# Effect of electrical parameters on conduction/switching losses calculated from thermal model

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I was calculating losses for a certain resonant converter converging to a nominal value of switching frequency of operation. I used thermal models available from the manufacturers (Wolfspeed, Dynex) and filled all the thermal parameters applicable to MOSFET+body diode (in primary side) and a secondary side diode (i.e. Rgoff, Rgon for MOSFETs, Tamb for all the three).

I ran two scenarios: one without any electrical parameters filled- Ron for MOSFET/ Vf and Ron for body and secondary diodes vs one with all these parameters filled. I got two different values of switching loss (though a small increase owing to the increase in the switching frequency of the converter because of the apparent shift of the closed loop frequency) and conduction loss (a substantial difference of around 7%). I was given to understand that both the electrical and thermal domain run effectively in parallel (Interpretation of conduction losses - PLECS User Forum (plexim.com)) and those electrical parameters are optional to fill (as per the instructional video for thermal modeling in PLECS website). Which approach is better to calculate losses? And why is there a discrepancy between the two if these run in parallel without any feedback of Rds,on/Vf/Rdiode,on (at least in the original model) affecting the other domain?

You are correct in that the thermal model does not influence the electrical behavior of the circuit.  However, the electrical circuit can impact the thermal behavior, in particular when you have a separate thermal description for the MOSFET and Diode.  By changing the electrical parameters you are influencing which devices are conducting and if there is any current sharing between the two. There also may be small impacts from the actual electrical operating condition of the circuit.

You can verify this by looking at the current through the MOSFET and Diode in your simulation. I suspect there will be a significant difference for the cases you are evaluating. In practice, when the gate is closed nearly all the current will flow through the channel instead of the body diode in the third quadrant. The current distribution will alter the losses as the body diode has higher losses.

The recommended approach that many vendors are moving towards is "integrated" models which provide the correct electrical parameters as part of the thermal description.  This reduces the likelihood of user error.

In the interim the use should determine the electrical parameters from the datasheet. I'll review the video on the website and see if it can be improved.  If you could point me towards the video and approximate time that would be helpful.
answered Sep 4 by (2,023 points)