Testing N-Channel

argonrepublic · 4th November 2009, 08:55 PM

I am working on a switch mode power supply. This a very basic question.
IC is STMicro B20NM50. Every combination of leads give no continuity. I am hoping someone will reinforce my conclusion that this IC is defective.
Thanks

Eva · 4th November 2009, 09:14 PM

This is STB20NM50

http://www.stmicro.fr/stonline/produ.../stp20nm50.pdf

Pinout is standard G D S. At least the D-S diode should show up when looking with a multimeter. Gate should appear open with respect to the other two terminals when using the diode test function. Finally, when a few volts are applied from G to S, a low resistance should appear between D and S.

argonrepublic · 4th November 2009, 09:27 PM

Wide open with multimeter on all pins in both directions.

Eva · 4th November 2009, 10:12 PM

It's very likely to be blown, but they usually fail shorted D-S and/or G-S (or G-D when S internal bonding wire blows).

If you are not used to checking MOSFET try with any other N-ch known to be ok (or blown too) and see if they behave as I explained.

star882 · 5th November 2009, 04:36 AM

A normal multimeter cannot be used to test a MOSFET.

One easy tester is to connect a car light bulb in series with the positive side of a 12v power supply. Connect positive to drain and negative to source for a N channel MOSFET. Turn it on and touch the gate and source terminals at the same time, making sure not to touch the drain. The bulb should not light up. (If it does, the MOSFET is shorted - most common failure mode or the connections are incorrect.) Then touch the gate and the positive side of the power supply at the same time. The bulb should light up. Let go and the bulb should remain on for at least a few seconds.

argonrepublic · 5th November 2009, 06:17 AM

I like that method. Sounds cool. Unfortunately, I used diode test on a different meter along with diagram of N channel and found the it is ok. Guess I have to keep looking. looks like my problem is going to be in surface mount components. Now I need glasses!
Thanks for help.

4th November 2009, 08:55 PM	#1
argonrepublic diyAudio Member Join Date: Apr 2005 Location: Simcoe Ont	Testing N-Channel I am working on a switch mode power supply. This a very basic question. IC is STMicro B20NM50. Every combination of leads give no continuity. I am hoping someone will reinforce my conclusion that this IC is defective. Thanks

4th November 2009, 09:14 PM	#2
Eva diyAudio Member Join Date: Oct 2003 Location: Near the sea	This is STB20NM50 http://www.stmicro.fr/stonline/produ.../stp20nm50.pdf Pinout is standard G D S. At least the D-S diode should show up when looking with a multimeter. Gate should appear open with respect to the other two terminals when using the diode test function. Finally, when a few volts are applied from G to S, a low resistance should appear between D and S. __________________ I use to feel like the small child in The Emperor's New Clothes tale

4th November 2009, 09:27 PM	#3
argonrepublic diyAudio Member Join Date: Apr 2005 Location: Simcoe Ont	N channel Wide open with multimeter on all pins in both directions.

4th November 2009, 10:12 PM	#4
Eva diyAudio Member Join Date: Oct 2003 Location: Near the sea	It's very likely to be blown, but they usually fail shorted D-S and/or G-S (or G-D when S internal bonding wire blows). If you are not used to checking MOSFET try with any other N-ch known to be ok (or blown too) and see if they behave as I explained. __________________ I use to feel like the small child in The Emperor's New Clothes tale

5th November 2009, 04:36 AM	#5
star882 diyAudio Member Join Date: Mar 2007	A normal multimeter cannot be used to test a MOSFET. One easy tester is to connect a car light bulb in series with the positive side of a 12v power supply. Connect positive to drain and negative to source for a N channel MOSFET. Turn it on and touch the gate and source terminals at the same time, making sure not to touch the drain. The bulb should not light up. (If it does, the MOSFET is shorted - most common failure mode or the connections are incorrect.) Then touch the gate and the positive side of the power supply at the same time. The bulb should light up. Let go and the bulb should remain on for at least a few seconds. __________________ "Fully on MOSFET = closed switch, Fully off MOSFET = open switch, Half on MOSFET = poor imitation of Tiffany Yep." - also applies to IGBTs!


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5th November 2009, 06:17 AM	#6
argonrepublic diyAudio Member Join Date: Apr 2005 Location: Simcoe Ont	I like that method. Sounds cool. Unfortunately, I used diode test on a different meter along with diagram of N channel and found the it is ok. Guess I have to keep looking. looks like my problem is going to be in surface mount components. Now I need glasses! Thanks for help.

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