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From: email@example.com (Bob Wilson)
Subject: Re: Question about linear derating factor for MOSFETs
Date: Tue, 12 Nov 2002 04:25:57 -0000
Organization: Your Organization
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References: <firstname.lastname@example.org> <email@example.com>
In article <firstname.lastname@example.org>,
>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.
The figure of 0.5C/W for a greased (and non-insulated) interface between case
and heatsink is pretty optimistic. You'd be lucky to get much less than 2 to
4 times that, and if you are planning on insulating the interface, the best
you can do is several times that. Thermal resistance from cast to heatsink is
going to be a problem with the hich power density you are looking at.
By the way, the very best insulating interface is a thick aluminum "washer"
(a rectangular one in this case), that has been heavily anodized (which is
what provides the electrical insulation), then a thin layer of thermal
compound is applied to both sides. These things are available commenrcially
but hard to find.
Another thing. You MUST forget about attempting to mount the TO247 using the
normal single-screw mount. At this power level mounting it with a single
screw to the heatsink will cause your thermal resistance to go right down the
Standard procedure for mounting TO220 and TO247 devices that are dissipating
high power, is to clamp them using a bar that presses in the center of the
device (i.e. on the plastic case). Usually this intails a bar that has screws
at each end, laying sideways across the center of the device. These packages
are hopeless for high dissipation when mounted with a single screw.
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