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Old 28th November 2005, 01:16 AM   #1
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Default Aleph 2 fet matching question

Just received my new adjustable supply and Iím in the process of fet matching. I took an initial reading at power up and throughout, let them sit at 15v for 5 minutes each. A few I let sit for 10 minutes, and they barely got warm; granted I was using a large heatsink, thermal grease, and the fets were clamped down with aluminum angle bar. Measured 11v across R1. NP's Mosfet matching article uses 56r for R1, am I off in using 150r? To get the best matched sets, do I need to match in situ? I understand this would be optimal, maybe not necessary. My aleph2: 1500VA Plitron 38v secondaries 49000uf - 2.2mH - 49000uf, around 50-53v unloaded coming off the PSU. In situ fet matching 45-50V?, R1?



11 of 50 fets tested (Irpf244, R1 150 ohm @ 15v)

3.75
3.80
3.80
3.76
3.74
3.83
3.79
3.84
3.90
3.83
3.83

regards,
John H.
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Old 28th November 2005, 08:50 PM   #2
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Looks plenty good enough to me. You don't need to
match in situ, which is usually a pain - rather you
simply need to test under identical conditions.
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Old 28th November 2005, 09:24 PM   #3
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I figured I was ok, but i re-matched with 50r. This was my first time matching, so I experimented.

looks pretty good:

23 fets between 3.97-4.07

Thanks NP,

John H.
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Old 30th July 2008, 07:59 PM   #4
kop89 is offline kop89  United States
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I am starting to build my Aleph2 clone, and in process of getting parts for it.

Questions (I am a newbie and does not have EE background):
1. Is it absolutely necessary to match the 12 IRF244 mosfets? What will happen if they don't match? What kind of tolerance (in mV)should all these 12 match?
2. Is it necessary to also match the three IRF9610?

Thank you!
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Old 30th July 2008, 08:22 PM   #5
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It's not absolutely necessary to match the output fets (irf244), but its good practice. Close matching to around 0.01mV will ensure each fet is sharing the load equally else distortion will rise. The input fets (irf9610) should also be matched as closely as possible to keep DC offset as low as possible, 0.01mV is fine, maybe even closer. The 9610 that's in the current source does not need to be matched to the two input fets.

-john
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Old 30th July 2008, 08:47 PM   #6
kop89 is offline kop89  United States
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Thanks Mega-Amp.

Just to confirm - do you mean 0.01V (10 millivolt) or really 0.01 millivolt (what kind of voltmeter are you using that reliably measures this small increment?)
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Old 31st July 2008, 01:30 AM   #7
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sorry about that, yes 0.01V or 10mV. You can use a True RMS meter, Fluke is good, but pricey. For around 50 bucks, you can get a decent digital, you don't have to go extravagant. Something like THIS would work.
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Old 31st July 2008, 02:40 AM   #8
kop89 is offline kop89  United States
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Whew.... matching to 0.01mV will be hard

What kind of circuit do you use for matching MOSFET? Could you show me your schematic, or refer me to some resources?

Thanks!
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Old 31st July 2008, 03:35 AM   #9
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Quote:
Whew.... matching to 0.01mV will be hard
As far as the output section of the Aleph2, you don't need to match 12 fets within 0.01V of each other. 6 closely matched for the current source, and another 6 closely matched for gain section. Within a batch of 50 fets, this should be feasible. I wouldn't say its hard, maybe tedious.

Lots of reading, including fet matching at the bottom:
passdiy
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Old 31st July 2008, 03:37 PM   #10
Lau700 is offline Lau700  France
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My notes :

I used this (Cheffdegaar) matching schematic :
Click the image to open in full size.
Left schematic (IRFP044N) is ok too for IRFP240 or 244.

Matching values:
IRFP9610 +/- 0.01V - Only for Q1/Q2
IRFP240 +/- 0.05V if possible (0,1V max)

With replace R11 (221R) by "100R + Pot200R" for example, itís possible to adjust offset to 0mV about (at running temp).

Laurent
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