Input voltage sense circuit in a line powered network element

1. A method of monitoring input voltage for a line-powered network element, the method comprising:

receiving an input voltage for the line-powered network element;

applying the input voltage across a primary winding of a flyback power converter during an on-time of a primary switch;

transferring the input voltage from the primary winding to a secondary winding of the flyback power converter during an off-time of the primary switch;

sensing the voltage of the secondary winding during the on-time of the primary switch; and

drawing minimal current from the flyback power converter.

2. The method of claim 1 , further comprising:

producing an output voltage from the sensed voltage of the secondary winding; and

determining when the input voltage is insufficient.

3. The method of claim 2 , wherein the output voltage is proportional to the input voltage.

4. The method of claim 2 , wherein the alarm signal indicates that the input voltage has fallen below a desired voltage.

5. The method of claim 2 , wherein determining when the input voltage is insufficient comprises:

comparing the output voltage to a reference voltage; and

when the output voltage differs from the reference voltage by a defined criterion, producing an alarm signal.

6. A method of monitoring an input voltage of a line powered sink network element, the method comprising:

receiving the input voltage;

reflecting the input voltage on a secondary of a power supply;

sensing the reflected voltage on the secondary while drawing minimal current form the power supply;

producing an output voltage that is proportional to the input voltage using the reflected voltage;

determining if the input voltage is close to a critical point;

producing an alarm signal when the input voltage is close to a critical point.

7. The method of claim 6 , wherein determining if the input voltage is close to a critical point includes comparing the output voltage to a reference voltage.

8. The method of claim 7 , wherein a critical point occurs when the output voltage falls below a defined amount.

9. The method of claim 6 , wherein the power supply is a flyback power converter.

10. A system employing a voltage sensing circuit, the system comprising:

a line powered network element having a voltage input, including:

a power converter adapted to receive an input voltage; and

a monitoring circuit coupled to the power converter, wherein the monitoring circuit selectively senses a voltage of a secondary winding of the power converter and provides an alarm signal when the input voltage falls below a reference voltage;

wherein the monitoring circuit is adapted to draw minimal current from the power converter.

11. The system of claim 10 , wherein the power converter comprises a flyback power converter.

12. The system of claim 10 , wherein the monitoring circuit includes a diode coupled to a secondary side of the power converter in an arrangement to conduct when a primary switch of the power converter is on.

13. The system of claim 12 , wherein the monitoring circuit further includes an output filter coupled to the diode.

14. The system of claim 13 , wherein the output filter comprises a resistance-capacitance circuit.

15. The system of claim 10 , wherein the monitoring circuit includes a comparator circuit that determines when the input voltage falls below a reference voltage.

16. The system of claim 10 , wherein the monitoring circuit provides an output voltage proportional to the input voltage.

17. An apparatus, comprising:

a flyback power converter;

a monitoring circuit coupled to a secondary side of the flyback power converter;

wherein the monitoring circuit selectively senses an input voltage to the flyback power converter from a reflected voltage of the input voltage, compares the input voltage to a reference voltage and provides an indication when the input voltage differs from the reference voltage by a defined amount; and

wherein the monitoring circuit is adapted to draw minimal current from the flyback power converter.

18. The apparatus of claim 17 , wherein the monitoring circuit includes a diode coupled to the secondary side of the flyback power converter in an arrangement to conduct when a primary switch is of the flyback power converter is on.

19. The apparatus of claim 18 , wherein the monitoring circuit further includes an output filter coupled to the diode.

20. The apparatus of claim 19 , wherein the output filter comprises a resistance-capacitance filter.

21. The apparatus of claim 17 , wherein the monitoring circuit includes a comparator to compare the input voltage to the reference voltage.

22. A network element, comprising:

a power supply; and

a monitoring circuit coupled to the power supply, including:

a voltage sensing circuit coupled to a secondary side of the power converter;

wherein the voltage sensing circuit monitors a reflected voltage of an input voltage to the power supply, compares the reflected voltage to a reference voltage and provides an indication when the reflected voltage differs from the reference voltage by a defined amount;

wherein the voltage sensing circuit monitors the input voltage without drawing current from the power converter.