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Old 10th September 2010, 09:52 AM   #11
AndrewT is offline AndrewT  Scotland
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Andy,
that 390mA across the 34r drops ~10V. That leaves ~7Vds on the DUT.
The DUT is consuming ~1.5W. The junction is heating quite rapidly even though the outside case temperature has hardly changed.

It will be very difficult to get consistent absolute results measuring single DUTs this way.
Vgs/Id is very dependent on Tj. How can you hold Tj constant?

That is why I recommend REF + DUT and use comparison for matching.
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Old 10th September 2010, 01:36 PM   #12
frags is offline frags  Canada
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This is how I matched IRFP240s measuring Vgs at 1A for :
At drain and gate I used resistor 1R0 connected to regulated power supply adjusted to about 5.4V to have about 1V drop on the resistor. The transistor was mounted at large heatsink w/o thermal paste. I measured the voltage drop not at gate/source of transistor (about 4.4V) but at the resistor (about 1V). It allowed me to use lowest voltage range at my tester with maximum accuracy. After that I was changing transistors keeping the same voltage at the regulated power supply and measuring the drops across the resistor.
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Old 10th September 2010, 02:24 PM   #13
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Andrew,

I have just verified my matching by your method with the first group of 6 x MOSFETs. During testing I have held them at a constant temperature (as best as possible). My initial results were all within 10-20mV Vgs of each other, with your suggested measuring, they are all within 1% of each other.
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Old 10th September 2010, 04:15 PM   #14
AndrewT is offline AndrewT  Scotland
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Quote:
Originally Posted by Andy5112405 View Post
with your suggested measuring, they are all within 1% of each other.
1% Id match at equal Vgs is incredible.
I don't think I have explained the DUT/REF method correctly.
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Old 10th September 2010, 04:42 PM   #15
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Quote:
Originally Posted by Andy5112405 View Post
Andrew,

I have just verified my matching by your method with the first group of 6 x MOSFETs.
Are you able post some pictures? I have always test mosfets individually. Wrong?
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Old 10th September 2010, 05:19 PM   #16
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Going through all the MOS-FETs I am getting completely different results which each method I'm using.

OK. Tj is important, I'm trying my utmost to bolt each DUT onto a huge heatsink which is temperature controlled. Each test is started at 25 Deg C and measurements are taken at 400mA and 40 Deg C. Using this method I can get the Vgs figures within groups of 6 to +/- 20mV. Please bear in mind that all my MOS-FETs are from a single production batch.

Using Andrew T's method it is tricky to stabilize Is sufficiently to take accurate readings. But reading each DVM across the 10R Drain Resistors, they are within 0.01V of each other - NOT IN ALL CASES.

Why does Nelson suggest his method of testing them if everyone else is poo-pooing that method ?
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Old 10th September 2010, 05:34 PM   #17
flg is offline flg  United States
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Quote:
Originally Posted by Andy5112405 View Post
poo-pooing ?
I need to go outside! Rough! Rough!
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Old 10th September 2010, 05:36 PM   #18
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Click the image to open in full size.
Here's the basic set-up.

The REF MOS-FET is on the LEFT in close thermal contact with the other 5 of it's group.

Two DVMs are used to measure Is across the 2 x 10R resistors. The whole lot is powered by a bench test PSU at 17V and Vgs can be varied with the pot.
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Old 10th September 2010, 05:38 PM   #19
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I use this method and test each mosfet individually. But when I put them together, they gave different readings and does not matched.
Attached Images
File Type: gif matching.gif (23.6 KB, 204 views)

Last edited by pmchoong; 10th September 2010 at 05:40 PM.
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Old 10th September 2010, 05:49 PM   #20
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That's very similar to the method that Nelson recommends, I've matched mine using that method but the other matching methods are not verifying my results - EXACTLY as you've stated.
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