I have a 3.7V LiPo battery, a usb charger this one and a solar MTTP LiPo charger (this one). The goal is to use the solar charger as default and the usb as a back up. Especially, when charging with usb, the sun charger should not interfere. I tried to do the switching circuit using P-MOS as a Normally-Closed switch and it opens once the usb is plugged in and 5V to the gate as shown below.


simulate this circuit – Schematic created using CircuitLab

Problem is, when the LIPO is fully charged, the Vgs for the PMOS is 5V-4.2V = 0.8V and it is less than the Vgs threshold (mostly 2V) to keep the MPPT disconnected. What change should I do to make it work? Can I get away with a combo of BJT and MOSFET, or I need an OR Controller to do this?

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