Electronics > Repair

strange behaviour of the damaged smps with uc3845

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excalibur_the_sword:
both are ok. 22ohms.
i attached picture of axillary winding output in three zoom levels.
and Vcc capacitor voltage waveform
when main caps are charged and i apply external 9 volts to controller, psu produces stable 12 volts.
when i slowly increase the input voltage, controller starts switching at 8.63V. But when it is connected to the mains, it will not continue to work, even the voltage reaches 8.77V.
i do not know why the burning sign is there. there is nothing there except those two resistors, and they are both ok. axillary winding is also, at the other side of transformer and it is unlikely to be damaged.
 so i think the problem is in startup circuit. but i don't know where it is.

minifloat:
22Ω seems a bit too high at the DC/secondary current sense (where the PCB is charcoal black).
For primary side current sense, the datasheet exemplary circuit uses 0.75Ω.

I did not find out what the transformer T2 is doing, looks a bit odd though...

br, mf

Harry_22:
Both 22Ω in series with capacitors. These are snubbers.

inse:
The normal supply voltage should be higher than barely above the UVLO, I would guess 12..15V.
Check whether it’s about the rectifier diode, the current limiting resistor or a zener diode, if present.
You have already ruled out the supply filter cap.
There have been cases with damaged transformers as well.
What is the second optocoupler for, are you able to reverse engineer that?
Something like over current feedback, maybe.

excalibur_the_sword:

--- Quote from: inse on October 24, 2024, 06:00:14 am ---The normal supply voltage should be higher than barely above the UVLO, I would guess 12..15V.
Check whether it’s about the rectifier diode, the current limiting resistor or a zener diode, if present.
You have already ruled out the supply filter cap.
There have been cases with damaged transformers as well.
What is the second optocoupler for, are you able to reverse engineer that?
Something like over current feedback, maybe.

--- End quote ---
thanks.
typical start voltage for 3845 is 8.4v.
almost checked every component and i think the last suspect is transformer.
i couldn't figure out what the second optocoupler is for.
if thats any issue except than startup circuit, it should act even when i apply external Vcc.

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