Abstract:
The present invention is directed towards a discrete protection circuit located on a circuit card, and systems and methods related thereto. The protection circuit protects circuit card components from an inrush current and overcurrent conditions. The discrete protection circuit includes a switch to control a delivered load current to an output port, an SCR which latches when an overcurrent condition is detected across a sense resistor, and a series of resistors and a capacitor that determines the retry delay subsequent to an overcurrent detection. Advantages of the discrete protection circuit of the invention over prior art integrated circuits include: lower parts counts, lower production costs, greater flexibility, and increased reliability.

Description:
TECHNICAL FIELD 
       [0001]    The present invention is generally related to a communications system and, more particularly, is related to systems, methods, and apparatus for connecting circuit cards with discrete hot swap and overcurrent-limiting circuits to a live backplane. 
       BACKGROUND OF THE INVENTION 
       [0002]    Integration of hot swap and overcurrent-limiting circuits are becoming an essential part of modern systems since any system downtime is unacceptable before and during any system hardware upgrades. Although there are many integrated circuits in the market today that handle these functions, they are expensive and single-sourced. Therefore, there is a need to address the issues of performing hot swaps as well as providing current-limiting protection with a simple and cost effective solution. 
     
    
     
       BRIEF DESCRIPTION OF THE DRAWINGS 
         [0003]    The invention can be better understood with reference to the following drawings. The components in the drawings are not necessarily to scale, emphasis instead being placed upon clearly illustrating the principles of the present invention. Moreover, in the drawings, like reference numerals designate corresponding parts throughout the several views. 
           [0004]      FIG. 1  is an abridged block diagram of a communications system that is suitable for use in implementing the present invention. 
           [0005]      FIG. 2  is a block diagram of a backplane for receiving and powering conventional circuit cards. 
           [0006]      FIG. 3  illustrates the drop in voltage across a backplane when unprotected circuit cards are connected thereto. 
           [0007]      FIG. 4  is an illustration of a schematic of the discrete protection circuit of the present invention. 
           [0008]      FIG. 5  is a block diagram of the live backplane for receiving and powering circuit cards including the discrete protection circuit of  FIG. 4 . 
           [0009]      FIG. 6  is a block diagram illustrating the current-limiting function of the discrete protection circuit of  FIG. 4 . 
       
    
    
     DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS 
       [0010]    The invention now will be described more fully hereinafter with reference to the accompanying drawings, in which preferred embodiments of the invention are shown. The invention may, however, be embodied in many different forms and should not be construed as limited to the embodiments set forth herein; rather, these embodiments are provided so that this disclosure will be thorough and complete, and will fully convey the scope of the invention to those skilled in the art. Furthermore, all “examples” given herein are intended to be non-limiting. 
         [0011]    The present invention is directed towards a discrete protection circuit that allows connection or removal of a protected circuit card from a live backplane without any service interruptions. Importantly, the power that is supplied by the backplane to other connected circuit cards is not affected by the connection or removal of a circuit card. More specifically, the discrete protection circuit is located on the circuit card and limits the current inflow to that circuit card. Due to the limited current flow, the voltage across the backplane remains constant. Additionally, the discrete protection circuit is used as an overcurrent-limiting circuit. A circuit card equipped with the discrete protection circuit can immediately detect a possible short circuit on the card, which may be caused by faulty component(s), and can limit the input current to protect the circuit card and the backplane; thereby avoiding a complete shutdown of the system. 
         [0012]      FIG. 1  is an abridged block diagram of a communications system  110  that is suitable for use in implementing the present invention. Typically, a communications system  110  includes a transport network  115  and a transmission network  120 . The transport network  115 , which is fiber optic cable, connects a headend  125  and hubs  130  for generating, preparing, and routing programs and other optical packets over longer distances; whereas a transmission network  120 , which is coaxial cable, generally routes electrical packets over shorter distances. Programs and other information packets received, generated, and/or processed by headend equipment racked in backplanes is either broadcasted to all subscribers in the system  110 , or alternatively, the programs can be selectively delivered to one or more subscribers. Fiber optic cable  135  connects the transport network  115  to an optical node(s)  140  that converts the packets from optical packets into electrical packets. Thereafter, coaxial cable  145  routes the packets to one or more subscriber premises  150   a - d.    
         [0013]    In the reverse, or upstream, direction, subscriber premises equipment, such as set-top boxes or cable modems, generate reverse electrical signals. The optical node  140 , which includes an optical transmitter, converts the reverse electrical signals into optical signals for further routing to backplane equipment at the hubs  130 . The backplane equipment in the hubs  130  then route the optical signals to the equipment in the headend  125  for further processing. 
         [0014]      FIG. 2  is a block diagram of a backplane, which may be located in the headend  125  and/or hubs  130 , for receiving and powering conventional equipment, such as circuit cards. A live backplane  205  is configured to accept a plurality of circuit cards  210 ,  215  via a connector  220 ,  225 . The circuit cards  210 ,  215  typically include many active components and circuits, such as microprocessors and Field Programmable Gate Arrays (FPGAs), which require power in order to generate the appropriate signals. In  FIG. 2 , circuit card  210  is connected to the live backplane  205  and circuit card  215  is about to be connected with the live backplane  205 . If the circuit cards  210 ,  215  do not include a circuit that limits the power, a rush of current is drawn from the backplane  205  through circuit card  215  when circuit card  215  is connected. The rush of current can cause in a voltage drop across the backplane  205 , thereby potentially disrupting the operation of the connected circuit cards and the overall system operation. 
         [0015]      FIG. 3  illustrates the drop in voltage when conventional circuit cards are connected to a live backplane. Prior to circuit card  215  being connected to the live backplane  205 , the voltage across the backplane, which powers all the connected circuit cards, is 24 Vdc. At the time  305  circuit card  215  is connected to backplane  205 , the voltage across the backplane drops significantly. As mentioned, active components on any of the previously connected circuit cards  210  are susceptible to the drop in voltage. Furthermore, the active components on the newly connected circuit card  215  may be adversely affected by the ensuing rush of current through the circuit card  215 . 
         [0016]      FIG. 4  is an illustration of a schematic of an exemplary embodiment of the discrete protection circuit of the present invention that provides a hot swap and over current-limiting circuit. The discrete circuit  400  is preferably included on each circuit card  505 ,  510  that will be connected to or removed from the live backplane  205  for ultimate protection as shown in  FIG. 5 . An input pin  405  to the discrete circuit  400  connects to the backplane  205  so that power passes through the discrete circuit  400  prior to any other components on the circuit card  505 ,  510 . In this manner, the discrete circuit  400  is able to limit the inrush of current when it connects to the live backplane  205 , thereby preventing a subsequent drop in voltage across the backplane  205 . Prior to a hot swap, an on/off pin  410  can be set to the on position in order to protect the circuit card  505 ,  510 . Alternatively, it can be used manually if a user wishes to turn on and off the power to the circuit card  505 ,  510  when it is inserted into a backplane  205 . 
         [0017]    As shown in the exemplary embodiment of  FIG. 4 , the discrete circuit  400  includes a sense resistor R 3  to detect an overcurrent condition, a discrete SCR  435  which latches when an overcurrent condition is detected, and a switch Q 5  to control an output load current, so that no load current is delivered when an overcurrent condition is detected. When the on/off pin  410  is turned on, for example a logic high is associated with the on/off pin  410 , and power is supplied to the discrete circuit  400 , a transistor Q 3  of the circuit  400  is turned on. When transistor Q 3  is turned on, it will initiate the charging of the capacitor C 1 . Once the voltage across capacitor C 1  exceeds the gate threshold voltage of a switch Q 5 , an input source  405  is connected to the output load  430  (i.e., to the load components on the circuit card) through the switch Q 5 . 
         [0018]      FIG. 6  is a block diagram illustrating the current-limiting function of the discrete circuit of  FIG. 4 . If, for example, a load circuit, such as a microprocessor or FPGA, fails on circuit card  505 , the discrete circuit  400  prevents the short from damaging the circuit card  505  by limiting the available current from the input  405 . In operation, resistor R 3  senses the load current at output  430 . More specifically, the load current passes through resistor R 3  and switch Q 5 . The load current through resistor R 3  develops a proportional voltage across it that is sensed by transistor gates Q 1  and Q 4  to detect a fault condition. If a voltage drop across resistor R 3  is greater than the emitter-base (E-B) diode drop of transistor Q 1 , then Silicon Controlled Rectifier (SCR)  435 , formed by transistors Q 1  and Q 4 , will go into a latch state. This will restrict the input voltage at the gate of switch Q 5 , and in turn will shut switch Q 5  off. Concurrently, capacitor C 1  will discharge through Q 1  and Q 4 . As soon as the capacitor C 1  is completely discharged, SCR  435  turns off. Switch Q 5  remains off until capacitor C 1  is charged back above the gate threshold voltage of the switch Q 5 . Capacitor C 1  charge time can be controlled by selecting appropriate values for resistors R 1  and R 2 . Additionally, capacitor C 1  charge time controls a retry delay following the detection of the over-current condition. 
         [0019]    Switch Q 5 , transistor Q 1  and transistor Q 4 , which form the SCR  435 , resistor R 1 , resistor R 2 , and capacitor C 1  form a circuit that has a fast initial response to changes in load current, for example, due to plugging the circuit card into a live backplane, and yet also allows a designer to set the retry delay. The retry delay is a predetermined time following a fault condition that the discrete circuit  400  takes before it retries to deliver current back to the load. In this manner, when the fault is cleared, the discrete circuit  400  then retries after the predetermined time and resumes normal operation. The retry delay is also useful during a cold start (i.e., an initial turn-on of the circuit card) where large load capacitors located on the circuit card are required to be charged with limited input current. Furthermore, the retry delay also keeps switch Q 5  dissipation under control during an output short circuit condition. 
         [0020]    The discrete protection circuit of the invention offers distinct advantages over prior art integrated circuits that are designed for hotswap and current-limiting applications. For instance, integrated circuit overcurrent protection circuits are relatively expensive as they are typically single sourced and designed for particular applications. The present invention, however, can be made of relatively inexpensive parts that are easily accessible from a variety of sources. Furthermore, because they are single sourced, most integrated circuits for hot swap applications are not compatible with each other. The present invention provides a hot swap overcurrent protection circuit that is suitable and cost-effective for a variety of applications. The protection circuit of the present invention also has a lower parts count and an increased reliability. 
         [0021]    Accordingly, systems and methods have been described regarding a discrete protection circuit that provides protection to circuit cards that are attached to a live backplane. It should be emphasized that the above-described embodiments of the present invention, particularly, any “preferred” embodiments, are merely possible examples of implementations, merely set forth for a clear understanding of the principles of the invention. Many variations and modifications may be made to the above-described embodiment(s) of the invention without departing substantially from the spirit and principles of the invention. All such modifications and variations are intended to be included herein within the scope of this disclosure and protected by the following claims.