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SMPS Always Trying to Restart

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xavier60:
The yellow waveform shows that the PSU is actually going into regulation briefly before the IC's UVLO is triggered.
Although the schematic shows the AUX winding phased correctly, maybe it isn't. As others have mentioned.

timenutgoblin:

--- Quote from: magic on October 28, 2021, 06:39:46 am ---R33 limits peak primary current and it should be set to a threshold below core saturation. If the PSU is designed to provide less power than the IC's maximum capability, the transformer may be smaller and R33 larger than in the reference design.

I humbly suggest that neither input power nor output load is the limiting factor, because output voltage rises from 2.5V to 4.5V instantly before it gets stuck.

It also falls down very distinctly when the PWM runs out of power, indicating that during the time when it's stable, the PWM is still operational and apparently in regulation.

--- End quote ---

It's interesting to note that the VCC voltage rises up to 20VDC, but then drops below 10VDC before the IC restarts. There isn't any indication that D9.b diode conducts to charge EC11.b capacitor. The capacitor only appears to be charging via the HV resistors. I wonder if R29.b resistor has gone open circuit?

As for the output voltage, I also noticed that it remains steady at 4.6VDC before collapsing to 2.5VDC then restarting.

Theoretically, if there was a 5k ohm resistor in parallel with R35.b resistor then that would explain the output voltage of 4.583VDC. The schematic suggests otherwise, though.

Interesting problem.

xavier60:
I have just noticed that it should be regulating at 15V. With the output being limited at 4.6V, the AUX winding will not develop enough voltage to sustain the IC.
Check to see what the REF pin on the TL431 comes up to.
If C14.b was shorted, the output would regulate at about what it appears to be.

xavier60:
If it was just a regulation problem, the output should be holding for longer at the 4.6V.
EC11.b is unusually low in value.

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