Abstract:
A modular packet network device has a chassis in which multiple logic cards mate to the front side of an electrical signaling backplane. Logic power for the logic cards is supplied from a group of power converter cards that convert primary power to the logic voltages required by the logic cards. The power converter cards lie in a separate cooling path behind the backplane. Advantages achieved in at least some of the embodiments include removing primary power planes from the signaling backplane or portion of the backplane, providing redundant, upgradeable power modules whose individual failure does not cause logic card failure, and providing cool air to power converter circuits that would be subject to only heated air if located on the logic cards. Other embodiments are also described and claimed.

Description:
BACKGROUND 
     1. Field of the Invention 
     The present disclosure relates generally to modular packet switch/router design and operation, and more particularly to internal power distribution for such devices. 
     2. Description of Related Art 
     Many packet switches/routers (hereinafter “switches”) are designed in a modular fashion, with exchangeable cards and modules. The heart of such a system is a backplane with signaling connectors for each available card slot, e.g., the exemplary backplane  100  of  FIG. 1 . Backplane  100  contains internal interconnections that route signals and primary power to a large number of external connector blocks. Two signal connector blocks RPSC 0 , RPSC 1  provide connections for switch management cards. Fourteen signal connector blocks (LCSC 1 , LCSC 14  are exemplary) provide connections for line cards, which provide external ports for receiving and transmitting the packets switched through the switch. Nine signal connector blocks (SFSC 1 , SFSC 9  are exemplary) provide connections for switch fabric cards, which actually pass packets between the line cards. 
     In addition to supporting signaling between the cards, backplane  100  distributes primary power to each attached card from two redundant banks of power supplies (not shown). One power supply bank supplies 48 V “A” power to a connection point  110 A, and the other power supply bank supplies 48 V “B” power to a connection point  110 B. Power is transferred through the backplane to “A” and “B”power connector blocks (LCPC 1 A and LCPC 1 B, adjacent signal connector block LCSC 1 , are exemplary) located adjacent the signal connector blocks. 
       FIG. 2  shows backplane  100 , partially populated with three switch fabric cards SF 5 , SF 6 , and SF 7 , and two line cards LC 11 , LC 13 , as a system  200 . Each card uses DC/DC power converters to convert the 48 V distribution power received through the backplane to lower voltages (e.g., 3.3 V down to 1.0 V) required by the various logic and memory devices present on each card. For instance, switch fabric card SF 7  is depicted with two DC/DC power converters PC 7 - 1 , PC 7 - 2  that supply power at two different low voltages to the logic and memory devices present on that card. Line card LC 13  (which actually is a combination of two cards, logic board LB 13  and port interface module PIM 13 ) is depicted with four DC/DC power converters PC 13 - 1 , PC 13 - 2 , PC 13 - 3 , PC 13 - 4  that supply power at four different logic voltages to the logic and memory devices present on the logic board and the ports (e.g., port device P 13 - 1 ) present on the port interface module. 
       FIGS. 3 and 4  show the general direction of power flow in the  FIG. 2  system. Referring first to  FIG. 3 , backplane  100  is shown without any cards inserted. Two four-ounce copper power planes (not visible) within backplane  100  connect 48 V A primary power from connection point  110 A to each “A” power connector block (connector block LCPC 1 A is exemplary). Two other four-ounce copper power planes (not visible) within backplane  100  connect 48 V B primary power from connection point  110 B to each “B” power connector block (connector block LCPC 1 B is exemplary). Primary power flow direction within the backplane is indicated generally by the groups of overlaid heavy arrows 48VA and 48VB. 
       FIG. 4  shows backplane  100  in side view, with a line card LC 13  and a switch fabric card SF 7  visible. 48V power distributed by the backplane reaches connector blocks SFPC 7 A and SFPC 7 B, where the power is transferred to internal power planes (not visible) within switch fabric card SF 7 . These power planes deliver primary power to DC/DC power converters PC 7 - 1  and PC 7 - 2 , which convert the primary power to two logic voltages LV. Power at logic voltages LV is then distributed on other internal power planes (also not visible) within switch fabric card SF 7  to logic and memory located on SF 7 . 48V power distributed by the backplane also reaches connector blocks LCPC 13 A and LCPC 13 B, where the power is transferred to internal power planes (not visible) within logic board LC 13 . These power planes deliver primary power to DC/DC power converters PC 13 - 1  to PC 13 - 4 , which convert the primary power to four logic voltages LV. Power at logic voltages LV is then distributed on other internal power planes (also not visible) within logic board LB 13  and port interface module PIM 13 . 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       The present invention can be best understood by reading the specification with reference to the following Figures, in which: 
         FIG. 1  shows a prior art backplane; 
         FIG. 2  shows the  FIG. 1  backplane, partially populated with prior art line and switch fabric cards; 
         FIG. 3  shows the  FIG. 1  backplane, indicating primary power flow; 
         FIG. 4  depicts the  FIG. 1  backplane and a connected switch fabric card and connected line card in side view, indicating primary and logic power flow; 
         FIG. 5  illustrates a three-piece backplane system according to one embodiment; 
         FIG. 6  illustrates the  FIG. 5  backplane system, partially populated with line cards, switch fabric cards, and power conversion cards according to an embodiment; 
         FIG. 7  depicts the  FIG. 5  three-piece backplane system, a connected switch fabric card, a connected line card, and a connected power conversion card in side view, indicating primary and logic power flow; 
         FIG. 8  depicts an embodiment similar to  FIG. 7 , but with logic power flow and signaling flow contained in the same backplane; 
         FIG. 9  depicts an embodiment similar to  FIG. 7 , but with primary power flow and signaling flow contained in the same backplane; 
         FIG. 10  depicts an embodiment similar to  FIG. 7 , but with the three-piece backplane system staggered vertically, allowing overlapping power delivery and signaling regions; 
         FIG. 11  depicts an embodiment similar to  FIG. 9 , but with a line card using an under-card bus to move logic power from the power conversion card to a more central location on the line card; 
         FIG. 12  depicts an embodiment similar to  FIG. 7 , but with primary power flow received via bus bars; 
         FIG. 13  depicts an embodiment similar to  FIG. 12 , but with power conversion cards also supplying logic power to the switch fabric cards; 
         FIG. 14  shows, in top view, a fully populated backplane system according to an embodiment with fourteen line cards, two management cards, and sixteen power conversion cards; 
         FIGS. 15 to 18  contain connection diagrams representing different embodiments for supplying logic power in the  FIG. 14  configuration; 
         FIG. 19  shows, in top view, a partially populated backplane system according to an embodiment; 
         FIG. 20  contains a connection diagram for supplying logic power in the  FIG. 19  configuration; 
         FIG. 21  shows, in top view, a fully populated backplane system according to an embodiment with fourteen line cards, two management cards, and eight power conversion cards; 
         FIG. 22  contains a connection diagram for supplying logic power in the  FIG. 21  configuration; 
         FIG. 23  illustrates a rear view of a chassis configuration according to an embodiment; 
         FIG. 24  illustrates a side view of the chassis configuration of  FIG. 23 ; 
         FIG. 25  shows, in rear and side views, a bus bar embodiment for distributing primary power; 
         FIG. 26  shows another bus bar feature that can be added to the bus bar embodiments; 
         FIGS. 27 and 28  show, in rear view, bus bar alternate embodiments; 
         FIG. 29  shows an exemplary circuit board stack for a logic power conversion card according to an embodiment; and 
         FIG. 30  contains an exemplary circuit block diagram for a logic power conversion card according to an embodiment. 
