How to improve the circuit with power switching?

How to improve the circuit with power switching?

In the design of power switching circuits, we should first focus on the that commonly used simple power switching circuits have. Taking the circuit shown in Figure 1 as an example, its application conditions have obvious restrictions and defects. On the hand, the circuit has strict requirements for battery voltage and external voltage, and the battery voltage VBAT cannot be greater than the external voltage VIN. For example, the common battery voltage is between 3.7 - 4.2V, it is no problem to use it with the external voltage of 5V USB, once the battery voltage rises to 7.2V, it cannot work normally, because its circuit structure will cause the high battery voltage to interfere with the power switching, resulting in the circuit cannot operate.

Although the Schottky diode can achieve one-way conduction, it still has a few millivolts of drop, and the external voltage of 5V will only be left with more than 4V after its voltage drop, which will affect the performance of the device Moreover, when the external power supply is powered, it will charge the battery irregularly through the body diode of the P-type MOS tube, which can beated by flipping Q4, but it can not completely solve the problem.

To this end, a more complex improved circuit will be adopted in the project ( shown in Figure 2). When the external power supply VIN is connected, the transistor Q7 conducts, Q6 cuts off, Q3 cuts off because gate-source is connected to the battery voltage VBAT, and the battery voltage cannot reach the output VCC. At the same time, VIN is conducted to output VCC through the parasitic diode of Q1, Q2 conducts to pull the gate of Q1 low, Q1 conducts and its parasitic di cuts off, VIN is conducted to VCC through Q1, Q5 and its body diode are both cut off, thus effectively preventing the external power supplyIN from charging the battery irregularly.

And when there is no external power supply VIN, Q7 cuts off, Q6 conducts, the gate voltage of Q3 becomes low level, its gate-source voltage is less than 0 and reaches the threshold level of conduction, Q3 conducts, and then through the parasitic di of Q5 to the output VCC, at this time the gate of Q5 is also low level so it conducts, and its parasitic diode cuts off the battery voltage to VCC.