Abstract:
A computer server rack for mechanically supporting and electrically powering computer servers is provided. The rack includes DC-to-DC conversion circuits which are configured to convert a relatively high DC facility voltage (e.g. 380 volts DC provided throughout a data center on busways and/or appropriate wiring) to DC voltage useable directly by a computer server (e.g. 12 volts DC). The conversion circuits are highly efficient and convert substantially all of power input to the circuits at the input voltage and input DC current to a high DC output current at a lower output voltage. The conversion circuits are distributed among the computer servers in an electrically parallel arrangement to provide a predetermined level of redundancy, to permit replacement of the circuits without disrupting power to the servers in the rack, to reduce variations in the voltage supplied to the servers along rack bus bars or power conductors as a result of the current flowing and the impedance of the bus bars or conductors distances between conversion circuits and servers distributed within the rack, and to increase the internal server volume dedicated to the servers.

Description:
CROSS-REFERENCE TO RELATED APPLICATIONS 
       [0001]    This application claims priority to U.S. Provisional Application No. 61/945,529, filed Feb. 27, 2014, the contents of which are incorporated by reference in their entirety. 
     
    
     BACKGROUND OF THE INVENTION 
       [0002]    The present invention relates generally to providing electrical power provided by a direct current (“DC”) power source to computer servers located in a data center. The present invention relates more specifically to the conversion of electrical power from an input voltage source to at least one output voltage and power distribution in space-efficient configurations. 
       BRIEF SUMMARY OF THE INVENTION 
       [0003]    In one aspect, the invention provides a rack frame with a plurality of support rails. The support rails are configured to support a plurality of computer servers in a stacked, parallel relationship. The rack frame also includes a first DC-to-DC conversion circuits including a DC-to-DC converter. The DC-to-DC converter includes a transformer having a primary winding and a secondary winding, first and second input terminals coupled to the primary winding, and first and second output terminals coupled to the secondary winding. The DC-to-DC converter reduces a voltage applied between the input terminals by at least a factor of 6 relative to the voltage applied to the output terminals by the DC-to-DC converter. The rack frame further includes a first power bus attached to the rack frame perpendicular to the support rails and mechanically supporting the DC-to-DC conversion circuit. The power bus includes first and second spaced, parallel bus bars connectable to the servers supported by the support rails to power the respective servers with DC power at substantially the voltage applied to the output terminals by the DC-to-DC converter. The first output terminal of the conversion circuit is electrically coupled to the first bus bar, and the second output terminal of the conversion circuit is electrically coupled to the second bus bar. 
         [0004]    In another aspect, the invention provides a power bus for powering servers connected in a spaced relationship along the bus. Power is applied to the servers with a direct current at a nominal voltage. The power bus includes a first metal bus bar supported relative to a second metal bus bar in a parallel spaced relationship. The power bus also includes a first DC-to-DC conversion circuit having a first DC-to-DC converter with a transformer having a primary winding and a secondary winding. First and second input terminals are coupled to the primary winding, and first and second output terminals are coupled to the secondary winding. The DC-to-DC converter reduces a voltage applied between the input terminals by at least a factor of 6 relative to the voltage applied to the output terminals by the DC-to-DC converter. The first metal bus bar is coupled to the first output terminal at a first location, and the second metal bus bar is coupled to the second output terminal at a second location. The power bus also includes an insulated housing which covers the bus bars and the conversion circuit. The insulated housing provides access openings to permit coupling of electric loads to the bus bars and to permit access to the conversion circuit, thereby permitting replacement of a conversion circuit while the bus bars remain powered. The insulated housing including thermal conduction to transfer heat energy from the conversion circuits. 
