SMPS Output voltage slowly goes down

SMPS Output voltaje slowly goes down

Hii,


I am a biomedical engineer, I not an specialist with smps, but I have to repair one that is not working properly.

It is a "CONDOR D.C. POWER SUPPLIES, INC." 32.5V 7.70A, SMPS.

At the beginning, there was no voltage at the output after pluging until like 10 minutes of being plugged, I changed all the caps but the high voltage one and i was still the same, then I replaced the uc2844 Current-Mode PWM Controller IC with a uc2845 ic and it worked, I finally had the output availble inmediately after pluging in.

After I replaced the components, the voltage output of my SMPS slowly decreases from 33v to something like 28v and then the outputs suddenly goes to 0v. If I unplug it for about 5min and plug it again, the output is 33v and starts goign down.

Is the problem of the smps that I didn´t replace the IC with an exact one? or is there another problem?, I already replaced the high voltage cap but it is still doing the same.


Thank you for all your time and support.
Sergio David Mercado Vera

Re: SMPS Output voltaje slowly goes down

ingMercado said:

decreases from 33v to something like 28v and then the outputs suddenly goes to 0v. If I unplug it for about 5min and plug it again, the output is 33v and starts goign down.

Click to expand...

The problem sounds like it's heat related. Probably overmuch current through one or more components (maybe due to load drawing too much power, thermal runaway, counterfeit IC, etc.). This results in heat build-up, and the control IC's safety feature responds. It shuts down the system (slowly or quickly).

After components cool, the power supply is able to operate but only for a while.

Re: SMPS Output voltaje slowly goes down

Replacing the PWM with a different part type is not good.
"Worked" as in "got anything at all", and "worked" as in
"I didn't have to ask questions about this new problem"
are obviously different.

I don't think you can distinguish what it's doing. You
could get some rough clues with basic DMM, more with
a 'scope. But you'd need to understand what the supply
design is, to make sense of the measurements.

Changing all parts is not the way to go - just change
the supply, you might as well....

Re: SMPS Output voltaje slowly goes down

As well as the above, consider that the smps may have a microcontroller in it...and if it senses that theres no load, it may shut down the smps.

Oh....So you changed the control ic and it worked...hmmmm.....maybe you look at the schem, and see if maybe they loaded the uc2845 and its supplying loads of current to something connected to its vref output and so the controller is overheating.............the uc284x draws a lot of bias current...try a ucc2xc4x, as they draw less bias current...but if the controller has some big load connected to its vref (5v) pin, then its just gonna overheat.

Maybe the controller is getting a too high voltage..and overheating due to that (vxi).

Maybe you measure the voltage to the controller in operation...maybe you also measure the current drawn by the controller in operation.

Maybe the main power fet is esd damaged and drawing big current from the controller's gate drive....thence the controller is overheating.

Switch it on.........get it running as you did before....then switch it off...and put your finger on the uc... thingy and see if its very hot.

Maybe the gate source resistor is too low ohms value.