I have this BUK9M12-60E MOSFET.
I have calculated the maximum power dissipation of the MOSFET to be 14W (Conduction loss, Switching Loss and Gate Loss). I believe that I don't require any heatsink.
I'd like to calculate the the Junction temperature of the device when it is working and when my ambient temperature is 85degC.
But in order to find, I don't have the Thermal Resistance (Junction to Ambient value). So, how to proceed with the calculation to find the junction temperature.
Best Answer
You will find detailed data on the LFPAK33 package in this document LFPAK MOSFET thermal design guide.
As a rule of thumb, in an automotive environment with 80°C max Ta, the maximum power dissipation on FR4 laminate is 1W. At 85°C it will obviously be less.
So you can forget about your 14W without a very, very substantial heat sink or equivalent structure to remove the heat.
The 79W is not only with a heatsink, it's with an extremely unrealistic heatsink that holds the mounting base temperature at 25°C despite 79W flowing into it. You might be able to accomplish that by fast-flowing chilled water through a copper heatsink. In short, it's not a realistically useful number except as kind of a theoretical extrapolation.
As an aside, I fear we are losing the intuition of power dissipation with the phasing out of incandescent bulbs. Anyone who has unscrewed a 60W or 100W incandescent bulb just after it has been illuminated for some time will know what I mean.