Why doesn't PSE circuit maintain power to PD?

Thread Starter

Cristian M

Joined Sep 14, 2017
I am trying to make a PoE 802.3af solution. This are my schematics:


The problem is that the power does not maintain. It only provides 48V for about 300ms, then it drops. Also, the above statement is valid when I remove the DC converter input capacitor; I tried a 22uF, 47uF, 22uF, 100uF capacitor, but every time I connect the capacitor, the PSE (LTC4263) does not supply power, instead it indicates low signature resistance by flashing the LED with a specific pattern.

This iare the oscilloscope capture of the voltage across D1 diode and PG output.
voltage across D1 diode_Cristian M.jpg
PG output_Cristian M.jpg

What is causing this behavior and how can it be resolved? What capacitor should I use?

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