Previously, VBsemi discussed the conduction conditions and current direction of MOSFETs, briefly mentioning that MOSFETs actually have two current directions, but without going into detail. Today, we will discuss the other current direction of MOSFETs.

In our conventional use, the current flow direction of NMOS and PMOS generally follows one direction, as follows:

NMOS: D—S

PMOS: S—D

In fact, the current direction of a BJT is fixed, unlike a MOSFET. We know that there is a parasitic diode, namely the body diode, and the current direction of the MOSFET is opposite to its polarity.

This means that when the MOSFET is used for switching, the conduction current will only pass through the body of the MOSFET. There will be no additional current passing through the body diode.

When the MOSFET is turned off, it will be in a completely off state (because no current is flowing).

What about if the current direction is reversed? Is it the same?

Let's look at the following diagram, where the body diode of the MOSFET is represented by Di:

[Diagram 1: Current flows from S to D in NMOS and from D to S in PMOS]

The current direction is opposite to the above. When the current direction of the MOSFET is:

NMOS: S—D

PMOS: D—S

We will see that the current direction is the same as the polarity of their respective body diodes.

When the MOSFET is used for switching, during conduction, the current flows through the body of the MOSFET, and there will also be additional current passing through the same polarity body diode (but only a small amount).

When the MOSFET is turned off, the MOSFET will not be completely off, and there will be a larger current passing through the body diode.

This is similar to what we discussed before: when VGS satisfies the driving condition, the drain potential is higher than the source potential, and the current flows from D to S; when the source potential is higher than the drain potential, the current flows from S to D. This mainly depends on the potential between the drain and the source.

So, why do we often see the current direction in Diagram 1 in switching scenarios?

Taking NMOS as an example of another current direction, S—D:

When NMOS is used with a current direction from S to D for switching, when NMOS is turned off, there will be a larger current passing through the body diode, Di, in the diagram. This cannot achieve the switching effect in practical applications.

This is why we usually switch NMOS using D—S.

Does S—D have no practical significance then?

As the saying goes, everything has its purpose.

In reverse connection protection circuits, it can play a special role.

We won't go into deep research here, just a brief explanation of how it works.

Taking a VBsemi N-MOSFET as an example of a reverse connection protection circuit:

[Diagram 2: Reverse connection protection circuit using an NMOSFET]

When the positive pole of the power supply is connected to the rear stage load circuit through the body diode, the body diode will conduct, and the voltage at the S terminal will be approximately 0.7V (the forward voltage of the body diode).

At the same time, the G terminal (gate) is connected to the positive pole of the power supply, so Vgs=Vcc-0.7V>Vgsth, and the NMOSFET will conduct.

After the NMOSFET conducts, the conduction voltage drop is basically 0, so Vgs equals Vcc, and the MOSFET remains in a conductive state.

Therefore, the overall power supply path is on, supplying power to the rear stage load, and the rear stage circuit works normally.

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The above images and information are compiled from the internet.