         [0005]    In yet another aspect, the invention provides a rack frame having a plurality of support rails configured to support a plurality of computer servers in a stacked, parallel relationship. The rack frame includes at least first and second DC-to-DC conversion circuits each including a DC-to-DC converter. The DC-DC converter includes a transformer having a primary winding and a secondary winding, first and second input terminals coupled to the primary winding, and first and second output terminals coupled to the secondary winding. The DC-to-DC converter reduces a voltage applied between the input terminals by at least a factor of 6 relative to the voltage applied to the output terminals by the DC-to-DC converter. The rack frame also includes frame supports mechanically attaching the DC-to-DC conversion circuits to the rack frame. The rack frame further includes first and second power conductors connectable to servers supported by the support rails. The first and second power conductors power the respective servers with DC power at substantially the voltage applied to the output terminals by the DC-to-DC converter. The first output terminals of the conversion circuits are electrically coupled to the first power conductor in a spaced relationship, and the second output terminals being electrically coupled to the second power conductor in a spaced relationship. The spaced relationships are configured such that electric power from the DC-to-DC conversion circuits is applied to the power conductors to reduce voltage variations on the power conductors when the conductors are powering a plurality of servers supported by the rack frame. 
         [0006]    Other aspects, objectives and advantages of the invention will become more apparent from the following detailed description when taken in conjunction with the accompanying drawings. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0007]    The accompanying drawings incorporated in and forming a part of the specification illustrate several aspects of the present invention and, together with the description, serve to explain the principles of the invention. In the drawings: 
           [0008]      FIG. 1  is s system diagram illustrating supply of electrical power to computers (e.g. data servers) in a data center, where the electrical power is converted from alternating electrical power (“AC”) to DC power which can be more efficiently used by the computers; 
           [0009]      FIG. 2  is a front perspective view of a DC-to-DC converter according to the present invention; 
           [0010]      FIG. 3  is a rear perspective view of a DC-to-DC converter according to the present invention; 
           [0011]      FIG. 4  is an front view of a rack frame having DC power distribution according to the present invention; 
           [0012]      FIG. 4A  is an front view of another embodiment of a rack frame having DC power distribution according to the present invention; 
           [0013]      FIG. 5  is a front detail view of a rack frame having DC power distribution according to the present invention; 
           [0014]      FIG. 6  is front isometric view of a rack frame having DC power distribution according to the present invention; 
           [0015]      FIG. 7  is a front view of view of power supply bars and a DC-to-DC converter according to the present invention; and 
           [0016]      FIG. 8  is a front perspective view of power supply bars and a DC-to-DC converter according to the present invention. 
           [0017]    While the invention will be described in connection with certain preferred embodiments, there is no intent to limit it to those embodiments. On the contrary, the intent is to cover all alternatives, modifications and equivalents as included within the spirit and scope of the invention as defined by the appended claims. 
       
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
       [0018]    The embodiments illustrated and described are representative computer rack systems and equipment suitable, for example, for use with a direct current uninterruptible power system (DC UPS). 
         [0019]    Referring to  FIG. 1 , an embodiment of a DC UPS suitable for use with the present invention is illustrated. The DC UPS shown includes: 
         [0020]    Alternating current input source  2 , which for a majority of data centers would be 480 volt (hereinafter “volt” or “v”), three-phase, alternating current (AC); 
         [0021]    Alternating current input connection or distribution  4 , which would typically include an AC load center including appropriate circuit protection (e.g. three-phase circuit breakers) coupled between the AC supply and AC wiring or an AC power busway; 
         [0022]    Direct Current Un-interruptible Power System (DC UPS)  6 , which in a preferred embodiment for a data center would be constructed in modular form to include multiple systems each supported within a rack (represented by the rectangle indicated at the arrow from no.  6 ) having form factor and frame construction the same as or similar to the racks supporting the servers in the data center; 
