Dear friends,
I have a power supply DC to DC input range (-48VDC to -72VDC) the output is +5VDC and +12VDC ).
the problem is the output +5.4VDC with fluctuation arround 0.2VDC and +8.2VDC(it should +12VDC).
I examined it and found
the IC for PWM is UC3845B on pin8 (Vref) the voltage fluctuation is between 1.45v to 1.9v and it should be +5V.
presumed that the problem from it ( due to the UC 3845B being responsible for generating Vref).so I changed it but the problem is still the same.
so my friends kindly anyone have any suggestions?
thanks in advance to all of you my friends.
Best regards

 

I suspect something connected to Vref is drawing too much current and pulling it down.

 

I suspect something connected to Vref is drawing too much current and pulling it down.

Good morning sir,
Thanks I will check that.
Best regards

 

The first suspect would be any filtering capacitors tied to the reference voltage line. The second suspects would be the supply voltage for the reference device. You may need to use an oscilloscope in the search, as the problem might even be some noise that should not be present. Or even an oscillation.
Questions: Did the supply function correctly previously? And what input voltage are you using for the testing?

 

The first suspect would be any filtering capacitors tied to the reference voltage line. The second suspect would be the supply voltage for the reference device. You may need to use an oscilloscope in the search, as the problem might even be some noise that should not be present. Or even an oscillation.
Questions: Did the supply function correctly previously? And what input voltage are you using for the testing?

I'm using -48VDC, I checked all electrolytic capacitors by LCR ( it was fine ) and I resoldering many points, especially the transformers, Mosfets and shotcky diodes, and I was wondering when I test the resistance between two leads of this small capacitor 3.3nf it was 19.2 ohme even I test the same capacitor in another power supply it was the same measurement( do you know why it is like that?

 

The measured resistance across any component connected in a circuit is affected by the rest of the circuit that it is connected to. That is basic to all electronic circuits.
What that means is that it requires the interaction of many components to provide the required circuit function.
It also means that checking individual components on a PC board assembly is a very inefficient method of finding a failed component causing a malfunction. It is even less efficient when not understanding the function of the components.
From my own experience, switcher power supplies are some of the most challenging systems to diagnose problems in, unless one has a circuit schematic to follow and understands how the supply works.

 

Thanks my friend for your reply and help, and about ( The measured resistance across any component connected in a circuit is affected by the rest of the circuit that it is connected to. That is basic to all electronic circuits. ) I know this point my friend, but the location and the value of the resistance made me wonder or suspect is the reason for the voltage dropped and fluctuated.
also As what you said it is better and more comfortable when you have a schematic. again thanks my friend I really appreciate your cooperation