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Power Over Ethernet Interoperability Guide by Sanjaya Maniktala

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Inrush Behavior
One of the key events constituting Power-up phase is inrush. As the port powers up, the PD can draw significant amount of inrush current as it tries to achieve steady state. The IEEE standard tries to protect the cabling infrastructure and also the PD by regulating this inrush. But it also wants to achieve steady state in the best possible manner (with no ringing), and reasonably fast too.
The AF and AT specs on inrush current are just a little different. But they do seem to be very different at first sight. On closer examination, the “differences” are actually only in the first 2 ms of operation. The AT spec is a bit more restrictive in this region. In the discussions leading up to the standards, there were many discussions ...

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