Common anti-reverse connection circuits in power supply circuits include diode anti-reverse connection circuit and self-recovery fuse. The diode anti-reverse connection circuit uses the unidirectionality of the diode to achieve the effect of protecting the circuit. The self-recovery fuse uses the large current during short circuit to blow the fuse to cause an open circuit to protect the circuit. There is also a common way to use MOS tubes to protect the circuit from reverse connection.
PMOS tube anti-reverse connection circuit:
The anti-reverse connection circuit of the PMOS tube is shown in the figure below. The condition for the PMOS tube to turn on is VGS<0. In the figure below, the G pole of the PMOS tube is short-circuited to GND through a resistor. When the input end is connected to the positive voltage of the power supply, the current of the power supply flows to the S pole through the body diode of the PMOS tube and returns to the negative pole of the power supply.
The figure below shows the traditional PMOS anti-reverse circuit. The voltage across the diode in the MOS tube is the voltage across the PMOS tube. When the PMOS tube is turned on, the diode in the body is short-circuited. At this time, there is no voltage drop across the body diode, that is, the voltage of the D pole and the S pole of the MOS tube is equal. When the power supply is connected normally, the LED is lit, and R1 is the current limiting resistor. When the power supply is reversed, that is, VIN inputs a negative voltage and GND is connected to the positive pole, at this time, VGS is greater than 0, and the PMOS tube is cut off. Resistor Rg is a buffer voltage. When the voltage across GS is too high, the diode will be reversely broken down. Within a certain reverse current, the reverse voltage will not change with the reverse current. When the input voltage is too high, the diode is damaged, causing the MOS tube to be damaged, thereby achieving the role of the protection circuit when the power supply is reversed. However, there are also problems with the use of PMOS tubes for anti-reverse connection: first, PMOS tubes are relatively expensive in terms of cost. There is a reverse current in the protection circuit. When the input voltage drops, PMOS still remains unblocked, which may cause system power interruption.
NMOS tube reverse connection protection circuit:
NMOS tube has a lower cost than PMOS tube for reverse connection protection circuit. NMOS tube generally uses lower tube, and PMOS tube uses upper tube.
Working principle: When the input connection of the power supply is correct, due to the switching characteristics of the NMOS tube, when the G pole of the NMOS tube inputs a high level, VGS is greater than 0, and the MOS tube is turned on, and the circuit can work normally; when the input power supply is reversed, due to the switching characteristics of the NMOS tube, the G pole input level is low, VGS is less than 0, the NMOS tube is cut off, and no current passes through the D and S poles of the NMOS tube, and the direction of the body diode is opposite to the current input direction. At this time, the NMOS tube is disconnected, thereby protecting the subsequent circuit when the power supply is reversed.
NMOS tube reverse connection protection circuit diagram
Advantages of NMOS tube for circuit reverse connection protection: First, in terms of price, NMOS is cheaper than PMOS, and the internal resistance of MOS tube is at the milliohm level, with small loss.
Disadvantages of NMOS tube for circuit reverse connection protection: The circuit structure is complex; when the input voltage is high, it is necessary to appropriately change the resistance value of the G pole series-parallel resistor, or to add a voltage regulator diode between GS to protect the MOS tube.
Scenario: Applicable to low voltage occasions, VDS should meet the voltage requirement of the highest reverse voltage, otherwise the body diode may be broken down. When the current is large, the current resistance characteristics of the NMOS tube and whether a heat sink is needed to dissipate heat should be considered.