         [0023]    Alternating current to direct current conversion stage  8 , which, for purposes of the preferred embodiment, would be the type of a conversion system commonly used in data centers to convert the AC supply to DC voltage to power the energy storage with DC power; 
         [0024]    Internal DC power bus  10   a  and  10   b;    
         [0025]    Energy storage system  12 ; 
         [0026]    Earth Ground terminal  32 ; 
         [0027]    Direct current to direct current conversion stage  14  which has its ground terminal  16  coupled to terminal  32  with a ground impedance  18 ; 
         [0028]    Output power conductors (+190 volts DC, neutral, −190 volts DC)  20   a,    20   b,    20   c  of stage  14 , wherein neutral conductor  20   b  is coupled to terminal  32  by impedance  24 ; 
         [0029]    Distribution system  26 , which may include a load center including appropriate circuit protection (e.g. two phase DC-rated circuit breakers for each branch circuit) coupled between the conductors  20   a,    20   b  and  20   c,  and DC wiring or DC power busways which define each branch circuit  27   n   1 ,  27   n   2 ,  27   nn;    
         [0030]    Output power conductors (+190 volts DC, neutral, −190 volts DC)  29   a,    29   b,    29   c  for each branch circuit wherein the neutral conductor  29   b  is coupled to terminal  32  by an impedance  30 ; 
         [0031]    loads  28   n   1 ,  28   n   2 ,  28   n,  which in the preferred embodiments are DC to DC conversion circuits which convert the +/−190 volts DC to 12 volts DC for provided 12 volts DC power to computers and servers supported by server racks, as discussed in further detail below; 
         [0032]    parallel bus bars  140 ,  142 ; and 
         [0033]    one or more DC loads  160 , for example computer servers. 
         [0034]    The component and circuit symbols used in  FIG. 1  are industry standard symbols. The DC UPS shown in  FIG. 1  may be the DC UPS disclosed in U.S. application Ser. No. 14/632,274, the entire contents on which are incorporated by reference. 
         [0035]    Referring to  FIGS. 2-3 , a DC-DC conversion circuit  28  is shown. In the embodiment shown, DC-DC conversion circuit  28  includes a front board  30  and a rear board  32 . This configuration is provided to meet limitations to width  152  of DC-DC conversion circuit  28  associated with the bus bar and housing configurations discussed in further detail below. In other embodiments, a single board may be employed. 
         [0036]    Conversion circuit  28  includes a DC-DC converter  40 . In one embodiment, converter  40  is a single package DC to DC converter of the type used for certain electric automobile applications when combined with appropriate output power conditioning/filtering. For example, the DC-DC converter  40  may be provided as a dual in-line package. DC-DC converter  40  may be a current-fed forward converter as described in U.S. Pat. No. 4,675,797, the entire contents of which are incorporated by reference herein. DC-to-DC converter  40  includes a transformer having a primary winding and a secondary winding. DC-DC converter  40  also includes first DC input  42 , shown as positive terminal  44 , and second DC input  46 , shown as negative terminal  48 . The input terminals  44 ,  48  of multiple DC-DC conversion circuits  28  may be coupled to a common source of DC power, such that the first input terminals  44  of multiple DC-DC conversion circuits  28  are electrically connected, and second input terminals  48  of multiple DC-DC conversion circuits  28  are electrically connected. Inputs  44 ,  48  of each DC-DC conversion circuit  28  are coupled to the primary winding of their respective DC-DC converter  40 . In some embodiments, DC-DC converter  40  or other components of DC-DC conversion circuit  28  may be provided with a heat sink and/or active cooling elements (e.g., a fan) to remove heat energy away from the conversion circuit  28 . 
         [0037]    DC-DC converter  40  further includes first and second output terminals  54 ,  58  coupled to the secondary winding. In one embodiment, output terminal  54  is a positive DC output, and output terminal  58  is a ground DC output. In the embodiment shown, positive output  54  is a bus bar clip  56 , and ground output  58  is a bus bar clip  60 . As shown, bus bar clips  56 ,  60  extend generally perpendicularly from front board  30 . Bus bar clip  56 ,  60  may be engaged to bus bar  140  of rack  100 , and bus bar clip  60  may be engaged to bus bar  142  of rack  100  (as shown in  FIGS. 4-6 ). When bus bar clips  56 ,  60  are engaged to bus bars  140 ,  142 , DC-DC conversion circuit  28  and DC-DC converter  40  may be mechanically supported by bus bars  140 ,  142 . 
