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Old 4th April 2015, 01:01 AM   #1
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Default mosfet matching wide variations/swings

Purchased 20 irfp140 and 20 irfp9140 mosfets.
Matching them has been less than fun.

Setup:

Mastech 30v 10A power supply
Large-ish blank anodized heat sink 4x6x1 inch
Sheathed alligator clip leads
Silicon iso pad.
15v, 60 ohm power resistor .13A

I thought I was getting good readings but if the dmm lead are touched or if I reach over the setup, the voltage swings but never settles to it was originally.
If i re-test, the Vgs is different.
If i touch the heat sink the voltage changes.

I don't trust the readings so I can't continue with the build. I don't remember it being this difficult.
I tried 4 meters and different meter leads.

How is matching done properly?
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Old 4th April 2015, 01:56 AM   #2
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Old 4th April 2015, 02:39 AM   #3
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Gate to drain
15v to drain
Ground to source pin
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Old 4th April 2015, 02:49 AM   #4
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Are you using a gate stopper resister?
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Old 4th April 2015, 02:49 AM   #5
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First, don't put a lot of trust in the meters which come with the regulated supplies.

Second, you may be seeing some parasitics - use a 470 to 1Kohm Gate resistor.

If this doesn't work, try an analog meter.

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Old 4th April 2015, 02:54 AM   #6
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From http://www.firstwatt.com/pdf/art_mos_test.pdf :
"P4 doesn’t matter here , as the DC is blocked by the
capacitor, but it is always nice to have a few ohms in series
with the Gate of any Mosfet to prevent high frequency
(parasitic) self-oscillation, which will alter the voltage
reading."

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Old 4th April 2015, 02:57 AM   #7
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Sorry, I'm a little slow sometimes...
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Old 4th April 2015, 03:00 AM   #8
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Thanks everyone.

So the output reading on the regulated supply is not to be trusted. Check.
Use a gate stopper. Check.
Will recheck tomorrow. Check.
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Old 4th April 2015, 01:04 PM   #9
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Appears to be solid now. Moving the cables doesn't move the meter.

Used 1/2w 750 ohm.

Thanks!
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Old 6th April 2015, 10:32 AM   #10
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You must keep Tj at the same value for every test/DUT, that is really difficult !
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