Design and process analysis of anti backflow circuit for MOS transistor

Anti backflow circuit design -MOS Knowledge sharing of pipe anti backflow circuit
MOS Pipe anti backflow circuit
MOS The design of pipe anti backflow circuit is shown in the figure below ： In some applications , Such as battery charging circuit ,B Point is the charger interface ,C The point is the battery interface , To prevent the charger from unplugging , The battery voltage appears at the charging interface .
（Q1、Q2、Q3 Together, it forms an anti backflow circuit ） Be careful Q3 Of DS Reverse connected to the circuit , This is to prevent MOS The body of = The effect of a transistor on a circuit （ If Q3 Connected to the circuit in a conventional way ,C The point power supply will be connected in B Point occurrence voltage ）

Automatic power switching

Anti backflow circuit design
Oring Circuit introduction
Oring Circuit can know its function from its name , It can be understood as a unidirectional conductive circuit . at present Oring Circuits are used in many applications , Its function is to ensure that each individual power supply is independent of each other 、 No backflow , One of the most common is used in current sharing circuit .
The diode has single conductivity , So he is natural Oring circuit . The most basic Oring The circuit is to add a diode at the output , Pictured 1, Select the diode according to the output current of the single power supply and the highest voltage that may appear at the output port of the single power supply connected in the system .

Because the forward voltage drop of diode is relatively large , When the output current is large （ Such as 100A）, chart 1 Diode in Oring The loss of the circuit is very large （70W about ）, Obviously, it is not suitable for large current Oring, At this point, it is necessary to apply Mos Tubular Oring circuit , A little more complicated , Pictured 2（ The most basic Mos tube Oring circuit ）

because Mos The transistor can be seen as a switch and a diode , Control this Mos Turn off the switch tube when there is no output current 、 Block by body diode , When there is output current, the switch tube is turned on , Thus, the unidirectional conduction can be ensured and the loss during the forward conduction can be reduced .

chart 2 And graph 1 Compared to the need for an additional auxiliary voltage .（ In some cases, you can put Oring The circuit is placed at the negative end of the output , At the same time, the output voltage is used as the auxiliary voltage ）. One of the key factors of this circuit is how to correct 、 Control properly Mos Tube switch . And strictly speaking, pictures 2 Medium circuit is generally incompetent , because Q2 Of BE Junction voltage and D1 The conduction voltage drop of is generally not equal , How to ensure that their voltages are equal , So that Mos What about the right switch , chart 3 Is the improved circuit .

In Figure 3 Q2,Q3 It's the same tube , This ensures that the two voltages are basically equal , Or select the device integrating the two triodes , So it's almost the same , So as to ensure that the switch can be properly . This circuit is often found in many products .
chart 3 It is a basically mature circuit , But if we consider the high reliability of the pipe , Additional circuits are needed , chart 4 It is a circuit that is often used .

chart 4 Our circuit is basically a very reliable one Oring Circuit ,R1,R2 You can choose between specific circuits , Because if the value is too large, the efficiency will be improved , But when there is reverse high voltage outside, some current will be injected back , If the value is too small, the efficiency will be reduced . In order to avoid the use of patents and other special places , chart 4 There are always some changes to the circuit of .

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