Abstract:
A battery return disconnect (“BRD”) circuit for use with a battery supply having a plurality of electrically parallel inter-connecting batteries. The BRD circuit includes a plurality of DC input ports, a set of OR-ing diodes, a plurality of polarity detectors and a plurality of switches. Each input port includes a DC input terminal and a DC return terminal. Each OR-ing diode is connected to a DC input terminal or a DC return terminal and is forward-biased to when the BRD circuit is connected to a plurality of batteries having substantially the same voltage level. Each polarity detector monitors the bias of an OR-ing diode electrically connected to a corresponding DC input terminal. Each switch is connected to a DC return terminal and a polarity detector. Each switch is closed as long as the monitored OR-ing diode is forward-biased and open when the monitored OR-ing diode is reverse-biased.

Description:
CROSS-REFERENCE TO RELATED APPLICATION 
     The present invention is related to and claims priority to U.S. Provisional Patent Application No. 61/075,894, filed Jun. 26, 2008, entitled “BATTERY RETURN DISCONNECT: SOLUTION TO STOP RETURN CURRENT FLOWING INTO DISCONNECTED POWER PORT OF AN NE POWERED BY DUAL BATTERY PLANT,” the entire contents of which is incorporated herein by reference. 
    
    
     STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH OR DEVELOPMENT 
     n/a 
     FIELD OF THE INVENTION 
     The present invention relates generally to a battery disconnect switch, and more specifically to a solid state disconnect switch to prevent excessive current on battery return leads of a network element powered by a dual battery supply. 
     BACKGROUND OF THE INVENTION 
     The telecommunication industry provides customers with constant, around the clock access to communication services. Downtime is not permitted or tolerated in this industry. As such, most communication systems require some form of redundancy for all or most of the underlying equipment and components. The actual power supply and back-up power supply is no exception. Thus, many telecom and/or networking equipment are required to operate from a dual back-up battery or power supply (as referred to as a “plant”). 
     An example of a prior art telecommunication device  10  is shown in  FIG. 1 . Telecommunication device  10  includes a network element  12  connected to dual back-up batteries  14   a ,  14   b  (collectively referenced as dual batteries  14 ). The dual batteries  14  are connected in a parallel circuit configuration, with each input feeding into one of four “OR-ing” diodes  16   a ,  16   b ,  16   c  and  16   d  (collectively referenced as OR-ing diodes  16 ). The OR-ing diodes  16  conduct current in the forward direction and prevent the flow of return current back through the dual batteries  14 . Current flows through the circuit in the direction indicated by arrows in  FIG. 1 . Current flowing from the positive terminals is indicated by solid arrowheads, while current flowing into the negative terminals is indicated as a line arrowhead. 
     The dual batteries  14  supply power to DC/DC converters  18  and transfer this energy to other components (not shown) of the network element  12 . The dual batteries  16  are connected to the building ground  20  via a “vertical riser”  22 . The vertical riser  22  is generally a cable, around 750 cm in length, connecting the positive terminals of each battery  14  to each other, to building ground  20  and to the chassis ground  24  of the network element  12 . When both batteries are connected and working properly, i.e., at virtually the same voltage level, both the vertical riser  22  and the chassis ground  20 , ideally, should carry virtually no current due to a balanced distribution of current flow in all feeds and returns. In other words, the currents flowing through the vertical riser  22  from the positive terminals of the batteries  14  are of equal magnitude and in opposite directions, thereby, effectively cancelling each other out and allowing no current to flow. Minor imbalances, if any, are due to slight resistance deltas in feeders, junctions, taps and the voltage and/or current properties of diodes. 
     Although the current load through ground is negated when both batteries  14  are connected and working properly, problems arise when one battery becomes disconnected or suffers a capacity/stored voltage loss. For example, as shown in the block diagram of  FIG. 2 , if the negative terminal of battery  14   a  is disconnected, then no current flows from the positive terminal of battery  14   a  to offset the current from the positive terminal of battery  14   b . Thus, a direct path to building ground is established from the positive terminal of battery  14   b  through the vertical riser  22 . The vertical riser  22  current may be up to half of the total plant current, if all network elements are powered off the same set of dual back-up batteries, and may exceed the rating of the vertical riser cable. The vertical riser  22  is generally not rated high enough to support this excess current and can potentially cause overheating of the cable. A similar problem is also created when the batteries  14  are not at the same voltage level, reverse biasing (i.e., disconnecting) one of the OR-ing diodes  16  causing excessive current to flow into the vertical riser  22 . 
     Therefore, what is needed is a battery return disconnect switch for network elements powered by a dual battery supply that prevents an excess current from affecting the building ground. 
