First channel of Alpeh 4 is working, however...

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Edwin Dorre

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Joined 2002
#1
Hello,

my first channel of my Aleph 4 (soon when all is working converted to a Aleph 2) is finished, however...

It plays, It sings etc. It runs great in a 8 ohm resistor at full power etc. But when I feel temperature of the mosfets (IRFP240) some are colder than others. When I measure voltage over the 1.5 ohm resistors, they are different, the colder ones are lower.

I have a difference of about 5.78 V tot 6.01 V in one sixpack and 5.74V to 5.94 in the other sixpack...

I did the 10 second 'Nelson Pass' way of testing the mosfets and they were all within 0.01V ...

I use 221 1% metalfilm and 1.5 ohm 2% 4W metal film resistors.

After about 15 minutes of running the coolelements are about 44 degrees celcius. Not very high! Measured with a thermocouple placed next to one of the mosfets.

Should I worry, and do my matching again? All mosfets were from the same lotcode...

Let me know,

Edwin
 
#3
Mosfet diff. temp. Aleph4 ?

Check mosfet matching again, there is nothing else except that you may want to check your mica isolators (or whatever you using)and the way you have fixed your mosfets on the heatsinks.

I am collecting parts as well for my Aleph 4 and mostly have everything except that I am waiting for my toroids from Plitron.
 
#6
Re: Mosfet diff. temp. Aleph4 ?

trigon said:
Check mosfet matching again, there is nothing else except that you may want to check your mica isolators (or whatever you using)and the way you have fixed your mosfets on the heatsinks.

I am collecting parts as well for my Aleph 4 and mostly have everything except that I am waiting for my toroids from Plitron.
Click to expand...

Have someone else used the 'Nelson Pass 10 seconds is enough' for one to settle way? Or do you all go for the 5 minute wait till it settles way?

Edwin
 
#8
I will use 6 seconds😀

Just kidding, I think I will go with 5 sec. as well. But just to
confirm what HPotter has mentioned and I agree with him and
that is that you have to be sure that you match the resistors as
well.
Good point HPotter 😎
 
#9
Given my experience with matching various IRF and Motorola (aka OnSemi) MOSFETs, I'd have to say give it until it settles down. Try a few in your test rig, watching until they give fairly consistent readings. Note the time. Then test all others for that same length of time.
I ran IRF644s for ten minutes each, which seemed to amuse/horrify some people. The IRFP044s I got in for the Aleph-X project settled down in less than a minute. If memory serves, the Motorola parts were more like 2 or 3 minutes each. My assumption is that it's a thermal issue, but I still haven't decided for sure.
This is all a long-winded way of saying that I vote for going back through and matching the output devices again.
By the way, I'm assuming that you read the current across all the Source resistors. You should have noted a fair difference in current draw from the hot devices to the cold ones. If you're reading zero, you've got a bad part or a bad solder joint.

Grey
 
#10
GRollins said:
This is all a long-winded way of saying that I vote for going back through and matching the output devices again.
By the way, I'm assuming that you read the current across all the Source resistors. You should have noted a fair difference in current draw from the hot devices to the cold ones. If you're reading zero, you've got a bad part or a bad solder joint.

Grey
Click to expand...

Oops, major typo, My current draws (drop in voltage) are:

I have a difference of about 0.578 V tot 0.601 V in one sixpack and 0.574V to 0.594 in the other sixpack...

This makes more sense!

Hmm... one camp says; 5 seconds is enough, and the other go until it settles... what to do!

I can not match the source 1.5 resistors, as I do not have the gear precise enough to do it. Come to think of it, perhaps put it between a precise power supply and measure the voltage drop instead of using the ohm-meter. What do you think?

Edwin
 
#11
Since you have fairly low restistance it is better to measure the voltage across the resistor rather than the resistance itself (for us folks that do not have super gear). If you can measure current and/or voltage accurately then that works just fine but if you can't then you can match using a bridge set-up like a wheatstone bridge. Simply put you make two voltage dividers by connecting two resistors in series in parallell with another pair in series. Swap around your resistors until you find a pair with a reading of 0 volts between the voltage divider outputs. When that happens you have two dividers with the exact same ratios but not necessarily the same values. By swapping one the lower end of on of your dividers with the other lower end you can find a set which has similar restistance as they would not be producing a 0 reading unless they were the same resistance. Swap away and find the lowest readings (I remember something about easy matching to 0.1 % doing this but I could be wrong). You have no idea what the value is (which is irrelevant as you know it close to the marked value but you just wanted to match)

/UrSV
 

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#12
Hello Edwin,

I wouldn´t worry about the small differences in bias current. I´ve got even bigger differences in my Aleph5 (matched for 5 minutes to 0,1V). As long as not one Mosfet does most of the work this can´t be dangerous and you´ve only got 5% of mismatch!!

groetjes,

william
 
#13
Hello,

some info for all you matcher out there:

I did first the 10 second test, so I read the VGS after 10 seconds of running current through them. From these readings I made 4 sets of 6 IRFP240 (all within 0.01) and 2 sets of 9610 (within 0.001).

I re-tested 2 sets of IRFP240 and 2 sets of 9610's with the 5 minute test to see how this turns out:

5 minutes:

3.91
3.90
3.90
3.89
3.90
3.90

and

3.871
3.879
3.877
3.874
3.885
3.884

and

3.821
3.823

and

3.803
3.803

So pretty close to the 10 seconds matching ... So perhaps Nelson is right after all 😛

I will build the second channel with these mosfets so I will let you know. I am testing my resistors tonight.

Edwin
 
#14
Second channel does work also !!! I still have a minor bias difference between mosfet's however I did not match the resistors. My DC offset is very low: 0.012V . So I am going to put another matched IRF9610 pair in the first channel to see if this helps!

Edwin

P.S. YEAH!!!!!!!!!!!
 
#15
Edwin Dorre said:
Second channel does work also !!! I still have a minor bias difference between mosfet's however I did not match the resistors. My DC offset is very low: 0.012V . So I am going to put another matched IRF9610 pair in the first channel to see if this helps!
Click to expand...

Just changed the IRF9610's in the first channel....... A DC offset of 0.007V it is a miracle!

Edwin
 
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