Negative turn-off losses of diode

Question

I'm trying to simulate the losses of the Infineon power module of which you can find the data sheet attached.
According to the specs I made a thermal model for both the IGBT and the diode. When testing it in the demo model "Buck Converter with Thermal Model" I got the following error (using FS200R12PT4_Diode_1):

Negative turn-off loss (E = -0.00135368). The relevant variables are:
v = -500
i = 20.3552
T = 0.0877919

I saw that the thermal model of the model had turn-off losses for two temperatures and tried interpolatinglinearly for a lower voltage, but the result was nearly the same.

Error using FS200R12PT4_Diode:

Negative turn-off loss (E = -0.000398819). The relevant variables are:
v = -500
i = 20.3552
T = 25.0038

Does someone have a clou how PLECS interpolates energies for non-defined states or how I can get around  this problem?

Thanks in advance
Fabian

Answer 1

PLECS uses linear inter- and extrapolation to determine the switching losses for a given operating point.

In your case the problem is that you've specified losses for 150 and 125 degrees but are operating at 25 degree. The specification for the losses in your thermal model basically tell PLECS that losses increase linearly over temperature. They are zero around 35 degree and negative for lower temperatures.

To avoid extrapolation you can repeat the losses for 125 degrees at a lower temperature (e.g. 0 or 100 degrees). This tells PLECS that losses are not dependent on temperature below 125 degrees and increase linearly above 125 degree.

Comments

Thank you for your suggestion, but the problem is, that the calculated losses would be much to high.

I found one value for the turn-off losses at 25 °C in the data sheet under characteristic values at the nominal point. At this point I devided the loss values at 125 °C by those at 25 °C. Then I devided all values at 125 °C by the resulting ratio and took the results as the loss values at 25 °C.

Now the simulation works fine.