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.
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?