What are the characteristics of MOS diodes?

What are the characteristics of MOS Adiodes? How to use the characteristics to design anti-reverse connection circuits?

In the design of supply anti-reverse connection circuits, although diodes can use their unidirectional conduction characteris tics to prevent power supply reverse connection from damaging the circuit, the problem of voltage across the diodes causes significant losses. However, MOS diodes have the excellent characteristic of low conduction voltage. So, how to use this excellent characteristic of MOS diodes to design-reverse connection circuits?

Taking NMOS as an example, when the power supply input is positive at the top and negative at the bottom, as shown the yellow lines in the figure, the current path passes through R1, R2, and the parasitic diode of the MOS to the ground. At this, the voltage division of R1 and R2 makes the voltage at the GS pole greater than the MOS conduction voltage Vgs, which makes the MOS diodes conductive and the entire circuit loop indicated by the red lines is connected, and the circuit works normally.

When the input is negative at the top and positive at the, the current path indicated by the yellow lines is cut off due to the reverse cutoff of the MOS parasitic diode, and the MOS diodes is turned off due no voltage at the GS pole, the entire circuit loop is broken, and the system circuit is effectively protected.

For the PMOS anti-reverse connection circuit its anti-reverse connection principle is similar to that of the NMOS.

But it should be noted that, usually, if the circuit system is common ground,OS anti-reverse connection is used, and if it is common source, NMOS anti-reverse connection is used.