Power Over Ethernet Interoperability Guide

Lower Detection Threshold: Practical Concerns in PSEs and PDs

Here is another problem: The lower threshold of the detection resistor is 2.8 V as per the standard. Many have pointed out that this is an oversight on the part of the engineers behind it. The reason is that if the signature resistor is being disengaged, it needs a high-voltage FET inside the PD IC to accomplish that. But the diode-bridge offset is supposed to be about 2 V. In other words, when the PD-side PI voltage is 2.8 V, the PD IC only “sees” 2.8 – 2 = 0.8 V. Therefore, the small, high-voltage detection-resistor FET inside the PD chip has to turn on at 0.8 V. But that is hardly possible, since we need to apply a couple of volts across its Gate and Source typically. Therefore, ...