Electronics > Repair

strange behaviour of the damaged smps with uc3845

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inse:
Sorry, the last sentence left me confused:
if thats any issue except than startup circuit, it should act even when i apply external Vcc.

I thought it was working with Vcc supplied

excalibur_the_sword:
sorry. that's magic of my awful english.
as i mentioned in previous posts, when i apply external Vcc to controller and bulk caps are cherged, psu generates stable 12V until the cas are discharged. so if there is any problem in current sense or other parts, that cause restart, controller should restart not only when connected to the mains, but also when started using external voltage.
so, i think the issue, whatever it is, should be in startup circut. i checked every component from winding to the Vcc pin. everything was ok. just the winding itself remained unchecked.

inse:
How much current is the controller drawing from external supply?
Does that match to the data sheet?

excalibur_the_sword:
i don't remeber. but i tried new one. result was the same.

shakalnokturn:
If you've already checked or replaced the diode that supplies UC3845 from the auxiliary winding, try loading the PSU's 12V output with maybe 10 Ohms, some PSU's just won't run correctly unloaded.
Maybe you're at the limit of UVLO and the ripple is bothering the UC3845, loading the regulated output will bring the UC3845's Vcc higher if the switching kicks in at all.

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