Secondary Power Supply of ATX faulty

[debjit625]

Two days ago when I was starting my PC ,the PSU got

Well my PSU was from Zebronic http://www.zebronics.com/ ,it was 450W ATX PSU 20pins + 4pins , 220V AC.

I just powered my PC by turning on the UPS's power and heard a (very light)blown sound and a smell of burning component for some time.

When I opened the PSU,and checked the secondary section,it was gone,it was not burnt but if carefully looked some resistors was burnt very light trace of burn,the fuse was ok but the thermistor was short ,the mosfet switcher for the secondary section was short,and the biasing resistors for the mosfet was open. An opto isolator was also damaged on the high side (of secondary power supply side).Else every thing is ok.

Do you people have any idea ,what could cause this kind of problem. In my idea it may be the mosfet got short first and the whole thing happened or could it be a power surge as the thermistor is short ,I was running my PC with a UPS but I haven't check the UPS if its working properly or not.

[Khron]

Re: Secondary Power Supply of ATX faulty

Let's get a few things straight:

"Thermistor was short" means... what? Was it cracked or exploded?

"Mosfet switcher for the secondary section" means what? The main switching transistor(s), or the secondary rectifier diodes? Or perhaps the 3.3v is regulated by a mosfet in your psu? Are you certain about this? Can you maybe provide the part number?

Also, what capacitor brands are in your psu? Are any of them vented or blown-up?

[c_hegge]

Re: Secondary Power Supply of ATX faulty

Zebronics? Never heard of them. This could well be a cheap gutless wonder made by HongKongFlyApart Company.

Do you have any pictures of the PSU's internals?

[debjit625]

Re: Secondary Power Supply of ATX faulty

@c_hegge
Well it is cheap...I have used their motherboard and its good so I tried their PSU too
Why you need a picture? I already repaired it,I asked this question because I wanted to know is it a common thing in PSU's

@Khron666
In electronic when we say something is short we mean its short circuit i.e.. it will allow no resistance to current,the opposite is open circuit when we say something is we mean open circuit i.e.. it will not allow to current to flow through it (very high resistance).
Have a look : http://en.wikipedia.org/wiki/Short_circuit

Mosfet switcher for the secondary section" means what?

Have you ever worked with an ATX PSU?
ATX PSU have two SMPS inside them one is primary SMPS which provides +5V,+12V,-12V,-5V,+3.3V. The other is secondary SMPS which powers the control section ,+5V_USB ...when you connect the power cord to mains the secondary PS get started and start the control section, when you turn on the power button of your PC ,control section turns on the primary PS of your ATX PSU.

The secondary PS is itself a SMPS,and any SMPS needs a switching element like BJT's or MOSFET's ,in my case its a MOSFET "2N60" as the switching element of secondary PS.

Hope you got it...

[Pentium4]

Re: Secondary Power Supply of ATX faulty

Even if it's repaired, we would want to see pics because we're all electronics nerds and love to see the build quality of PSU's, good or bad

[Khron]

Re: Secondary Power Supply of ATX faulty

Yes, i've worked with ATX's before, no need to explain how they work, nor electronics basics The way you put your description together sounded a bit confusing, i just wanted to get all the details correctly.

A healthy input thermistor should typically be around 10ohm @ 25c, tops, if i'm not very mistaken. So on the ohms setting on the multimeter, is it showing 0 (zero)? On the "beep" setting, on my multimeter at least, anything under 60ohms "beeps".

"Secondary section" = 5Vsb = 5 volt stand-by, ok, glad we cleared that up. So, "fried" mosfet and surroundings, sounds like a "two-transistor switcher" gone bad. The safest route would be a "DM311 mod" with an FSDM311 chip from FairchildSemi, or a similar mod with something like a ViPer12A chip from ST (like i did in my resurrection of my Antec SP-500P).

You could try replacing the mosfet and the other damaged parts in the current 5Vsb section in your power supply, but i've got a hunch it's one of those "critical cap"-based designs, so... You'd be running the risk of it failing again, maybe not as quickly as the original, but it's a bit risky anyway.

Have you been able to check anything in the secondary of the "main" section of the power supply? If that cheap 5Vsb design failed, it might've gone "overvolt" (putting out way more than 5V), and damaged something on the mainboard, or even the PWM chip...