signal. 
   The circuit board  47  has mounted thereon a vertical column of transmit/receive modules, one of which is identified in  FIG. 2  by reference numeral  51 . Each of the transmit/receive modules is associated with a respective one of the antenna elements  21 . The transmit/receive modules are identical, and therefore only the module  51  is discussed below in further detail. 
   More specifically, the module  51  includes a ceramic base plate  52 , which is hermetically, sealed along its peripheral edge to a ceramic cover  53 , thereby defining a module housing. The module  51  includes within this housing a circuit board, which has a GaN MMIC  56  mounted thereon. The side of the MMIC  56  which is visible in  FIG. 2  is the side commonly referred to as the active-junction side. Aside from the MMIC  56 , the circuit board within the module  51  is generally not visible in FIG.  2 . As mentioned above, the circuitry provided on the circuit board within each module has a configuration of a type which is known in the art, and which is therefore not described here in detail. 
   The cover  53  of the module  51  has a rectangular opening  61 , which is aligned with the MMIC  56  and serves as an inlet port through which coolant can enter the module  51 , as discussed in more detail later. The cover  53  has near its opposite lower corners two output ports  62  and  63 , through which coolant exits the module  51 , as discussed later. 
   The circuit board within the module  51  is electrically coupled to the multi-layer circuit board  47 . In the enclosed embodiment, the facing surfaces of the circuit board  47  and the base plate  52  each have a plurality of not-illustrated pads, and respective pairs of these pads are electrically coupled to each other by a grid of solder drops or balls, in a configuration of the type generally known in the art as a ball grid array. The base plate  52  of the module  51  has a plurality of not-illustrated vias, which each extend through it in a manner facilitating a hermetic seal. One end of each via is coupled to one of the pads provided on the base plate  52 , and the other end of each via is electrically coupled in some suitable manner to the circuit board in the module  51 . 
   In the assembled configuration of the slat  16 , a seal exists between the cooling plate  42  and each of the modules provided on the slat member  41 , including the module  51 . In the disclosed embodiment, this is effected through the use of a low-temperature solder provided between the cooling plate  42  and each of the modules. However, the seal could alternatively be effected in some other suitable manner, for example with a compression seal. Still another alternative approach is to use a seal which extends along the periphery of the cooling plate  42  and the periphery of the group of modules, in a manner so that the inlet and outlet ports  61 - 63  are not sealed relative to each, but are all sealed relative to structure external to the seal. 
   The cooling plate  42  has, on the side thereof opposite from the slat member  41 , a configuration of shallow channels or grooves that are formed through machining, casting or molding. This network of channels includes an inlet port  72 , which can be operatively coupled to a coolant supply line located within the frame  12  (FIG.  1 ), for example through use of a quick-disconnect coupling of a type which is known in the art and therefore not shown and described in here detail. The use of quick-disconnect couplings permits a slat to be removed and replaced without draining coolant from the entire system. The inlet port  72  leads to a high-pressure inlet channel  73 , which in turn communicates with a plurality of orifices, two of which are designated in  FIG. 2  by respective reference numerals  76  and  77 . In the disclosed embodiment, there is one orifice for each of the modules provided on the circuit board  47 , including the module  51 . 
   In  FIG. 2 , the illustrated orifices are all the same size. However, as shown diagrammatically by the broken line  78 , it would alternatively be possible for some of the orifices to be larger or smaller than others, so that some of the modules on the circuit board  47  intentionally receive more or less coolant than other modules, thereby supplying more or less coolant to modules which need proportionally more or less cooling. 
   After a portion of the coolant from the inlet channel  73  passes through any one of the orifices, it reaches a respective group or array of small feed openings  81 , which extend transversely through the cooling plate  42 . Each group of feed openings  81  is aligned with the opening  61  in a respective one of the transmit/receive modules. The cooling plate  42  also has several pairs of return openings  83  and  84 , which each extend transversely through the cooling plate  42  and which each communicate with a respective one of the outlet ports  62  and  63  in a respective one of the modules. Each pair of return openings  83  and  84  communicates with a respective low-pressure return channel  86 . The return channels  86  all lead to a somewhat deeper channel  88 , which serves as a low-pressure collection chamber. Coolant from the collection chamber  88  travels from the cooling plate  42  to a not-illustrated outlet port, which is coupled to the frame  12  ( FIG. 1 ) by a not-illustrated quick-disconnect fitting of a known type. 
   In the assembled configuration of the slat  16 , the cover  43  has its peripheral edges sealed with respect to peripheral edges of the cooling plate  42 , for example by brazing or welding, or by a not-illustrated gasket. Further, the peripheral edges of the cooling plate  42  are sealed in a similar manner to peripheral edges of the slat member  41 . 
