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Power Over Ethernet Interoperability Guide by Sanjaya Maniktala

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Stored Energy and Core Saturation
If PoE currents start causing core saturation of the data transformers, the inductance (OCL) will fall, and that can greatly affect signal integrity and cause data corruption. We had started to explain all this in Chap. 1 (see Fig. 1.12). In PoE, because of center-tapping, the current splits up equally in the two winding halves—in an ideal case. If the two windings were exactly equal (and the rest of the circuit identical too), we will get zero DC-bias contribution from the PoE currents.
However, we do need to be very clear that even if that were true, in an Ethernet transformer, the current as “seen” by the core may in fact be very different from the current as “seen” by the copper (windings). For example, ...

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