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porfi4ever 28th July 2005 11:17 PM

Matching Mosfets
 
Hello everyone, sorry to bother you with this question the more I read about this more confused I got. I want to macht IRFP244 and 9610 for my aleph2 that I'm going to start building, but I don't know how to get the value of my resistor, I'm using 2.2k resistor with regulated power supply for 5 sec. for both transistors.Is this setup ok? please help me with this. thanks Rommel

Brian Donaldson 28th July 2005 11:29 PM

2.2K is about right for the 9610 @ 15V, but I would use something around 20 ohms for the 244's so you'll be matching at around .5A where they'll be used. You'll need to time the measure closely (5 sec within +- .5 sec) because 1.5W will heat them quickly and skew your measures). And don't try to read it again for at least 20 min so it can return to ambient temp.

I set up everything (supply, resistor and DVM). Clip an aligator clip to the drain and gate, then the other to the source, count to 3(one one thousand two one thousand three one thousand) then read the measure.

Mattyo5 28th July 2005 11:49 PM

If you use a 20 ohm resistor for the 244's, make sure that its at least 10 watts.... the resistor will heat up. I match for 30 seconds per mosfet....the meter isn't moving as much after 30 seconds...i don't know how easy it would be to get mosfets accurate at only 5 seconds. On the other hand.... they don't have to be super accurate matched..... close is just fine.

Make sure the mosfets are on a flat surface... i use a glass table....so that they all start at a similar temp, and just like Brian said, if you mess up a reading...wait 20-30 min before you rematch.

Lots of different opinions on matching.... who knows who's right. If Nelson says something....stick with that :) good luck man!

-Matthew K. Olson

porfi4ever 29th July 2005 01:30 AM

matching mosfets
 
Thank you both to reply, I'm going to get a 20 ohms resistor to match the 244's for 5 seconds and I was matching the 9610 with 2.2 k 1 w and was getting numbers like 14.99, 15.00, 14.98,15.14. Are this numbers fine?

Brian Donaldson 29th July 2005 02:09 AM

You need to be measureing the voltage accross drain-gate and source and it should be 3.5 to 4 volts.


If you match the 244 after 30 sec under juice, you've heated them up a bit, and you're assuming that they reached the same final temp, but if the vgs is different, then they're under different amounts of heat, and not always the same temp.

But on the otherhand, they are closer to operating temp. One idea I had for matching was to keep them in boiling water untill testing them so they would be closer to the temp where they'll run, but testing burning hot fets would be a PITA.

Mattyo5 29th July 2005 03:03 AM

I guess it'd be interesting to do some experiments... do the 5 sec test....then do a 30 sec test... and see if the fets actually match up... doesn't matter....good luck porfi! whatever you do should be close enough for alephs. :) Later!

-Matthew K. Olson

Brian Donaldson 29th July 2005 10:50 AM

the bigger your source resistor, the less important the matching.

hifimaker 29th July 2005 10:11 PM

Did you see Nelson's article on Mosfet matching?

http://www.passdiy.com/howto/matching.htm

-David

steenoe 29th July 2005 11:06 PM

2,2k is way too much resistance! You need like 10r for the irfp240 and something like 560r for the irf610 (or9610)!! I remember NP said that they just measured long enough to get a reading! I normally "Juice" them for 30 sec's. The resistor needs to be at least 10 watts. And a steady supply at that!

Steen:cool:

BTW 15v for the irfp240 and 10v for the irf9610!!!
Take a closer look at the link DW gave!!

porfi4ever 2nd August 2005 08:18 PM

Quote:

Originally posted by dw8083
Did you see Nelson's article on Mosfet matching?

http://www.passdiy.com/howto/matching.htm

-David

yes,1 million times, but still confused.


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