   The embodiment of  FIGS. 1 and 2  effects what can be referred to as “front side” cooling of the MMICs  56  in each module, in that the coolant directly contacts the active-junction side of each MMIC. This requires a coolant which is thermally conductive but electrically non-conductive. To meet these criteria, the embodiment of  FIGS. 1 and 2  uses as the coolant a fluorinert of a known type, such as that commercially available as FC- 77 . Due to the fact that the coolant directly engages the MMIC, without intervening structure, the temperature differential between the coolant and active-junction of the MMICs is minimized, which helps to keep the junction temperatures of the MMICs low, which in turn increases the operational lifetime and reliability of the MMICs. 
   In operation, coolant in a liquid phase is supplied through the frame  12  to each of the slats, including the slat  16 . The portion of the coolant which enters the slat  16  flows into the cooling plate  42  through the inlet  72 . This coolant then travels through the inlet channel  73 , where it splits so that a respective portion of it passes through each of the orifices, including the orifices shown at  76  and  77 . After passing through these orifices, respective portions of the coolant each reach and pass through a respective group of the openings  81 , and then each flow through the opening  61  in a respective one of the modules, as indicated diagrammatically in  FIG. 2  by the broken line  96 . 
   After entering each module through the associated opening  61 , the coolant directly contacts the exposed active-junction side of the MMIC  56  in that module, and absorbs heat from the MMIC  56 . Due to this absorption of heat, part or all of the coolant in that module boils and changes from its liquid phase to its vapor phase. This phase change is inherently associated with absorption of a substantial amount of heat by the coolant. The coolant inside the module, including vapor and possibly some liquid, then flows within the module from the region of the opening  61  toward the outlet ports  62  and  63 , absorbing heat from other components on the circuit board within the module. In each module, respective portions of the coolant exit the module through the outlet ports  62  and  63  of that module, and then respectively pass through the associated pair of return openings  83  and  84  in the cooling plate  42 , as indicated diagrammatically in  FIG. 2  by the broken lines  97  and  98 . The portions of the coolant passing through each pair of the return openings  83  and  84  then enter a respective one of the channels  86 , where they travel to the collection chamber  88 . 
   From the collection chamber  88 , the coolant travels to the not-illustrated outlet port for the slat  16 . The coolant which exits the slat  16  through the outlet port flows through a channel provided within the frame  12 , and then travels along the path  26  through the cooling and condensation system  27 , where it is cooled and returned to a liquid phase. This cooled liquid coolant is then routed back into the frame  12 , where it is again distributed among the inlet ports  72  of the slats. 
   In  FIG. 2 , the cooling plate  42  is shown as being sandwiched between the slat member  41  and the cover plate  43 . However, in an alternative configuration, the cover plate  43  can be replaced with a cooling plate and a slat member which are mirror images of the cooling plate  42  and slat member  41 , resulting in a single slat that has a central cooling arrangement sandwiched between and serving two vertical columns of antenna elements and transmit/receive modules. 
   As discussed above, the embodiment of  FIGS. 1 and 2  uses a fluorinert as the coolant, because a fluorinert is thermally conductive but electrically non-conductive. As also discussed above, a significant part of the cooling effect is obtained by the phase change of the coolant, from its liquid phase to its vapor phase. The amount of heat absorbed by a coolant as it changes from a liquid to a vapor is known as its latent heat of vaporization. 
   There are known coolants which have a latent heat of vaporization substantially higher than that of the fluorinert used in  FIGS. 1 and 2 . In other words, when these other coolants change from a liquid to a vapor, they absorb substantially more heat per unit of volume than a fluorinert. For example, water has a latent heat of vaporization which is higher than that of virtually any other suitable coolant, and which is about 20 times the latent heat of vaporization of a typical fluorinert. Stated differently, the volume of a fluorinert which must flow in order to remove a specified amount of heat is about 20 times the volume of water that must flow in order to remove the same amount of heat. In addition to water, coolants that have latent heats of vaporization greater than a fluorinert include methanol, ammonia, a mixture of water and ethylene glycol, and a mixture of water and methanol. 
   Due to the fact that they have higher latent heats of vaporization, these other types of coolant have the potential to cool more efficiently than an equal amount of a fluorinert. On the other hand, all of these various other coolants are electrically conductive, and thus are not suitable for use in the embodiment of  FIGS. 1 and 2 , because they cannot be brought into direct contact with a MMIC. Therefore, in order to provide for use of one of these other types of coolant (or even a fluorinert), a further embodiment is shown and described in association with  FIG. 3 , where efficient cooling is achieved without direct contact between the coolant and a circuit component such as a MMIC. 
