In the design of power switching circuits, we must first pay attention to the problems existing in common simple power switching circuits. Taking the circuit shown in Figure 1 as an example, its application conditions have obvious limitations and defects. On the one hand, the circuit has strict requirements on the battery voltage and external voltage, and the battery voltage VBAT cannot be greater than the external voltage VIN. For example, when the common battery voltage is 3.7-4.2V, it is no problem to use it with the 5V USB external voltage, but once the battery voltage rises to 7.2V, it will not work properly because its circuit structure will cause the high battery voltage to interfere with the power switching logic, causing the circuit to fail to operate.

Although Schottky diodes can achieve unidirectional conduction, they still have a voltage drop of a few tenths of a volt. After the 5V external voltage is stepped down, it only has more than 4V. The power loss caused by this will affect the performance of the device. In addition, when the external power supply is used, the battery will be improperly charged through the body diode of the P-type MOS tube. Although it can be alleviated by flipping Q4, it cannot completely solve the problem.

 For this reason, a more complex improved circuit will be used in the project (as shown in Figure 2). When the external power supply VIN is connected, the transistor Q7 is turned on, Q6 is turned off, and Q3 is turned off because the gate source is connected to the battery voltage VBAT, and the battery voltage cannot reach the output terminal VCC. At the same time, VIN reaches the output terminal VCC through the parasitic diode of Q1, Q2 is turned on to pull the gate of Q1 down, Q1 is turned on and its parasitic diode is turned off, VIN reaches VCC through Q1, Q5 and its body diode are turned off, thus effectively preventing the external power supply VIN from charging the battery irregularly.

When there is no external power supply VIN, Q7 is turned off, Q6 is turned on, the gate voltage of Q3 becomes low level, its gate-source voltage is less than 0 and reaches the conduction threshold level, Q3 is turned on, and then passes through the parasitic diode of Q5 to the output terminal VCC. At this time, the gate of Q5 is also low level, so it is turned on, and its parasitic diode is turned off, and the battery voltage reaches VCC.