     
    
    
     DETAILED DESCRIPTION 
     The modular switch design described in  FIGS. 1 to 4  delivers clean, redundant power to each connected card. As computing power and heat dissipation requirements typically increase with each new switch generation, however, it has now been discovered that a number of disadvantages inherent in scaling the prior art switch design can be addressed using a novel power distribution design. 
     In the prior art backplane and line cards, four very thick copper layers are used to carry primary power from the redundant power sources. Due to the copper thickness of these power planes, dielectric fill during manufacturing is difficult—complicated by the fact that the best dielectric materials for the high-speed signaling layers are not the best materials for filling voids in thick layers, nor necessarily the best for optimal power plane performance. Sophisticated multi-step board assembly procedures are used to produce both power and signal layers with good quality. Furthermore, the presence of these thick power distribution layers adds as much as 30% to the board thickness and therefore to signaling through-hole and via lengths, which undesirably increases stub effects (reflections, radiated noise, etc.) on the signal traces. 
     The presence of the DC/DC power converters on the line cards creates challenges as well. The power converters are generally placed at the extreme top of the line cards, preserving prime routing room and cooler air flow for the critical logic components and paths on the card. This placement puts the power converters last in the cooling air stream, and thus they receive air that has been heated by first passing across all the logic components. Thus as logic power demands increase, it becomes increasingly difficult to cool the power converters, which decreases their efficiency or may even cause them to shut down on overtemperature. In a negative pressure chassis, the power converters may also be close to the air movers, creating hot spots on the fans if the power converters run too hot. Finally, if a power converter fails, due to heat or otherwise, the entire board is brought down with it, and must be completely disconnected from the system and all network cables, and returned to the vendor for repair. 
     It has now been discovered that a different approach can at least partially mitigate each of these difficulties, in a given embodiment. In an exemplary embodiment, a set of power conversion cards are placed behind the backplane, and populated with DC/DC power converters. Primary power flows vertically through the power conversion cards instead of through the backplane, allowing the thick, dedicated power planes to be removed from the backplane. As logic voltages for the line cards are generated on the power conversion cards, no primary power planes are required on the line cards either. The power conversion cards do not necessarily require the same thickness power layers as the prior art backplane, as each card carries but a fraction of the total power distributed by the system. A portion of the cooling air is drawn behind the backplane to cool the power conversion cards separate from cooling of the logic cards, providing more effective cooling for the power supplies than that afforded by placing them after all logic devices in the same vertical cooling stream. Also, should a power conversion card fail, it can be replaced much more easily than a line card, and it can be upgraded without having to upgrade any line card. 
     In some embodiments, sharing mechanisms allow one power conversion card to provide power to two different line cards, providing logic power redundancy and/or efficiency. These and other achievable advantages will become clear during the detailed description presented below. 
       FIG. 5  illustrates, in perspective view, a three-circuit-board backplane system  500  according to one embodiment. The first circuit board in system  500  is a signaling board  510  that contains line card, management card, and switch fabric card signaling connector blocks, e.g., arranged similar to those in backplane  100 . Unlike backplane  100 , however, signaling board  510  does not contain primary power distribution planes or power connector blocks. 
     Instead, a second circuit board—primary power distribution board  520 —distributes primary power. Primary power distribution board  520  fits underneath signaling board  510 , presenting two rows of primary power connector blocks (SFPC 1 A and SFPC 1 B, for one switch fabric card position, are typical) for connection of switch fabric card primary power. Further, the back side of primary power distribution board  520  contains two additional rows of primary power connector blocks (PPPC 16 A and PPPC 16 B, for one power conversion card position, are typical) for connection of power conversion card primary power. 48 V “A” power is supplied to a connection point  522 A on primary power distribution board  520 , and 48 V “B” power is supplied to a connection point  522 B on primary power distribution board  520 . Four internal power planes in board  520  route power and return current from the connection points to the respective “A” and “B” power connector blocks on both sides of board  520 . 
     A third circuit board—logic power distribution board  530 —routes logic power as produced by power conversion cards mounted behind the backplane to logic cards mounted in front of the backplane. Logic power distribution board  530  fits above signaling board  510 , presenting two rows of logic power connector blocks (LCLP 1 A and LCLP 1 B, for one line card position, are typical) for connection of line card logic power. Further, the back side of logic power distribution board  530  contains two additional rows of logic power connector blocks (PPLC 16 A and PPLC 16 B, for one power conversion card position, are typical) for connection of power conversion card-generated logic power. Various embodiments will be presented below, after the presentation of further physical backplane system embodiments, for routing logic power between the power conversion card logic power connector blocks and the line card logic power connector blocks to achieve different operational advantages. 