   More specifically,  FIG. 3  is a diagrammatic fragmentary sectional top view of a portion of a slat which is an alternative embodiment of the slat  16  shown in FIG.  2 . In  FIG. 3 , a cooling plate  110  made of AlSiC has several passageways extending through it, one of which is visible at  111  in  FIG. 3. A  coolant is caused to flow through the passageway  111 , as indicated diagrammatically by broken lines in FIG.  3 . The distribution of coolant to respective passageways  111  is effected through respective not-illustrated orifices, in a manner similar to that described above in association with the embodiment of FIG.  2 . In  FIG. 3 , the coolant is water, but could alternatively be ammonia, methanol, a mixture of water and methanol, a mixture of water and ethylene glycol, a fluorinert, or any other suitable two-phase coolant. The cooling plate  110  has openings  116  and  117  provided through opposite sides thereof. 
   A highly thermally conductive spreader part  121  is fixedly securing within the opening  116  by a ring of a sealant  123 . The sealant  123  can be any suitable known sealant, and prevents coolant from leaking out of the passageway  111  around the edges of the thermally conductive part  121 . In a similar manner, an identical spreader part  122  is fixedly secured within the opening.  117  by a ring of sealant  124 . In the embodiment of  FIG. 3 , the thermally conductive parts  121  and  122  are each made of synthetic diamond, which is highly thermally conductive, but they could alternatively be made of any other suitable material, one example of which is beryllium oxide (BeO). 
   The thermally conductive parts  121  and  122  each have a plurality of fins or projections  126  or  127  that extend inwardly, in a direction toward the passageway  111 . The ends of the projections  126  and  127  may be disposed outwardly of, even with the edges of, or within the passageway  111 . The length and position of the projections  126  and  127  ensures that the coolant will be properly distributed so as to make contact with the projections  126  and  127 . The projections  126  and  127  result in a substantial increase in the amount of surface area on the thermally conductive parts  121  and  122  which engages the coolant, in comparison to a not-illustrated configuration in which each of the thermally conductive parts has just a flat inner surface that engages the coolant. Since the projections  126  and  127  significantly increase the amount of surface area on the parts  121  and  122  which engages the coolant, they also significantly increase the rate at which heat can be transferred from the thermally conductive parts  121  and  122  to the coolant. 
   The thermally conductive parts  121  and  122  each have a flat surface on a Bide thereof opposite from the projections  126  and  127 . A respective MMIC  131  or  132  is fixedly mounted on each of these flat surfaces, with the back side of each MMIC  131  or  132  facing toward and the active-junction side of each MMIC facing away from the associated thermally conductive part  121  or  122 . The MMICs  131  and  132  can be secured in place by a known epoxy which is highly thermally conductive, or in any other suitable manner which ensures that the MMICs  131  and  132  are in substantially direct thermal communication with the thermally conductive parts  121  and  122 , so as to achieve a very high level of thermal conductivity between each MMIC and the associated thermally conductive part. 
   Although  FIG. 3  shows the thermally conductive parts  121  and  122  which are disposed between the coolant and the MMICs  131  and  132 , it would alternatively be possible for the coolant to directly contact the back sides of the MMICs, even if the coolant is electrically conductive. For example, the thermally conductive parts could have openings therethrough which expose the back sides of the MMICs to the coolant, in which case the parts  121  and  122  could be either thermally conductive or non-conductive. Moreover, where the back sides of the MMICs are exposed directly to the coolant, the back sides could be roughened to facilitate heat transfer, and/or could have integral projections similar to the projections  126  and  127  on the thermally conductive parts. 
   In the embodiment of  FIG. 3 , two multi-layer circuit boards  138  and  139  are supported on opposite sides of the cooling plate  110 . Each of the circuit boards has an opening therethrough, and a respective one of the thermally conductive parts  121  and  122  extends through each such opening. Two transmit/receive modules  146  and  147  are disposed on opposites of the cooling plate  110 . The modules  146  and  147  are each generally analogous to the type of transmit/receive module shown at  51  in  FIG. 2 , although there are some differences. 
   In this regard, each of the transmit/receive modules  146  and  147  includes a housing defined by a ceramic base plate  148  or  149  that has its peripheral edges fixedly and sealingly secured to edges of a ceramic cover  151  or  152 , for example by brazing. The base plates  148  and  149  each have therethrough a stepped opening  153  or  154 , which matingly engages a corresponding stepped opening in a respective one of the thermally conductive parts  121  and  122 . The base plates  148  and  149  are fixedly and sealingly secured to the respective thermally conductive parts  121  or  122 , for example by brazing. Thus, the housings of the modules  146  and  147  are each hermetically sealed. 
