Abstract:
A front-to-back cooling system allows cooling of an apparatus containing two orthogonal sets of modules. A vertical set of modules is cooled with vertical air flow across the modules that enters from a front of the apparatus and exits from the back of the apparatus. A horizontal set of modules is cooled with air flow that passes through openings in a midplane connecting the two sets of modules.

Description:
TECHNICAL FIELD 
     The present invention relates to the field of cooling systems, and in particular to cooling of a modular chassis with orthogonal modules. 
     BACKGROUND ART 
     Systems that require very high bandwidth any-to-any connectivity among a set of modules typically use an orthogonal mid-plane configuration. In this configuration, a set of cards are plugged into the front side of the mid-plane in vertical configuration and another set of cards are plugged into the rear side of the mid-plane in horizontal configuration. This layout enables each front card to be directly connected to each rear card, and makes it possible to eliminate the use of PCB signal traces on the mid-plane to carry high speed signals. 
     However, the orthogonal configuration also creates a cooling challenge, especially in applications where front-to-back cooling is required. Vertical cards can be cooled using conventional cooling mechanisms with front air intake and rear air exhaust, but cooling the horizontal cards while maintaining overall front-to-back air flow is challenging. 
     If front-to-back cooling is not required, the horizontal card cage can be cooled using side-to-side air flow. However, many rack mount environments require front-to-back air cooling. One solution has been to divert air taken from a front intake to the back and run it up in a column next to the horizontal cards. Such a mechanism typically uses a set of fans or blowers to create the air pressure across the horizontal cards. However, the amount of air flow that is provided in such a system is typically limited due the number of turns in the air path. Also, the placement of one or two fan blades along the sides of the horizontal cards can severely limit the PCB area and panel surface that is available. 
     SUMMARY OF INVENTION 
     In one embodiment, a method of cooling an apparatus, comprises defining a plenum, positioned between a pair of a first plurality of modules, forming an opening in a midplane, the midplane electrically connecting the first plurality of modules with a second plurality of modules oriented orthogonally to the first plurality of modules, the opening in fluid communication with the plenum, moving air from a front of the apparatus through the plenum, through the opening, and across a surface of a module of the second plurality of modules, and exhausting the air at a rear of the apparatus. 
     In another embodiment, an apparatus, comprises a first plurality of modules, a second plurality of modules, mounted orthogonally to the first plurality of modules, a midplane, positioned between and electrically connecting the first plurality of modules and the second plurality of modules, the midplane having an opening formed through the midplane and adapted to allow air movement through the midplane, a plenum defined between a pair of the first plurality of modules, in fluid communication with the opening, and a fan configured to move air through the opening from the plenum and across a surface of a module of the second plurality of modules. 
     In yet another embodiment, an apparatus, is disclosed comprising a chasis a first plurality of modules mounted in the chassis, a second plurality of modules mounted in the chassis orthogonally to the first plurality of modules, a midplane, positioned between and electrically connecting the first plurality of modules and the second plurality of modules, means for providing fluid communication from a front of the chassis through an opening formed in the midplane, and means for moving air through the opening. 
    
    
     
       BRIEF DESCRIPTION OF DRAWINGS 
       The accompanying drawings, which are incorporated in and constitute a part of this specification, illustrate an implementation of apparatus and methods consistent with the present invention and, together with the detailed description, serve to explain advantages and principles consistent with the invention. In the drawings, 
         FIG. 1  is a cutaway perspective view illustrating a chassis and plenums according to one embodiment; 
         FIG. 2  is a cutaway perspective view illustrating a chassis and plenums according to a second embodiment; 
         FIG. 3  is a cutaway perspective view illustrating a chassis and plenums according to a third embodiment; and 
         FIG. 4  is a top perspective view of the embodiment of  FIG. 1 . 
     
    
    
