I have 3 paralleled MOSFETs which will be attached to a single heatsink.
I am trying to calculate maximum possible power dissipation and required heatsink thermal resistance Rsa(sink-ambient).
I start from the general equation:
- Rja(junction-ambient) = Rjc(junction-case) + Rcs(case-sink) + Rsa(sink-ambient).
- Rsa = Rja - Rjc - Rcs
- Rsa = (Tj-Ta)/Pd - Rjc - Rcs
Ta is ambient temperature and is choosen to be 45 °C due to forced air cooling and enclosed device.
Tj is arbitrarily choosen to be 150 °C with 25 °C headroom (Tjmax = 175 °C) which should be enough? Or should I go for more headroom?
Rcs includes resistance of thermal compound and insulating pad.
And here is where I have some doubts. Since all MOSFETs will be attached to the same heatsink how do I calculate Rsa?
Lets say that power dissipation of each MOSFET will be 20W thus 60W in total. Do I plug 60W into the above equation and multiply Rjc and Rcs by 3 (as their values are for single MOSFET)?
Or do I plug in 20W, calculate Rsa for single MOSFET but divide final result by 3 (due to 3 MOSFETs)?
The latter method seems more "right" to me, dont know why tho...