Hey gdiyers,
I'm building a power amp and I'm not sure how to calculate the junction temperature of my transistors so as to determine how big a heatsink I need.
Right now I have 12" wide extruded aluminum heatsink from HeatsinkUSA. They claim that 3" will be roughly 0.85 c/w. Let's say its 6" long so Rha will be 0.85*.707 (inverse square law for length) = 0.6
I'm using 5 IRFP240 mosfets with a junction-to-case of 0.83 c/w, and 5 IRFP9240 mosfets with a junction-to-case of 0.83 c/w.
I have micas and thermal grease so maybe Rch is 0.5 c/w
The amp produces 350 watts.
Lets say it is just one transistor:
P*(Rjc + Rch + Rha ) + Ambient (20c)
35* (0.83 + 0.5 + 0.6) + 20 = 78.888 degrees
Ok that's fine.
But, if I need 10 transistors dissipating 350 watts vs 35, do I just change P to 350?
Then the junction temperature rises to 608 degrees
Not good!
If the heatsink is shared then its ability to dissipate heat remains unchanged. Perhaps I divide Rjc by 10?
I still get 346 degrees which is still way too hot.
I must be misunderstanding something.
Thanks so much for any help!
Josh
I'm building a power amp and I'm not sure how to calculate the junction temperature of my transistors so as to determine how big a heatsink I need.
Right now I have 12" wide extruded aluminum heatsink from HeatsinkUSA. They claim that 3" will be roughly 0.85 c/w. Let's say its 6" long so Rha will be 0.85*.707 (inverse square law for length) = 0.6
I'm using 5 IRFP240 mosfets with a junction-to-case of 0.83 c/w, and 5 IRFP9240 mosfets with a junction-to-case of 0.83 c/w.
I have micas and thermal grease so maybe Rch is 0.5 c/w
The amp produces 350 watts.
Lets say it is just one transistor:
P*(Rjc + Rch + Rha ) + Ambient (20c)
35* (0.83 + 0.5 + 0.6) + 20 = 78.888 degrees
Ok that's fine.
But, if I need 10 transistors dissipating 350 watts vs 35, do I just change P to 350?
Then the junction temperature rises to 608 degrees
Not good!
If the heatsink is shared then its ability to dissipate heat remains unchanged. Perhaps I divide Rjc by 10?
I still get 346 degrees which is still way too hot.
I must be misunderstanding something.
Thanks so much for any help!
Josh