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I am designing an automatic Power switch between a LIPO battery and a USB as a power input. The system should use the USB but if the LIPO cable is plugged it has to use that as a power source instead of the USB. The USB voltage is 5V and my LIPO Voltage is 8.4V. The MOSFET power selector used in the Arduino micro looks very simple and easy. I just want to confirm if it suits my application.

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  • \$\begingroup\$ When vin is OFF and VUSB present, the 5V LDO is in a strange case where input is disconnected and output is connected to 5V. I don't think the datasheet can give you how much current will be drawn from the OUT terminal in this case. This is a small unknown that I see. \$\endgroup\$
    – sai
    Mar 16, 2023 at 11:27

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Yes. T1 is a P channel mosfet, so a gate-source voltage greater than its threshold will close it. T1 has Vin at its base, so if Vin > (5V + Vth) the transistor will close, disconnecting Vusb from 5V.
T2 is the same, but in reverse so if 5V > (Vusb + Vth) this will close. I would guess this is there to completely disconnect the input to U2 if not using Vin. IIRC, there can be some leakage back to Vin, so this is probably to stop that tring to power U2 and creating a problem there.

This works nicely here because Vusb and Vin dont both directly connect to the same point. Vin powers U2 to create 5V, whereas Vusb drives 5V directly. If in you're application you need to power your system from the same point just using these two inputs, or if you want something simpler, you may prefer to use a diode. circuit diagram showing single diode

In your system, if you can guarantee

  1. When in use Vbat > Vusb + Vfd
  2. Your system can run from Vusb - Vfd
  3. The power lost in the diode is acceptable
  4. Nothing other that the battery will be connected to Vbat

Then this is a simple low-cost option. You may also want to include a diode on Vbat for a bit of extra safety.

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  • \$\begingroup\$ thanks for the explanation, i prefer not to have the 0.5V drop caused by the diode, that's why i want to use a MOSFET, i can have a arrangement where the diode in on the vbat and the MOSFET on the Vusb if you can propose one. \$\endgroup\$ Mar 18, 2023 at 5:30
  • \$\begingroup\$ Well, it's your design, so consider whether the voltage drop is a problem and make your decision. Schottky diodes can have very low forward voltages, lower than 0.5V. \$\endgroup\$
    – LordTeddy
    Mar 19, 2023 at 17:15

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