         [0038]    In a preferred embodiment, the DC-to-DC converter  40  of DC conversion circuit  28  reduces the voltage applied between the input terminals by at least a factor of 10 relative to the voltage applied to the output terminals. In a particularly preferred embodiment, the DC-to-DC converter  40  of DC conversion circuit  28  reduces the voltage applied between the input terminals by a factor of  32  relative to the voltage applied to the output terminals. In a preferred embodiment of circuit  28  the output voltage is proportional to the input voltage within a predetermined range of input voltages. Accordingly, the differential voltage input=K*differential voltage output. For a K of 32, a differential input voltage of 380 volts =a differential output voltage of 11.875 volts. In other embodiments, DC-DC converter  40  is a regulated DC-DC converter with a variable DC input voltage and a constant DC output voltage. In still other embodiments, DC-DC conversion circuit  28  may supply a nominal output ranging from 6-60 volts, for example a nominal output of 48 volts. In embodiments where a higher nominal output is required, DC-DC converter  40  of DC-DC conversion circuit  28  may reduce the voltage applied between the input terminals by at least a factor of 6 relative to the voltage applied to the output terminals. 
         [0039]    The DC-DC converter  40  of DC-DC conversion circuit  28  includes a switching circuit and a capacitor coupled to the primary winding, the switching circuit being switchable to transfer electrical energy applied to the input terminals at a voltage in the range of 260 volts to 410 volts to the output terminals at a voltage in the range of 8.1 to 12.8 volts. In a preferred embodiment, the switching circuit is switchable at a frequency which transfers electrical energy applied with a first DC amperage having a nominal voltage differential of 380 volts at the input terminals  44 ,  48  to a second DC amperage greater than the first DC amperage and having a nominal voltage differential of 12 volts at the output terminals  54 ,  58 . 
         [0040]    As shown, rear board  32  is electrically coupled to front board  30  by a plurality of male connectors  34  and female connectors  36 . Rear board  32  includes an energy storage device  62 , shown as capacitors  64 . In the event of a momentary disruption of high-voltage DC inputs  42 ,  46 , energy storage device  62  can provide stored DC power to DC outputs  54 ,  58 . Rear board  32  may also include a reverse-current prevention diode  68  and bleed resistor  66 . Depending upon system use and requirements, the DC-DC conversion circuit  28  may not require an energy storage device  62 . 
         [0041]    DC-DC conversion circuit  28  may optionally be provided with an operation indicator, such as a visual indicator. The visual indicator may be a light-emitting diode. An operation indicator can be configured to indicate the operational state of the DC-DC conversion circuit  28 , for example when the DC-DC conversion circuit  28  is powered and operating normally, or to indicate that the DC-DC conversion circuit  28  is not operating properly. In some embodiments, the operation indicator may be configured to provide an indication of a failure type of the DC-DC conversion circuit  28 . 
         [0042]    Referring to  FIGS. 4-6 , a rack  100  is shown. Chassis components of rack  100  includes front vertical frame posts  102 , rear vertical frame posts  104 , base  106 , and top  108 . Rear vertical frame posts  104 , base  106 , and top  108  define a rear wall area  110  of rack  100 . Additionally, a rack volume  112  of rack  100  is defined by the space within base  106 , front vertical frame posts  102 , rear vertical frame posts  104 , and base  108 . In some embodiments, rack volume  112  of rack  100  may divided into a plurality of vertically-stacked power zones, shown as a bottom zone  120 , middle zone  122 , and a top zone  124 . In some rack embodiments, a power shelf volume  126  defined within rack volume  112  by left and right power shelf supports  127  may be associated with each power zone. However, in a preferred embodiment, power shelf volumes  126  are not present in rack  100 . In such an embodiment, DC-DC conversion circuits are not located in a vertically-stacked relationship with computer servers  160 , that is, DC-DC conversion circuits are not located in power shelf volumes  126 . For example, DC-DC conversion circuits  28  may be disposed on the rear wall area  110  of rack  100 . When positioned on the rear wall  110  of rack  100 , DC-DC conversion circuits  28  do not reduce the internal volume  112  available for servers  160 . In another embodiment shown in  FIG. 4A , rack  100  may be configured to have a single power zone extending from base  106  to top  108  of rack  100 . Chassis components of rack  100  may be grounded to an earth ground. 
