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Power Over Ethernet Interoperability Guide by Sanjaya Maniktala

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Rise-Time Limits
We also need to limit the minimum rise time of the rising voltage, to avoid making it too “snappy.” That can cause significant noise in the lines, and in the RJ-45 integrated magnetics, affecting other ports too. There is also a well-known industry case that caused a Type 2 PD to “forget” it saw two fingers, which would have identified a Type 2 PSE to it. So, despite offering its classification circuit twice, it “thought” it was connected to a Type 1 PSE and did not power up in high-power mode. All because of the steep dV/dt. For such reasons, the standard asks that the rise-time “TRISE” be more than 15 μs. How is TRISE supposed to be measured? From 10 percent of the final port voltage to 90 percent of the final port voltage. ...

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