       FIG. 6  illustrates a system  600  including a partially-populated backplane system including signaling board  510 , primary power distribution board  520 , logic power distribution board  530 , three switch fabric cards SF 6 , SF 7 , SF 8 , two line cards LC 9  and LC 13 , and three power conversion cards PC 11 , PC 15 , PC 16 . Although the switch fabric cards could, in a given embodiment, be the same as those found in the prior art systems, their connections to the system differ in that primary power connector blocks are provided on a different backplane system circuit board than signal connector blocks. Line cards LC 9  and LC 13  differ from prior art line cards at least in that neither line card contains primary-to-logic-power converters or primary power distribution planes or connectors. Instead, connectors to logic power distribution board  530  supply power at the needed logic voltages from one or more power conversion cards. Each power conversion card contains power converters (e.g., power converters PC 16 - 1 , PC 16 - 2 , PC 16 - 3 , and PC 16 - 4  on power conversion card PC 16 ) to generate various logic power voltages needed by the line cards. 
       FIG. 7  shows a side view section of system  600 , with the three backplane circuit boards  510 ,  520 ,  530 , switch fabric card SF 8 , line card LC 13 , and power conversion card PC 16  visible. Primary power flow in the system is represented by thick bolded arrows, and logic power flow in the system is represented by thinner bolded arrows. As stated above, primary power from redundant “A” and “B” supplies flows through primary power distribution board  520  and out connector blocks to the switch fabric cards and power conversion cards. Thus primary power flows through power connector blocks SFPC 8 A and SFPC 8 B and through internal power planes on switch fabric card SF 8  to reach power converters PC 8 - 1  and PC 8 - 2 . Power converters PC 8 - 1  and PC 8 - 2  generate logic power at voltages used by the logic devices mounted on switch fabric card SF 8 , and distribute that power back through internal conductive layers on switch fabric card SF 8  in order to power the switch fabric card logic devices. 
     Primary power from redundant “A” and “B” supplies also flows through primary power distribution board  520  and out power connector blocks PPPC 16 A and PPPC 16 B on the backside of board  520  to reach power conversion card PC 16 . Primary power then flows through internal layers (or patterned portions of layers) on the PC 16  circuit board to reach the power conversion circuitry on power conversion card PC 16 , including power converters PC 16 - 1 , PC 16 - 2 , PC 16 - 3 , and PC 16 - 4 . Power converters PC 16 - 1 , PC 16 - 2 , PC 16 - 3 , and PC 16 - 4  generate logic power at voltages used by the logic devices mounted on line card LC 13  (and possible one or more other line cards, as will be discussed below), and distribute that power through internal layers on the PC 16  circuit board to power connector blocks PPLC 16 A and/or PPLC 16 B. 
     Power connector blocks PPLC 16 A and/or PPLC 16 B connect through one or more internal layers of logic power distribution circuit board  530  to power connector blocks LCLP 13 A and/or LCLP 13 B on the opposite side of circuit board  530 . This couples logic power to internal layers of line card LC 13 , which distributes the logic power to logic and/or port transceivers on LC 13 . 
       FIG. 8  shows a side view section of an alternate system embodiment  800 , with switch fabric card SF 8 , line card LC 13 , and power conversion card PC 16  visible. Primary power flow in the system is represented by thick bolded arrows, and logic power flow in the system is represented by thinner bolded arrows. In system  800 , the signaling backplane and logic power distribution backplane of the prior embodiment have been combined into a single signaling/logic power distribution backplane  810 . Because the logic power distribution connector blocks (PPLC 16 A, PPLC 16 B, LCLP 13 A, LCLP 13 B) are positioned above all signaling connector blocks (LCSC 13  and SFSC 8  shown), the signaling layers in circuit board  810  are not needed to route signals in the region of the logic power distribution connector blocks. Thus the signaling layers can be retasked, in the upper region of backplane  810 , to form logic power distribution planes, with one or more signaling layers assigned to each logic voltage in an exemplary embodiment. 
       FIG. 9  shows a side view section of a second alternate system embodiment  900 , with switch fabric card SF 8 , line card LC 13 , and power conversion card PC 16  visible. Primary power flow in the system is represented by thick bolded arrows, and logic power flow in the system is represented by thinner bolded arrows. In system  900 , the signaling backplane and primary power distribution backplane of the  FIG. 7  embodiment have been combined into a single signaling/primary power distribution backplane  910 . Because the primary power distribution connector blocks (SFPC 8 A, SFPC 8 B, PPPC 16 A, PPPC 16 B) are positioned below all signaling connector blocks (LCSC 13  and SFSC 8  shown), the signaling layers in circuit board  910  are not needed to route signals in the region of the primary power distribution connector blocks. Thus the signaling layers can be retasked, in the lower region of backplane  910 , to form logic power distribution planes. Due to the current distribution requirements of the primary power region of backplane  910 , multiple signaling and/or digital ground layers present in the signaling region can be ganged in the primary power region of the backplane to transmit the primary power. For instance, if the signaling region contains 12 signaling layers interspersed between 13 digital ground layers, the frontmost six signaling layers can be used for “A” power delivery and the digital ground layers sandwiched between those six signaling layers can be used for “A” power return in the primary power distribution region of the card. The backmost six signaling layers can be used for “B” power delivery and the digital ground layers sandwiched between those six signaling layers can be used for “B” power return. The two outermost digital ground layers can remain as shielding layers to protect the power layers from external noise. 
     Although not shown in a separate drawing, the concepts of  FIGS. 8 and 9  can also be combined in a single backplane design, with the middle section of the backplane providing signaling and digital ground layers, the upper section retasking those same layers to logic power delivery, and the lower section ganging those same layers to provide primary power delivery. 
     In  FIG. 7 , the three circuit boards that make up the backplane system are essentially coplanar. This feature is not necessary, however, as  FIG. 10  illustrates. In  FIG. 10 , an embodiment  1000  uses a backplane system that comprises a signaling board  1010 , primary power distribution board  1020 , and a logic power distribution board  1030 . Unlike  FIG. 7 , neither power distribution board is limited in size to the available space under and above the signaling board. Instead, the power distribution boards are slightly offset backwards from the signaling board to allow them to pass behind the signaling board. This allows an increase in size of the power distribution boards, which may assist in routing power in some embodiments without heavy resistive losses. The increase in size is potentially offset by a decrease in size of the power conversion cards, as shown in  FIG. 10 . Further, referring to  FIG. 6 , the notches in the lower corners of the signaling backplane circuit board—used to accommodate primary power connection points  522 A and  522 B on primary power distribution board  1020 —can be eliminated, increasing signal routing room on the board. As the location of the power conversion card power connector blocks PPPC 16 A and PPPC 16 B (and the entire row of similar connectors) is shifted upwards and placed behind the signaling board  1010  on primary power distribution board  1020 , the connection points  522 A and  522 B can be accommodated on primary power distribution board  1020  below the level of signaling board  1010 , essentially at the level of and to the sides of the row of connector blocks including SFPC 8 A and SFPC 8 B. 
