# Understanding MOSFET usage

In the schematics of a Firebeetle ESP32 board (first PDF page) we can see this part for the 3.3V power:

This board can be powered in different ways:

• From 5V USB (5V in the schematic)
• From a 3.7V battery (VB in the schematic)
• From a external power supply (Vcc in the schematic)

My question is what is the mode of the P-channel MOSFET SI2301 in case of power from battery? In this scenario, VB = 3.7V, Vcc and 5V are not connected. Thus, gate of MOSFET is at 0V across pull-down resistor RES1A (10k), drain is at 3.7V from battery, but what is at source?

What is the value of Vgs in this scenario?

Another similar circuit is this one (from here):

If first power supply is not-connected and second power supply is +5V, gate is at 0V across pull-down 1k resistor, drain is at +5V, but what is in source (what is Vgs value)?

• @V.V.T: in the first circuit the symbol for D10 is for a Schottky of 6.2v and D2 normal diode, in the second circuit D1 symbol is for a Zenner (I expected both D1 and D2 being Schottky and D10 Zenner). Sep 1, 2021 at 10:54
• Note that if VCC (ext PSU) is present and 5V (USB) is not, Vg = 0V and VCC is thus connected to the battery ... boom!
– user16324
Sep 1, 2021 at 13:54
• @user_1818839: yes, good point. Moreover, using Vcc it is not possible to charge the battery. Better think on Vcc only as output. Something not nice if other power sources than USB wants to be used, because there are not easy pin to connect it. Sep 1, 2021 at 19:10