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Several implementation schemes of anti reverse connection protection of positive and negative poles of power supply!
2022-07-05 08:06:00 【Learning notes of hardware Xiaobai】
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 .
- 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.
- 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 .
- 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 .
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