MOSFET testing

Is there a relatively simple way of checking power MOSFETs out of circuit? I've not had much to do with them, and can't find any information on checking them.

Reply to
Dave Plowman (News)
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Have a play with

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look for app not an-558

Reply to
James Salisbury

I usually test them with a diode check function on a dmm, only gives a basic go/no go check, but that's all I'm usually looking for.

Normally it's SMPS or or HV regs in monitors I'm checking them in, so I don't usually care, above shorted or open.

Lee

Reply to
Lee

Have then ever been in-circuit ? If so they may have been degraded in removal.

But in anycase, look in the datasheet for the device and there will usually be some simply parameter that can be measured (Rds(on) whilst gate connected to +9/12v is popular) which you can do with a good meter and battery.

Reply to
G&M

Do you have an oscilloscope by any chance? If you do then, yes... otherwise I can't think of an easy way anyway.

Reply to
Scott Mills

Obtain one 9V battery, and a digital multimeter. Assuming an enhancement mode device (will not conduct with 0V) Connect gate to source. One way round source-drain should look like a diode, and the other way almost an open circuit. Now, connect 9V battery between source and gate (positive to gate for N channel). This should turn fully on nearly all power MOSFETs without damaging them. (Much over 10V, and you risk permenant damage with some).

Now, you can measure the drain-source resistance, which should be about the same either way round.

As an additional test, you can measure the current through the drain-source with 9V across them. This will give you (low voltage) leakage.

Anything with more than a few nanoamps should probably be discarded. (on 200mv range, a DMM with an impedance of 1Mohm will read up to 200nA.)

Reply to
Ian Stirling

To add to the earlier message - MOSFETS are static sensitive, if the gate is charged to over the maximum (15-20V is common, though it can be lower) it may arc over and do irreperable damage.

They should really be stored with pins shorted, often you'll get away with it due to the large capacitance of the gate-source junction.

(Hmm, technically I suspect it's repairable, but certainly not economically)

Reply to
Ian Stirling

economically)

Not sure how. The failure is usually blown oxide from gate to body. If we could repair oxide failures semiconductor fab yields would be nearly 99%

Reply to
G&M

Gawd, this thread made me jump.

MOSFET is the name of some software at work ...

Reply to
Huge

Strip the packaging (conc nitric + sulphuric acid pumped around the top with fluorine rubber gaskets to keep it away). Remove die. Now, mill away the failed spot, and passivate the die again.

Put back into package.

Should only take a few hours. (assuming you have the several million pounds worth of equipment).

Reply to
Ian Stirling

hi dave, this may be of help

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bob

Reply to
burbeck

Good idea but would only work for old style planar VDMOS and LDMOS devices, even if you could avoid damaging the metallisation. More modern LDMOS, trench and super-junction devices push the gate deep into the die so you'll be almost out the back of the die before you've removed the short.

Reply to
G&M

Reply to
Dave Plowman (News)

Yup - a pristine Tek 465B

Reply to
Dave Plowman (News)

Thanks Ian, I'll store this for future reference if I need a more sophisticated test than the one on the site that burbeck gave.

Reply to
Dave Plowman (News)

Why do you think it might be dead anyway ? These things are quite robust and usually obvious when they've failed in-circuit from the hole in the middle where the die used to be :-)

Reply to
G&M

I should also have mentioned that probably the first test to do is to measure the resistance of the gate-source, which should be infinity. Most FETs (that don't explode) will short out as they stop working.

Reply to
Ian Stirling

Not always. Those with gate zeners leak like f**k. And if you're meter is high impedance, you spend some time charging those with deliberately very high gate capacitance such as motor drivers.

Reply to
G&M

It's an invertor that has been coupled up the wrong way round to a battery

- possibly with the fuse replaced with a bigger one.

I've fixed all the burns on the PCB. ;-)

There are two groups of power MOSFETS - four low voltage high power and four high voltage lower current. I've found a couple that are toast. And an opto-isolator. And a couple of caps.

Keeps me out of the pub. ;-)

Reply to
Dave Plowman (News)

In article , Dave Plowman (News) writes

It sounds pretty comprehensively buggered to me :) Is it an in-car inverter? You can buy these new very cheaply now, probably for not much more than the cost of the components you're replacing; we're talking ukp25 for a (claimed) 1kW inverter.

There is that...

Reply to
Mike Tomlinson

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