Sawtooth on output
We have 16 identical power supplies built on PCBs. About half of them, even when very lightly loaded (maybe 10 milliamps), generate about 200mV p-p of noise in a sawtooth waveform on a 9VDC output.
The schematic is attached. We think it's some type of feedback oscillation that occurs because we might happen to have the incorrect combination of components. When not loaded, there is no problem.
We examined the waveform from the TOPSWITCH source pin to drain pin, and we can see the duty cycle varying rythmically on the power supplies that have this issue. On the power supplies that do not have this problem, the duty cycle is rock solid.
We've tried increasing capacitance, changing resistor values, nothing fixes it.
Comments
Sorry about the lack of the schematic. I work with the original poster and will attach it.
Please ignore. There were layout issues on the PCB. Specifically Ct had a long trace to the TOPswitch.
Good to hear that you found the root cause of the problem in the poor layout.
I cannot see any attached schematic.
If you follow our design guide as shown in our reference designs using TL431 or zener feedback, there should be no issue..