From: John Larkin
Subject: Re: Question about linear derating factor for MOSFETs
Date: Sat, 09 Nov 2002 16:10:45 -0800
Organization: Posted via Supernews, http://www.supernews.com
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On Sun, 10 Nov 2002 10:17:59 +1100, "Phil Allison"
>"John Muchow" wrote in message
>> The IR datasheet for a MOSFET I was considering (for a larger version
>> of a constant-current circuit that this group is giving me great tips
>> on) has the Linear Derating Factor listed as 5.0W/degree C. The max
>> power dissipation rating is 625W (SOT-227 case) at Tc = 25 degrees C.
>> I can't find what "Tc" (subscripted "c") means anywhere on the
>> datasheet...is it case temperature?
> ** Yep.
>For device derating purposes, can
>> I assume that the heat sink temperature (using the delta-Temp
>> calculated using the load and sum of the thermal resistances from the
>> junction to air) will be the same as the case temperature?
> ** No, the case temp will need to be measured. It will have a
>significant thermal loss depending on mounting arrangement and the surface
>temp of the heatsink immediately under the device pack is the one that
>> If that's true, I think the derating is applied this way:
>> If the heat sink temp is 80 degrees C. under load, then I have a
>> 55-degree difference (from 25-degrees). Taking the 5.0W/degree C.
>> derating factor gives me a 275W derating. I would subtract that from
>> the 25-degree rating of 625W to arrive at the 80-degree rating of 350W
>> for the device.
>> This means, for this device, that I cannot safely have a load greater
>> than 350W with the heat sink I'm using? Assume that I've maximized
>> the effectiveness of the other factors involved...fan, possible
>> insulator (grease, Kapton, Sil-Pad, etc.), etc.
> ** I suspect it is going to be difficult to safely achieve 350 watts of
>dissipation from one device unless liquid cooling is used.
> .............. Phil
a few months ago we tested a lot of TO-247 power fets to destruction.
Bolted to a solid copper block, it was hard to find one that could
stand 350 watts dissipation for 150 milliseconds (our target), even
for parts rated for a lot higher DC dissipation.
Good parts: IXYS IXTH11P50 P-channel
IR IRFPS37N50A N-channel