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

 

Sounds like something is overheating.
Can you check the various part temperatures with a finger (keep one hand in you pocket and avoid the high voltage stuff ) to see if anything appears hot?

 

Tje difference between the 44 and the 45 is the startup and shutdown thresholds for the ICs own power supply. They are lower for the 45, which suggests it might be a problem with that part of the circuit.

Usually the way those chips are operated is that a capacitor is charged through a fairly high value resistor directly from the rectified AC input. The capacitor holds sufficient charge to get the supply started, then continued operation is from a winding on the main transformer. Usually if there is a problem in that part of the circuit the supply will start up then shut down in tens to hundreds of milliseconds. I wouldn't normally expect a slow reduction in output voltage. The '45 part will allow the circuit to run longer if there is a problem in that part of the circuit. It may be that the voltage drop is happening because the main switching FET (assumed, might be a bipolar but not too likely) is getting inadequate gate drive and not switching cleanly. This would make the FET get very hot and possibly be destroyed if the supply were connected to its normal load, but might lead to the observed behavior if there is no load on the supply. The startup and shutdown thresholds are usually selected for the very purpose of making sure the FET is driven properly.

Have you looked on the web for a schematic? I don't know if Condor publishes schematics.

Do you have the actual model number? A photo or two of the circuit board might be helpful.

Crutschow's reply popped up while I was typing. If overheating of something is happening, blowing air over the board with a fan will likely make it run longer.

Do be very careful. All of the primary circuitry should be considered to be connected directly to AC mains "hot" in terms of safety. Biomedical engineers should not suffer injurious shocks

EDIT: The fact it would start up after a long delay after application of AC power also points to a problem with the IC's startup power circuit. Figure 17 of the TI data sheet shows a typical startup circuit. The capacitor will be a small electrolytic type in the range of probably 10 to 50 microfarads.
http://www.ti.com/lit/gpn/UC2845

 

Sounds like something is overheating.

That's what I was going to suggest. They also sell cooling spray (like a spraypaint) you can spray onto a suspected part. If keeping it cool prevents the shutdown, you know where the problem is. Well, maybe not the cause of the problem.

I think it's worth pointing out that a SMPS with those ratings shouldn't be very expensive. It's probably not worth the time and effort to repair if you don't already have the equipment and the chops.

 

For the sake of £17 it’s not worth wasting your time & effort trying to fix the PSU.

Although the output of this unit is a nominal 36V, if you look closely at the images, the PSU has an output adjustment pot that should allow it to be trimmed to 32.5V.

https://www.ebay.co.uk/itm/Universa...sformer/142588654015?var=&hash=item2132f231bf

You mention that you are a biomedical engineer – it is unlikely that this PSU has medical grade isolation between the primary circuit and the dc output. However it is likely that the isolation meets the requirements of IEC/EN 60950-1. If the PSU involves patient connected circuits, and you understand the principles of MOOPs & MOPPS (IEC/EN 60601-1) – you could assess the suitability of the PSU in your application.

 

I would not touch a power supply from China from an ebay vendor. It might be OK, but far too many either lack safety agency certification or flat out lie in claiming that they are certified. I've seen lots of stuff that claims safety compliance that would not be approved by any safety agency anywhere in the world. Condor is reputable.