# Current Direction in Mosfet

As I know, the direction of current in N channel mosfet is from drain to source. Now, how this current can be pass in this circuit???? SO, why most of the reference books say, in n-channel current flows from drain to source, and for p channel from source to drain??

• The LM5050 has fast response comparator to turn off the FET when current flows in the reverse direction. Does it not trigger, OP? – Huisman Apr 11 '19 at 11:29

The reason reference books say that N-channel current flows from drain to source is that when the transistor is OFF the intrinsic diode of the transistor pevent the current from flowing from source to drain. When the transistor is ON, the current can flow in either direction as the diode is effectively shorted by the Rds(ON) of the drain to source channel.

Similar effect takes place in a P-Channel transistor.

In the shown circuit the N-Channel FET diode prevents current to flow from the output back to the input in a paralleled supplies configuration. The FET get's turned OFF when the input voltage drops below the output voltage minues some small voltage. In normal operation the FET is ON, bypassing the internal diode and ensuring minimal losses.

• N-channel current flows from drain to source is that when the transistor is OFF the intrinsic diode of the transistor prevent the current from flowing from source to drain. How is it possible? in the datasheet has been explained, the mosfet is off and the current passes through body diode from source to drain – Nakh0d4 Apr 11 '19 at 14:46
• So taking the diagram in your question the SOURCE is at VIN, DRAIN at VOUT. When VIN is within a few mV of Vout, the LM5050's internal charge pump turn the external FET ON. If for some reason VIN goes low (a short on the PSU's output) the transistor is turned OFF and the intrinsic diode of the FET blocks any current from flowing from VOUT into VIN. Hope this clarifies your question. – Jean Rubillon Apr 12 '19 at 13:47

No, current can flow either way between drain and source, as long as VGS is above the threshold voltage.

Next to the answer of Dave Tweed, the body diode also conducts from source to drain, independent of the gate voltage.

The body diode cannot handle huge currents, therefore in shown application it is 'shorted' by a better diode which will take most current into account.
As soon as the LM5050 senses current is flowing through the diode(s), the LM5050 opens the mosfet, allowing for smaller voltage drop of $$\I \cdot R_{DSon}\$$ instead of the diode's voltage drop.
The LM5050 has fast response comparator to turn off the FET when current flows in the reverse direction.

For applying ideal diodes, it is important to sense if the input voltage drops below the output voltage and to close the mosfet. Otherwise, the ideal diode will act less ideal than you thought, i.e. conducting the current both ways instead of only one way like an ideal diode would.

Regarding the textbook stating current will flow from drain to source:
I don't know if that statement is true.
Mosfets are normally applied such that the body diode will not conduct. This is the case when the drain voltage is higher than the source voltage. The latter implies current will flow from drain to source when enough gate-source voltage is applied.