     Also shown in  FIG. 10  is a noise shield  1040 , which can be optionally inserted between the electrical signaling backplane  1010  and the power conversion cards, primary power distribution backplane  1020 , and logic power distribution backplane  1030 . Noise shield  1040  can be, e.g., a bulkhead in the switch chassis, a wire mesh grounded to the chassis, etc. 
       FIG. 11  illustrates yet another embodiment  1100 , which is similar in all respects to embodiment  900  ( FIG. 9 ) except for the implementation of the line card LC 13 ′, which differs from line card LC 13 . Whereas line card LC 13  delivers logic power received from connectors LCLP 13 A and LCLP 13 B directly to internal power planes on LC 13 , linecard LC 13 ′ does not. Instead, a connector LCLP 13  delivers logic power to a multiconductor logic power ribbon LPR 13 , located under card LC 13 ′ but above the card carrier (not shown). Logic power ribbon LPR 13  terminates, approximately central to card LC 13 ′, at a terminal connector LPTC 13 , which delivers logic power to internal layers of card LC 13 ′. With logic power ribbon LPR 13  constructed to provide extremely low resistance, the inclusion of thinner logic power planes and/or traces on the line card is possible. The ribbon can also be designed to tee and deliver power to multiple terminal connectors, located as desired relative to the line card layout, if so desired. 
       FIG. 12  shows a side view section of an alternate system embodiment  1200 , with switch fabric card SF 8 , line card LC 13 , and power conversion card PC 16  visible. Primary power flow in the system is represented by thick bolded arrows, and logic power flow in the system is represented by thinner bolded arrows. In system  1200 , the logic power distribution backplane has been replaced by two bus bar assemblies BBAA and BBAB, respectively providing primary “A” power and return channels and primary “B” power and return channels. Although specific implementation details for the bus bar assemblies are provided at a later point in this disclosure, visible in  FIG. 12  are switch fabric card power connectors SFPC 8 A and SFPC 8 B, and power conversion card power connectors PPPC 16 A and PPPC 16 B, which bolt to the bus bar assemblies to tap power and supply that power, respectively, to SF 8  and PC 16 . 
       FIG. 13  shows a side view section of an alternate system embodiment  1300  that, like embodiment  1200 , uses bus bar assemblies BBAA and BBAB. Primary power flow in the system is represented by thick bolded arrows, and logic power flow in the system is represented by thinner bolded arrows. In system  1300 , only the power conversion cards (e.g., PC 16 ) connect to bus bar assemblies BBAA and BBAB. The power conversion cards contain additional power converters (e.g., PC 16 - 5  and PC 16 - 6 ) on power conversion card PC 16  to convert primary power to logic power required by the switch fabric cards. Logic power passes through a lower set of logic power connector blocks, PPSF 16 A and PPSF 16 B, to a lower section of signaling backplane  1310 . Signaling backplane  1310  distributes the logic power, e.g., to logic power connector blocks SFLP 8 A and SFLP 8 B on switch fabric card SF 8 . The logic on SF 8  is therefore powered without the requirement for power converters on the switch fabric card. Power conversion card PC 16  in this embodiment provides logic power to both one or more switch fabric cards, through backplane  1310 , and one or more line cards, through logic power distribution card  1320 . If desired, backplane  1310  and logic power distribution card  1320  could be merged as exemplified in  FIG. 8 . 
       FIG. 14  shows an embodiment  1400  in top view, showing line cards L 1  to L 14 , management cards RPM 0  and RPM 1 , logic power conversion cards P 1  to P 16 , and a backplane system  1410  in edge view. The arrangement of embodiment  1400  is compatible with all previously described backplane system embodiments. In embodiment  1400 , the number of power conversion cards is equal to the aggregate number of line cards and management cards. Thus one possible arrangement is for each power conversion card to provide logic power to the line or management card directly in front of it, e.g., P 1  provides power to L 1 , P 8  provides power to RPM 0 , P 16  provides power to L 14 , etc. 
     A logic power distribution backplane circuit board (or section of backplane circuit board) can route logic power in other ways than a simple one-to-one power/powered card correspondence. For instance,  FIG. 15  shows a connection diagram  1500 , with the power conversion cards appearing as their labels spaced around an outer circle and the line/management cards appearing as labels spaced around a smaller inscribed circle. A logic power connection from a power conversion card to a line or management card is represented by a solid line between the two. In  FIG. 15 , two power conversion cards supply power to two line/management cards, e.g., P 1  and P 4  power L 1  and L 4 . It is believed that ground loop noise can be better avoided by not having adjacent line cards powered by the same two power conversion cards, thus this pattern is continued around the circle, with L 2  and L 5  powered from P 2  and P 5 , and L 3  and L 6  powered from P 3  and P 6 . In a second grouping, L 7  and L 8  receive power from P 7  and P 10 , RPM 0  and L 9  receive power from P 8  and P 11 , and RPM 1  and L 10  receive power from P 8  and P 12 . In a third grouping, L 11  and L 13  receive power from P 13  and P 15 , and L 12  and L 14  receive power from P 14  and P 16 . 
     The “50” labels on each power connection indicate that each power conversion card is supplying 50% power to each connected line or management card. Should one or more power converters fail on one of the power conversion cards (or a power conversion card be removed), the paired power conversion card can attempt to supply 100% power to both cards that it powers, so that neither line/management card is brought down by a power failure on a single power conversion card. For this redundancy to be fully effective, each power conversion card should be rated to deliver the full maximum power requirements of two cards. Such an effort can also be aided by not placing two high-power-consumption cards (e.g., two Power Over Ethernet cards) in power-paired line card slots when possible. Different power conversion cards with different ratings can also be used, depending on the power requirements of the protected cards. 
