# Diode VS MOS tube, who should you choose for your anti-reverse polarity protection circuit?

#### BySipxmach

Feb 12, 2022

Anti-reverse connection protection circuit, under normal circumstances, the DC power input anti-reverse connection protection circuit uses the unidirectional conductivity of the diode to realize the anti-reverse connection protection.

Anti-reverse polarity protection circuit

Under normal circumstances, the anti-reverse connection protection circuit of the DC power input uses the unidirectional conductivity of the diode to realize the anti-reverse connection protection.

As shown in Figure 1 below:

Figure 1 A series diode protects the system from reverse polarity. The diode has a 0.7V drop.

This connection method is simple and reliable, but the influence of power consumption is very large when a large current is input. If the rated value of the input current reaches 2A, if the fast recovery diode MUR3020PT of Onsemi is selected, and the rated voltage drop of the tube is 0.7V, then the power consumption must at least reach: Pd=2A×0.7V=1.4W, which has low efficiency and large heat generation. , to add a radiator.

The input can also be rectified with a diode bridge so that the circuit always has the correct polarity. As shown in Figure 2 below:

Figure 2 is a bridge rectifier that works regardless of polarity, but with two diodes conducting and dissipating twice as much power as Figure 1.

The disadvantage of these schemes is that the voltage drop across the diode dissipates energy. When the input current is 2A, the power consumption of the circuit in Figure 1 is 1.4W, and the power consumption of the circuit in Figure 2 is 2.8W.

The anti-reverse connection protection circuit is designed by using the switching characteristics of the MOS tube and the conduction and disconnection of the control circuit. Due to the small internal resistance of the power MOS tube, the voltage drop and power consumption of the existing diode power supply anti-reverse connection solution are solved. Excessive consumption problem.

MOS tube type anti-reverse polarity protection circuit

Figure 3 uses the switching characteristics of the MOS tube to control the turn-on and disconnection of the circuit to design an anti-reverse connection protection circuit. Due to the small internal resistance of the power MOS tube, the MOSFET Rds(on) can now reach the milliohm level. The problem of excessive voltage drop and excessive power consumption existing in the existing diode power supply anti-reverse connection scheme is solved.

The reverse polarity protection connects the protection FET and the protected circuit in series. The protection field effect transistor is a PMOS field effect transistor or an NMOS field effect transistor.

If it is a PMOS, its gate and source are respectively connected to the ground terminal and power supply terminal of the protected circuit, and its drain is connected to the substrate of the PMOS element in the protected circuit.

If it is an NMOS, its gate and source are respectively connected to the power supply terminal and the ground terminal of the protected circuit, and its drain is connected to the substrate of the NMOS element in the protected circuit.

Once the polarity of the power supply of the protected circuit is reversed, the protection FET will form an open circuit to prevent the current from burning the FET components in the circuit and protect the overall circuit.

The specific N-channel MOS tube anti-reverse connection protection circuit circuit is shown in Figure 3.

Figure 3. NMOS tube type anti-reverse polarity protection circuit

The N-channel MOS tube is connected in series between the power supply and the load through the S pin and the D pin. The resistor R1 provides a voltage bias for the MOS tube, and uses the switching characteristics of the MOS tube to control the on and off of the circuit, thereby preventing the power supply Reverse connection will damage the load. When connected, R1 provides the VGS voltage, and the MOS is saturated and turned on.

When reversed, the MOS cannot be turned on, so it plays the role of anti-reverse. The Rds(on) of the power MOS tube is only 20mΩ. The actual loss is very small. The current of 2A, the power consumption is (2×2)×0.02=0.08W, and there is no need to add a heat sink at all. The problem of excessive voltage drop and excessive power consumption existing in the existing diode power supply anti-reverse connection scheme is solved.

Figure 4. Schematic diagram of input anti-reverse connection circuit using N-type power MOS tube

Key device: VMI N-channel MOS tube

Figure 5. Schematic diagram of input anti-reverse connection circuit using P-type power MOS tube

Key device: VMI P-channel MOS tube

VZ1 is a voltage regulator to prevent the gate-source voltage from being too high to break down the MOS tube. The on-resistance of NMOS tube is smaller than that of PMOS, so it is best to choose NMOS.

The NMOS tube is connected to the negative pole of the power supply, and the gate is turned on at a high level.

The PMOS tube is connected to the positive pole of the power supply, and the gate is turned on at a low level.

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