Tips on preventing reverse connection of power supply

1.Overview

For the input part of the power supply, in order to prevent misoperation, if the positive and negative poles of the power supply are connected reversely, which will cause damage to the circuit, it is generally protected, such as using fuses, diodes, MOS tubes, etc. Here is a brief summary.

2.Method introduction

2.1 Diode anti-reverse connection

Using diodes for protection, the circuit is simple, low cost and takes up little space. However, when the PN junction of the diode is turned on, there is a voltage drop <= 0.7V, which causes unnecessary losses to the circuit. For example, in battery-powered systems, circuits with larger currents will have a more obvious impact (in the circuit, Power consumption and heat are issues that cannot be ignored).

2.2 Fuse protection

For many common electronic products, after disassembly, you can see that the power supply part has a fuse. When the power supply is connected reversely and there is a short circuit in the circuit, the large current will blow the fuse to protect the circuit. However, this method Repair and replacement are troublesome.

2.3 MOS tube protection

Due to factors such as process improvements and their own properties, the conduction internal resistance of MOS tubes is often in the milliohm level or even smaller. This causes a very small loss in terms of voltage drop and power consumption of the circuit, and can even be ignored. Therefore, It is recommended to choose MOS tube to protect the circuit.

2.3.1 NMOS protection

As shown in the figure below: At the moment of power-on, the parasitic diode of the MOS tube is turned on, and the system forms a loop. The potential of the source S is about 0.6V, and the potential of the gate G is Vbat. The turn-on voltage of the MOS tube is extremely: Ugs = Vbat - Vs , the gate shows a high level, the DS of the NMOS is turned on, the parasitic diode is short-circuited, and the system forms a loop through the DS access of the NMOS.

If the power supply is connected reversely, the conduction voltage of NMOS is 0, the NMOS is turned off, the parasitic diode is connected reversely, and the circuit is disconnected, thus forming protection.

2.3.2 PMOS protection

As shown in the figure below: At the moment of power-on, the parasitic diode of the MOS tube is turned on, and the system forms a loop. The potential of the source S is approximately Vbat-0.6V, while the potential of the gate G is 0. The turn-on voltage of the MOS tube is extremely: Ugs = 0 -(Vbat-0.6), the gate shows a low level, the ds of PMOS is turned on, the parasitic diode is short-circuited, and the system forms a loop through the ds access of PMOS.

If the power supply is connected reversely, the conduction voltage of PMOS is greater than 0, the PMOS is turned off, the parasitic diode is connected reversely, and the circuit is disconnected, thus forming protection.

Note: The NMOS tube connects ds to the cathode, and the PMOS tube connects ds to the anode. The direction of the parasitic diode is towards the direction of the correctly connected current;

Access to the D and S poles of the MOS tube: When using an N-channel MOS tube, the current usually enters from the D pole and flows out from the S pole. For PMOS, S enters and D exits. When used in this circuit, it is just right. On the contrary, the voltage condition for MOS tube conduction is met through the conduction of parasitic diodes. The MOS tube will be fully conductive as long as a suitable voltage is established between the G and S poles. After it is turned on, it is like a switch is closed between D and S, and the current is the same resistance from D to S or S to D.

In practical applications, the G electrode is usually connected in series with a resistor. In order to prevent the MOS tube from being broken down, a Zener diode can also be added. The capacitor connected in parallel with the voltage divider resistor has a soft-start effect. At the moment when current starts to flow, the capacitor is charged, and the voltage of the G electrode gradually builds up.

For PMOS, compared to NOMS, Vgs needs to be greater than the threshold voltage to turn on. Since its turn-on voltage can be 0, the voltage difference between DS is not large, which has more advantages than NMOS.