Controlling power distribution among multiple wires in communication cable

1. A system for providing power over a communication cable having at least first and second pairs of wires, comprising:

a power supply device for supplying power over the communication cable to a powered device, and

power distribution circuitry for controlling power distribution between the wires of the communication cable so as to provide balance of power between the first pair and the second pair, the power distribution circuitry including circuitry for adjusting voltages applied to the first and second pairs so as to control distribution of current between the first pair and the second pair.

2. The system of claim 1 , wherein the power supply device is configured for supplying power over an Ethernet cable.

3. The system of claim 1 , wherein the power distribution circuitry is configured for controlling distribution of current between the wires of the communication cable.

4. The system of claim 3 , wherein the power distribution circuitry is configured for providing balance of current between the wires of the communication cable.

5. The system of claim 1 , wherein the power distribution circuitry is configured for providing balance of current between the first pair and the second pair.

6. The system of claim 1 , wherein the power distribution circuitry comprises a reference circuit for providing a reference voltage, first and second sense resistors, and first and second operational amplifiers for respectively comparing the reference voltage with first and second sense voltages across the first and second sense resistors.

7. The system of claim 6 , wherein the power distribution circuitry further comprises first and second driving elements respectively coupled to outputs of the first and second operational amplifiers for controlling current values.

8. The system of claim 7 , wherein each of the first and second driving elements includes one or more MOSFET transistors.

9. The system of claim 1 , wherein the power distribution circuitry comprises a reference circuit for providing a reference voltage, a first sense resistors, and a current limit amplifier for comparing the reference voltage with a first sense voltage across the first sense resistor to limit a current value in accordance with the reference voltage.

10. The system of claim 9 , wherein the power distribution circuitry further comprises a second sense resistor, and a balance amplifier for comparing a second sense voltage across the second sense resistor with the first sense voltage to provide a balance between current values at outputs of the current limit amplifier and the balance amplifier.

11. The system of claim 10 , wherein the power distribution circuitry further comprises first and second driving elements respectively coupled to the outputs of the current limit amplifier and the balance amplifier for controlling the current values.

12. The system of claim 1 , wherein the power distribution circuit comprises a pair of current sense elements and a first difference amplifier for producing a signal representing a difference between signals produced by the current sense elements.

13. The system of claim 12 , wherein the power distribution signal further comprises a pair of second and third difference amplifiers having first inputs for receiving the output of the first difference amplifier, and second inputs for receiving respective reference values, a reference value supplied to the second difference amplifier differs from a reference value supplied to the third difference amplifier.

14. The system of claim 13 , wherein the power distribution circuit further comprises first and second driving elements respectively coupled to outputs of the second and third difference amplifiers for controlling current values.

15. The system of claim 1 , further comprising a transformer for coupling the power supply device to the communication cable, the transformer having a first winding corresponding to a first wire in the communication cable, and a second winding corresponding to a second wire in the communication cable.

16. The system of claim 15 , further comprising a capacitive element provided between the first winding and the second winding to prevent a DC connection between the first and second winding.

17. The system of claim 1 , wherein the power distribution circuitry is configured for balancing sum of currents in wires of the first pair with sum of currents in wires of the second pair.

18. A method of supplying power over a communication cable having at least first and second pairs of wires, comprising the steps of:

supplying power over the communication cable, and

adjusting voltages applied to the first and second pairs so as to control distribution of current between the first pair and the second pair.

19. The method of claim 18 , wherein the power is supplied over an Ethernet cable.

20. The method of claim 18 , wherein the step of adjusting voltages includes the step of providing balance of current between the first pair and the second pair of the communication cable.

21. The method of claim 18 , wherein the step of adjusting voltages comprises balancing sum of currents in wires of the first pair with sum of currents in wires of the second pair.

22. A local area network comprising:

at least a pair of network nodes,

a network hub, and

communication cabling having at least first and second pairs of wires for connecting the network nodes to the network hub to provide data communications,

the network hub having a power supply device for providing power to a load over the communication cabling, the network including power distribution circuitry for controlling distribution of power between the wires so as to provide balance of power between the first pair and the second pair, the power distribution circuitry including circuitry for adjusting voltages applied to the first and second pairs so as to control distribution of current between the first pair and the second pair.

23. The network of claim 22 , wherein the power distribution circuitry is configured for balancing sum of currents in wires of the first pair with sum of currents in wires of the second pair.