Several implementation schemes of anti reverse connection protection of positive and negative poles of power supply!

Anti reverse connection of power supply , It should be that this series of designs will be adopted in many circuit scenarios .

A few days ago , Xiaobai is doing board verification , When the dummy battery is connected and the power is supplied , Accidentally connect the positive and negative poles of the fake battery with the input and output of the power supply , Cause the board to burn , In an instant, a wisp of smoke floated on my seat . Because our products use real batteries , So there will be no reverse connection , There is no design to prevent reverse connection of power supply , But it is in the stage of debugging and verification , True battery Limited , So the fake battery is used , So ,,, I can't help but feel the above situation .

Based on this question , today , I still want to sort it out simply , In some circuits , Circuit measures taken to prevent reverse connection of power supply .

1. Diode series reverse connection protection circuit
   At the input of the power supply , Connect a forward diode in series , It mainly uses the forward conduction of the diode , Characteristics of reverse cut-off .

When the circuit is connected normally , The diode is on , The circuit can work normally .
When the power supply is connected reversely , Diode cut off , The power supply cannot form a loop , The circuit board does not work properly , It can effectively prevent the harm caused by reverse connection .

But here's the thing , There is a voltage drop in the diode . The diode voltage drop of silicon material is generally 0.7V. The diode voltage drop of germanium material is generally 0.3V.

2. Use bridge rectifier circuit to prevent reverse connection protection circuit

Use bridge rectifier circuit , Whether the power supply is connected directly or reversely , The circuit can work normally .

But there are the same problems as the first method , There is a voltage drop in the diode , It will cause the input voltage of the subsequent circuit to be less than the power supply voltage .

3. Use MOS The tube is protected against reverse circuit

MOS The tube has conduction impedance , namely RDS(on)- Drain electrode / Conduction impedance between sources . So when designing this kind of circuit , Choose the one with smaller conduction impedance MOS tube . Generally, it is about a few milliohms or dozens of milliohms . At this time, the pressure drop is very small , Negligible .

（1） NMOS protective
In the moment of electricity ,MOS The parasitic diode of the tube turns on , The system constitutes a loop . The source voltage is about 0.6V. At this time, the grid voltage is Vbat,MOS The opening voltage of the tube Vgs=Vbat-0.6. As long as it is greater than the standard of the specification ,DS It can be connected , here MOS The parasitic diode of the tube is short circuited , The system goes through MOS Tubular DS Generate circuit .

If the power supply is reversed ,NMOS The conduction voltage of the tube is 0,NMOS end , Parasitic diode reverse connection , The circuit is disconnected , Unable to form a loop .

（2） PMOS protective （ It is recommended to use ）
Similar to the above , At the moment of power on ,MOS The parasitic diode of the tube turns on , The system constitutes a loop , The source voltage is Vbat-0.6V, However, the grid voltage is 0,MOS The opening voltage of the tube is Ugs=0-（Vbat-0.6）, The grid is low ,PMOS, Conduction , Parasitic diode is short circuited , The system goes through PMOS Of ds Connect to form a loop .

If the power supply is connected reversely ,NMOS The on voltage of is greater than 0V,PMOS end , Parasitic diode reverse connection , The circuit is disconnected , Thus forming protection .

among ,NMOS Connected in series to the negative pole ,PMOS Connected in series to the positive pole , The parasitic diode is oriented in the correct direction of current flow .

NMOS, The current flows from D Flow into the pole S Polar outflow .PMOS It is ,S Flow into the pole D Polar outflow .

Practical application ,G Generally, a resistor is connected in series , In order to prevent MOS Tube is broken down , You can also add a zener diode . Capacitance connected in parallel to the voltage divider , It has a soft start function . At the moment when the current begins to flow , Capacitor charging ,G The pole voltage is gradually established .

about PMOS, Compared with NMOS Conduction requires Vgs Greater than the threshold voltage , Because its opening voltage can be 0,DS The pressure difference between is not big , Than NMOS More advantages .