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Heat Sink Calculation

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aljamri

Member
Hi everybody,
I am trying to calculate the temp. of my MC7805 regulator after attaching it to Heat sink of type P/N 6073B according to POWER SUPPLY COOKBOOK page 17 ( attached), I found the first equation correct, but my calculator keeps telling me that the second equation = 197 not 84.4.

What's wrong with my calculation?
 

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dknguyen

Well-Known Member
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Huh, yeah. If you take their numbers and blindly plug it in you get 197 instead of the 84.4 that they have written. So according to their math the junction temperature is hotter with a heatsink. This is because the silicon insulator's thermal resistance is as high as the junction-to-ambient resistance without any heatsink.

THere is probably a typo somewhere. You get a closer answer if you use 6.5C/W or 0.65C/W for the silicon insulator, but still not exactly their answer. Either way, the formula is right. It's just that their math is wrong or their data for the silicon insulator is wrong, or the silicon insulator is so bad that you might as well not use a heatsink at all (or at least not use the silicon insulator).
 
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aljamri

Member
Thanks dknguyen,

if you assume that instead of 65 its 0.65, the answer will be correct. I've tried to get any other reference to make my calculations but failed to do so.
 

aljamri

Member
I tried to get another formual to calculate the difference between the temperature of the regualtor with and without the heat sink. Any Link / Tip / Help, please
 

Willbe

New Member
I tried to get another formual to calculate the difference between the temperature of the regualtor with and without the heat sink. Any Link / Tip / Help, please
Without the heat sink, you need to know θja for the regulator [or a similarly-shaped semiconductor package with the same surface area].

I get 65C/W for this TO-220 package. Knowing the max junction temp you can figure the watts to get there. It's probably 3 or 4W.
 
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aljamri

Member
do you mean to ommit 65 from the formula ? the equation will be like the attached?

yes doing that gives more reasonable figure, or at least shows some reductions.

Thanks for all for your help.
 

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audioguru

Well-Known Member
Most Helpful Member
The term "65" is used in the calculation when you do not use a heatsink. It is the thermal resistance of the chip's junction to the ambient.
The book is wrong when they included it in the calculation with a heatsink but their answer is correct.
 
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