How to correctly set up a heat sink for MOSFET and Body Diode Submodel in 3-LVL-ANPC-VSI

Question

Hello,

I'm new to PLECS and am wondering how to correctly implement a heatsink in my circuit, a 3-Level ANPC SiC VSI. For my switches (Wolfspeed C3M0021120K), I use the thermal description provided by the manufacturer, one for the MOSFET and one for the intrinsic body diode. I have compiled both into a masked Switch submodel.

In the thermal description, under the thermal chain, the same thermal RC chain in the form of a 4th-order Cauer network is specified for both the MOSFET and the intrinsic body diode. 

• This Cauer network represents the dynamic Zth_junction_to_case of the MOSFET switch, right?

Furthermore, In your demo models, sometimes each individual switch submodel (consisting of a MOSFET and its  body diode) has its own heatsink with Rth_Case_to_Heatsink (thermal grease) implemented. But sometimes there is noch extra heatsink...

• So What would be the correct approach here? Option a) or b) ? (see photos)

Best regards

THK

Answer 1

Hello, Wolfspeed is in the process of updating their thermal descriptions at this time. And actually there will be a new format where this exact topic will be more intuitive and the user will not need to figure out the best approach. Feel free to email support@plexim.com to access these files now. Or if you want to use the files you already have then the approach I recommend is a third one - you should take the thermal chain information and copy it into a series-connected RC Chain block in the circuit itself (in series with the thermal grease resistance in your last image). Then, remove the thermal chain data from both thermal descriptions. Since this is a MOSFET, there is not a separate Zth for the channel and body diodes and having the two in parallel is technically not correct. Let me know if this is clear or you have further questions.

Comments

Hello Kris,

thank you very much for your fast answer. Since I already set up all my switches with the wolfspeed data, I went for your third approach.  I think I'm almost clear. There is only one question regarding the parameters Thermal Capacitance and Initial Temperature (see screenshots)

• For the new heatsink I leave the value for thermal capacitance at 0?
• And I leave the initial temperature blank (since I specified T_init in the MOSFET switch and its Body Diode)?
• Apart from that, is my schematic correct so far?

best regard THK

 

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Indeed, that is all correct!