Power loss of SiC Mosfet and bodydiode for 3rd quadrant operation

[BrianJoe]

Hello,

I have some basic entry-level questions regard the power loss of MOSFET with bodydiode and hope someone could provide some clarification - thank you in advance for patience!

1, When both current and voltage are reversed, it is called "3rd quadrant" operation, correct?

2. My understanding is that when 3rd quadrant operation, only bodydiode will conduct the current(reversed) and thus has CONDUCTION loss. But why (for PLECS simulation) the given thermal model/datasheet also include the CONDUCTION loss of the MOSFET itself? So, MOSFET itself is also able to conduct the reversed current? How?

3. Even if MOSFET itself can also conduct the reversed current, why there's no SWITCHING loss (but only CONDUCTION loss) involved in the given thermal model?

4. The given thermal model of the bodydiode has no SWITCHING loss. Why is it so?

Thank you for patience.

[Anshika_G · Accepted solution]

Hello BrianJoe,
Thank you for posting on Infineon Community.

As per your questions:
1. Yes, when both current and voltage are negative, it is 3rd quadrant operation, also known as synchronous rectification(SR) mode.

2.  SiC Being a wide bandgap PN junction, the body diode has a high forward voltage drop. In this quadrant, i.e., (Vds < 0) and (Ids < 0), current flows in the opposite direction compared to the first quadrant. In this state of operation, there are two possible paths for the current to flow. The first is the inversion channel induced
MOSFET path. The second is the body diode. 
Attaching two images for reference:

For Vgs higher than the threshold voltage, the inversion layer exists in the channel region, and current flows through the channel path following the MOSFET characteristics. In this range of operation, when the voltage drop across the p-n junction is not sufficient to forward bias, the MOSFET characteristics dominate.

In this mode, MOSFET is operating in the SR mode. The current only flows through the body diode during the dead time and transfers to the channel once the MOSFET is ON. The body diode has to operate during dead-time operation when MOSFETS is off.  It helps to reduce the on-state conduction losses due to the body diode. 

3. The PLECS model does not calculate negative values, so  it will show 0 losses.

Regards,
Anshika

[BrianJoe]

Thank you Anshika!

So in conclusion, MOSFET is able to conduct both way current, not like IGBT which can only conduct forward current, right?

[Anshika_G]

Hello,

Yes, SiC MOSFETs(or Power MOSFETs) can conduct in either direction when fed with a positive bias at the gate electrode(refer to the picture).

Thanks and Regards,
Anshika