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From: jmuchow@SPAMMENOTcamlight.com (John Muchow)
Subject: Re: Question about linear derating factor for MOSFETs
Date: Tue, 12 Nov 2002 05:23:11 GMT
Organization: MindSpring Enterprises
References: <firstname.lastname@example.org> <email@example.com>
X-Server-Date: 12 Nov 2002 05:28:39 GMT
X-Newsreader: Forte Free Agent 1.21/32.243
>>>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.
For the TO-220/247 devices, I agree. But, the SOT-227 cased device I
used in the example has a very low thermal resistance to the heat sink
(0.20-degrees C/W). This, along with the 0.34-degrees C/W thermal
resistance of the heat sink I was considering allows me to come in at
about 0.7 (with the more realistic number of 0.2 for the thermal
grease, not 0.05). I've since switched to a heat sink that has a
0.18-degrees C/W thermal resistance to air. This keeps the total
resistance to about 0.5-0.6. The SOT-227 case is already isolated so
all I need is some high performance thermal grease between it and the
>>>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.
I've seen them mentioned in Wakefield and Thermalloy app notes and
tech papers. I'm lucky enough to have an isolated case with the
>>>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
Thanks for the tip. I've read that overtightening can cause the case
to tip up and significantly increase the thermal resistance. It's
nice to have confirmation that the single screw mount hole isn't the
best thing for high-power applications.
>>>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.
I've seen the clips that go across the TO-220/247 devices to hold them
against a heat sink (Thermalloy has an huge line of clip-only heat
sinks for high-power apps) but haven't seen screw mount bars. Seems
like that would be a great idea to help pull heat from the top of the
tab too (if machined accurately enough). Are these bars commercially
available as a standard product?
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