Power Over Ethernet Interoperability Guide

Semiconductors for Protection and Some PCB Recommendations

So far, we have deliberately avoiding mentioning the 58-V transient voltage suppressor (TVS) habitually placed in the PD and PSE. The question is how much good does that really do?

Despite common belief, this has rather limited use. In fact, it cannot even work on its own. The reason is that this diode has a max peak-current rating of only 4.3 A, whereas the measured surge currents are closer to 20 to 40 A at 2-kV, depending on amount of Y-capacitance used (it can theoretically be as high as 2000 V/42 Ω ≈ 50 A with higher Y-capacitances). Further, with only 4.3 A max passing through it, this TVS clamps not at 58 V as commonly assumed but at 93.6 V. So not only is its current rating ...