A reverse connection protection circuit is often required at the power input end. Common solutions include connecting a Schottky diode in series or using a PMOS or NMOS tube.

1. Using Schottky diodes

 When the power supply is connected correctly, the current flows through the diode in the forward direction; when the power supply is connected in reverse, the diode is reverse biased to block the current and protect the load. If the 12V input suddenly changes to -20V, the output remains stable due to the large-capacity capacitor and can power the load for a short time.

Although the use of diodes has a simple circuit and can always block reverse current, there are problems such as large forward loss, increased leakage current at high temperatures, and startup surges that easily exceed rated values.

(II) Using MOS tube

In order to reduce the forward voltage drop of the diode, a PMOS tube can be used instead of a Schottky diode as a power supply reverse connection protection circuit.

 When the power supply is normal, the PMOS tube body diode is turned on briefly, and when the gate voltage is lower than the source voltage, the PMOS tube is turned on; when the power supply polarity is reversed, the gate voltage becomes positive, the PMOS tube is turned off, and the subsequent circuit is protected. Its advantage is low power consumption, but it cannot block reverse current, which will cause the holding capacitor to discharge (because the PMOS tube is turned off when the power supply voltage is close to the threshold voltage Vth, the capacitor will reversely charge the power supply, causing the output voltage to drop sharply)

 The principle of NMOS tube is similar to that of PMOS. However, it should be noted that because the gate-source (GS) voltage of MOS tube cannot exceed 20V, in high-voltage power supply systems such as 24V and 36V, voltage regulator diode protection is required to prevent MOS tube damage.

(III) Comparison of the two circuits

When the input is short-circuited, the Schottky diode is quickly reverse biased to block the reverse current, and the output holding capacitor maintains the load power supply. After the input is restored, the diode continues to supply power. However, the PMOS tube is turned off when the gate-source voltage exceeds Vth. Since it has no reverse current blocking capability, the output voltage will drop significantly.