even more lateral fets

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ed
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#16 Re: even more lateral fets

Post by ed »

a bit more....

I don't like tapping blind holes, I have tried and failed in the past...so my M.O over the last 20 years has been to drill through heatsinks between the fins and attach circuit boards to the sinks with nuts and bolts and spacers.

this build was slightly different because I had the pcbs made by pcbway and the mounting holes didn't match the fin spacing on the heatsinks, which already had lots of blind holes already tapped......

so....I cut some 6mm Nylon which I drilled and screwed to the heatsinks using the tapped holes that were already on the sinks, the nylon shapes provided a base to screw the pcbs to.

the pics show the nylon sheet which I cut on the bandsaw...
nylon cuts1.jpg
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nylon cuts2.jpg
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#17 Re: even more lateral fets

Post by pre65 »

ed wrote: Thu Apr 06, 2023 10:41 pm a bit more....

I don't like tapping blind holes, I have tried and failed in the past...
If you are not requiring full thread strength, then using a drill a gnats bigger than specified can make things easier.

And/or, check the drill with a vernier (or micrometer) to make sure they are what you think they are, it's caught me out in the past. :oops:
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#18 Re: even more lateral fets

Post by ed »

moving on....
mofo2 panels1.jpg
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mofo2 pcb1.jpg
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mofo2 ps1.jpg
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#19 Re: even more lateral fets

Post by pre65 »

Looks good. :)
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#20 Re: even more lateral fets

Post by IslandPink »

"Shiny shiny"
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#21 Re: even more lateral fets

Post by ed »

bit more....

got it running in biased at 2 amps
here's the square wave at 10k.....
mofo sq1a.jpg
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at this point it clips at 2.52v i/p - 6.74v o/p - 5.67 watts
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#22 Re: even more lateral fets

Post by ed »

waily waily.....

major disappointment......plugged into the main system and switched on.....serious buzz....
scratched head a bit...
took out the earth lift for the balanced supply to the audio circuit, bearing in mind that the meanwells only have chasis ground direct to the incoming earth point. So everything is connected to the incoming earth point.
even bigger buzz.

the volume masks the buzz but....we know it's there.

with hand on the chassis the buzz is ever so slightly reduced.

immediate thoughts suspect the smps, which is very close to the boards....in the last incarnation the smps was underneath the main amp so it was slightly further away from the bits that matter. In addition the wiring is a bit of a rat's nest.

back on the bench
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#23 Re: even more lateral fets

Post by ed »

a bit more..

I've taken the smps out of the chassis and I'm running one channel with the smps well away from the chassis.
This is a shot of the input and output on the scope with the input shorted. The input trace has me well curious.
If I put my hand on the chassis both traces go to a flat line for about 3 or 4 seconds and then resume the traces shown with my hand still on the chassis.
mofo test1.jpg
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Anybody got any suggestions.....could I have excited the opamp? what about the output trace being inverted?
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#24 Re: even more lateral fets

Post by Mike H »

I doubt the SMPS runs at 9.1 MHz ... I'm suspecting parasitic oscillation? You've banged up the DC, and the chokes are different...? Are the amps working into a load?
 
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#25 Re: even more lateral fets

Post by ed »

I spent a lot of time trying to understand what constitutes a capacitive load and thought I'd avoided it...but... I suspect that might be the problem...

I'm going to cut the capacitors out of the feedback loop to see if it makes any difference....any further suggestions would be gratefully received..
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#26 Re: even more lateral fets

Post by ed »

more head scratching

with respect to this circuit, same as the previous mofo except larger chokes and opa2134 instead of ltc1150...
mofo cct.jpg
mofo cct.jpg (82.41 KiB) Viewed 4921 times
here is the ouput from the opamp. With opamp powered but the fet unpowered with input shorted...this looks acceptable to me..
mofo opamp no fet.jpg
mofo opamp no fet.jpg (133.17 KiB) Viewed 4921 times
here is the output from the opamp. With opamp powered and fet powered with input shorted....
mofo opamp fet on.jpg
mofo opamp fet on.jpg (120.27 KiB) Viewed 4921 times
I've double checked and the sk1058 has input capacitance of 600pf..I didn't think this was much and it was the same device as the last version which had no problems.

there are reports on the web that the opa2134 show instability for no apparent reason..mmmm...
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#27 Re: even more lateral fets

Post by Nick »

What does the squarewave response look like on the output of the op amp? The general response you showed earlier looked a little rolled off on the leading edges. That may be the result of the 220R (r7) gate resistor OR the 15p (c3) compensation cap. Looking at the output of the op amp would hopefully show which is rolling that off.

My gut feeling is that its worth increasing the size of c3 and see if that helps the op amp to be stable. The mosfet gate capacitance is probably not going to be there when its not passing current.

Also, I have found that gate resistors can be double edged swords in that they will damp the HF response, but they can also allow the gate to do its thing less connected to the driver.

Not sure how it would work out, but my gut suggests that it would be nice to build the gate voltage from p2 into the op amp stage, so you could directly couple the op amp output to the mosfet avoiding C4.
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#28 Re: even more lateral fets

Post by ed »

I'm unable to reproduce the conditions at first switch on...something has popped/changed

here is the 10k square at the output of the opamp with no power to the fet...
mofo 10k fet off.jpg
mofo 10k fet off.jpg (120.94 KiB) Viewed 4907 times
and here with power to the fet...
mofo 10k fet on.jpg
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#29 Re: even more lateral fets

Post by Nick »

I would try increasing R7 and see if it makes it any better.

Also I would experiment with a 100R between the op amp output and C4.
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#30 Re: even more lateral fets

Post by ed »

Thanks for looking

At the moment I don't think I'm going to acheive much because something fundamental has changed since first switching on. At first I had a buzz, but it produced a valid output, both sine and square, and it played music.....

now there is serious corruption. The output from the opamp looks valid as long as there is no power to the fet but the fet is now doing something it didn't do when first switched on....

I'm going to take the chassis apart and put 1 channel on a bread board and do some substitutions.....I think testing the fet might be first in line.

edit: got the lhs sink off the chassis and tested before taking the pcb off...and....the 220R gate stopper smoked..it's gone a lovely shade of brown but it still registers 219R. Not sure why so much current should run through the gate but the fet does look suspect...

onward
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