Need Help to replace PWM Controller

[Escort Eagle]

Hi,
Currently PSU has SD6109 PWM Controller . But it is not working . I have a HS8108 PWM controller in old board . Can I use this HS8108 in place of SD6109 ??

I have upload both datasheet . Plz suggest me can I use this ?

[domas]

Re: Need Help to replace PWM Controller

i am no expert, but due to same pinouts, i would say it would be safe to use it.

[momaka]

Re: Need Help to replace PWM Controller

I didn't read the data sheets in detail, but besides the matching pinouts, both ICs also have 2.5V reference voltage, so I think it should be okay too.

[Agent24]

Re: Need Help to replace PWM Controller

I read in a repair article (Jestine Yong) about replacing the controller IC, sometimes it will work but blow up a few days later if not matched to the original completely.

He reckons hook the thing up to a ammeter on mains side, run it a few hours with a constant load, if the current draw increases slowly over time then the replacement IC will fail.

If the current draw stays steady, the replacement IC is suitable.

Apparently.

[Escort Eagle]

Re: Need Help to replace PWM Controller

Thanks everybody . I will give a try .

[Escort Eagle]

Re: Need Help to replace PWM Controller

How can I Force Start this PWM controller ?

[Agent24]

Re: Need Help to replace PWM Controller

I take it you swapped the controller and you still have the same problem?

In that case the fault is probably not the controller - check startup resistor(s), capacitor, run diode etc. You may also have a secondary-side problem (like shorted rectifiers) stopping it from starting.

You may find this useful http://www.repairfaq.org/sam/smpsfaq.htm

[domas]

Re: Need Help to replace PWM Controller

if it is not TL494 type, it is difficult to force start it, as it has those protection comparators inside. I would say fix the p/s.

[Escort Eagle]

Re: Need Help to replace PWM Controller

It's problem was 5VSB going to 8.5V . After replacing IC it is 5.2V . Now standby voltage is OK but power supply is not turning on . Just fan spins little bit and then shut down . I checked the whole output but did not find any faulty components . I also checked in controller section diodes and capacitors are OK . Then what is the problem there ??


In another power supply all outputs +12V, +5V, +3.3V, PG 5V are OK but that's problem is -12V is going to -22V . -12V is taken from the 12V rail . Before the rectifier I measured it is 12V AC . After the rectifier it is showing -22V . There is 470mF/16V capacitor in the -12V line . I changed that capacitor but still it is -22V . Why it is crossing capacitor's voltage and reaching to -22V ??

[Agent24]

Re: Need Help to replace PWM Controller

I quickly looked at the datasheet, it doesn't appear the SD6109 has anything to do with 5vSB voltage?

Where does 5vSB come from in your PSU?

Stupid question, but are you measuring -12v with respect to ground?

[Escort Eagle]

Re: Need Help to replace PWM Controller

Normally TL431 use to feedback for 5VSB . But in this IC Fb2 and Vref2 giving feedback for 5VSB . [See at page 9]

Stupid question, but are you measuring -12v with respect to ground?

Yes, I always do that . Do you have any different method ?

[everell]

Re: Need Help to replace PWM Controller

Since your 5vsb was giving problems, I would recommend that you replace ALL electrolytic capacitors in that 5vsb circuit. You might have a bad "critical"capacitor which damaged the chip you replaced.

Are you putting a load on the voltage rails. Many on this forum suggest using a hard drive or two, or a CDROM or two for a load. This is a great idea EXCEPT that it does not put a load on the 3.3 volt rail. I had one stubborn power supply with a chip similiar to yours which refused to start until I put a pair of parallel 3.3 ohms as load. It then started right up.

You probably have a voltage divider between +12 volts and -12 volts. At some point on this voltage divider, the voltage goes to the NVP (negative voltage protection) pin. The voltage on this pin should be about zero, otherwise the chip will not start up. Since you are measuring -22 volts with respect to ground, you may have lost a ground on either the -12 volt circuit or on the voltage divider feeding the NVP pin. Check all of the surface mount resistors labeled 0 to make sure that none are open. If your power supply does not have any surface mount resistors (I did not see any pictures) then ignore this last suggestion.

Measure the voltage on the NVP pin and let us know what you find.

[Agent24]

Re: Need Help to replace PWM Controller

Yes, I always do that . Do you have any different method ?

No, that's how you should be doing it. I thought you *might* have accidentally measured from +12v instead, in any case it sounds like the problem everell describes is more likely.

[Escort Eagle]

Re: Need Help to replace PWM Controller

Since your 5vsb was giving problems, .......... you replaced.

I checked them before replacing IC and replaced Electrolytic at first .

Are you putting a load on the voltage rails......... started right up.

Already I have done that . I have another good running PSU with same IC, same circuit and same design . It is starting up with no load . I have tried with that running IC also but same result .

You probably have a voltage divider between +12 volts and -12 volts....... voltage divider feeding the NVP pin.

It is a different PSU, not above . I have checked all zener diodes and resistios which going to comparator IC through -12V Line . It has two IC in controller section - one is TL494 and another is LM339 .

[everell]

Re: Need Help to replace PWM Controller

Did you try putting a load resistor on the -12 volt rail?

[Escort Eagle]

Re: Need Help to replace PWM Controller

Load will drop some voltage, but you have to accurate the voltage first . I will change all zener and resistors in -12V line and comparator IC also . Now I am busy with another three Power supplies . I will do that at free time .

[Escort Eagle]

Re: Need Help to replace PWM Controller

Can I use a LM7912 for -12V ? I think -12V does not need more than 200 or 300mA current . It is a 300W old PSU . Is LM7912 enough for that ?

[lti]

Re: Need Help to replace PWM Controller

You can, but the PSU is supposed to output -12V already.

Maybe the circuit that produces -12V isn't very well regulated. Did you try putting a load on it? The voltage might drop down to -12V as soon as there is a load.

[everell]

Re: Need Help to replace PWM Controller

I have seen several power supplies that use a 7912 chip for the -12 volt rail.

[Agent24]

Re: Need Help to replace PWM Controller

Can I use a LM7912 for -12V ? I think -12V does not need more than 200 or 300mA current . It is a 300W old PSU . Is LM7912 enough for that ?

You *could* but I agree with lti, the thing may well be faulty, plonking in a totally different regulator without fixing the root problem is just hiding the symptoms, and it will come back later to bite you in the ass.