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Power Over Ethernet Interoperability Guide by Sanjaya Maniktala

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Recommendations for AC Disconnect
We have realized from Figs. 11.5 and 11.7 that the problem with AC disconnect is that because of the AC-disconnect diode blocking in a reverse direction, access to the bulk cap of the PSU (and input 58 V TVS, not shown) is not available. The surge waveform can therefore play havoc with the front end, especially the 0.1-μF port caps and from there to the port pins or FETs.
The easiest way to handle this is to have a small Y-capacitance. The idea behind that is that the applied surge waveform has a certain dV/dt, and if the Y-capacitance charges up at the same rate or a little faster, we will be in a “comfort zone,” where on an instantaneous basis, the applied voltage will almost equal to the voltage across the ...

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