         [0043]    Rack  100  is provided with a plurality of left  128  and right  130  support rails. Support rails  128 ,  130  are adjustably coupled to front  102  and rear  104  frame posts. In a preferred embodiment, support rails  128 ,  130  are L-shaped brackets that snap into frame posts  102 ,  104 . Accordingly, the plurality of support rails  128 ,  130  may be configured to permit installation and support of computer equipment  160  (e.g. servers) or other equipment having different chassis heights in a vertically stacked, parallel relationship. In a preferred embodiment, rack  100  is dimensioned to permit installation of servers  160  conforming to the Open Rack standard and having an OpenU rack slot height of 48 mm, as described in “Open Rack Hardware v1.0” by P. Sarti and S. Mills (2012), the contents of which are incorporated by reference in their entirety. The Open Rack rack has a nominal width of 600 mm wide and an equipment bay width of 538 mm, and depth between 350 mm to 1220 mm (corresponding to a maximum computer equipment depth of 914 mm). In some embodiments the total rack height may be 2100 mm. Generally, the rack  100  may be constructed from zinc-plated sheet metal or another suitable material. Front  102  and/or rear  104  frame posts may be a stamped sheet metal configured with “C” channels to provide cabling runs. Flanges are built in to the channels that can retain the cables, routed vertically, using for example velcro straps or cable ties. 
         [0044]    In the embodiments shown, rack  100  is provided with a power bus, shown as a first bus bar  140  and a second bus bar  142  located at the rear wall area  110  of rack frame  100 . Parallel bus bars  140 ,  142  are attached to rack  100  at rear wall  110 , and are supported by rear crosspieces  132 . In the embodiment shown, bus bars  140 ,  142  are attached perpendicular to support rails  128 ,  130 . One or more DC-to-DC conversion circuits  28  are electrically connected to bus bars  140 ,  142  by bus bar clips  56 ,  60  of DC-to-DC conversion circuits  28 . Bus bars  140 ,  142  also mechanically support the DC-DC conversion circuits  28  at the rear wall  110  of rack  100 , thereby eliminating the need for power shelf volumes  126  in the rack volume  112  of rack  100  and increasing the volume within rack volume  112  available for computer equipment  160 . DC-DC conversion circuits  28  a voltage differential between bus bars  140 ,  142 . The DC-DC conversion circuits  28  thereby supply DC power to the power bus at the same voltage supplied to the bus bar clips  56 ,  60  by the DC-DC converter  40 . 
         [0045]    As shown in  FIG. 4 , three pairs of bus bars  140 ,  142  are provided at the rear of each of the three power zones  120 ,  122 ,  124  of rack  100 . In a typical embodiment, the longitudinal axis of each bus bar  140 ,  142  is oriented vertically. Bus bars  140 ,  142  are electrically isolated from rack  100 . In embodiments where one bus bar is supplied with a ground voltage, the grounded bus bar may be electrically coupled to a grounded chassis of rack  100  Computer equipment (e.g., computer servers  160 ) are connectable to bus bars  140 ,  142  when the computer equipment is slidingly supported by support rails  128 ,  130 . Bus bars  140 ,  142  thereby supply power to the computer equipment  160  at substantially the same voltage applied to the output terminals  54 ,  58  of DC-DC converter  40  of DC-DC conversion circuit  28 . Where two or more DC-DC conversion circuits  28  are electrically coupled to bus bars  140 ,  142  in a spaced-apart relationship, the supply of DC voltage at multiple discrete locations minimizes the resistance between the power source and load, thereby minimizing the voltage drop between DC-DC conversion circuits  28  and the computer equipment  160  coupled to the bus bars  140 ,  142 . In a preferred embodiment, each of a plurality of DC-DC conversion circuits  28  are placed at approximately the center of equal-length segments of bus bars  140 ,  142 . 
