Generally, the direction of the current from NMOS is from D to S. However, in actual application circuits, the current from NMOS may flow from S to D, such as the following NMOS tube anti-power reverse connection circuit.

 When the power is connected normally

The positive electrode VCC of the power supply is connected to the body diode through the subsequent load circuit, and the body diode is turned on, so the voltage of the S pole is about 0.7V at this time.

At the same time, the gate G is connected to VCC, Vgs = Vcc - 0.7V, when Vgs is greater than the turn-on voltage Vgsth, the NMOS tube is turned on . At this time, the current of the MOS tube is from S to D

 When the power supply is connected reversely (power and ground are connected reversely)

 The gate G is connected to the negative pole of the power supply 0V, and the S pole is connected to the negative pole of the power supply (0V) through the load, so Vgs=0V, and the MOS tube is not turned on.

At the same time, the D pole is Vcc, the S pole is 0V, the body diode is reverse biased and is not conducting, so no current can flow through the NMOS tube.

 Let's look at this Buck circuit again. The lower tube is an NMOS tube. When there is a dead time in the switch switching (neither the upper tube nor the lower tube is turned on), and the inductor current cannot be interrupted, the inductor current will flow through the body diode of the lower tube, and the current direction is from S to D. This shows that after the MOS tube is turned on, the current direction can flow in both directions, from D to S, and from S to D.

How much current can the body diode of a MOS tube carry?

The instantaneous current of the MOS tube body diode is generally equivalent to the instantaneous current after the NMOS tube is turned on, and is generally not likely to become a bottleneck in use. The continuous current can be calculated based on the power consumption limit of the MOS tube. When selecting a model for special scenarios, you need to pay attention to its current parameters to avoid circuit failures.