Checking an IC...

[Shagreobe]

Hi everyone, Im wondering if there is a way to test this IC preferably in circuit. Its L4981AD Power something. Any ideas?

 

[Nigel Goodwin]

No, either try it in a known working circuit (where if it's faulty it may well destroy that circuit), or try a new one in your existing circuit - and again, if something else in the circuit has destroyed the chip, the new one will be destroyed as well.

It's a PFC chip, so essentially a SMPSU type circuit, so you need to replace ALL faulty components, before you power it up again, or it's likely that everything will just blow again. The chip is one component you replace as a matter of course, as it's likely to have been killed by the failure, even if it wasn't itself at fault.

 

[danadak]

Yes you could chk parts of the chip for gross functionality. That's how the part is tested in manufacture.
Both at wafer sort and then at final test. But takes detailed understanding of internals of part. I think you
could test the OSC, some of the amps, Vref, maybe the drive path to the external MOSFETs......but tedious.

Regards, Dana.

 

[Shagreobe]

I've tried following the block diagram, and even the sample circuits in the datasheet, but nothing of real value. Im replacing them anyway. Im not paying for them so i'll do anything
On the pcb, there is a shorted diode, 4 blown resistors, and one or more mosfets shorted, along with the chip. Im changing everything

 

[Nigel Goodwin]

I've tried following the block diagram, and even the sample circuits in the datasheet, but nothing of real value. Im replacing them anyway. Im not paying for them so i'll do anything
On the pcb, there is a shorted diode, 4 blown resistors, and one or more mosfets shorted, along with the chip. Im changing everything

I've repaired thousands of SMPSU's, it's essential that you change ALL damaged components, otherwise it just goes 'bang' again when you turn it on. I presume you don't have a circuit diagram?, in which case downloading the datasheet for the chip should give you a suggested circuit, and show obvious components to check or replace.

 

[Shagreobe]

I've checked everything I can find to check. nothing else seems to be bad

 

[Nigel Goodwin]

I've checked everything I can find to check. nothing else seems to be bad

Good luck - keep your fingers crossed

 

[Shagreobe]

Yeah, we already blew up one again so thats why im looking for help lol

 

[Nigel Goodwin]

Yeah, we already blew up one again so thats why im looking for help lol

As I've mentioned here before, I used to make 'repair kits' for popular power supplies I repaired a lot of, and included all parts that failed in the kits - then just replaced the kits rather than waste time fault finding. There were a few devices though that had occasional failures - such as the large reservoir capacitors in some supplies (which on some models caused the PSU to self destruct), and even the SMTX, which on specific models sometimes went O/C, S/C or even short between windings. So on those specific models I'd check those as well as fitting the kit.

VERY occasionally, you'd fit the kit and it'd go BANG again - in that case take all the 'new' bits out, fault find to locate the cause, replace the faulty part and fit a new kit. If all was well, add that faulty part to the kits

So the kits did evolve a little, but it didn't take long to end up with a pretty fool proof kit.

Bear in mind, I was probably repairing in excess of 1000 PSU's every year.

 

[Shagreobe]

Nigel, Thats basically what we are doing. So far out of the 12 or so bad ones we have, the resistors, mosfets, and diode have been cooked. Maybe I had a solder bridge on the chip? who knows. Just have to keep on trying I guess

 

[Nigel Goodwin]

Nigel, Thats basically what we are doing. So far out of the 12 or so bad ones we have, the resistors, mosfets, and diode have been cooked. Maybe I had a solder bridge on the chip? who knows. Just have to keep on trying I guess

If you haven't got a circuit, then draw one out - it will help you greatly.

Then studying the circuit, just think what happens when one of the output transistors goes S/C, and what might potentially be damaged. Often it's fed from the IC (so that dies), but there's usually a resistor or two, and perhaps a diode, these can often get destroyed as well.

 

[BR-549]

I used to repair unknown and un-doc circuits boards. I found that an octopus saved a lot of time. It not only indicated the quality of the board components, it displayed the condition of all PN junctions of the solid state devices. Including IC chips.

I repaired about a years worth of old dogs which had been given up on, received a large bonus and then trained all the other techs on how to use it.

We had no docs, so couldn't test. But we still gave a 90 day warranty, and seldom had any return.

This was industrial repair($$$)....and we charged 1/2 of replacement board cost.

And actually made some money. Which is hard to do repairing electronics now days. Today it's replacement, not repair.