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dantwomey 15th June 2003 11:34 PM

My first FET matching
Here's some of my observations from my first try at FET matching. I followed Kristijan's FET matching diagram using a Lambda 15v regulated power supply, a Fluke 23, and one of Peter's ceramic insulators to help hold the IRFP244's to a spare heatsink I had. Here's a pic of my setup:

My first reservation is the values I measured. Kristijan's sheet suggests values from 3.8 to 4.2. Mine went all the way from 3.30 to 3.60vdc. Also, I'm wondering about so-called drift i've read about. Even when left on for an hour none of my FETs drifted more than 10mv. If fact the majority of them didn't drift at all.
Does this 'match' the experiences of my fellow forum members?


Peter Daniel 15th June 2003 11:38 PM

What resistor value did you use. It seems like the current was too low and they didn't change temp. much.

dantwomey 16th June 2003 12:39 AM

Kristijan's diagram suggests a 150 Ohm/2w resistor. That's what I used. Here's his suggested setup:


moe29 16th June 2003 12:56 AM

... i thought you get good enough matching by just taking a
reading until the meter settles down. You don't need to go
through the pain of attaching the fets to a heat sink. It takes
more than 30 seconds for them to get warm and you should
have a reading well before then. As long as you obtain your
values in a consistant manner you should have groups that
match well enough.

i don't see a resistor in your set up there.

Peter Daniel 16th June 2003 12:57 AM

150 ohm resisor and 15V PS gives about 73mA current per FET. I think it should be higher, like 0.5A. Which requires a resistor in a range of 20 ohm or so.

Also, you don't need a heatsink as the time involved in attaching ea FET to the sink and your handling changes the steady state of a FET and the reading. Just put them all on a table, wait a while and then mesasure one by one quickly. Usully, the first 3 seconds are enough and a socket to insert FETs comes useful, so you don't have to connect wires every time.

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