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From: jmuchow@SPAMMENOTcamlight.com (John Muchow)
Subject: Re: Question about linear derating factor for MOSFETs
Date: Mon, 11 Nov 2002 06:05:40 GMT
Organization: MindSpring Enterprises
X-Server-Date: 11 Nov 2002 06:06:30 GMT
X-Newsreader: Forte Free Agent 1.21/32.243
Thanks for the feedback. In my first repsonse to Phil, I mentioned
that I'm limiting my load to 125-150 watts (I've since settled on 125
watts) and was only setting up an example to ask if I understood the
math for derating calculations. Sorry, I didn't make that clear
The actual numbers I have for my application and the SOT-227 cased
device I'm considering are:
Max power dissipation = 625W
Max operating junction temperature = 150-degrees C.
Linear Derating Factor = 5.0W/degree C
theta-jc, junction to case = 0.20-degrees C./watt
theta-cs, case-to-sink (flat, greased surface) =.05-degrees C./watt
theta-sa, heat sink-to-air = 0.34-degrees C./watt (assuming 18 CFM
Tj = junction temp, 100-degrees C. max for this example.
Ta = ambient temp, 25-degrees C. for this example.
The calculation I used to get the junction temperature used for the
Power dissipation for 100-degrees C. junction temp =
(Tj - Ta) / (theta-jc + theta-cs + theta-sa) =
(100 - 25) / (.2 + .05 + .34) = 127 watts
I interpreted this as stating that with the device and heat sink I've
selected I can have a 127W load and the junction temperature will be
limited to 100-degrees C.
Taking the 100-degrees C. junction temp, subtracting the ambient temp
and I get a 75 degree rise in temp. Multiplying the temp rise by the
5.0W/degree-C derating factor and I get 375 watts.
The max dissipation for the case is 625W.
Subtracting 375W from that and I get 250W.
I interpreted this as stating that the max. allowable power
dissipation level (for this device) with a 100-degree C. junction
temperature = 250W
This was confusing though...what's my max load to limit the Tj to
100-degees....250W or 127W? Should I have used the actual Tj and not
the temp rise over ambient for the derating factor calculation? If
so, that gives me a 500W derating which allows for a 125W load with
the device I'm considering (625W max). Seems to be the right way to
do the calculation, but I wasn't sure.
There's no SOA chart for continuous use in the datasheets. It seems
to only go up to a 10mS pulse. The max drain temp. vs. case temp
chart does show the max drain current rating dropping from 57A at
25-degrees to 35A at 100-degrees and 25A at 125-degrees....a 40%-60%
drop in allowable power. But the derating drops it about 80%...more
To simplify things I picked 125W max for the power level of the load
knowing that if I exceeded it a bit, or if the thermal resistances
were a bit off, that I'd have a bit of headroom before I hit the max
Tj of 150-degrees.
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