From: John Larkin
Subject: Re: Question about linear derating factor for MOSFETs
Date: Sun, 10 Nov 2002 13:41:14 -0800
Organization: Posted via Supernews, http://www.supernews.com
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On 10 Nov 2002 05:45:32 -0800, Winfield Hill
>> 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
> Hey, John, a nice story, and nice parts.
> Yep, 150ms is a long time to a semiconductor. Glancing at the
> 37N50A's Effective Transient Thermal Impedance curve, fig 11, it's
> clear that 150ms is nearly the same as infinity to the junction.
> It's interesting that IRF touts "low gate charge" for the 37N50A,
> even though the FET's gate capacitance is a staggering 5600 pF.
> I suppose we're to be thankful it's not 15,000pF. :-)
> However, one should note that older parts with higher capacitance
> also had much bigger dies, and therefore lower thermal resistance.
> One should seek out these older parts for maximum transient-power-
> handling capability. While newer more compact FETs are appealing
> for easier fast switching and less switching loss due to their
> low-gate charge, the are dramatically less suited to non-switched
> applications where high heat flux is the name of the game. For
> semiconductor companys, the name of the game is reduced silicon
> area, more dies per wafer, lower manufacturing cost, more profit.
> - Win
yeah, the actual junction thermal time constants are pretty short. But
there is a (complex) composite time constant that is the junction, the
small heat spreader in the package, and the copper (or whatever) in
the heat sink itself. The heatsink situation is one of those nasty 3D
thermal diffusion things that is hard (at least for me) to begin to
quantify. In the 150 msec sort of domain, I'm thinking the thermal
dynamics of the heatsink matter.
I've often wondered what might be...
The thermal resistance and time constant of a TO-220
bolted to an infinite sheet of 0.062 aluminum
Ditto, a TO-247 bolted to half-a-universe full of copper
And a few similar situations that would be handy starting points.
Too bad the serious thermal simulation software is so expensive.
And right, some of the older semiconductor processes, like the triple
diffused bipolars and huge ole fets, are tougher than a lot of the
newer stuff. I never considered gate charge to be positively
correlated with thermal beefiness, but it makes sense.