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Issues regarding infinite temperature error for thermal model of converter IGBTs [closed]

0 votes


I'm trying to evaluate the thermal performance of this converter (attached) by analysing the IGBT switch losses. Within subsystem 2 (Sub 2) there is an interleave full bridge converter design. Inverter 1 contains the thermal network that is giving me trouble.

I'm attempting to use the Infineon IKW40N120CS7 IGBT and Diode thermal models (attached). Based on the datasheet specifications it should be a suitable device for the application. 

When I attempt to run the model I get the following errors:

"Thermometer Converter_Termal_Model/Sub2/Inverter1/IGBT1:Tj may measure infinite temperatures caused by the impulsive heat sources listed below. These sources should be connected to a thermal capacitance or a fixed temperature. - Converter_Termal_Model/Sub2/Inverter1/IGBT1:Q"

My initial thoughts were that the thermal circuit (heatsink and thermal resistor) parameters were too large for the IGBT to realise that there was a thermal network in place. But despite trying a range of thermal resistance and thermal capacitance I am unable to get the system working.

Could someone please let me know what I've got wrong? 

Many Thanks.


closed with the note: Correct answer provided below.
asked Apr 27, 2023 by Brian Strugnell (21 points)
closed May 4, 2023 by Brian Strugnell

1 Answer

+1 vote
Best answer

The thermal models you shared have a "Convert to Cauer at simulation start" option in the Thermal chain tab.  You should click this option.

PLECS, by default, always converts the values from Foster to an equivalent Cauer network.  However, these models include an experimental option that leaves the thermal chain as a Foster network and is the exception to the rule noted in the documentation excerpt below.

Foster networks are valid only for the measured conditions. If the thermal interface resistance changes then the whole foster chain must be recalculated to be accurate.  From the PLECS perspective, a Foster network connected to a series resistance has a resistance-only path between the junction and temperature source.  The impulse of energy added during a switching event then passes through this resistive-only current path, resulting in an impulsive/infinite junction temperature (leading to your error message).

The also is an underlying state-source dependency issue with your model due to the zero on-resistance of all your switching devices along with a mix of forward voltages for the different IGBTs and diodes.

answered Apr 27, 2023 by Bryan Lieblick (1,949 points)
selected Apr 27, 2023 by Brian Strugnell