We found that after feeding according to the schematic diagram, the circuit did not output；But after canceling U21, the POE outputs a 12V power supply，However, in this case, it was found that the Vds of MOS reached 220V。Please help analyze it，Thanks。
Thanks for reaching out! We saw some concerns in your schematic. Resolving them may help you get a correct Vout. Besides, you can refer PMP9563 (www.ti.com/.../PMP9563) for a 12-V 2.5-A ACF design.
1. The AUX winding design is better to be another forward converter rather than a rectifier. Since the rectifier AUX voltage is related to Vin, and Vin may vary from 37V to 57V for PoE standard which makes VCC a wide range. While forward converter based circuit (D15, D16, L7 in the below picture) can make VCC voltage a fixed ratio the regulated Vout.
2. The APD needs to be tied to RTN if not use. Leave it open may cause PD’s hot swap turning off if VADP gets coupled by VDD.
3. Sometimes the large capacitor value of GAT2 could cause startup issue. You may try with 0.22 – 0.47 uF to see if any improvements.
Best regards,
Diang
