Simulation of overvoltage protection (OVP) circuit based on PMOS

          The circuit comes from sun, a male teacher of Engineering ,B Station video link ： Click the jump

          The use of protues It is verified by simulation , The following figure shows the circuit model

 1. Normal voltage input 5V 

          Circuit input 5V when , The circuit is normally connected , The input voltage is equal to the output voltage . At this time, the input voltage is less than the regulated value of the regulator , The zener diode does not work in the stabilized voltage state .PNP Transistor's BE The pressure difference between is 0, Do not meet the conduction voltage , So at this time, the triode is off , therefore PMOS Of G Polar quilt R3 Pull down resistor pull down , At the moment PMOS Conduction , because Vgs<Vgs(th). What needs to be noted here is , During normal operation Vgs It should be equal to the input voltage , So for PMOS The type selection of the needs to pay attention to its parameters Vgs(th), Ensure complete conduction during normal operation .

2. Input overvoltage 12V

           Circuit input 12V when , The circuit is cut off .（ I don't know why there is still 0.95V Output , Is there a problem with the simulation software ？） At this time, the zener diode is normally stabilized , Triode voltage Veb= Input -- Voltage stabilizing value . as long as Vbe Greater than the on voltage , The triode will turn on , Input 12V when ,Veb=12--6.2=5.8V, The display meets the conduction conditions , Therefore, when the triode is turned on, the resistance R3 Act as a pull-up resistor , take PMOS Pull up to the input voltage , Lead to Vgs=0,PMOS No conduction , Realize circuit disconnection .

         Overall analysis of the circuit , In fact, the voltage value of overvoltage shutdown is related to the voltage stabilizing value of Zener diode , Its overvoltage shutdown value = Voltage stabilizing value + Triode on voltage . The conduction voltage of triode is basically 0.6 To 0.7V, So for the design of overvoltage protection circuit , It mainly changes the voltage stabilizing value of the voltage stabilizing tube to realize . about R1、R2 Also pay attention to the value of , When the regulator tube stabilizes the pressure , That is, when the circuit is over-voltage , The current through the zener diode should be guaranteed >1mA, Because the current cannot be too small when the voltage regulator is normally stabilized .

        The picture below is BZX84C Pressure stabilizing curve of series stabilizer , When the current is small, it shows the trend of linear voltage stabilization .