Hi All,
I'm trying to measure the thermal performance of an existing buck converter using hyperlynx thermal. To perform the simulation correctly I need to know the power dissipations of the critical components including the switching mosfets.
First I tried using the power dissipation calculated in my spice software. This dissipation yielded a result that was too low, as confirmed by experiment. I then tried to measure the current through the mosfets by measuring the sense resistor voltage and the duty cycle. This still yielded temperatures that were too low. Then I simulated the circuit again and I multiplied the RMS current squared by the on resistance. This still yielded a low result.
Next, I multiplied the RMS Ids and Vds from the simulation together, yielding a 5W dissipation. This 5W is way too high, because with the particular thetaJC a junction temperature of 1000C was found.
Finally, I multiplied the average Vds and Ids together, which yielded the most realistic temps I have seen yet.
Ultimately, my main question is can the power dissipation of a mosfet be calculated by multiplying the average VDS by the average IDS? Is this the most accurate power calculation for thermal purposes? From all of the different combos I've tried this is the method that has yielded the closest results.