Electronics > Projects, Designs, and Technical Stuff
MOSFET instead of Diode for Solar panel?
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Ikthyoid:
In order to do this, you need to use two "back-to-back" P-channel MOSFETs (PFETs) with their sources connected together and sharing a common gate connection. It's common to see a pull-up resistor from the common gate to the common source in order to ensure stability when the gate is not actively being driven.

Just like using any PFET as a high-side load switch, you close this switch by lowering their common gate voltage to something below the "gate threshold voltage", often 0V or logic LOW; bringing the gate voltage higher than the "gate-source threshold voltage" will open (or break continuity of) the switch.

This back-to-back PFET topology is necessary (vs. using only one PFET) because the body diode of just a single PFET will allow reverse current flow.

You can also use NFETs instead of PFETs, but this can require voltage boosting to drive the gate high enough. There are also "high-side load switch" ICs available that encapsulate all of this functionality into a single IC.
Seekonk:
Everyone loves an experiment. Why don't you measure the leakage from a fully charged battery to the solar panel.Just see if this is worth the over thinking.
boB:

Seems to me that if this does work and turn on during the day, and the PV terminals of that PCB has battery voltage appearing at those PV terminals, the that voltage will still appear there at night, turning on the FET.

Back-to-back FETs as was mentioned might work if you operate it off of the PV voltage itself which will only be there during the day.

But you will also want to make sure that the FETs are not partially on due to low G-S voltage.

But maybe this PV module is so dang small that it cannot hurt anything ?  No idea on that part.
panoss:
Ok guys, I have some data which I think answer to my question:



In the picture you can see the battery voltage from 07/08/2019 (Day/Month/Year) up to 09/08/2019 (today).
07/08/2019
10:33   3.38V
20:45: 3.53V

08/08/2019
09:54  3.52V
20:30  3.61V

09/08/2019
09:07  3.59V
10:13  3.63V

So we see that when no sun (e.g from 20:45 of 7th of August till next day in the morning, 09:54) practically, the voltage remains stable.
So, I conclude, the solar panel consumes no (or VERY little) energy from the battery when outputs no current! (that is when there is no sun)

(the circuit I use is the one in the picture posted in my first post without the diode D1)
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