Precautions for single chip microcomputer anti reverse connection circuit

How to design anti reverse connection circuit

 

If there is a charger circuit that does not consider the anti stupidity design , There is no protection circuit designed , Then if someone accidentally plugs the charging head upside down , The circuit may explode , It is not only dangerous but also harmful to property safety .

If the engineer doesn't want to see the board smoke and catch fire , It is necessary to know how to design anti reverse connection circuit . The simplest way to prevent reverse connection circuit is to connect a diode in series in the circuit , In this way, even if the circuit is connected reversely, there will be no current flowing through the diode .

 

But this design has a big drawback , When the circuit works normally , Current flowing through the diode will produce a voltage drop .

With IN5400 For example , The working current of the circuit is 2A Under the circumstances , The voltage drop generated on the diode is 0.85V, That is to say, the power of diode is 1.7W, Such a design circuit is not only inefficient , The fever is also severe , The working life of the diode will also decrease .

 

In order to improve circuit efficiency , We can choose Schottky diodes with smaller phase voltage drop STPS2L60. At the same working current 2A Under the circumstances , The forward voltage drop of diode is only 0.55V, The reverse leakage current is lower than 100uA.

 

This parameter of leakage current must be paid attention to , The leakage current of some Schottky diodes may reach several milliamps , Choosing Schottky diodes can reduce our power consumption to 1.1 watts ,0.55V*2A=1.1W. Although the power consumption is reduced , But it obviously hasn't met our expectations . Want to further improve the working efficiency of the circuit , You need to use MOS tube , Here we choose FQP47P06,P Ditch MOS FET.MOS The on voltage of the tube VGS near -4V when ,MOS It can be completely connected .

 

When the power supply is normally powered , The voltage passes through PMOS The body diode reaches S level , The voltage drop of the bulk diode is 1V, here S Pole voltage is 11V,VGS be equal to -11V,MOS Can be fully opened , The current flows from DS Flow through , The body diode is shorted ,MOS After conduction DS The internal resistance between is very small , The pressure drop is basically negligible .

 

When the power supply is reversed ,VGS The voltage of is 0,MOS The tube doesn't work , The circuit is protected . When it comes to calculating MOS When the power consumption of the tube , We need to focus on one parameter -RDS, In circuit MOS Tubular RDS by 0.026 ohm , The corresponding power consumption is 0.107W, Compared with using diodes , The efficiency is increased ten times .

When selecting the type of seemingly pipe , We need to focus on a few parameters

 

The first is VDS The voltage should be higher than the power supply voltage , Avoid breakdown MOS tube , The second is RDS The resistance should be as small as possible , To reduce unnecessary power consumption .

And that is VGS, Our choice PMOS Of VGS The voltage range of is positive and negative 25V, But a lot of MOS Tubular VGS The voltage is only 15 Volts even lower , At this time, we need to re optimize the circuit design .

 

In this circuit , The voltage of the power supply is 30 v , We can connect a zener diode between the grid and the source stage , In this way, the voltage between the grid and the source stage is embedded in 10V, At the same time, a large current limiting resistor should be connected in series in the circuit , Ensure that the zener diode works at normal current , In this way, we can guarantee MOS The tube works stably , No breakdown .

 

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