Matching Power Mosfet

I Build an Aleph 3 few days ago, without matching that MOSFET (IRFP240). The voltage across SOURCE RESISTOR (3W, 2%) is little difference between them. So I try Matching & Group them under 100mV range, but this time I use IRFP250 & 3 pair of them. I also see some of them are very difference. 0.1 / 0.2 / 0.3 voltage amount them.

How this happen after matching up?

And this time the sounding is better then last time. If it is because I matching the differential pair of IRF9620 or just IRFP250 is better then IRFP240?

P.S. I use +/-45V DC on Power rail.
 
Vgs after Run-in of Amp

I funded another interesting thing about Vgs that we are matching the MOSFET.
Since I use un-Matching pair of IRFP240 & IRF9630 in my first version Aleph 3 project. After that, I use well match pair of IRFP250 & IRF9620 in second Version.
I recheck the Vgs of IRFP240 & IRF9630 to matching it. Since I prepare 2ch part and install 1ch only, then I have Run-in parts and unused parts.
Interesting is it seem to be all Run-in Parts have very good matched Vgs. In IRFP240, ¾ under 0.1V difference. One in 0.2V range. But this may be only lucky. Because unmatched pair also easy to match up.
In IRF9630, unused pair show 0.1V of difference amount them. So I Get another 10 of IRF9620 to match up. But in Run-in Pair show only totally 4mV difference in all 3 of them. I think this may not be lucky. Because I get 10 IRF9620 to get 4 perfect matched pair in 4mV range.

Does this meaning after Run-in, those parts become stable & matched?
Does any on have same experience?
 

Nelson Pass

The one and only
Paid Member
2001-03-29 12:38 am
Something else is going on. We have used probably
200,000 matched devices in the past few years and
have not seen a case where the devices aged so as to
match, nor do I recall the converse case.

The match does depend very much on temperature, and
if you test the devices in a different ambient temperature
or leave them in the test rig for different periods of
time, you will get different numbers. We always bring
the whole batch up to room temperature and use a consistent
test period.

One advantage we have is that we can generally keep the
devices in the same lot codes which aids in matching the
temperature tracking of the match.

Keep in mind that a similar problem can occur if the
devices in the amp don't all get the same heat sinking
(including the same amount of thermal grease).

The spec is .1 volt variation, but with a decent
population, we get .01 V in production.

In an Aleph 3, the Source resistor voltages are about
.5 volts. If the devices are within .1 volts of each
other, this will work just fine, as it represents a
current sharing variation of about 20%, which is good
enough.
 
Thanks you answering.

1. I use about 0.03A for IRF96xx, 0.97A for IRFP2xx to match up.
2. I did monitoring the tempt rue in surface of IRFP2xx till 40 degree, then mark the Vgs. and MOSFET clip on an CPU heatsink. I read Vgs by Digital Scope in my Working Lab.

Of course that ageing of MOSFET make it matching is very strange. I think maybe only Lucky.

Another Question is what current should we use to do the matching? Same as we used in circuit? Because I fund those matched MOSFET did not have same voltage in Source resistor in circuit.

Another question too, I Just change IRFP250 to IRFP 240 with 0.47 source resistor. But have oscillation & clipping sound on speaker. It looks like my transformer cannot supply enough current for circuit. So I change source resistor from 0.47 to 0.75. Then problem disappear. Did IRFP240 will draw more current then IRFP250?

I still try to find out why I just have 0.2V on source resistor. And matching MOSFET really give better sound quality.

P.S. In the setup I use for now. My rail voltage is 41.5V.
 
There is some result & question during these few days.

1. I see another oscillation when I change R104 from 10k to 22.1k. Try to increase Total Gain of AMP. Then I solve this by add a 63p cap in parallel with R104. After that I change back to 10k without cap. I think last time I may have something wrong on connection of circuit.

2. I increase current by change R111 from 1.5k to 4.75k. Then I got 0.42V on Source Resistor. A little bit smaller then Nelson tells us. But I think I will leave it as for now.

3. Question: the sequence of R114 & C103. I found in Aleph Design have two setup. Resistor come first from Output, some time is Capacitor. And sounding little difference by listen some music. I like Resistor first from output. It seems to be more Clearly & Purity. Why there is two kind of setup?

I like Aleph 3 circuit because it is so simple, use as less as possible on parts. Especially the capacitor and I only need Unbalanced Input. But some modify to C102, C103, and R114 will same as Aleph 4.