Hello.
Here are answers to your questions:
-
Actually 3rd quadrant operation is when the current through the device is negative (either through the channel or body diode). So, the voltage could be positive or negative.
-
See above- Yes, MOSFETs are bi-directional and conduct current in both directions when sufficient gate voltage is applied (unlike the IGBT which can only conduct current in the forward direction).
-
What thermal model are you speaking about? The conduction loss will correspond to third quadrant operation, not switching loss. Turn-on and turn-off losses for the MOSFET channel could be for (before or after) forward or reverse conduction.
-
Diodes typically have negligible turn-on losses, but have more substantial turn-off losses, e.g., due to the reverse recovery effect. If your diode thermal model has no switching loss it may have been left out or not considered substantial enough.
If you reference a specific part I can give more feedback.
Regards,
Kris