Power Over Ethernet Interoperability Guide

Types of Power-Up Behavior and Power-On

We now know how to tell when inrush is complete as per the standard, based on the monitored port voltage. We also realize that event can actually happen well before the inrush timer runs out. For example, if we have a very small bulk capacitor inside the PD, the cap will charge up in say only 10 ms, whereas the inrush timer is 65-ms nominal. So the question is: Do we say Power-up is complete at the 10-ms mark or at the 65-ms mark? There is a difference between the AF and AT standards in this respect. The AT standard says: wait till the inrush timer runs out, then come in and check the port voltage and current limit conditions, as implied in the previous para. If all is “kosher” at that moment, then consider ...