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Experiment: Which mosfet thermal interface material is best?
Experiment: Which mosfet thermal interface material is best?
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Old 2nd November 2018, 09:32 PM   #11
vdi_nenna is offline vdi_nenna  United States
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Experiment: Which mosfet thermal interface material is best? Send a message via Yahoo to vdi_nenna
I once exploded a mosfet and watched the Keratherm pad come down like confetti. Wasn't fun.
Wasn't the pads fault, but just the same. The thin stuff isn't reassuring to me.
At any rate, good to know the ceramic pads work well.
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Last edited by vdi_nenna; 2nd November 2018 at 09:35 PM.
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Old 2nd November 2018, 11:48 PM   #12
MEPER is offline MEPER  Denmark
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How would only thermal grease (the white stuff) compare?
If you did not care about electrical isolation?
MoFo could function this way.
I like housing which is already electrical isolated so only thermal grease is needed but the thermal resistance in the housing is probably larger than metal housing. Can probably be verified from data sheets. So far only "small and medium power" semiconductors uses the electrical isolated housing?
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Old 3rd November 2018, 01:49 AM   #13
PRR is offline PRR  United States
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OK, I'll play dumb-dumb.

Why do we care about the temp of the sink? the screw?? Are they going to melt?

The real temp is the Silicon. And since it is inseparable, the device mounting surface.

Yes, it is hard to find a consistent technique to even approximate temp in metal tab or epoxy body. I have seen detailed drilling instructions which may be fine at the factory (screw-up, grab another) but not DIY (order more parts *again*).

An alternate technique is to measure some property of the device. Assuming truly controlled current, a BJT's Vbe is a sweet test. MOSFETs maybe not so simple.

Where is the screw insulation? Is the screw "on" the device or "on" the sink? Or no little washer, so "on both"?
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Old 3rd November 2018, 02:26 AM   #14
Eric is offline Eric  United States
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Experiment: Which mosfet thermal interface material is best?
MEPER: Thermal grease only is an option I intended to measure, but didn't. I'll have to give it another run and see how it compares. As you point out, this would work only for the MoFo because it uses only a single output device. Don't go trying this with any other amps...

PRR: While imperfect, the temp of the screw is the best proxy for the temp of the mosfet case itself. Sure, a measure of junction temp would be ideal, but this is a little difficult to obtain. The temp of the heatsink is just a curiosity. Since there is often commentary of "use XZY" to mount your devices or, "If your devices are going to run extra hot, then use ABC", I wanted to see just how much of a difference various interface materials actually make. Turns out, the differences between materials seems worthwhile to pay attention to.
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Old 4th November 2018, 01:09 AM   #15
Nelson Pass is offline Nelson Pass  United States
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Experiment: Which mosfet thermal interface material is best?
Interesting.

Of course I use the grease and Mica because I like smearing it around.

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Old 4th November 2018, 09:28 AM   #16
danny_66 is offline danny_66  Belgium
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You still get that pleasure with AlO3 ceramics and grease
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Old 4th November 2018, 10:46 AM   #17
oreo382 is offline oreo382  Canada
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Did you happen to measure the mosfet body as well? I'm curious if there would be a difference between the screw and the body itself.The screw is embedded in the sink which would wick away heat.The head of the screw is the only part that is in contact with the mosfet itself.
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Old 4th November 2018, 03:47 PM   #18
kasey197 is offline kasey197
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Experiment: Which mosfet thermal interface material is best?
This is really great stuff Eric thank you! I have tried many different options including the exotic graphene interface material from Panasonic but keep coming back to keratherm or the ceramics. Now I understand why.

This would have taken a lot of work to produce and I am very grateful that you did.
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Old 4th November 2018, 04:11 PM   #19
Mark Johnson is online now Mark Johnson  United States
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Experiment: Which mosfet thermal interface material is best?
For steady state measurements, Thermal Trak devices provide quite accurate results. Inside the package, there is a diode which is thermally bonded to the same die-attach paddle as the power transistor. The diode is a temperature sensor which quite accurately tells you the temperature of the power transistor's collector. (Since the collector is the die-attach terminal).

Mount it with mica+goop. Shove in X watts. Measure transistor collector temperature.

Rip it off, remount it with Kapton tape. Shove in the exact same number X of watts. Measure temp.

Rip it off, remount it with Beryllium Oxide slab. Shove in X watts. Measure temp.

Rip it off, remount it with Keratherm tape. Shove in X watts. Measure temp.

Rinse and repeat as necessary.

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Last edited by Mark Johnson; 4th November 2018 at 04:15 PM.
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Old 4th November 2018, 08:24 PM   #20
rookakoma is offline rookakoma  Hungary
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Which Heatsink Finish Performed Best?
BLK: screw+34C; sink+23C
Brown: screw+37C; sink+22C
Alu: screw+41C; sink+22C

It seems to me, like a thermal resistance between the screw (mosfet) and the heatsink or measuring mistake. The sink temperatures were almost the same.
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