Power Over Ethernet Interoperability Guide

Analyzing the Inrush Phase

Going back to the inrush case, where the C_{DC_DC} cap is fully discharged initially, we ask: How quickly can a 180-μF cap be charged? Just the basic calculation only at first! Here we are depending on the PSE-side current limit of 0.4 to 0.45 A, and taking the lowest value of the range (0.4 A) we get the worst-case time as

Note that we have retained the starting voltage as 0 V above as a reminder that even though the port voltage may be 30 V or so when the PD’s pass-FET turns ON, the capacitor itself is fully uncharged.

A reasonable time is 26 ms. We know from the inrush specification that a PSE must support at least ...