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Old 6th July 2006, 09:32 AM   #1
TroelsM is offline TroelsM  Denmark
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Default Heatsink calculations

Hi.

I would like to start a debate about the good old "if-the-heatsink-burns-you-its-to-hot-rule".

I'm doing a commercial design with the IRFP260N mosfet. The thermal resistance from chip to heatsink is pretty low and without any isolators ("live" heatsink) the heat-transfer to the heatsink is very low.

My calc shows that the silicium is around 110 degrees celsius when the heatsink is 80 deg! That means that I've got a burning hot heatsink but the chip is safe, - or what do you think.

On a side-note, check out this heatsink http://www.aavidthermalloy.com/produ...onepiece.shtml (scroll down to the 78240 "Big Chief".)

It's extremely efficient when fan-cooled. I'm dissipating 40W in 60mm of this in an ~ 2m/s airstream!

Any comments?

Regards TroelsM
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Old 6th July 2006, 10:49 AM   #2
TroelsM is offline TroelsM  Denmark
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One more thing:

Any idea on how to add heat sink compound when using the "big Chief" profile?

A lot of the compound will simply be smeared off when the mosfet is inserted in the gab.

One more more thing... : If someone wants to see the calculations I can upload them later.

If these heatsinks are as good as the date suggest they should be the **** for low-profile high-power amps...

Regards TroelsM
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Old 6th July 2006, 03:24 PM   #3
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Try here:

http://www.bergquistcompany.com/thermal_materials.cfm

http://www.chomerics.com/products/do...mselguide2.PDF
and very interesting here:

http://www.kerafol.de/jml/
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Old 6th July 2006, 03:51 PM   #4
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always a good tool:
http://www.aavidthermalloy.com/technical/thermal.shtml
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Old 6th July 2006, 06:16 PM   #5
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IIRC there are safety specs on how hot heatsinks can get. also make sure to move electrolytic caps or heat sensitive parts away from the very hot heatsinks!
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Old 7th July 2006, 10:25 AM   #6
TroelsM is offline TroelsM  Denmark
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A related thing: How hot is your mosfets?

If a fet is rated at 175 deg. what temp do you allow the junction to operate at? 100Deg- 125 150? (!)

Regards TroelsM
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Old 7th July 2006, 12:10 PM   #7
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Default Re: Heatsink calculations

Quote:
Originally posted by TroelsM
Hi.

I would like to start a debate about the good old "if-the-heatsink-burns-you-its-to-hot-rule".

My calc shows that the silicium is around 110 degrees celsius when the heatsink is 80 deg! That means that I've got a burning hot heatsink but the chip is safe, - or what do you think.

Any comments?

Regards TroelsM
Comments:

There is nothing to debate with regards to the safety of an 80C heatsink. You'll burn flesh with it and you'll cook your semiconductors (and probably destroy the speaker connected to the amp that is heatsinked at 80C).

It is simple physics- run semiconductors hot and they will die early. You can't change that by using thermal grease or funky heatsink profiles. Too hot is too hot.

I don't know about your customers, but when I buy a commercial product, I expect not to get burned by it and I expect it to work for a long time. Your design fails on both counts.

Skimping on a heatsink is a really bad way to economize.

I_F
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Old 7th July 2006, 12:27 PM   #8
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Default Re: Re: Heatsink calculations

Quote:
Originally posted by I_Forgot


I don't know about your customers, but when I buy a commercial product, I expect not to get burned by it and I expect it to work for a long time.
I guess you don't work for Samsung -- my LCD TV power supply failed in 1 year and it took 6 months to get a new one from Korea -- failure was heat related.

My car's electronic lock system instrument cluster, GPS system -- MB engineers say that the CPU/MCU is prone to failure due to thermal inadequacies.

I can recall that period in the distant past when controls in cars went from purely mechanical to hydraulically assisted -- door locks, speed controls, etc. -- the inadequacies of the technology and limited "learning curve" experience led to a flood of failures -- this when the Feds didn't enforce any recalls -- you were just stuck with it. Same thing is happening with MCU assisted devices and switching power supplies.

OK, off the soapbox -- a bigger heatsink is almost always a better solution -- but a little fan action will have an even more profound effect than square inches of surface area -- but put a fan into any device and you have to concern yourself with mechanical failure -- perhaps more important if you are running a server farm, or in a hospital ICU or operating room.

One thing which drives me nuts is folks trying to squeeze a 40 or 50 watt Class AB amplifier into a tea-tin. I guess it's a "guy-thing" in which case the product folder has only been delved in to the extent that there's a cookbook schematic.
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Old 7th July 2006, 12:33 PM   #9
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Hi I_forgot

I see your point with the 80 deg surface, but this heatsink is internal and cannot be touched.

My post was meant to start a debate in regards to the "old rule" vs the calculations.

What matters to the silicon is the junction temperature. My calculations (and measurements) indicate that the die is about 110deg and that should not be a problem.

The thing is that with a low thermal resistance from junction to case and no isopad ( live heatsink) there not much difference between junction and heatsink.

Therefore the heatsink could be 80 while the junction is a healthy 110 deg...

I have attached a sim of an IRF530 and a IRFP260N disapating the same W.

The first example is an IRF530 that's isolated from the heatsink. The Die-temp is 137 deg and thats high!

The last example is a IRFP260N that's not isolated from the heatsink. The die is only 103 deg, and that's not dangerous in any way.

The difference is that the IRFP260N is way better at transporting heat to the sink than the IRF530. Of course its not a "fair" comparison when the IFR530 is isolated. - I only did that to illustrate my point.

Hope this makes sense.

If there´s anything wrong with my model please say so.

Regards TroelsM
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Old 7th July 2006, 04:08 PM   #10
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Calculations and sims are a good tool for estimating what to do. Often they simply do not reflect the real world operating environment. Customers have a really good oportunity to destroy all your work by doing the unplanned to your creation.
Have you actually bought the pieces and mounted them together and put it into operation? Put a thermometer on it and measure. Now you will know, not guess.
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