     SUMMARY OF THE INVENTION 
     The present invention advantageously provides a method and system for preventing return current from flowing into a disconnected power port of a device powered by a dual battery. The present invention also prevents current from flowing through a return port when the battery connected to the return port is at a substantially lower voltage than the other battery. 
     In accordance with one aspect of the present invention, a battery return disconnect circuit is provided for use with a battery supply having a plurality of electrically parallel interconnecting batteries. The battery return disconnect circuit includes a plurality of DC input ports, a set of OR-ing diodes, a plurality of polarity detectors, and a plurality of switches. Each DC input port is connectable to a corresponding one of the batteries. Each DC input port includes a DC input terminal and a DC return terminal. Each OR-ing diode of the set of OR-ing diodes is electrically connected in series with either a corresponding DC input terminal or a corresponding DC return terminal such that each OR-ing diode is forward-biased to conduct current when the battery return disconnect circuit is connected to the plurality of batteries and the batteries are at substantially the same voltage. Each polarity detector monitors the bias of an OR-ing diode electrically connected to a corresponding DC input terminal. Each switch is electrically connected in series with a DC return terminal and electrically coupled to a polarity detector. Each switch operates in a closed position when the corresponding polarity detector determines that its monitored OR-ing diode is forward-biased and operates in an open position when the corresponding polarity detector determines that its monitored OR-ing diode is reverse-biased. 
     In accordance with another aspect of the present invention, a fuse/breaker panel is provided for use with a battery supply having a plurality of electrically parallel interconnecting batteries. The fuse/breaker panel includes a plurality of DC input ports, either a plurality of breakers or a plurality of fuses, and a battery return disconnect circuit. Each DC input port is connectable to a corresponding one of the batteries. Each DC input port includes a DC input terminal and a DC return terminal. The plurality of breakers or the plurality of fuses is directly connected to the plurality of DC input ports. The battery return disconnect circuit is electrically connected to the plurality of DC input ports. The battery return disconnect circuit includes a set of OR-ing diodes, a plurality of polarity detectors, and a plurality of switches. Each OR-ing diode is electrically connected in series with either a corresponding DC input terminal and a corresponding DC return terminal such that each OR-ing diode is forward-biased to conduct current when the battery return disconnect circuit is connected to the plurality of batteries and the batteries are at substantially the same voltage level. Each polarity detector monitors the bias of an OR-ing diode electrically connected to a corresponding DC input terminal. Each switch is electrically connected in series with a DC return terminal and is electrically coupled to a polarity detector. Each switch operates in a closed position when the corresponding polarity detector determines that its monitored OR-ing diode is forward-biased and operates in an open position when the corresponding polarity detector determines that its monitored OR-ing diode is reverse-biased. 
     In accordance with yet another aspect of the present invention, a method is provided for preventing current flow through a DC return port of a device powered by a battery supply. The device has a plurality of DC input ports. Each DC input port is connectable to one of the batteries and each DC input port includes a DC input terminal and a DC return terminal. The battery supply has a plurality of electrically parallel interconnecting batteries. A battery return disconnect circuit is electrically connected between the battery supply and the plurality of DC input ports. The battery disconnect circuit includes a set of OR-ing diodes electrically connected to a corresponding DC input terminal, a plurality of polarity detectors, and a plurality of switches electrically connected to a corresponding DC input terminal. A bias of an OR-ing diode electrically connected to a corresponding DC input terminal is monitored. IF the monitored OR-ing diode is reverse-biased, one of the switches electrically connected to a corresponding DC return terminal is switched to an open position. 
    
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
       A more complete understanding of the present invention, and the attendant advantages and features thereof, will be more readily understood by reference to the following detailed description when considered in conjunction with the accompanying drawings wherein: 
         FIG. 1  is a block diagram of a prior art telecommunication device powered by dual batteries; 
         FIG. 2  is a block diagram of a prior art telecommunication device powered by dual batteries, illustrating the flow of return current though ground when one battery is disconnected or operating at a lower voltage relative to the other battery; 
         FIG. 3  is a block diagram of an exemplary battery return disconnect (“BRD”) circuit constructed in accordance with the principles of the present invention; 
         FIG. 4  is a block diagram of an exemplary BRD circuit with active OR-ing diodes, constructed in accordance with the principles of the present invention; 
         FIG. 5  is a block diagram of an exemplary BRD circuit embedded in a fuse/breaker panel for use with network elements containing integrated OR-ing diodes, constructed in accordance with the principles of the present invention; and 
         FIG. 6  is a block diagram of an exemplary BRD circuit embedded in a fuse/breaker panel for use with network elements without integrated OR-ing diodes, constructed in accordance with the principles of the present invention. 