         [0046]    Other arrangements of bus bars  140 ,  142  and DC-DC conversion circuits  28  are contemplated by the present invention. For example, where rack  100  is configured to have a single power zone, one or more pairs of bus bars  140 ,  142  may extend substantially the entire height of rack  100 , that is, from base  106  to top  108 . A pair of bus bars  140 ,  142  may be provided with two, three, or four or more DC-DC conversion circuits  28 . Alternatively, each power zone of rack frame  100  may be configured with two pairs of bus bars, for example at the left and right of the rack, or even with a single pair of bus bars, for example at the center of rear wall  110  the rack  100 . In other embodiments, DC-DC conversion circuits  28  may be mechanically supported by rack  100 , for example by rack crosspieces  132  at rear wall  110  of rack  100 . In such embodiments, the DC-DC conversion circuit may each include a wire connecting first and second output terminals  54 ,  58  respectively first and second power conductors, for example bus bars  140 ,  142 . 
         [0047]    In a typical embodiment, computer equipment  160  is plugged directly into the bus bars using a blind-mating, hot-pluggable bus bar connector clip or jaw connector assembly when the computer equipment  160  is horizontally inserted into rack volume  112  of rack  100  while supported by support rails  128 ,  130 . Such bus bar connector clips or jaw connectors are spaced to connect with bus bars  140 ,  142 . Computer equipment  160  is thus hot swappable from the front of the rack  100  without disturbing the DC-DC conversion circuits  28  of rack  100 . In a typical embodiment, connector clips are rated  80 A after 20% derating, are nickel plated, and ensure a contact resistance less than lmOhm when mated with the bus bar. Each bus bar pair  140 ,  142  may be mechanically protected by a cage or housing  144 , perforated for air circulation. 
         [0048]    In a preferred embodiment, the positive and ground bus bars  140 ,  142  are nickel plated, 3 mm thick and have a bus bar separation width  154  of 14 mm. Ground bus bar  142  may be a small distance, for example 2.5 mm, closer to the front of rack frame  100 , thereby allowing a pre-mating of the 12 volt ground connector clip of computer equipment  160  to the ground bus bar  142 . Generally, bus bars  140 ,  142  may be provided with a knife shape or taper on the front side to facilitate the mechanical mating to a connector clip assembly of computer equipment  160 . 
         [0049]    In other embodiments, DC-DC conversion circuits  28  may supply electrical power to other power bus configurations. For example, DC-DC conversion circuits  28  may supply electrical power to a plurality of spaced-apart electrical sockets or connectors spaced along rack  100 , thereby allowing DC loads (for example, computer servers  160 ) to be plugged into the sockets, either directly or via an extension cable and matching plug. Each of a plurality of DC-DC conversion circuits  28  may supply electrical power to the spaced-apart arrangement of electrical sockets, such that electrical power is also supplied at space-apart locations relative to the sockets, where the distances are selected to reduce the voltage variation among the electrical sockets when electric loads are plugged in to multiple spaced-apart electrical sockets. In another example, DC-DC conversion circuits  28  may be supplied with a plurality of terminal lugs for direct connection of DC loads. 
         [0050]    As best shown in  FIGS. 7 and 8 , each pair of bus bars  140 ,  142  is protected by a housing  144 . Housings  144  may be perforated to permit air flow through and around bus bars  140 ,  142 . Housings  144  may be formed of a metal or a plastic material. Where DC-DC conversion circuits  28  are provided with a visual indicator, the housing  144  may be include an opening to permit visualization of the indicator from outside of the housing  144 . Housings  144  may be coupled to rack  100  by rear crosspieces  132  or other supports. In a typical embodiment, housings  144  are grounded to the rack  100  and are electrically insulated from bus bars  140 ,  142  having a non-ground voltage. Housings  144  include parallel side walls  146 , rear cover  148 , and open side  150 . Parallel side walls  146  are spaced apart to accommodate the width  152  of panel  30  of one or more DC-DC conversion circuits  28 . Open side  150  exposes bus bars  140 ,  142  from the front side of rack  100 , thereby permitting access from the front side of rack  100  and permitting the coupling of computer equipment to bus bars  140 ,  142  as discussed above. Rear cover  148  may be provided with access doors or removable panels to permit replacement of a DC-DC conversion circuit  28  while bus bars  140 ,  142  remain powered by other DC-DC conversion circuits  28 . Accordingly, DC-DC conversion circuits  28  are hot-swappable within rack  100  to permit uninterrupted operation of computer equipment  160  installed in rack  100 . 
