Patent ID: 8643218

Claim:
A powered device that receives DC power and data over a communications link, the powered device comprising: a transformer configured to provide isolation between said communications link and said powered device, said transformer having a first winding coupled to said communications link and a second winding coupled to a data transceiver; a flux detection circuit, coupled to said second winding of said transformer, that detects a primary magnetic flux associated with said DC power provided over said communications link, wherein said flux detection circuit is configured to: inject a first test current into a first part of said second winding to saturate said transformer and measure a first voltage across said second winding, inject a second test current into a second part of said second winding to saturate said transformer and measure a second voltage across said second winding, and determine said primary magnetic flux using values of said first and second test currents that saturated said transformer or said first and second measured voltages; and a compensation circuit, coupled to said second winding of said transformer, configured to produce a DC cancellation current in said second winding of said transformer, said DC cancellation current causing a secondary magnetic flux in said transformer having a polarity opposite that of said primary magnetic flux.