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Old December 13th 03, 03:23 PM
TOM
 
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Default 2SC1947

Neil:

Attached is a link to the Mitsubishi datasheet for the 2SC1947.
Regarding the thermal
specifications, the maximum junction temperature is 175C. The thermal
impedance is
15C-per-watt junction-to-case, and 150C-per-watt junction-to-ambient.

What this means is that the junction temperature will be hotter than the
case temperature
by 15C times the numer of watts dissipated. For example, if the transistor
is dissipating 4 watts,
the junction will be 60C hotter than the case temperature. Thus the maximum
case temperature
should be limited to 175-60 = 115C (pretty darn hot).

There will be some thermal impedance between the heatsink and the case,
but if well designed,
and well attached, it should be only 1 or 2 C/watt, almost negligible in
this example.

The 150C/watt junction-to-ambient spec means that the metal case of the
transistor is not
too effective at rediating heat by itself. Without any heatsink, and
assuming 1 watt dissipation, the
ambient temp would have to be limited to 175-150 = 25C (room temperature).

http://www.mitsubishichips.com/data/.../ds/sc1947.pdf

-- Tom, N5EG




"Neil" wrote in message
...
How safe is it to run these without forced air cooling?

I've got one as a 1 - 4w driver (feeding a BLW60C) and although has a

finned
TO5 heatsink, it gets up to 70+ degrees Celsius when running. The

Mitsubishi
spec says it is safe up to 175 degrees Celsius but am not sure if what I'm
doing will do the device damage?

Any thoughts anyone?



Neil






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