POE power will not interfering on Data travelling on Ethernet , Whats the logic

Question

I was design POE+ circuit , so I read lot of articles on

POE(power over Ethernet) IEEE802.af 15.4W on PSE side , For 10/100 base T can use Alternative A method where it use to deliverer power on data pair or can use Alternative B method where it use to deliverer power on data pair. And for 10/100/1000 base T it uses all four data pair to deliver the power.

Also

POE+(power over Ethernet) IEEE802.at 30W on PSE side , For 10/100 can use Alternative A method where it use to deliverer power on data pair or can use Alternative B method where it use to deliverer power on data pair. And for 10/100/1000 base T it uses all four data pair to deliver the power.

And

UPOE/POE++ IEEE802.bt 60W on PSE side , For 10/100 and for 10/100/1000 base T it uses all four data pair to deliver the power.

I am little bit in need od some solid explanation what make to possible to deliver power on data pair and its not corrupting the data, since POE power voltage is 48V and data voltage is 3.3V. how two different level of voltage sharing the same line.

My understanding is Center tapped transformer play a key role here, and data is in differential mode and power will be in common mode. And power is on 0 Hz and data is on couple in Mhz. different frequency band.

but how , whats the logic, magic behind this.

I am thinking same concept can be used for RS485, CAN , USB or any differential line by using a Transformer. ( understand also need same type of PD at opposite end)

Answer 1

The data is modulated differentially onto the wire pair. Ie the current flows always anti-parallel in the pair. The transformers job is to ensure that there is no DC current (for the signal) and thus that the current is truly differential on the sender's end and that any DC component (from interference) is blocked before the data signal is passed to PHY on the receiver side. By using twisted wires, it is ensured that any interference is common mode on both wires (ie induced current flows into the same direction on both wires). Hence even large interference that would result in multiple volts at the receiver side get canceled out at the transformer and only the differential data signal is left. The PoE current is now put onto both wires of the pair in parallel, ie it flows in the same direction on both, ie looks like a large interference to the transformer. Thus it cancels out the same way.

The cancellation in the transformer works by the current flowing from the wires into the center tap. If the current is the same on both wires and has the same direction, the magnetic field they form in the transformer is the same size, but opposite sign (or direction or whatever your way of looking at it is), because they enter from different sides of the transformer winding.