We designed board with PoE capability using ST PM8800 chip. We encountered some problems.
Issue #1
When our board is powered from external power supply (Vin(AUXII)=32V) we get output voltage with high ripples and PM8800 nPGD is inactive (High). Ripple Vpp voltage with ~6.4W resistive load is Vpp_ripple = 1.3V (picture vin=32V).
Issue #2
When we power board from PoE switch, switch PoE LED is lit and PM8800 nPGD is active (Low). Ripple Vpp voltage with ~6.4W resistive load. Ripple Vpp voltage with ~6.4W resistive load is Vpp_ripple = 2.0V (picture vin=poe).
Issue #3
Another interesting PoE schematic behavior was noticed when it is operating without load and is powered from external power supply (Vin(AUXII)=32V) we get Vpp_ripple = 0.5V. But when we connect one end of Ethernet cable (another end is not connected) to our board and another end of Ethernet cable is not connected, ripples increases to Vpp_ripple = 1.2V
I attached board schematic measurements results.
We tried to add more capacitance to output (100uF and 1000uF), but there was no noticeable improvement.
We also tried to change feedback compensation values, MOSFET gate control circuit (diode + resistors), RT resistor value (124k), RC snubber according to AN4377 Fig. 2 (AM03487V1).
Is there any errors in our schematic?
What can cause these ripples on the output?
Is there any requirements for transformer wingdings directions?
Maybe there is problem with feedback compensation circuit? How I can calculate values for feedback compensation?
I also opened case (ID: 00069919) on 14/12/2018 but I haven't received a reply so far.