     Other load-sharing logic power backplane designs are also possible. For instance,  FIG. 16  shows a connection diagram  1600  for a different power load-sharing map. In this map, to the extent possible, three power conversion cards power three line/management cards as a group, with two of the three power conversion cards supporting each one of the powered cards in the group. An exemplary group L 1 , L 4 , L 7  is powered from the group P 1 , P 4 , P 7 , such that no two line cards in the first group receive power from the same two power conversion cards. Thus if one power converter goes down, two power converters can still share the load for the three powered cards. Each power conversion card in such a grouping need only be designed to generate 150% of a single line card&#39;s power requirements in this design, as opposed to 200% in the prior embodiment. 
     This same load-sharing concept can be applied to group sizes greater than three cards.  FIG. 17  shows a connection diagram  1700  for a power load-sharing map in which the group size encompasses all sixteen power conversion cards and all sixteen line/management cards. Each power conversion card powers two logic cards that are three slots apart, in a single linked pattern. In diagram  1700 , the failure or removal of a single power conversion card can be compensated by balancing a slight increase (6.6%) in power drawn from each of the remaining 15 power conversion cards, with each adjusting its power share to the cards it powers accordingly. 
     Such a failure scenario is illustrated in the connection diagram  1800  of  FIG. 18 . Connection diagram  1800  is identical to connection diagram  1700 , except P 1  is represented as shut down by dashed connection lines with “0” showing the amount of power supplied from P 1 . P 1  previously powered L 3  and L 14 . L 3  was previously powered 50% by P 4  as well, and P 4  must now supply 100% power to L 3 . P 4 , however, reduces the percentage of power it supplies to its other powered card, L 6 , to 6.7%. P 7  compensates by supplying 93.3% power to L 6 , and reduces the percentage of power it supplies to its other powered card, RPM 1 , to 13.3%. P 10  compensates by supplying 86.7% power to RPM 1 , and reduces the percentage of power it supplies to its other powered card, L 10 , to 20%. This load-shifting behavior cascades until P 9  is reached—P 9  supplies 53.3% power to both RPM 0  and L 9 . Further load shifting continues along the chain, until P 14  is reached. P 14  supplies 100% of the power to L 14 , since P 1  is no longer available. Thus one benefit of this design is that a single power failure can be compensated by distributing the power load across all remaining power conversion cards, with only a slight increase in the power drawn from each remaining power conversion card. 
     Connection diagram  1800  suggests that efficient power redundancy is also possible with a partially-populated backplane.  FIG. 19  depicts such a situation, from a top view of an embodiment  1900 . Embodiment  1900  comprises: backplane system  1410 ; two management cards RPM 0  and RPM 1 ; nine line cards L 2 , L 3 , L 5 , L 6 , L 8 , L 10 , L 11 , L 13 , and L 14 ; and twelve power conversion cards P 1 , P 3 , P 4 , P 6 , P 7 , P 8 , P 9 , P 10 , P 11 , P 13 , P 14 , and P 16 . 
     The particular arrangement of line card and power conversion card insertion allows N+1 power conversion cards to provide redundant power to N line and management cards in a partially populated system, as long as the selection of line and power conversion card slots follows the power distribution mapping of  FIG. 17 .  FIG. 20  shows a connection diagram  2000 , which is the same power distribution mapping of  FIG. 17 , except shown with “X” markings for the unused line card and power conversion card slots of  FIG. 19 . By selecting sets of empty line card slots served by the same empty power conversion card slots, the remaining slots can be served by a supporting chain of power conversion cards. The percentages shown on connection diagram  2000  illustrate the power sharing used with all remaining power conversion cards active—each operates at less than 92% load, with at least two power conversion cards providing power to each line and management card. 
     With the power distribution arrangement of  FIG. 20 , a customer can be supplied with a chart instructing them as to how to populate a chassis to achieve best power redundancy protection. An exemplary chart is presented below as Table 1, which assumes that both management card slots are populated as well: 
                                           TABLE 1               # of Linecards   Place Linecards in Slots   Place Power Conversion Cards in Slots                                1   L5   P3, P6-P7, P9-P10       2   L5, L10   P3, P6-P7, P9-P10, P13       3   L2, L5, L10   P3, P6-P7, P9-P10, P13, P16       4   L2, L5, L10, L13   P3, P6-P7, P9-P10, P13, P16       5   L2, L5-L6, L10, L13   P3-P4, P6-P7, P9-P10, P13, P16       6   L2-L3, L5-L6, L10, L13   P1, P3-P4, P6-P7, P9-P10, P13, P16       7   L2-L3, L5-L6, L10, L13-L14   P1, P3-P4, P6-P7, P9-P10, P13-P14, P16       8   L2-L3, L5-L6, L10-L11, L13-L14   P1, P3-P4, P6-P7, P9-P11, P13-P14, P16       9   L2-L3, L5-L6, L8, L10-L11, L13-L14   P1, P3-P4, P6-P11, P13-P14, P16       10   L2-L3, L5-L8, L10-L11, L13-L14   P1, P3-P11, P13-P14, P16       11   L2-L8, L10-L11, L13-L14   P1-P11, P13-P14, P16       12   L1-L8, L10-L11, L13-L14   P1-P11, P13-P16       13   L1-L8, L10-L14   P1-P16       14   L1-L14   P1-P16                      FIG. 20  and Table 1 are exemplary; many other power conversion card mappings and population patterns can be developed according to the teachings herein.
 
     Another use of the teachings herein is to supply logic power for several line and/or management cards from a single power conversion card.  FIG. 21  illustrates this concept in a top view of an embodiment  2100 . Embodiment  2100  includes a backplane system  2110 , two management cards RPM 0  and RPM 1 , fourteen line cards L 1  to L 14 , and eight power conversion cards P 1  to P 8 . Each power conversion card supplies logic power to two line and/or management cards. Each power conversion card is thus rated to supply at least 200% of a line or RPM card&#39;s maximum power rating, and can contain internally redundant power converters to safeguard against partial power loss on the power conversion card. Each power conversion card can also be configured to cut power preferentially to one of its two powered cards over the other one in the case of the power conversion card not being able to meet the power requirements of both cards. 
