Author Topic: How to test MOSFET's ( IRF740 ) PROPERLY ?  (Read 1396 times)

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Offline Mike JungTopic starter

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How to test MOSFET's ( IRF740 ) PROPERLY ?
« on: May 12, 2021, 09:25:54 pm »
Can anybody please either explain or point me to some text on how to test MOSFETs properly ? I have a constant problem with a power supply using 4 IRF740. I also wonder if I could replace those with something better but without any other mods.
 

Offline RichC

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Re: How to test MOSFET's ( IRF740 ) PROPERLY ?
« Reply #1 on: May 12, 2021, 09:43:20 pm »
You can certainly replace it, better depends on why it is failing currently.
They're relatively high Rds for a power mosfet I assume because of the high voltage rating. If it didn't need that high voltage tolerance you could reduce the power dissipation by swapping to a lower resistance part.

I would normally test by putting them in a circuit with an led as a low side switch and testing that they switch when the gate voltage is changed.
 

Offline Jwillis

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Re: How to test MOSFET's ( IRF740 ) PROPERLY ?
« Reply #2 on: May 12, 2021, 10:34:04 pm »
This test won't tell you if the MOSFET is getting weak because the development of hot spots can slowly deteriorate the MOSFET die . This is caused by prolonged stress from over heating . This can reduce the current rating of the MOSFET because the die heats up faster and can no longer  pass the rated amount of current.  Excess heat can be  slow death for a MOSFET .

This test  can tell you if the MOSFET has completely failed .

Step 1  Diode check

Set DMM to diode  and connect negative to drain and positive to source .  Reading for a N-Channel should be around 0.3V to 0.9V . On a P-Channel it should have no reading . Diode is OK
Reverse the DMM test leds   .  N-channel should have no reading and a P-Channel should read around 0.3V to 0.9V  . Diode is OK
0V in either direction means diode is shorted

Step 2 Resistance and continuity check

Set DMM to Ohms   Drain to source should be very high resistance with test leds in either direction .  Drain to source is OK
Very low to 0 \$\Omega\$  the Drain to source is shorted . MOSFET is defective

Gate to Drain should read very high resistance . In fact you may even see the resistance climb fairly quickly to very high . The is means the gate is charging . Reverse the leds and you may see the gate discharge then charge again quickly .  Gate to Drain is OK.
Do the same from gate to source . If that checks the Gate to Source is OK  . Don't worry you will not hurt the MOSFET .

If the gate to drain and/or gate to source is shorted or very low resistance the MOSFET is defective.

 

Offline james_s

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Re: How to test MOSFET's ( IRF740 ) PROPERLY ?
« Reply #3 on: May 12, 2021, 10:35:08 pm »
A better test might be to use a test load that requires a bit more power than an LED. It's trivial to wire up a mosfet to switch power to something like a 12V automotive light bulb, feed the gate through a resistor, anywhere from 100 ohms to a few k ought to be fine. The lamp should turn on when the gate is connected to 12V and off when the gate is grounded.
 


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