DISCRETE HOT SWAP AND OVERCURRENT-LIMITING CIRCUIT

INVENTOR: Vinitkumar S. Adi

TECHNICAL FIELD

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

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 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.

FIG. 1 is an abridged block diagram of a communications system that is suitable for use in implementing the present invention.

FIG. 2 is a block diagram of a backplane for receiving and powering conventional circuit cards.

FIG. 3 illustrates the drop in voltage across a backplane when unprotected circuit cards are connected thereto. FIG. 4 is an illustration of a schematic of the discrete protection circuit of the present invention.

FIG. 5 is a block diagram of the live backplane for receiving and powering circuit cards including the discrete protection circuit of FIG. 4.

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 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. 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.

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 150a-d.

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.

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. 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.

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.

As shown in the exemplary embodiment of FIG. 4, the discrete circuit 400 includes a sense resistor R3 to detect an overcurrent condition, a discrete SCR 435 which latches when an overcurrent condition is detected, and a switch Q5 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 Q3 of the circuit 400 is turned on. When transistor Q3 is turned on, it will initiate the charging of the capacitor Cl. Once the voltage across capacitor Cl exceeds the gate threshold voltage of a switch Q5, an input source 405 is connected to the output load 430 (i.e., to the load components on the circuit card) through the switch Q5.

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 R3 senses the load current at output 430. More specifically, the load current passes through resistor R3 and switch Q5. The load current through resistor R3 develops a proportional voltage across it that is sensed by transistor gates Q 1 and Q4 to detect a fault condition. If a voltage drop across resistor R3 is greater than the emitter-base (E-B) diode drop of transistor Ql, then Silicon Controlled Rectifier (SCR) 435, formed by transistors Q 1 and Q4, will go into a latch state. This will restrict the input voltage at the gate of switch Q5, and in turn will shut switch Q5 off. Concurrently, capacitor Cl will discharge through Ql and Q4. As soon as the capacitor Cl is completely discharged, SCR 435 turns off. Switch Q5 remains off until capacitor Cl is charged back above the gate threshold voltage of the switch Q5. Capacitor Cl charge time can be controlled by selecting appropriate values for resistors Rl and R2. Additionally, capacitor Cl charge time controls a retry delay following the detection of the over-current condition.

Switch Q5, transistor Ql and transistor Q4, which form the SCR 435, resistor Rl, resistor R2, 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 Q5 dissipation under control during an output short circuit condition.

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. 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.