A computer switched mode power supply.

PC SMPS REPAIRED

Hi friends once again welcome to the wonder full world of electronics repair articel.Now i am going to reveal how i repaired A computer switched mode power supply.

I think everyone know what is SMPS & its application.

I don’t want to pull you to the brief theory of power electronics.

Just i am reminding you its operation & applications, let's begin

One of my repair friend brought this high wattage SMPS for repair.

The unit is completely dead there is no any fan movement at all, I opened the chassis & visually inspection made. Now all the necessary testing including input fuse & bridge at primary side of smps components found ok

To become smps repair expert one should have brief knowledge on comparator ic , pwm ic,terms like vref,feed back,CT,RT etc.What are all this term i think new comers may 

 confuse but i hope professional technicians may understand what are all I am talking.

The entire working principle of smps i cannot cover in this article because i am dealing only the problem & its root cause for the above equipment. 

look inside view of smps.

By looking at inner view it is messed with wire bunches .The place for rework is so congested & even no places for proper ventilation. Looking at the inner views I don't know why the SMPS designer has designed like this. The first clue what i got is because of no proper ventilation possibility of electrolyte capacitors failure is more. So in my mind I hoped I can fix this easily & opened my capacitor police ESR meter to check all the secondary filter capacitors. After testing I got 5Volts main filter capacitor 470mfd/16Volts shows wrong reading. I felt very happy to see this suddenly I replaced this and re-powered to see the result.
After re-check there is no good result .Fan has moved for two seconds & tripping off  & no 5volts. I was not happy to see this. I searched an internet to see the circuits for further analysis. Go through the circuit below.

Since it seems to be critical one must have proper schematics to resolve the issues.(Note: Above circuit is Typical circuit choose to make you understand)
After deep looking in to the circuit LM393 is the main circuits which is making to trip off. Sure I will explain what is  happening here for tripping off. Before one should know  the data sheets for both comparator ic LM339 & PWM IC TL494.  

 

TL494

 Please concentrate at pin # 5 of LM339 in above circuit.Because of R33 4.7 K ohms Out of tolerance when smps is ON at no load condition pin # 2 of LM339 is going to high & pulling the DTC (Dead time control )pin #4 to high & causing TL494 to stop releasing PWM pulses.This is the reason why the SMPS is tripping off. I think we got the  result for tripping off. Yes,the second culprit is R33 4.7K Ohms out of tolerance which is reduced to 2.5kohms .After replacing  R33 problem solved . SMPS is now working fine which is removed from the gaming PC.Happy to hear good opinion from the customer end.That's the climax for the SMPS repair story.Let us meet in the next session with one more interesting story.


With best regards
Basavaraj:BM
basavarajradiotech @gmail. Com