Other Parts Discussed in Thread: TL431, LM51551
Hi Team,
I following PMP22764 design a 28Vin/56Vo/100W flyback converter. The schematic is attached below.
28 to 55V schematic with CMC.pdf
During the EVT build, I find 3 of 480 pcs have issue which couldn't loading over 60% at output.
I check the Gate and CS signal, observe the PWM signal is unstable at 50-60% load (The duty will cahnge at each cycle) cause VCS has chance to trigger the 100mV threshold.
I trying to swap same components (Transformer, TL431, Photo-coupler, Rsense) to check if it's components failure. I also trying to modify the Rsense from 7mOhm to 5mOhm to see if it's design margin issue.
But, those changes doesn't work. The failure symptoms still exist.
I measure the impedance of the LM51551 pins between failures and good unit. They looks the same.
Please help to recommend debugging suggestion. thank you.
Muhsiu.