MOS transistor realizes the automatic switching circuit of main and auxiliary power supply, with "zero" voltage drop and static current of 20ua

See a circuit abroad , It is also a circuit for automatic switching of main and auxiliary power supply , The design is very clever .

The circuit design above is also very good , If VCC The voltage required at the terminal is not necessarily equal to VUSB, Then this circuit is ok , So here comes the question , If the main and auxiliary input voltages are equal , At the same time, it is required to output the same voltage , There should not be too much pressure drop , How to design ？
The above circuit is definitely not enough , because D1 The minimum pressure drop is also 0.3V, Let's look at the following circuit .

At first glance, this circuit is much more complicated , It's very simple , Clever use of MOS When the tube is connected, it is low Rds Characteristics of , Compared with the way of diode , In the case of low cost control , Greatly improved efficiency .
This circuit realizes , When Vin1 = 3.3V when , No matter Vin2 Is there voltage , All by Vin1 adopt Q3 Output voltage , When Vin1 When disconnected , from Vin adopt Q2 Output voltage . Because of the choice MOS Tubular Rds A very small , The resulting pressure drop is almost dozens mV, therefore Vout Basically equal to Vin.

Principle analysis

1、 If Vin1 = 3.3V,NMOS Q1 Conduction , Then it pulled down PMOS Q3 Grid of , then Q1 Also began to turn on , here ,Q2 The voltage between the grid and the source of is Q3 On voltage drop of , The voltage is about dozens mV, therefore Q2 close , External power supply Vin2 To break off ,Vout from Vin1 Power supply ,Vout = 3.3V. At this time, the static power consumption of the whole circuit I1+I2 = 20uA.

2、 Now? ,Vin1 Disconnect the ,Q1 end ,Q2 The grid of has R1 Drop down of , therefore Q2 Conduction ,Q3 The grid passes R2 Pull up , therefore Q3 Also cut off , The whole circuit ,Q1 Follow Q3 end ,Vout from Vin2 Power supply ,Vout = 3.3V. At this time, the above circuit I1 Follow I2 The static power consumption of does not exist .

Analysis complete . When there is a main power supply , The static power consumption of the circuit is 20uA, otherwise , Is almost zero . So the battery is suitable for external power supply .

MOSFET Q1、Q2 Follow Q3 Should choose to have low-voltage grid and very low on resistance characteristics . for example ：Q2 = Q3 = PMN50XP , stay V gs = 3.3V when R dsON by 60mΩ. The transistor T3 It could be flow 2N7002, For reference only , Actually, choose the right one according to different situations MOSFET.

One of the advantages of this circuit is , There is almost no voltage drop in the whole circuit , Of course, a large current is suitable for another , Skillfully control three MOS Opening and closing of tubes , Realize the automatic switching of main and auxiliary power supplies with maximum efficiency .

Reference to the original ：《 Very exquisite automatic switching circuit of main and auxiliary power supply , also “ zero ” Pressure drop , you GET To the essence of it ？》