Please see the section “Semiconductor Loss Specification” on Page 141 of our current user manual. Depending on whether you are using separate switch and diode components, or a combined switch with diode, there are certain considerations regarding the voltage and current regions for defining conduction and switching losses (see the table we provide on the next page).
Otherwise, you may need to clamp the loss values to zero at the lower axes limits. This is due to that fact that the engine linearly extrapolates past the limits you specify and during simulation startup there could be small numerical discrepancies leading to a negative voltage or current being used in the lookup table. Therefore, you might add a voltage row at e.g., -1V, and current column at e.g., -1A.
If these ideas don’t help, you should post your model and thermal description files.