Help identity blown part in PC psu

[roadrash]

Just had this PC power go off with crack noise and all i can see is the top blown off a small square chip (next to bright blue ceramic). My wife hoovered up and i cant see it anywhere. Does anyone know what it is and what may have caused it to blow?

[R_J]

Re: Help identity blown part in PC psu

Without a make, model or board number, you won't get much help. it is very likely an 8 pin smps standby ic, too many to choose from.
IE ncp1060, mc44608....

Look in the power supply case to see if you can find the top with the number

[stj]

Re: Help identity blown part in PC psu

oh hell, it's Roadrash.

he's gonna say it's from an old wang 8086 or something!

[CapLeaker]

Re: Help identity blown part in PC psu

are there any pieces of the blown off lid from the IC floating around somewhere?

[PeteS in CA]

Re: Help identity blown part in PC psu

Line-filter-free zone! Since it was built around March, 2006, I'm going to guess it might be a Power Integrations standby regulator, or maybe a 384x PWM.

[roadrash]

Re: Help identity blown part in PC psu

oh hell, it's Roadrash.

he's gonna say it's from an old wang 8086 or something!

Trust you STJ but you are right. It was from Unamed chassis which I had a PChips M590 mothertboard in I used for testing. It no great problem as I have found a spare AT PSU in my spares. I dont even know why it should have done that as there was no shorts or anything. I plugged the spare PSU in and it powered up fine. Unfortunately it blew with a loud "Crack" like a cap gun and its clean blown the top off the chip. I will take a look and see if I can find some ID somewhere.

[stj]

Re: Help identity blown part in PC psu

work out where the pins go.
and maybe look at danyk's site.
http://danyk.cz/s_atx_en.html

[R_J]

Re: Help identity blown part in PC psu

Might be a FSDM311, some power supplies use it. Check trace side of the board, see if pins 6,7,8 (drain) are common, pin 1 ground
Does the ic drive the transformer direct or does it drive a fet that drives the transformer

[stj]

Re: Help identity blown part in PC psu

it's much older than a 311 i'm sure.
more likely a 3842 or similar

it would be worth desoldering it, it may have markings under it or on the pcb
it has to go anyway.

[roadrash]

Re: Help identity blown part in PC psu

Ive desolderd the chip and there are no markings on it and just what looks like U1 on the PCB under it. There are two transistors right beside it on the heatsink (Q1 & Q2). I found the following on the PCB & number/barcodes on the metal chassis.
NE0646 (on pcb)
101VC612504400711
EL1312S500ATXH00295

I have uploadeded some more pictures now you can see more with the heatsink removed. If you look at the solder side you cab see where the 2 transitors were fitted and the chip that blew. On the component side you can see that the chip was getting hot by the burnt colour on the pcb.
I dont know if its worth repairing but its my last AT type psu (comes with on/off switch attached) and all the other spare psu's I have are ATX type.
I thought I had found the bit that blew off in my bin but it was a bit off a TOP200 switch chip that blew quite a few months ago in another PSU.
Anyway take a look and if you can recognise whats caused this and if its easly fixed that would be great but if its going to be awkward and not worth doing I will just canabalise it for bits etc.

[stj]

Re: Help identity blown part in PC psu

well obvious thing, pin4 is not used.
so it's not a 3842

and from the pic, it's got 2 parts in it - probably a chip and a fet.
so it is an all-in-one power i.c.

[R_J]

Re: Help identity blown part in PC psu

It sure looks like a FSDM311
Pin#1 ground, pin#2 vcc via diode from transformer secondary, pin #3 connected to opto, pin#4 n/c, pins 6,7,8, connected together.pin #5 connected to B+ via resistor

The IC is only for standby, the fets are for the main power supply

[roadrash]

Re: Help identity blown part in PC psu

Hey thanks RJ how do you do you know all that? To identify that so easily, I wish I had some of the skill you have. Do you think this just failed because of age or was it maybe overloaded or something more serious. It really shouldn't have been overloaded as it was only powering a basic Cyrix processor motherboard and one 5.25" floppy drive so I doubt it was that.
Anyway thanks.

[R_J]

Re: Help identity blown part in PC psu

That ic only powers the standby circuit, so it's on all the time, I know some atx power supplies use that ic so I looked up the specs and it seems to match.
I would check the secondary diode to make sure its not shorted, and there is a cap. on the ic's vcc line I would check/change. As for why it went, who knows, being standby its on 24/7/365
If your lucky its only that standby ic that whet and the main supply is likely ok.

[momaka]

Re: Help identity blown part in PC psu

Hey thanks RJ how do you do you know all that?

Well, even if you didn't know all of that RJ knows, here are the hints that pretty much concluded this chip should be for the 5VSB:
1) The fact that there are no line filters means the PSU is a cheap-o unit. And most cheap "gutless" PSUs (especially the older ones) are regularly half-bridge designs. See point point #2 below.

2) You have a pair of 13007 BJTs, which is very common in low-end PSUs (and not MOSFETs, like you would see in a more-modern 2-transistor forward design.) This means the PSU (main PS portion... i.e. 3.3V/5V/12V rails) uses the oldschool half-bridge topology. The controller for those two 13007 transistors is on the secondary side - likely a TL494/DBL494, KA7500, SG6105, ATX2005/AT2005, or if it's a Deer PSU - "COTY" (chip-of-the-year as we like to call them ) like "2003"/"2005"/"2012".

All of that above means the 8-pin DIP chip must be for the 5VSB power supply. From these, FDSM311 is one popular chip that is often used. But others are TinySwitch(R) like the TNY253/TNY254/TNY255 series, TopSwitch TOP221 through TOP227 series , and ICE3A__ or ICE3B__ series.

So really it just comes down to keeping a copy of the datasheet of the above mentioned chips and seeing which one of those matches the pins on the board in the same way as the sample schematic in the datasheet.

...
And that info above is more or less a "condensed" version of what you should know about controllers and basic design of low-end / cheap / and many old power supplies.

Do you think this just failed because of age or was it maybe overloaded or something more serious. It really shouldn't have been overloaded as it was only powering a basic Cyrix processor motherboard and one 5.25" floppy drive so I doubt it was that.

Since that blown chip is for the 5VSB... most likely there is either a fault with the 5VSB circuit -OR- it was just a random failure occurrence ... which isn't too uncommon for these off-line switch ICs, actually.

RJ mentioned some very good points: check the output rectifier and also the small electrolytic cap next to the 5VSB chip. While at it, it's probably not a bad idea to pull out the 5VSB output filter caps and check them. And given that it's a cheap / low-end PSU, it's probably also not a bad idea to replace them anyways, since they are likely to be cheap junk caps. If they are failed, that would likely well be the reason why the 5VSB chip blew. And if they aren't failed... well, don't wait for them to fail and make the PSU "dead" again.

By the way, when you get the new DM311 chip, make sure to use the "dim incandescent bulb trick" to limit current on the primary side of the PSU. That way, if there is something wrong to make the DM311 chip blow up, at least you won't end up with a big bang and lots of other blown parts. Might even possibly save your new DM311 chip from blowing. So it's worth testing PSUs this way... at least without a load / for a basic "smoke test" (i.e. will it blow up as soon as it's plugged it?)

[stj]

Re: Help identity blown part in PC psu

replace the small caps.
a lot of psu chips will go into a kind of startup-loop and make a chirping or hickup sound if the startup cap is poor - eventually they go boom.

i used to replace soo many topswitch devices because of this that you wouldnt believe it!