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Power Over Ethernet Interoperability Guide by Sanjaya Maniktala

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Recommendations for Common-Mode Filter Position
In DC disconnect, access to the bulk capacitor of the PSU is available to absorb the huge surge energy. See Figs. 11.5, 11.7, and 11.10. This is fortuitous, but it also means we should not place an unnecessary impedance or barrier between the PSE chip (that we wish to protect) and the bulk cap (its protection). For this reason, it is not advisable to place any exotic inrush-limiting circuitry, or even common-mode filters, between the PSE chip and the bulk cap of the PSU. A common-mode filter in the port has pros and cons. We now discuss some key points regarding a common-mode filter in order to make the best decision.
1.  It is usually added to just make up for poor filtering in the PSU, so it ...

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