From: Winfield Hill
Subject: Re: Question about linear derating factor for MOSFETs
Date: 10 Nov 2002 05:45:32 -0800
Organization: Rowland Institute
X-Newsreader: Direct Read News 4.10
> 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.