Discrete Hot Swap and Overcurrent-Limiting Circuit

• US 20090027818A1
• Filed: 07/27/2007
• Published: 01/29/2009
• Est. Priority Date: 07/27/2007
• Status: Active Grant

• Current Assignee: Cisco Technology, Inc. (Cisco Systems, Inc.)
• Original Assignee: Scientific-Atlanta LLC (Cisco Systems, Inc.)
• Inventors: Adi, Vinitkumar S.
• Granted Patent: US 7,813,095 B2
• Time in Patent Office: Days
• US Class Current: 361/75
• CPC Class Codes: G06F 13/4081 Live connection to bus, e.g. hot-plugging current or voltage limitation during live insertion H02H9/004

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.

Claims

1. A system comprising:

a live backplane adapted to provide a voltage to one or more circuit cards connected thereto; and

at least one protected circuit card adapted to couple to said live backplane, said protected circuit card including a discrete protection circuit adapted to limit the current provided to said protected circuit card when said protected circuit card is coupled to said backplane, said discrete protection circuit comprising;

a first capacitor adapted to receive an input and accumulate a charge accordingly;

a sense resistor adapted to sense a load current and have a voltage proportional to said load current so that an overcurrent condition can be detected by sensing the voltage across said sense resistor.

a switch adapted to control a load current delivered to an output port;

a discrete SCR adapted to latch when an overcurrent condition is detected, thereby preventing the flow of said load current to said output port; and

a series of two resistors coupled between said first capacitor and ground, the series of resistors adapted to set a charge time for said first capacitor, said charge time controlling the retry delay subsequent to the detection of an overcurrent condition.

2. The system of claim 1, wherein said SCR comprises a first transistor having an emitter-base voltage, said SCR adapted to turn off said switch when said voltage across said load current resistor exceeds said emitter-base voltage.

3. The system of claim 1, wherein said first capacitor discharges through said SCR when said switch is turned off.

4. The system of claim 1, said discrete protection circuit further comprising:

an on/off pin adapted to turn said discrete protection circuit on and off; and

a control transistor, said control transistor adapted to turn on when said on/off pin is in an on position, thereby initiating the charging of said first capacitor.

5. The system of claim 1, wherein at least one circuit card is connected to said live backplane.

6. The system of claim 5, wherein said voltage provided to said at least one connected circuit card remains constant while a protected circuit card is being connected to said live backplane.

7. The system of claim 1, wherein at least one protected circuit card is coupled to said backplane.

8. A method of providing a discrete protection circuit, comprising:

providing a first capacitor adapted to receive an input and accumulate a charge based on said input;

providing a sense resistor adapted to sense a load current and have a voltage proportional to said load current so that an overcurrent condition can be detected by sensing the voltage across said sense resistor;

providing a switch to control a load current delivered to an output port;

providing a discrete SCR adapted to latch when an overcurrent condition is detected, thereby preventing the flow of said load current to said output port; and

providing a series of two resistors coupled between said first capacitor and ground, the series of resistors adapted to set a charge time for said first capacitor, said charge time controlling the retry delay following the turning off of said switch due to the detection of an overcurrent condition.

9. The method of claim 8, further comprising:

providing an on/off pin adapted to turn said discrete protection circuit on and off; and

providing a control transistor, said control transistor adapted to turn on when said on/off pin is in an on position, thereby initiating the charging of said first capacitor.

10. The method of claim 8, further comprising providing an input from a live backplane to said protection circuit.

11. The method of claim 8, further comprising coupling said protection circuit to electrical components to form a protected circuit board.

12. A discrete protection circuit, comprising:

a first capacitor adapted to receive an input and accumulate a charge based on said input;

a sense resistor adapted to sense a load current and have a voltage proportional to said load current so that an overcurrent condition can be detected by sensing the voltage across said load current resistor;

a switch adapted to control a load current delivered to an output port;

a discrete SCR adapted to latch when an overcurrent condition is detected, thereby preventing the flow of said load current to said output port; and

a series of two resistors coupled between said first capacitor and ground, the series of resistors adapted to set a charge time for said first capacitor, said charge time controlling the retry delay subsequent to the detection of an overcurrent condition.

13. The protection circuit of claim 12, further comprising:

an on/off pin adapted to turn said discrete overcurrent protection circuit on and off; and

a control transistor, said control transistor adapted to turn on when said on/off pin is in an on position, thereby initiating charging of said first capacitor.

14. The protection circuit of claim 12, said SCR comprising a first transistor with a base-emitter voltage and adapted to turn off said switch when said load current resistor voltage exceeds said base-emitter voltage.

15. The protection circuit of claim 12, wherein said first capacitor discharges through said SCR when said switch is turned off.

16. The protection circuit of claim 12, wherein the charge time of said capacitor can be controlled by selecting the appropriate values of said series of two resistors.

17. The protection circuit of claim 12, further comprising an output port adapted to provide said load current to electrical components on a circuit board.