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Power Over Ethernet Interoperability Guide by Sanjaya Maniktala

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The Inrush Timer and the Real End of Inrush
Now we consider some timing issues. The end of inrush is marked by the fact that the PD’s bulk cap is fully charged and neither the PD’s pass-FET or the PSE’s pass-FET are in current limiting anymore. This supposedly marks the entry into Power-on state (with some qualifications as discussed later).
How do we know the capacitor has been fully charged? The standard says the inrush is considered complete when the port capacitor voltage is 99 percent of the final settling value. This number may however prove difficult to measure on a test setup with any precision: 1 percent of 50 V is 0.5 V. The University of New Hampshire Interoperability Lab (UNH-IOL) asks to monitor the port voltage on an oscilloscope ...

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