Tool/software:
Hi
Customer use UCC3818A, and find abnormal noise in some board, at their MP statge.
we had summited FA, but the FA shows there is no issue on those IC (first FA returns IC).
Could you help check this issue? and check there is design issue on schematic and layout ?
related files as attachment. thanks
thanks
Hello Jonas,
It is clear that the gate-drive PWM output of the NG IC board is abnormal for some unknown reason.
The PWM duty cycle is generated by comparing the CAOUT signal to the Oscillator ramp, so in my opinion, there seems to be some interference with either the ramp or the CAOUT signal. The oscillator ramp comes from 2 pins: RT and CT. The CAOUT signal comes from a combination of 6 signals: SS, VAOUT, VSENSE, VFF, IAC, MOUT, and CAI.
I have circled these signals in the image below. (I forgot to circle CAI, pin 4.)
I suspect that maybe one of these 8 possible signals may have some noise or corruption that results in the abnormal PWM.
Since you have already the components on pins 3 and 5 (CAOUT, MOUT) I suggest to probe each of the other 8 pins and compare to the same pin waveforms on the OK IC board.
See if one of them has an abnormal input waveform that can eventually corrupt the PWM signal. Then investigate the cause of that abnormal input.
I cannot determine any possible cause for this problem from the pcb layout that you provided. Since the layout works okay on the OK IC board, I assume this is not a layout problem. But it could be a bad component or a bad or intermittent solder joint on one of the components attached to one of the pins. Please check this.
Regards,
Ulrich