   Each of the transmit/receive modules  146  and  147  has therein a respective circuit board  161  or  162 , which in turn has transmit/receive circuitry of a known type thereon, and which has therethrough an opening that accommodates the associated MMIC  131  or  132 , and an end of the corresponding thermally conductive part  121  or  122 . The MMICs  131  and  132  are each electrically coupled to the associated circuit board  161  or  162  by a plurality of gold wires, for example as indicated diagrammatically at  163  and  164 . 
   The facing surfaces of the base plate  148  and the multi-layer circuit board  138  each have a plurality of pads. A plurality of balls of solder, arranged in what is commonly known as a ball grid array, are provided between the base plate  148  and the circuit board  138 . The solder balls  171  of the ball grid array are surrounded by an underfill  173  of a type known in the art, which is not electrically conductive. In a similar manner, facing surfaces of the base plate  149  and the multi-layer circuit board  139  have pads which are electrically coupled to each other by solder balls  172  of a further ball grid array. The solder balls  172  in this second ball grid array are surrounded by an associated underfill  174 , which is electrically non-conductive. 
   The above-mentioned pads on the base plate  148  are electrically coupled to the transmit/receive circuit board  161  in a suitable manner, for example by not-illustrated vias which extend through the base plate  148  in a manner that effects a hermetic seal for the housing of the module  146 . In a similar manner, the pads on the base plate  149  are electrically coupled to the transmit/receive circuit board  162  in a suitable manner, for example by not-illustrated vias which extend through the base plate  149  in a manner that effects a hermetic seal for the housing of the module  147 . 
   In operation, the MMICs  131  and  132  each generate substantial amounts of heat, the majority of which is transferred to a respective one of the highly thermally conductive parts  121  and  122 . Coolant in a liquid phase enters the passageway  111  from the left side in  FIG. 3 , and travels rightwardly to the thermally conductive parts  121  and  122 . The coolant directly engages the thermally conductive parts  121  and  122 , where it absorbs enough heat so that part or all of the coolant boils and is vaporized. The resulting coolant vapor, along with any remaining liquid coolant, continues rightwardly through the passageway  111 . It is then eventually routed through a cooling condensation system of the type shown at  27  in  FIG. 1 , where it is cooled in a manner that causes it to revert from its vapor phase to its liquid phase. After that, this coolant can again be supplied to the passageway  111 . 
   Although only two transmit/receive modules  146  and  147  are visible in  FIG. 3 , a number of the transmit/receive modules are provided on each side the cooling plate  110  and circuit boards  138  and  139 , in a manner comparable to the depiction in  FIG. 2  of the column of transmit/receive modules (including the module  51 ). 
   The present invention provides a number of technical advantages. One such technical advantage results from the use of a two-phase coolant which engages a heat-generating circuit component, either directly or substantially directly through a highly thermally conductive part, in order to extract heat at a high rate from the circuit component while minimizing the temperature differential between the coolant and the heat-generating structure within the circuit component. This permits the extraction of heat at rates substantially greater than the rates available with pre-existing cooling techniques, and in fact the rates achieved through use of the invention can be as much as five to ten times the existing rate. Since cooling is effected primarily by coolant vaporization, which occurs at a specific temperature, application of the invention to a phased array antenna system offers the advantage of providing a cooling effect at substantially the same temperature throughout the antenna array. This helps to eliminate temperature gradients within the phased array antenna system. As is well known, temperature gradients within a phased array antenna system are disadvantageous because they produce undesirable phase shifts across the array, and these undesirable phase shifts degrade accurate operation. As the operational frequency selected for a phased array antenna system is progressively increased, there is a progressive decrease in the magnitude of permissible temperature gradients within the array. The present invention is highly effective in minimizing temperature gradients, in a manner that permits accurate operation even at high frequencies. 
   A further advantage results from the use of orifices that control the flow rate or flow volume of coolant to respective heat-generating circuit components, which permits the cooling system to be configured to more efficiently remove different levels of concentrated heat from different circuit components. Still another advantage is that, because the present invention provides highly efficient cooling, it is possible to provide phased array antenna systems which are smaller and lighter than pre-existing systems, and which provide performance that equals or surpasses the performance of larger and heavier preexisting systems. Yet another advantage is that the significant features of the disclosed cooling technique are internal to the system in which the techniques are applied, such as a phased array antenna system. 
   Although selected embodiments have been illustrated and described in detail, it will be understood that various substitutions and alterations are possible. For example, the invention has been presented in the context of a phased array antenna system, but could alternatively be utilized in some other type of high thermal density electronic system, such as a cell phone ground base station, a high power directed energy weapon, a high power processor device, or a high density power supply. Various other substitutions and alterations are also possible without departing from the spirit and scope of the present invention, and as defined by the following claims.