     The powered cards can be, in a given embodiment, the two logic/management cards physically closest to a power conversion card. Alternately, backplane system  2110  can contain connections that distribute power in other arrangements.  FIG. 22  contains a connection diagram  2200  for one such arrangement, where each power conversion card supplies power to two line or management cards that are at least two slots apart, according to the following mapping: P 1 -&gt;L 1 , L 4 ; P 2 -&gt;L 3 , L 6 ; P 3 -&gt;L 5 , RPM 0 ; P 4 -&gt;L 7 , L 8 ; P 5 -&gt;RPM 1 , L 10 ; P 6 -&gt;L 9 , L 12 ; P 7 -&gt;L 11 , L 14 ; and P 8 -&gt;L 2 , L 13 . 
     When the logic cards receive power through a logic power distribution circuit board that is separate from the signaling backplane circuit board, different logic power distribution circuit boards can be inserted in a chassis to achieve different effects, including those described above, combinations of the embodiments above, and many others. Further, upgrades in power converter technology can be implemented merely by switching power conversion cards. 
       FIG. 23  illustrates, in rear view, the physical arrangement of the interior of an unpopulated switch chassis embodiment  2300 . Switch chassis  2300  includes a chassis frame  2302 , e.g., for mounting in an equipment rack. The backplane system includes a signaling backplane  2310 , a logic power distribution backplane  2320 , and “A” and “B” primary power distribution bus bars  2330 A and  2330 B. Bus bars  2330 A and  2330 B extend downwards behind a cooling air intake/filtering zone plenum  2350  to connect, respectively, to “A” and “B” primary power supplies  2340 A and  2340 B. At the extreme top of chassis frame  2302 , a fan tray  2360  pulls cooling air through the switch chassis  2300  and exhausts the heated cooling air. 
       FIG. 24  illustrates switch chassis  2300  in left side view, populated with a switch fabric card SF 1 , a line card LC 1 , and a power conversion card PC 1 . Superimposed on switch chassis  2300  are bolded arrows representing air flow through switch chassis  2300 . Starting at the bottom of chassis frame  2302 , primary power supply  2340 A mounts in an AC/DC primary power zone. Primary power supply  2340 A has its own cooling fans, which draw cooling air in the front and exhaust spent cooling air out the back of chassis frame  2302 . 
     Immediately above the AC/DC primary power zone, a cooling air intake/filtering zone includes plenum  2350 . Cooling air is drawn into plenum  2350  through openings in the front of chassis frame  2302 . The cooling air is turned approximately vertically, e.g., with assistance from a series of vanes or louvers (not shown), and preferably passes through a filter (not shown) that inhibits the introduction of dust into the electronics. Although bus bars  2330 A and  2230 B extend through plenum  2350 , a large area between the bus bars allows cooling air to enter the region behind the backplane system. 
     Cooling air next passes vertically through the DC distribution power zone, where switch fabric card SF 1  and power conversion card PC 1  connect to bus bars  2330 A and  2330 B. In front of the bus bars, electronics of SF 1  will likely exist and require cooling. Behind the bus bars, there may or may not be electronics mounted on PC 1 . If not, cooling air will continue upward across the surface of PC 1 . 
     Cooling air next passes vertically on both sides on both sides of signaling backplane  2310 , in the backplane signaling zone. In front of signaling backplane  2310 , cooling air in this zone cools the electronics on the remainder of SF 1 , and cools electronics on LC 1 . Behind signaling backplane  2310 , power converters on PC 1  receive cooling in this zone. 
     The logic power delivery zone roughly coincides with the location of logic power distribution board  2320 . Cooling air passing in front of board  2320  cools the electronics on the top portion of LC 1 . Cooling air passing behind board  2320  cools any remaining power converters on PC 1 . 
     After the cooling air passes out of the electronics, it enters an upper plenum  2362 . In plenum  2362 , the cooling air is drawn backwards to fan tray  2360 . Thus the cooling air that cools the logic boards is separated from the cooling air that cools the power boards until after all electronics have been cooled. 
       FIG. 25  depicts, in an enlarged view, the primary power distribution system  2500  of  FIGS. 23 and 24 . Two primary power supplies,  2340 A and  2340 B, connect respectively to two bus bar assemblies,  2330 A and  2330 B. Each bus bar assembly is shown in a front view (as seen from the vantage point of a power conversion card mounted in a chassis) and a side cross-section view. Features of each bus bar assembly are largely similar, except bus bar assembly  2330 A is taller, and jogs behind the end of bus bar system  2330 B. 
     In common system assemblies, chassis ground, primary power return connections, and digital ground connections are tied together at each primary power supply. Further, chassis ground and digital ground tie together at a single point on the backplane. Control connections between the backplane and one or more power supply units may exist as well, with a digital ground reference connected between the power supply units and the backplane to serve the control connections. Each power conversion card also contains a plurality of logic power converters that reference to primary power return voltage on the input side, reference to digital ground on the output side, and contain at least one chassis ground connection as well. These distributed multiple ground connections present the potential for undesirable ground loops in the system. Further, it has now been found that radiated noise from the electronics can intrude on the primary power through the leads from the power supplies to the backplane. Bus bar assemblies described herein have now been developed that combat these problems. 
     Each bus bar assembly comprises a supply conductor (AS and BS) and a return conductor (AR and BR), e.g., fabricated from (111) copper. At the power supply ends of the conductors, each conductor contains a bare section to facilitate establishing an electrical pathway to a power supply. The remainder of each conductor is preferably covered with a dielectric, e.g., 6 mils of polypropylene in one embodiment. The polypropylene is removed in specific locations along the length of the bar to allow connection to the power supply connector blocks. 
     As the supply and return conductors of a bus bar assembly ascend through the chassis from their respective power supply connections, the two conductors&#39; positions collapse against each other such that the conductors overlie each other in close proximity (with insulation in between). The conductors ascend upwards together, surrounded by a shield material that is tied to chassis ground (the shield material in the Figures is only shown behind the bus bar assemblies for clarity of other features, but in fact surrounds the bus bar assembly). The conductors separate at one point along their upward path in the illustrated embodiment before rejoining. The separation allows ferrite chokes  2510  and  2512  to be inserted respectively around BS and BR, and similar ferrite chokes (unlabeled) to be inserted around AS and AR. 
