Hi everyone,
i was looking at the power supply part of an Adafruit Metro ESP32-S3 board out of curiosity, where three different power supplies coexist: 12V, VBAT and VBUS.
What I can't understand is why in many circuits I see P-channel MOSFETs, in this case Q2 and Q1 are oriented in such a way as to polarize the body diode directly even when the mosfet is off.
The only advantage I can see is that it prevents polarity inversion, but the disadvantage is that a large quiescent current will flow even when the mosfet is off.
Especially in the diagram where there is Q2, this will mean that even if the voltage is 5V, current will still flow from VBAT. Why?

 

In a free country fools are still allowed to talk. And they are also allowed to publish circuits on the internet which are incorrect. Then folks with a bit of understanding look at those circuits and ask how can that work. Often the answer is "NO"