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Power Over Ethernet Interoperability Guide by Sanjaya Maniktala

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Dissipation during Classification
How dissipative is the classification phase really? In fact the dissipation during classification can even be more than when the port is powered up and the pass-FET is fully conducting and delivering full AF/AT load current to the PD. For example, if the incoming supply is 48 V, the voltage across the pass-FET of the PSE during classification (class-event range ~18 V) is roughly 48 V – 18 V = 30V. The rest, 18 V, falls across the PD. With a Class 3 current of 30 mA, the dissipation is 30 V × 30 mA = 900 mW (~ 1W) in the PSE pass-FET, and 18 V × 30 mA = 540 mW (~1/2 W) in the PD pass-FET. This is all really significant dissipation. So the maximum classification time during 1-event classification is constrained ...

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