         [0051]    In preferred embodiments, attachment of DC-DC conversion circuits  28  to a pair of bus bars  140 ,  142  within housing  144  causes minimal occlusion of cooling airflow through servers  160  in the direction from front of rack  100  towards rear  110  of rack  100 . Accordingly, the width  152  of DC-DC conversion circuits  28  is minimized when DC-DC conversion circuit  28  is mounted to vertically-oriented bus bars  140 ,  142  to thereby reduce the potential restriction of airflow horizontally adjacent to an installed DC-DC conversion circuit  28 . In one embodiment, DC-DC conversion circuits  28  are dimensioned with a conversion circuit width  152  that is less than 150 mm. In another embodiment, DC-DC conversion circuits  28  are dimensioned with a converter width  152  that is less than 100 mm. In another embodiment, conversion circuit width  152  is not more than 8 times bus bar separation width  154 . In a preferred embodiment, conversion circuit width  152  is not more than 6 times bus bar separation width  154 . 
         [0052]    DC-DC conversion circuit  28  may optionally include one or more cooling fans. Alternatively, housing  144  may include cooling fans. However, in many embodiments, servers  160  installed in rack  100  include cooling fans directing a flow of air from the front of rack  100  and through the servers towards the rear wall  110  of rack  100 . In such embodiments, housing  144  may be configured to further channel the airflow through servers  160  across DC-DC conversion circuits  28 , thereby providing a cooling benefit to DC-DC conversion circuits  28  without requiring an active cooling component (e.g., powered fans) on DC-DC conversion circuits  28  or within housing  144 . 
         [0053]    All references, including publications, patent applications, and patents cited herein are hereby incorporated by reference to the same extent as if each reference were individually and specifically indicated to be incorporated by reference and were set forth in its entirety herein. 
         [0054]    The use of the terms “a” and “an” and “the” and similar referents in the context of describing the invention (especially in the context of the following claims) is to be construed to cover both the singular and the plural, unless otherwise indicated herein or clearly contradicted by context. The terms “comprising,” “having,” “including,” and “containing” are to be construed as open-ended terms (i.e., meaning “including, but not limited to,”) unless otherwise noted. Recitation of ranges of values herein are merely intended to serve as a shorthand method of referring individually to each separate value falling within the range, unless otherwise indicated herein, and each separate value is incorporated into the specification as if it were individually recited herein. All methods described herein can be performed in any suitable order unless otherwise indicated herein or otherwise clearly contradicted by context. The use of any and all examples, or exemplary language (e.g., “such as”) provided herein, is intended merely to better illuminate the invention and does not pose a limitation on the scope of the invention unless otherwise claimed. No language in the specification should be construed as indicating any non-claimed element as essential to the practice of the invention. 
         [0055]    Preferred embodiments of this invention are described herein, including the best mode known to the inventors for carrying out the invention. Variations of those preferred embodiments may become apparent to those of ordinary skill in the art upon reading the foregoing description. The inventors expect skilled artisans to employ such variations as appropriate, and the inventors intend for the invention to be practiced otherwise than as specifically described herein. Accordingly, this invention includes all modifications and equivalents of the subject matter recited in the claims appended hereto as permitted by applicable law. Moreover, any combination of the above-described elements in all possible variations thereof is encompassed by the invention unless otherwise indicated herein or otherwise clearly contradicted by context.