Analysis on the safety measures of MOS tube anti-reverse connection protection circuit

This kind of connection is simple and reliable, but the influence of power consumption is very large when the input current is large. The rated input current reaches 2A. For example, if you choose Onsemi’s fast recovery diode MUR3020PT and the rated tube voltage drop is 0.7V, then the power consumption must be at least: Pd=2A×0.7V=1.4W, so the power is low and the heat is large , To add a radiator.

1. Under normal circumstances, the DC power input anti-reverse connection protection circuit uses the unidirectional conductivity of the diode to complete the anti-reverse connection protection. As shown in Figure 1 below:

This kind of connection is simple and reliable, but the influence of power consumption is very large when the input current is large. The rated input current reaches 2A. For example, if you choose Onsemi’s fast recovery diode MUR3020PT and the rated tube voltage drop is 0.7V, then the power consumption must be at least: Pd=2A×0.7V=1.4W, so the power is low and the heat is large , To add a radiator.

2. Others can also use a diode bridge to rectify the input, so that the circuit will always have the correct polarity (Figure 2). The disadvantage of these solutions is that the voltage drop across the diode consumes 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.

Analysis on the safety measures of MOS tube anti-reverse connection protection circuit

figure 1

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

Analysis on the safety measures of MOS tube anti-reverse connection protection circuit

figure 2

Figure 2 is a bridge rectifier, which can work normally regardless of polarity, but with two diodes conducting, the power consumption is twice that of Figure 1.

MOS tube type anti-reverse connection protection circuit

Figure 3 uses the switching characteristics of the MOS tube to control the on and off of the circuit to plan the anti-reverse protection circuit. Due to the small internal resistance of the power MOS tube, the MOSFET Rds (on) can now be achieved at the milliohm level. , To deal with the problems of voltage drop and excessive power consumption in the existing anti-reverse connection scheme of the diode power supply.

The reverse polarity protection connects the protective field effect transistor and the protected circuit in series. The field effect tube for protection is a PMOS field effect tube or an NMOS field effect tube. In the case of PMOS, its gate and source are respectively connected to the ground and power terminals of the protected circuit, and its drain is connected to the substrate of the PMOS element in the protected circuit. In the case of NMOS, its gate and source are respectively connected to the power terminal and 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 protective field effect transistor will form an open circuit to prevent the current from burning the field effect transistor components in the circuit and protect the overall circuit.

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

Figure 3. NMOS tube type anti-reverse protection circuit

Analysis on the safety measures of MOS tube anti-reverse connection protection circuit

image 3

The N-channel MOS transistor is connected in series between the power supply and the load through the S pin and the D pin. The resistor R1 supplies the voltage bias for the MOS transistor. The switching characteristics of the MOS transistor are used to control the on and off of the circuit, and then prevent the power supply. Reverse connection causes damage to the load. During the positive connection, R1 supplies the VGS voltage, and the MOS is fully turned on. The MOS cannot be turned on when it is connected reversely, so it plays a role in preventing reverse connection. The Rds(on) of the power MOS tube only needs 20mΩ, and the practical loss is very small. With a current of 2A, the power consumption is (2×2)×0.02=0.08W. There is no need for an external heat sink. The problems of voltage drop and excessive power consumption in the existing anti-reverse connection of the diode power supply are dealt with.

Analysis on the safety measures of MOS tube anti-reverse connection protection circuit

Analysis on the safety measures of MOS tube anti-reverse connection protection circuit

VZ1 is a voltage regulator tube to prevent the gate-source voltage from breaking down the MOS tube. The on-resistance of the NMOS tube is smaller than that of the PMOS, and 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.

The Links:   G156HAN021 NL10276BC20-07Y

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