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Old 8th February 2002, 06:28 AM   #1
diyAudio Member
Join Date: Dec 2001
Default MOSFET matching--Question on Vgs!

Hi there,

I measured the output MOSFET(IRFP140) last night by using A75's diagram. The total output current is 200mA. The Vgs I got from my IRFP140 are all in 3.1-3.6V range, not 4-5V range as Mr. Nelson Pass stated in A75 diagram.

I am wondering if I did anything wrong??

The DC power supply showed 13.3V and the resistor in series is 47Ohm. From the equation I=(V-4)/R I got I=197mA which makes sence to me.

If we look at the A75's setup, we have +15V total voltage. 11V will be dissipated by resistor and 4V by the MOSFET. How come we can get the MOSFET Vgs in 4-5V range?? Could anyone please explain this to me??

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Old 8th February 2002, 01:01 PM   #2
PassFan is offline PassFan  United States
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Join Date: Aug 2001
Location: Central FL
Mine matched out low as well and my A75 works fine. I think mine came in around 3.7 vgs. I think it has more to do with the batch than anything else. Yes my power supply sagged down to around 11 volts as well. Might have something to do with Kirchoffs Law. As long as you follow Nelsons procedure you should be alright. All my devices are reading within 10 deg.F of each other. Good luck
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