Estimating MOSFET + diode conduction loss with separate thermal description for each device

Question

Hello,

I am trying to estimate conduction loss and switching loss of a MOSFET. MOSFET and the antiparallel Bodydiode both have separate thermal description files. Plecs currently has a bug where it ignores MOSFET gate conduction state and  divides negative current through MOSFET + Body diode such that Body diode conduction loss is over estimated. There is a solution when the device is modeled as MOSFET+Diode with a single thermal description file:

https://forum.plexim.com/1168/mosfet-conduction-overestimated-current-negative-mosfet

My question is: how to apply this solution when MOSFET and diode have separate thermal descriptions. I have attached a demo project showing my problem. I feel the solution somehow involves changing the conduction loss calculation tab in the body diode from "Lookup table" to "Lookup table and formula" but I need help from someone at Plecs to figure this out.

Comments

Hi Perry, this would work much in the same way with the formula overlay in the conduction loss tab to account for the state of the gate. You did not attach files it seems, but I have here to demonstrate how one could do this. Note that you could also move the custom tables for MOSFET and diode conduction losses to their Conduction Loss tabs directly. Also, note that both the MOSFET and Diode have the same Zth chain in their respective thermal descriptions, which may not be true for your device. But there are ways to pull the chain values out to an actual thermal chain component that is connected to the shared junction of the MOSFET and diode. Let me know if you have any questions.

Kris

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Hello Kris,

Thanks for the detailed reply and example model. I looked at your model and you are using the Formula computation method for conduction loss, the formula for v_on(i,T,gate) depends on a custom table 'BodyDiode' and 'MOSFET'. There is also a lookup table for the components if I change the computation method - is there a way to reference this look up table in the formula instead of the custom table? This matters because some device manufacturers provide thermal description files with lookup tables and thermal impedance already filled out. They may or may not have custom tables. I want to use the tables already provided instead of creating custom tables for every device.

A second query I have is- if the thermal description file is in same folder as the plecs file, will it get automatically referenced in the simulation even if the simulation folder is not in the Thermal description folder path?

I apologize that my example model didn't get uploaded in my previous message. I have attached a simple model with separate device thermal description to illustrate what I am looking for.

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Hello Perry,

Yes, actually you do not need to use Custom Tables for this purpose. You can rather use the data tables given directly by the manufacturer for the channel and parallel diode and then include formulas that reference the state of the gate signal to determine which data table is active in the third quadrant. For example, the formula for conduction loss for the channel could be v*gate, and the formula for conduction loss for the diode could be v*(1-g). Does that make sense?

And the way to automatically reference a thermal description file without including it in the Thermal Description folder path is to place it in a directory next to (in the same folder as) the model file, with the directory naming syntax as filename_plecs. If you add the .xml files to this folder with the model open you will need to close and reopen the model file to re-initialize the paths search or else the thermal descriptions won't be found.

Let me know if you have further questions.

Kris