     As each bus bar assembly rises to the level of its connectors for the power and switch fabric cards, the bus bar assembly bends and runs horizontally. The supply and return conductors remain partially overlapped over the horizontal run, although the conductors are staggered to allow power connectors to access both conductors from each side. Each power conversion card connector (PPPC 3 A and PPPC 3 B are exemplary) bolts through the insulating material covering each conductor to holes aligned with the respective power conversion card slot. On the opposite side of the bus bar assemblies, switch fabric card primary power connectors (SFPC 1 A and SFPC 1 B are exemplary) bolt through the insulating material in the opposite direction from the power conversion card connectors. 
     In one embodiment, it is desirable to reduce noise in the bus bar assembly over at least the frequency range 10 kHz to 100 MHz. In  FIG. 25 , differential mode noise is combated by the proximity of and the capacitance existing between the two conductors of each bus bar assembly. The capacitance between the conductors is enhanced by the dielectric material and by the close spacing of the conductors. Differential-mode noise is also combated by the ferrite chokes on each conductor. Both differential-mode and common-mode noise rejection are aided by several features of the bus bar assemblies, including: the grounded shielding surrounding the conductor pair (this shielding can also be replaced by or used with a ferrite choke surrounding both conductors); and discontinuities in overlap that create differences in the incremental impedance between the conductor pairs, e.g., where the conductors bend away from each other to allow the positioning of the ferrite chokes. Other features, such as a widened section  2600  of the bus bars shown in  FIG. 26 , can also be incorporated to create impedance discontinuities that lower noise. When considered together, these elements provide an extremely low-noise, low-resistance DC connection between the power supplies and the power conversion cards. 
       FIG. 27  illustrates another primary power distribution system embodiment  2700  for use with eight power conversion cards instead of sixteen. Two bus bar assemblies  2730 A and  2730 B connect, as in the previous embodiments, to two primary power supplies  2340 A and  2340 B. Four outboard power conversion card power connectors (PPPC 1 A, PPPC 2 A, PPPC 7 A, and PPPC 8 A, for the bus bar assembly  2730 A; PPPC 1 B, PPPC 2 B, PPPC 7 B, and PPPC 8 B, for the bus bar assembly  2730 B) bolt to the power conversion card side of the bus bar assemblies. The remainder of the power conversion card power connectors are paired with switch fabric card power connectors in a common assembly that bolts across the bus bar assemblies. On bus bar assembly  2730 B, these paired connectors are labeled SFPPPC 3 B, SFPPPC 4 B, SFPPPC 5 B, and SFPPPC 6 B. In between the paired connectors, individual power connectors SFPC 1 B, SFPC 3 B, SFPC 5 B, SFPC 7 B, and SFPC 9 B serve the remainder of the switch fabric cards. 
     Particularly with the lower density connector pattern of  FIG. 27 , additional bus bar features can be incorporated to further reduce noise propagation through the primary power supply distribution system. One example of additional features is illustrated in  FIG. 28 , which shows yet another primary power distribution system embodiment  2800  with the same power connector pattern as embodiment  2700 . Embodiment  2800  shows two bus bar assemblies  2830 A and  2830 B. Each bus bar assembly, at the power connector level, contains two power conductors that completely overlap except for tabs that extend from each conductor at the locations of the power connectors. For instance, bus bar assembly  2830 B comprises a supply conductor BS and a return conductor BR that overlap along their horizontal run. At the location of power connector PPPC 2 B, supply conductor BS contains a tab  2850 S projecting upwards, and return conductor BR contains a tab  2850 R projecting downwards. The tabs can be bare copper, or coated with an insulator except for where a power connector must make contact with the copper. 
     An additional feature on the bus bar assemblies  2830 A and  2830 B is the presence of ferrite chokes placed between some of the power conversion card connectors. Bus bar assembly  2830 B has two such ferrite chokes,  2852  and  2854 , placed around BS and BR between the first and second power conversion card connectors on each end of the bus bar assembly. 
     Many options exist for fabrication of bus bar assemblies according to an embodiment. The two conductors of a bus bar assembly can be separated by an “H”-shaped plastic extrusion inserted between the conductors where they overlap. The plastic extrusion can snap, screw, clip, etc. to the conductors to form a solid assembly, and can also snap, screw, clip, etc. into the chassis. 
     In an alternate embodiment, the assembly comprising the two conductors and the “H”-shaped plastic extrusion (and possibly ferrite chokes, insulated from the conductors), with the contact ends covered, is dipped into a bath of thermoplastic material. The thermoplastic material, once cooled and hardened, provides additional insulation for the assembly. Alternately, the two conductors can be held in a desired relative position in a fixture during the dipping, cooling, and hardening operation, with no plastic spacer at all. A supportive, non-conductive scrim cloth material can also be wrapped around the conductors in either method, thereby adding strength and stiffness to the assembly and helping to maintain the desired conductor gap. Each conductor may also be dipped separately, and then used in any of the above operations with an “H”-shaped plastic extrusion and/or a second dipping operation. 
     As one alternative to dipping, the pair of conductors can be fixtured at the correct spacing, with ends covered, and wrapped in a pre-impregnated thermo-set cloth material, and then cured. As a second alternative to dipping, the pair of conductors can be fixtured at the correct spacing, with ends covered, and surrounded with heat shrink tubing lined with a hot-melt adhesive. When the tubing is heated and shrunk, the adhesive lining adheres itself to and fills the gap between the two conductors, forming a flexible insulated assembly. As a third alternative, the conductor pair can be inserted into a mold that protects the ends of the conductors, and then over-molded with a thermoplastic or thermo-set material. 
     When shielding is placed over the conductor pair, the shielding can be assembled to the conductor pair in several ways. For instance, the shield may consist of: a wire mesh tube that is slid over the insulated conductor assembly; metallized tape applied over the insulated conductor assembly; a metal coating applied, e.g., by vapor deposition, electro-deposition, or plasma spray; or a metal mesh sheet cut to shape and wrapped around the insulated bus bar. A drain wire, which may consist of a copper braid, is soldered to the shield material. A ring lug is crimped onto the other end of the drain wire, and secured to chassis ground by a backplane mounting screw, preferably placed close to the backplane end of the bus bar. 
     Once the shielding is in place, an optional final step can add further insulation over the assembly, including the shielding. Any of the insulating methods described above can be used to add this further insulation. The drain wire may be insulated as part of this step, or can be separately insulated if so desired. 
