F6 heatsink Calculations

[dantwomey]

Revisiting the neverending discussion of heatsink calculations........

Refering to this webpage
ESP - Heatsink design and transistor mounting

I have,

You will use the following terms -

Tr - Temperature rise
Ta - Ambient temperature (example 22°C)
Th - Heatsink temperature (example 54°C)
Vh - Voltage to heater (example 12V)
Ih - Current to heater (example 3.5A)
Ph - Power applied to heatsink
Rth - Thermal resistance (in °C/W) so ...

Tr = Th - Ta = 54 - 22 = 32°C
Ph = Vh * Ih = 12 * 3.5 = 42W
Rth = Tr / Ph = 32 / 42 = 0.76°C/W

I'd to like mount three or four of these heatsinks vertically on a piece of 1/4" plate aluminum. At there 100mm length they are rate at .85K/C.

How do I calculate the effect of multiple heatsinks bolted to a base?

Regards,
Dan

 

[Eric]

If you orient this sink on its "edge" (closest surface in your image) with the fins running up and down and place multiple sinks side by side, you simply divide the c/w figure by the number of sinks you are using. For example, at 0.85k/w each, having 2 of them side by side will result in net dissipation of 0.42c/w. Having 4 of them will result in 0.21c/w dissipation.

However, is you take these sinks and stack them one on top of the other (effectively making a column of them), the impact of adding more is significantly decreased. One will dissipate 0.85c/w. Two (one of top of the other -effectively making it look like a single sink that is twice as tall) will increase your cooling by 0.85 / sqrt(2) or about 0.60c/w. Thus it is much less efficient to stack them than to place them side by side.

You might want to see the "heatsinking" section of my Aleph-X page for a more thorough description...