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Power Over Ethernet Interoperability Guide by Sanjaya Maniktala

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Lowering the PD Undervoltage Lockout
The AT standard does say that the PD must turn OFF before the port voltage falls to 30 V. Though there is a valid concern if this is an unintended error in the standard as discussed in Chapter 5. That aside, it also does not specify that the PD must continue operating down to at least, say 32 V or even 36 V. And that is the problem, because as we go to longer cable lengths we maximize power delivery provided we allow the PD to continue to function down to close to half the PSE-side port voltage (25 V for VPSE = 50 V or 30 V for VPSE ≈ 60 V). In that sense, the IEEE committees seem to have missed an opportunity to future-proof the standard for longer than 100-m cable lengths. The logic should have been based ...

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