     Although bus bar features are shown on a bus bar that delivers power directly to the power conversion cards, the disclosed bus bar noise rejection/suppression features can also be used, e.g., on a bus bar for a  FIG. 4  type backplane, which bolts to the power delivery end of the bus bar conductors and then internally distributes power to the power conversion cards. Such a bus bar can alternately be used with a prior art backplane, e.g., like the one shown in  FIG. 1 , to improve noise rejection on the primary power delivery network of the prior art. When cooling air does not need to pass behind the backplane, the portion of the bus bars extending between the power supply cavity and the electronics cavity can be shielded from electronic noise by enclosure in a separate cavity within the chassis. 
       FIGS. 29 and 30  illustrate further features of power flow through a logic power conversion card according to an embodiment.  FIG. 29  shows a side view of a logic power conversion card LPCC, including two magnified cross-sections showing the use of internal layers of the card&#39;s circuit board. LPCC receives primary power through two connector blocks PPCBA and PPCBB. The two connector blocks feed power to internal conductive layers of the LPCC circuit board. Cross-section  2910  shows one possible layer assignment near the primary power connector blocks. All labeled layers are conductive layers constructed of one-ounce copper, separated by FR-4 dielectric layers. 
     Cross-section  2910  depicts  12  conductive layers. The two outermost conductive layers are DGND digital ground layers (digital ground connections to card LPCC are made, e.g., through two logic power connectors LPCBA and LPCBB). Each DGND layer provides radiated noise shielding for the interior conductive layers of the circuit board. Each DGND layer is separated by a dielectric layer from a conductive signaling layer (SIG A, bottom, and SIG B, top). The signaling layers are used to route control and monitoring signal traces on the LPCC circuit board, e.g., from logic power connectors LPCBA and LPCBB. As shown, the signaling layers can be placed much closer to the outer DGND layers than to the next innermost conductive layers, such that the signaling impedance is dominated by the DGND proximity, and return signaling currents flow primarily in the outer DGND layers. 
     Below the SIG B layer, the next four insulated conductive layers carry 48V B RTN, 48V B supply, 48V B RTN, and 48V B supply, respectively. Likewise, above the SIG A layer, the next four insulated conductive layers carry 48V A RTN, 48V A supply, 48V A RTN, and 48V A supply, respectively. The innermost 48V A and 48V B supply layers face each other, separated by a dielectric layer. Capacitance between the return-supply-return-supply layer stacks is enhanced, providing further noise attenuation in the primary power supply circuits. 
     Primary power passes from connectors PPCBA and PPCBB to isolation and filtering block IFB (described further below in conjunction with  FIG. 30 ), where power from the A and B supplies is combined, further filtered, and distributed to logic power supply conversion circuits (PS 1  and PS 2  are visible in  FIG. 29 ). 
     At the output side of the logic power conversion circuits, the conductive layers that were used on the primary power supply side of the logic power conversion card to carry primary power are now retasked to carry logic power to the logic power connectors LPCBA and LPCBB. On the logic power delivery side of the logic power conversion card, for instance, the 12 conductive layers of the LPCC circuit board can be assigned, top to bottom as shown in cross-section  2920 , as DGND, SIG B, DGND, LV 1 , DGND, LV 2 , LV 3 , DGND, LV 4 , DGND, SIG A, and DGND, where “LV” denotes a logic voltage. Depending on the current needs of each logic voltage, more conductive layers or less of a conductive layer can be assigned to each logic voltage. A layer may also be split, if isolation circuitry SBR is included on the card and the same power supply conversion circuit supplies power to two or more logic cards. 
       FIG. 30  contains a block diagram for the power conversion circuitry  3000  on a logic power conversion card according to an embodiment. Circuitry  3000  includes isolation and filtering block IFB, four logic power supplies PS 1  to PS 4 , and split bridge SBR. 
     Isolation and filtering block IFB contains a diode bridge DB, a differential/common mode filter DCM, and four common mode filters CM 1  to CM 4 . Diode bridge DB allows the 48V “A” and “B” supplies to be connected together at the input of filter DCM, while preventing loops between the primary supplies as a result of the common connection. Filter DCM works, in conjunction with the capacitance of the primary power planes in the card itself, to reduce noise in the combined primary power supply. Each of common mode filters CM 1  to CM 4  connects the output of filter DCM to a respective input of one of power supplies PS 1  to PS 4 . The common mode filters reduce noise from each power supply feeding into the inputs of the other power supplies. 
     Each power supply PS 1  to PS 4  is set to output a corresponding logic voltage LV 1  to LV 4 . In other embodiments, two or more power supplies can team to produce current at the same logic voltage, or a different number of logic voltages can be generated. The “CTL” input to each power supply can comprise voltage feedback signals, or instructions from the chassis controller, from the supplied line cards, or from an onboard controller located on the power conversion card. 
     When the power conversion card is designed to supply power to multiple line cards, a split bridge SBR can provide isolation and/or common mode filtering for each supplied voltage, as desired. In  FIG. 30 , split bridge SBR is shown as a simple diode inserted in each leg of a split output for each power supply. The split bridge can optionally employ further noise filters, or even controlled solid-state switches to dynamically route logic power and/or combine logic power from different power supplies. 
     Those skilled in the art will appreciate that the embodiments and/or various features of the embodiments can be combined in ways other than those described. The specific embodiments can be adapted to other backplane chassis arrangements with different logic card numbers and/or functionality, positive pressure cooling, side-to-side cooling, etc. Cooling air for logic cards and power converter cards need not enter from the same cooling air intake, exit through the same exhaust, or use common air movers. When different backplane circuit boards are used for signaling and for primary power and/or logic power delivery, noise-defeating structures and shields can be employed between the circuit boards. Noise shielding can also be employed between the signaling portion of a backplane and the power conversion cards running behind the backplane as well. Not all logic voltages supplied by a power conversion card need be supplied to the same group of logic cards, where a power conversion card provides power to two or more logic cards. 
     Although the specification may refer to “an”, “one”, “another”, or “some” embodiment(s) in several locations, this does not necessarily mean that each such reference is to the same embodiment(s), or that the feature only applies to a single embodiment.