     DESCRIPTION OF EMBODIMENTS 
       FIG. 1  is a cutaway perspective view of a chassis  100 . A plurality of modules  110 , typically circuit boards, are mounted in a vertical orientation in the front of the chassis  100 . A plurality of modules  150 , also typically circuit boards, are mounted orthogonally to the plurality of modules  110  and a horizontal orientation in the rear of the chassis  100 . A midplane  140  is positioned between the modules  110  and the modules  150  to electrically connect the modules  110  and  150 . A plurality of openings or holes is formed through the midplane  140  to allow air flow to the rear modules  150 . 
     As shown in  FIG. 1 , the plurality of openings  130  is distributed evenly across the midplane  140 , but this arrangement and number of holes is exemplary and illustrative only and any desired arrangement and number of openings  130  can be formed in the midplane  140  as needed because of heating characteristics of the modules  150 . For example, if some of the modules  150  generate more heat than others of the modules  150 , then the midplane  140  may have more openings  130  to provide more air flow to the hotter modules  150 . Similarly, if some portions of one of the modules  150  require more cooling than others, more or bigger holes could be placed at or near that portion of the modules  150 , while fewer or smaller holes  130  could be placed near the cooler portion of the modules  150 . At least one opening  130  must correspond to each of the rear modules  150 . The openings  130  are preferably sized to provide sufficient air flow across the rear modules  150  to provide appropriate cooling for the modules  150 . More than one opening  130  may be formed for each horizontal module  150  as desired and as electrical circuit positioning constraints on the midplane  150  allow. 
     Heated air from the first and second plurality of modules is exhausted rearwards from the horizontal modules  150  and the vertical modules  110  through exhaust outlets  160  on the rear of the chassis  100 . In  FIGS. 1-3 , air flow across the vertically oriented modules  110  is driven by a lower fan tray  180  containing fans that take air from a lower air intake  170  and blow air vertically across the modules  110 . An upper fan tray  190  can provide additional air flow to take heated air from the vertical modules  110  and exhaust the air to the rear of the chassis  100  through a plenum  240  through exhaust outlets  160 . 
     The embodiment of  FIG. 1  provides a plurality of plenums  120  that allow air flow from a plurality of front inlets  175  of the chassis  100  through one or more of the openings  130  in the midplane  140 . Two walls, closed at the top and bottom form a rectangular plenum  120  open to the front of the chassis  100  through upper air intakes  175 . The pitch of the modules  110  must be sufficient to allow placement of the plenums  120  between a pair of modules  110 . In some embodiments, the plenums  120  provide at least a one quarter inch air gap, but preferably, the plenums  120  provide at least a one half inch air gap. In this embodiment, the cooling of the front vertical modules  110  is independent of the cooling of the rear horizontal modules  150 . Preferably, in embodiments similar to that illustrated in  FIG. 1 , negative pressure is used to pull air from the upper front intakes  175  through the plenum  120  and openings  130 . The air is then drawn across the surface of the rear modules  150  and exhausted through rear outlets  160 . 
     Not shown in  FIG. 1  are the fans or blowers used to move the air for the rear modules  150  when negative pressure is used.  FIG. 4  is a top perspective view of the chassis  100  illustrating an exemplary fan configuration. As shown in  FIG. 4 , two fans or blowers  410  on each rear module  150  pull air from the openings  130  in the midplane  140 . In other embodiments, a single fan or blower  410  could be used, typically in a central rear position. Alternatively, one or more fans could be mounted distal to a line from one of the openings  130  to the rear of the chassis  100  at the sides of the modules  150 . At least one fan  410  would need to be mounted with each rear module  150  in embodiments such as illustrated in  FIGS. 1 and 4 . 
     Although positive pressure and front fans could be used in the embodiments illustrated in  FIG. 1  to push air through the plenums  120 , the openings  130 , and across the rear modules  150 , size constraints generally make the negative pressure embodiments shown in  FIG. 4  preferable. 
       FIGS. 2 and 3  illustrate alternative embodiments that divide air flow between the plenums  120  and the vertical front modules  110 , using fans in fan tray  180  to push air through the plenums  120  and through the openings  130  to the rear modules  150 . In such embodiments, the fans  410  of  FIG. 4  may not be necessary to achieve sufficient air flow, but in some embodiments can be used to improve air flow and cooling across the rear modules  150 . 
     In the embodiments illustrated in  FIGS. 2 and 3 , the plenums  210  and  310  are not open to the front, like the plenums  120 , so the upper front air intakes  175  of  FIG. 1  are not required. Instead, the plenums  210  and  310  are open to the bottom to allow air to be pushed through the plenums  210  and  310  by the fans of fan tray  180 . In embodiments as illustrated in  FIG. 2 , the plenums  210  are closed on top with vertical walls forming a rectangular plenum  210  where the air makes a turn to exit the plenums  210  through openings  130 . As with the embodiments illustrated by  FIG. 1 , the pitch of the modules must be sufficient to allow insertion of the separate plenum units  210  between a pair of modules  110 . 
     Embodiments illustrated by  FIG. 3  are similar to those illustrated by  FIG. 2 , but instead of using separate plenum units  210 , they take advantage of the common practice of mounting modules  110  in a carrier or tray  320  for insertion into the chassis  100 . By leaving an air gap between the carriers  320  and a surface, typically an underside, of the modules  110 , the carriers  320  and modules  110  together form plenum  310  that can be aligned with openings  130  to provide cooling air flow through the openings  130  to cool the rear modules  150 . As with the embodiments illustrated in  FIG. 2 , air flow is provided by the fans of the fan tray  180  and the air flow is divided between cooling the modules  110  and the plenum  310 . In some embodiments, the air gap forming the plenum  310  can be as small as one quarter inch, but preferably the air gap is at least one half inch. The size of the plenums  310  is engineered based on the cooling needs of the rear modules  150 . Use of embodiments similar to that illustrated in  FIG. 3  avoids the need for separate plenum units  210  and can allow for closer spacing of the modules  110 . 
     In both  FIGS. 2 and 3 , air passed over the vertical front modules  110  is exhausted through a plenum  240  formed or defined above the horizontal modules  150  and exhausted through upper air exhausts  160 . 
     While certain exemplary embodiments have been described in details and shown in the accompanying drawings, it is to be understood that such embodiments are merely illustrative of and not devised without departing from the basic scope thereof, which is determined by the claims that follow. By way of example and not limitation, the specific electrical components utilized may be replaced by known equivalents or other arrangements of components which function similarly and provide substantially the same result.