     
    
    
     DETAILED DESCRIPTION OF THE INVENTION 
     Before describing in detail exemplary embodiments that are in accordance with the present invention, it is noted that the embodiments reside primarily in combinations of apparatus components and processing steps related to implementing a system and method for battery return disconnect switch for network elements powered by a dual battery supply that prevents an excess current from affecting the building&#39;s vertical riser and ground when one battery is disconnected or the batteries are at unequal voltage levels. Accordingly, the system and method components have been represented where appropriate by conventional symbols in the drawings, showing only those specific details that are pertinent to understanding the embodiments of the present invention so as not to obscure the disclosure with details that will be readily apparent to those of ordinary skill in the art having the benefit of the description herein. 
     As used herein, relational terms, such as “first” and “second,” “top” and “bottom,” and the like, may be used solely to distinguish one entity or element from another entity or element without necessarily requiring or implying any physical or logical relationship or order between such entities or elements. 
     One embodiment of the present invention advantageously provides a battery return disconnect that eliminates current flow in the vertical riser of a dual battery system. The battery return disconnect effectively eliminates vertical riser current when one battery is disconnected, as well as when the batteries operate at different voltage levels. 
     Referring now to the drawing figures in which like reference designators refer to like elements, there is shown in  FIG. 3 , a battery return disconnect circuit  26  provided in accordance with the principles of the present invention. Battery return disconnect circuit  26  is a solid state switch that may be integrated inside a network element  12 , as shown in  FIG. 3 , or may be an external “stand-alone” device such as a rack mountable fuse/breaker panel. The network element  12  is powered by a dual battery plant  14  connected to each other and to building ground  20  through a vertical riser  22 . The batteries  14  are connected in an electrically parallel relationship, such that, in theory, if one of the batteries  14  should fail, the second battery  14  is able to power the network element  12  alone. The battery return disconnect circuit  26  is normally “ON” and turns “OFF” whenever it detects a disconnected power feed or a reversed feed OR-ing diode  27 , indicating that one of the power plants  14  is at a lower voltage than the other. The battery return disconnect circuit  26  automatically turns back on whenever the disconnected feed is reconnected and both feeds are on and carrying current. 
     The battery return disconnect circuit  26  includes a pair of polarity detectors  28   a ,  28   b  (referenced collectively as polarity detector  28 ), and a pair of DC input ports  29   a ,  29   b  (referenced collectively as DC input port  29 ). Each DC input port  29  includes an input terminal  30  and a DC return terminal  36 . Each polarity detector  28  may be implemented as an op-amp comparator whose inputs are connected across an OR-ing diode  27   a ,  27   d  that is in series with an input terminal  30   a ,  30   b  of the battery return disconnect circuit  26 . Each input terminal  30   a ,  30   b  is connected to a negative terminal  32   a ,  32   b  of one of the batteries  14 . The positive terminal  34   a ,  34   b  of each battery  14  is connected to a DC return terminal  36   a ,  36   b  of the battery return disconnect circuit  26 , as well as connected to each other through a vertical riser  22  connected to building ground  20  and chassis ground  24 . Each DC return  36  is connected to an N-channel metal-oxide-semiconductor field-effect transistor (“MOSFET”)  38   a ,  38   b  configured as a switch, which conducts current when both batteries  14  are connected across the DC inputs  30  and DC returns  36 , or when the battery connected across the corresponding DC input  30  and DC return  36  is at the same or higher voltage level than a battery connected across the other DC input  30  and DC return  36 . In order to reverse bias the OR-ing diode  27   a ,  27   d  and turn off the battery return disconnect, the voltage difference between the two batteries  12  should be greater than the “On” voltage of the OR-ing diode  27 . 
