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Why do we connect the power (48 V) and ground (return path for PoE power) on the center taps of transformer for PoE applications?

Can't it be directly connected to TD+ & TD- pins of the transformer? If it is legitimate, what is the difference between two topologies?

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  • \$\begingroup\$ Putting power directly onto a data line means data is shorted to power. \$\endgroup\$
    – Andy aka
    Commented May 31, 2018 at 10:29

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Here is a schematic diagram of the ethernet magnetics.

The DC current flows from the PoE injector into the center tap where it splits.

Half of the current flows through the transformer winding in one direction, and the other 1/2 of the current flows in the opposite direction, thereby no net magnetic field is created by any changes in the DC current.

No net magnetic field means that no signal is injected into the ethernet data stream.

Similar thing happens at the other end.

enter image description herefrom: avnet.com

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  • \$\begingroup\$ seems non-standard. the standard has the powered device accepting power on the spare pair also. \$\endgroup\$ Commented May 31, 2018 at 8:43
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    \$\begingroup\$ @Jasen AFAIK it is the standard for the old 802.3af Mode A. \$\endgroup\$ Commented Feb 11, 2019 at 15:38
  • \$\begingroup\$ some magnetic field is injected by the mismatch of the two legs of the pair, the key thing is the sum of the fluxes through the two windings is sufficiently small not to saturate the core. \$\endgroup\$
    – james
    Commented Oct 20, 2019 at 9:13

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