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FET self-heating calculation

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allanvv

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I'm trying to do a simple DC self-heating calculation. The design manual for my process says:

Tself = P * Rth
P (mW / um) = power through device = Id * Vd
Rth (degC/mW / um) = 100

The units are confusing to me. It seems like to get Tself to be units of temperature, Rth should be degC * um / mW. They use Tself later on as a pure temperature unit since it's added to Tamb... is it just a mistake in the manual or am I missing something? The way they spaced it (degC/mW / um) made it seem like it was done purposely.
 
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(degC/mW / um) ≡ (degC * um/mW)

I think they prefer the first notation because Rth usually is expressed in units of degrees/power .
 

what is the part number of the transistor? the calculation is not straightforward, and depends on the air flow, emmisivity etc.
the delta junction to ambient will give you a fair idea, (power X junction to ambient)
 

The thermal impedance of a small transistor (and most are small in relation to
wafer thickness) is a messy thing to calculate, and that's if there are no other
local heaters messing with you. It's a spreading geometry problem (at simplest)
unless your transistor is wider and longer than the wafer is tall.
 

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