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M^3 MOSFET question (replacing and cooling)

post #1 of 19
1/1/09 at 5:05am
Thread Starter 
Hi guys,

As I've poster last week, a friend of mine gave me a M^3 + STEPS which were faulty. I've managed to get them both working well (still have a couple of changes to make, like upload the voltage to 32V or so).

Unfortunately mt friend soldered the MOSFET's very close to the board, and the heat sinks had to be drilled so they will mount securely (which of course means they's a big hole in it and the heat transfer isn't too good). Anyway, I want to replace the MOSFET's (dont want to add wires between them and the board) and replace the heat sinks to new ones with no unneeded holes in them.

What MOSFET's would you guys recommend me using?
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post #2 of 19
1/1/09 at 5:31am
What makes you think the heat sinks aren't that good? They're just supposed to keep the MOSFETs within proper operating temperatures.

I believe the design calls for IRFZ24N/IRF9Z34N, not sure if you can really use others without changing other things (which might not be the best idea considering these MOSFETs are also using in the Beta22 and were chosen for a reason).

Do note that 32V is quite a bit (make sure your opamps support it) and I really don't think you will gain anything between 24V and 32V with 99% of headphones, if not 100% of them.
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post #3 of 19
1/1/09 at 5:36am
Thread Starter 
Hi,

I'm going to 32V simply because the PS regulator works very hard at the moment and increasing the voltage will allow it to run cooler the OPA's are the 637 and 627 so voltage is not a problem for them.

As for the MOSFET's I only want to replace the heat-sinks to larger ones since they get very hot, and this running at 80ma and not 115 as I want them to.

I only consider replacing the MOSFET's since the ones in the amp were soldered too close to the board so if I want them to have a good contact with the heat-sinks (at the moment only about half of the MOSFET's body is touching the sinks) I'm going to have to add some wire (very short, but wires all the same) and this is something I'm trying to avoid.
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post #4 of 19
1/1/09 at 6:31am
Quote:
Originally Posted by KT88 View Post
Hi,

I'm going to 32V simply because the PS regulator works very hard at the moment and increasing the voltage will allow it to run cooler the OPA's are the 637 and 627 so voltage is not a problem for them.

As for the MOSFET's I only want to replace the heat-sinks to larger ones since they get very hot, and this running at 80ma and not 115 as I want them to.

I only consider replacing the MOSFET's since the ones in the amp were soldered too close to the board so if I want them to have a good contact with the heat-sinks (at the moment only about half of the MOSFET's body is touching the sinks) I'm going to have to add some wire (very short, but wires all the same) and this is something I'm trying to avoid.

Air wiring is definitely not the best for them so I completely agree that they should have proper heat transfer and be soldered to the board. There are heatsinks that don't have the "gap" at the bottom which you can use. Honestly though, I think that you're worrying more than you need to; these MOSFETs have operating temperatures of 175C, that's quite hot and I'm fairly certain they will never get that hot with practically any heatsink attached. Also, MOSFETs have a positive temperature coefficient (as opposed to BJTs which have negative temperature coefficient), so they will run more current as they heat up. If you're really worried (and I'm sure it's more of a worrying thing than actual danger of damage), replace the heatsinks by all means. Personally, I wouldn't bother unless you have a VERY funky installation of them.
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post #5 of 19
1/1/09 at 6:43am
Thread Starter 
I guess I'll end up replacing the sinks to larger ones and that's all. I just dont like stuff that aren't 100% right, and the way these are mounted is just wrong.

Thanks
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post #6 of 19
1/1/09 at 6:54am
I think KT88's amp might have its MOSFETs mounted incorrectly (i.e., it was soldered to the board too low, and not properly aligned to the mounting hole on the heatsink). Otherwise the MOSFET body should be in full contact with the heatsink, even those with the "notch" cutout.

The correct procedure is to bolt the MOSFET to the heatsink first, and then the heatsink should be securely mounted to the board before soldering. The heatsink itself should seated against the surface of the board, not hanging in mid-air. If you have the pin-less heatsink like those I offer, it should be tapped and screwed down to the board. Those with mounting pins should have the pins tack-soldered to the board (I recommend only a small amount of solder to make future removal easier).

Replace with the recommended IRFZ24N and IRF9Z34N MOSFETs. Note that running a higher supply voltage will reduce the heat on the voltage regulator, but will increase the heat on M³'s MOSFETs. The case should be well ventilated.

M³ website
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post #7 of 19
1/1/09 at 8:39am
Thread Starter 
Hi Ti,

Indeed, my friend did the mistake of first soldering the MOSFET and than placing the sinks.

As for the heat, once I will have larger cooling sinks I think it'll handle the extra voltage.
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post #8 of 19
11/28/09 at 8:45pm
I dug this thread up from the dead as I have a similar question that relates to mosfet’s and heat transfer.

I’m slightly out with a measurement, which has left my mosfet’s hanging slightly off (about 1 - 2 mm) the edge of the off board heatsink’s I’m using as this picture shows.



Looking at back of the mosfet the heat transfer pad is still in total contact with the heatsink this image demonstrates how much overhangs, the coloured section represents the heatsink.



I’m assuming I’ll be ok, any advice would be welcome.

Cheers
John
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post #9 of 19
11/28/09 at 8:53pm
Almost 99% sure that that is fine as the plastic/whatever housing won't transfer much heat even if you were to seat it totally on to the heat sink.
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post #10 of 19
11/28/09 at 9:21pm
i assume you're going with the recommended 24V rail voltage? i wouldn't worry as long as your heatsink is adequate. btw, why the M3? not a knock on the amp, i love mine and anticipate replacing the lytics in 10 years or so .
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post #11 of 19
11/28/09 at 9:52pm
Quote:
Originally Posted by Emooze View Post
Almost 99% sure that that is fine as the plastic/whatever housing won't transfer much heat even if you were to seat it totally on to the heat sink.
Thanks Emooze, that made me feel better.

Quote:
Originally Posted by fishski13 View Post
i assume you're going with the recommended 24V rail voltage? i wouldn't worry as long as your heatsink is adequate. btw, why the M3? not a knock on the amp, i love mine and anticipate replacing the lytics in 10 years or so .
Thanks fishski13, sorry I’ve hijacked this thread purely to ask about my mosfet query I’m not building an M3 as such.

It’s a ±30V dual σ22, with Avel-Lindberg Y23 series 100VA toroidal power transformers. Heat sinks are from Conrad in Australia.



I’ll be using it as psu duties between my EHHA speaker/headphone amp (yet to be built) and my balanced β22.
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post #12 of 19
11/28/09 at 10:57pm
Sorry for further thread derailment, but that's a sick setup you're building john.
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post #13 of 19
11/29/09 at 2:11am
Quote:
Originally Posted by Anonanimal View Post
Sorry for further thread derailment, but that's a sick setup you're building john.
Thanks Anonanimal, hopefully I’ll get the mosfets installed this week and get the initial check stage completed.
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post #14 of 19
11/29/09 at 3:50am
John, those aren't heatsinks, they're thermal black holes.....
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post #15 of 19
11/29/09 at 6:24am
I don't get it - I don't see those heatsinks being in contact with the MOSFETs.
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