Apparatus for sensing an output current in a communications device

• US 20070174527A1
• Filed: 01/17/2007
• Published: 07/26/2007
• Est. Priority Date: 01/17/2006
• Status: Active Grant

• Current Assignee: Avago Technologies International Sales Pte Limited (Broadcom, Inc.)
• Original Assignee: Broadcom Corporation (Broadcom, Inc.)
• Inventors: Vorenkamp, Pieter
• Granted Patent: US 7,782,094 B2
• Time in Patent Office: Days
• US Class Current: 710/100
• CPC Class Codes: G06F 1/26 Power supply means, e.g. regulation thereof for memories G11C
• CPC Class Codes: G06F 1/266 Arrangements to supply power to external peripherals either directly from the computer or under computer control, e.g. supply of power through the communication port, computer controlled power-strips
• CPC Class Codes: H04B 3/44 Arrangements for feeding power to a repeater along the transmission line
• CPC Class Codes: H04L 12/10 Current supply arrangements
• CPC Class Codes: H04L 12/40045 Details regarding the feeding of energy to the node from the bus
• CPC Class Codes: H04L 25/02 Details ; arrangements for supplying electrical power along data transmission lines systems for transmitting signals via power distribution lines H04B3/54
• CPC Class Codes: Y10S 370/91 Carrier sense multiple access, e.g. ethernet, 10base-t

Abstract

Power over Ethernet (PoE) communication systems provide power and data communications over the same communications link, where a power source device (PSE) provides DC power (for example, 48 volts DC) to a powered device (PD). The DC power is transmitted simultaneously over the same communications medium with the high speed data from one node to the other node. The PSE typically includes a controller that controls the DC power provided to the PD at the second node of the communications link. The PSE controller measures the voltage, current, and temperature of the outgoing and incoming DC supply lines to characterize the power requirements of the PD. The PSE controller includes a resistorless switch to measure the current. The resistorless switch includes a sense transistor and a current mirror to allowing the PSE controller to calculate the current based upon a replica current.

Claims

1. An apparatus for sensing current output in a communications device;

comprising;

a main transistor configured to provide the current output to a communications medium;

a first amplifier coupled to the main transistor, wherein an output of the first amplifier is based upon a drain to source voltage of the main transistor;

a second amplifier coupled to the first transistor, wherein an output of the second amplifier is based upon a gate to source voltage of the main transistor; and

a sense transistor coupled to the first amplifier and the second amplifier, wherein a drain to source voltage of the sense transistor is substantially equivalent to the drain to source voltage of the main transistor and a gate to source voltage of the sense transistor is substantially equivalent to the gate to source voltage of the main transistor.

2. The apparatus of claim 1, wherein the sense transistor is a scaled version of the main transistor so that the main transistor generates a first current and the sense transistor generates a second current that is a scaled version the first current.

3. The apparatus of claim 2, wherein the first current flows over the communications medium so as to provide power to a device coupled to the communications medium.

4. The apparatus of claim 2, further comprising:

a current mirror coupled to the sense transistor to generate the second current that is applied to the sense transistor.

5. The apparatus of claim 4, further comprising:

a sense resistor coupled to the current mirror, wherein the current mirror sources a third current to the sense resistor so that the third current is based on the second current.

6. The apparatus of claim 5, wherein the sense resistor generates a voltage that represents the first current flowing through the main transistor.

7. The apparatus of claim 6, wherein the first current flowing through the main transistor is applied to the communications medium so as to provide power to a powered device coupled to the communications medium, the voltage across the sense resistor representing the current delivered to the powered device.

8. The apparatus of claim 4, wherein the current mirror comprises:

a third transistor having a gate and a source; and

a fourth transistor have a gate and a source, wherein the gate of the third transistor is connected to the gate of the fourth transistor, the source of the third transistor is coupled to a drain of the second transistor, and the source of the fourth transistor is connected to the resistor.

9. The apparatus of claim 8, further comprising:

a fifth transistor connected between the current mirror and the sense transistor.

10. An apparatus for sensing current output in a communications device, comprising:

a main transistor having a first current;

a first amplifier coupled to the main transistor, wherein an output of the first amplifier is based upon the first current; and

a sense transistor having a second current coupled to the first amplifier, the second current is based upon the first current.

11. The apparatus of claim 10, further comprising:

a second amplifier coupled to the main transistor, wherein an output of the second amplifier is based upon the first current, and the second current is based upon the output of the first amplifier and the output of the second amplifier.

12. The apparatus of claim 10, wherein the sense transistor is a scaled version of the main transistor so that the main transistor generates a first current and the sense transistor generates a second current that is a scaled version the first current.

13. The apparatus of claim 12, further comprising:

a current mirror coupled to the sense transistor to generate a third current, wherein the third current is based on the second current.

14. The apparatus of claim 13, further comprising:

a sense resistor coupled to the current mirror and receiving the third current, wherein the sense resistor generates a voltage that represents the first current flowing through the main transistor.

15. The apparatus of claim 14, wherein the first current flowing through the main transistor is applied to the communications medium so as to provide power to a powered device coupled to the communications medium, the voltage across the sense resistor representing the power delivered to the powered device.

16. The apparatus of claim 15, wherein the communications medium is an Ethernet communications medium.

17. A Power over Ethernet (POE) device capable of providing DC power over a communications medium, comprising:

a DC power supply;

a main transistor coupled to the DC power supply that controls an output current from the DC power supply to the communications medium; and

means for sensing the output current flow, thereby determining DC power applied to the communications medium.

18. The PoE device of claim 17, wherein the means for sensing includes:

a sense transistor configured to produce a current proportional to the current output of the main transistor.

19. The PoE device of claim 17, wherein a bias of the sense transistor is determined based on a bias of the main transistor.

20. The PoE device of claim 17, wherein the bias of the sense transistor is determined to replicate the bias of the main transistor.