My understanding is that, when open (off), a mosfet only blocks current in one direction (drain to source). Current can still flow in the opposite direction (source to drain).

Is that due to the body diode?

 

Even a body diode has a forward voltage drop.

 

Ya, the body diode is still presented. You can check it with multimeter (diode mode) without connecting mosfet to circuit. Gate has to be discharged/shorted with source during measurement.


When the mosfet is ON (conducting) the Vf of body diode fall to almost zero. As example the reverse conduction is used in synch. rectifiers to avoid losses. Or replacing the diode in buck converter.

 

When the mosfet is ON (conducting) the Vf of body diode fall to almost zero.

Specifically, when ON, the MOSFET conducts equally well in both directions, with the voltage drop equal to the current times the MOSFET on-resistance.

 

I can't tell which of these answers talks about when the mosfet is open (off). It seems most are talking about the mosfet closed (on). @Papabravo mentions drop, but doesn't say current flows then the mosfet is open.

 

Your understanding (post #1) is correct, provided Vds is less than the rated maximum.

 

Your understanding (post #1) is correct, provided Vds is less than the rated maximum.

Would Vds have to be negative to flow in the opposite direction?

 

Would Vds have to be negative to flow in the opposite direction?

In order for current to flow from Source to Drain the MOSFET channel must be OFF (Vgs <<Vgs(th)) and the source voltage must be greater than the drain voltage by the forward drop of the body diode. Saying the Vds must be negative is necessary, but maybe not sufficient.

 

Would Vds have to be negative to flow in the opposite direction?

For an N-MOSFET, yes.

Not to confuse things but, in general, the N-MOSFET can be ON or OFF, either way current will flow for a negative Vds.
It flows through the normal channel if the MOSFET is ON, with the voltage drop determined by its on-resistance and, if the MOSFET is OFF, through the substrate diode with a voltage drop determined by the substrate diode characteristics.

 

If Vds is greater than the rated maximum then transistor breakdown can occur and current will flow even when Vgs is zero and the transistor is supposedly Off.

 

Saying the Vds must be negative is necessary, but maybe not sufficient.

Must drain be negative with respect to ground? Or just with respect to source?

 

Must drain be negative with respect to ground? Or just with respect to source?

With respect to the Source