     The polarity detectors  28  detect when the corresponding input feed  30  is off, i.e. the negative battery terminal  32  is disconnected, or has a lower voltage than the other battery  14 . In other words, a polarity detector  28  is activated when the corresponding OR-ing diode  27  is reverse biased. When the polarity detector  28  is activated, its output triggers a gate booster  40   a ,  40   b  to switch the corresponding MOSFET  38  off, thereby opening the circuit path and preventing any current from flowing out the return  36 . The gate booster  40  boosts the gate voltage of the corresponding MOSFET  38  above the source voltage in order to fully turn on the MOSFET  38  with minimum drain-to-source resistance. The polarity detector  28  turns off the gate booster  40 , and hence, the MOSFET  38 , when it detects a reverse bias across the OR-ing diode  27 . N-channel MOSFETs are preferred for use as the BRD switch, as opposed to P-channel MOSFETs, to minimize “ON” resistance and improve power dissipation. Other switching devices, such as P-channel MOSFETs, junction field effect transistors (“JFETs”), bipolar junction transistors (“BJTs”), silicon-controlled rectifiers (“SCR”), etc. may be used in place of the N-channel MOSFETs; however, these devices may be more complex to implement and dissipate significantly more power than an N-channel MOSFET. Referring to the scenario depicted in  FIG. 3 , wherein the negative terminal  32   a  of battery  14   a  is disconnected, polarity detector  28   a  detects that OR-ing diode  27   d  is reverse-biased, i.e., the voltage drop across the OR-ing diode  27   d  is less than the forward “on” voltage of the diode, activating the output of the polarity detector  28   a , and enabling the gate booster  40   a  to “turn off” MOSFET  38   a , thereby preventing current flow through the return  36   a . Thus, no current flows in the vertical riser  22 . 
     Referring now to  FIG. 4 , an alternative embodiment of the present invention is illustrated. The alternative embodiment is the same as the embodiment shown in  FIG. 3  except that the OR-ing diodes are implemented using N-channel MOSFETs  42   a ,  42   b ,  42   c , and  42   d  for ease of implementation in a silicon die. 
     Referring now to  FIG. 5 , an alternative embodiment of the present invention is illustrated in which the battery return disconnect circuit  26  is implemented external to the network element  12  as a fuse/breaker panel  44 . Fuse/breaker panel  44  may be used in conjunction with pre-existing network elements  12  designed to operate from dual battery plants  14  and having OR-ing diodes  16  on the DC input and return ports. When the battery return disconnect circuit  26  is active, current from the dual power plants  14  flows through the battery return disconnect circuit  26 , out the DC output terminals  46   a ,  46   b , into the network element  12 , and returns through the DC return terminals  48   a ,  48   b.    
     As above, the battery return disconnect circuit  26  includes a pair of polarity detectors  28   a ,  28   b  connected across an OR-ing diode  27   a ,  27   d  that is in series with an input terminal  30   a ,  30   b  of the battery return disconnect circuit  26 . Each input terminal  30   a ,  30   b  is connected to a negative terminal  32   a ,  32   b  of one of the batteries  14 . The positive terminal  34   a ,  34   b  of each battery  14  is connected to a DC return  36   a ,  36   b  of the battery return disconnect circuit  26 . Each DC return  36  is connected to an N-channel MOSFET  38   a ,  38   b , which conducts current when both batteries  14  are connected across the DC inputs  30  and DC returns  36 , or when the battery connected across the corresponding DC input  30  and DC return  36  is at the same or higher voltage level than a battery connected across the other DC input  30  and DC return  36 . A fuse (or breaker)  50   a ,  50   b  is connected in series with each DC input terminal  30   a ,  30   b  for safety purposes. As known in the art, the fuse or breaker  50  disrupts the circuit when an excessive amount of current is detected. 
     The polarity detectors  28  detect when the corresponding input feed  30  is disconnected, or has a lower voltage than the other battery  14 . When the polarity detector  28  is activated, its output triggers a gate booster  40   a ,  40   b  to switch the corresponding MOSFET  38  off, thereby opening the circuit path and preventing any current from flowing out the return  36 . 
     Referring now to  FIG. 6 , an alternative embodiment of the present invention is illustrated wherein the battery return disconnect circuit  26  is implemented external to a network element as a fuse/breaker panel  52 . Fuse/breaker panel  52  may be used in conjunction with network elements designed to operate from a single battery or power plant  14 . Fuse/breaker panel  52  provides network elements having a single DC input port with the benefit of having redundant battery supplies  14  such that the network element continues to operate as designed in the event that one of the batteries  14  fail. When the battery return disconnect circuit  26  is active, current from the dual power plants  14  flows through the battery return disconnect circuit  26 , out the DC output terminal  54 , into the network element  12 , and returns through a single DC return terminal  56 . 
     The battery return disconnect circuit  26  of  FIG. 6  is identical to the battery return disconnect circuit  26  discussed above in relation to  FIG. 5 , except that the OR-ing diodes  16  of the network element  12  are integrated into the fuse/breaker panel  52 , allowing the battery return disconnect circuit  26  to be used when the network element  12  has a single DC input port. 
     Unless mention was made above to the contrary, it should be noted that all of the accompanying drawings are not to scale. Significantly, this invention can be embodied in other specific forms without departing from the spirit or essential attributes thereof, and accordingly, reference should be had to the following claims, rather than to the foregoing specification, as indicating the scope of the invention.