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I am following this video from EEV blog where he talks about calculating heatsinks and I noticed that he does not talk about multiple components sharing a single heatsink. I have lurked around the web but I have not been able to figure out how it is done.

This is the thermal circuit for a single transistor in a single heatsink as shown in the video enter image description here

How would the thermal circuit for two or three transistors mounted in the same heatsink look like?

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Something like this:

If there are multiple pads, then the branch would happen after the pads and there would be three pad resistors under the transistor thermal conductances.

schematic

simulate this circuit – Schematic created using CircuitLab

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  • \$\begingroup\$ Great!, so if I solve this circuit I can calculate the heatsink I need to get the transistors at a certain temperature, right?. thanks! \$\endgroup\$ – Joaquin Brandan Mar 3 at 20:43
  • \$\begingroup\$ Yep, as a reality check, you could compare it to the case of one transistor with 3x time heat, this result should be a lower temperature \$\endgroup\$ – Voltage Spike Mar 3 at 20:56
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    \$\begingroup\$ Actually now that I think about it, the pads should be branched... \$\endgroup\$ – Voltage Spike Mar 3 at 20:57

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