To protect a 3v/3w (mppt 3.2v/996mA) solar panel from back current (dark current) I would like to use a mosfet P-channel as it seems to limit voltage lost to 0. Could you advice which one should I choose? I have found the following one: Drain-source voltage -16.5V Drain current -1.25A Power 1W Gate-source voltage -1V On-state resistance 1.5Ω Thanks in advance for your help Andrea
The key to choosing a MOSFET as a switch is - ignore the current rating!
First decide how much voltage drop you can accept, and choose a FET with RDSON to match. For example if you want <= 0.1V drop at 1A then you need RDSON of 100mΩ or less. From this you can also calculate the FET's power dissipation, eg. 0.1V *1A = 100mW.
Next, make sure the Gate drive voltage is sufficient to turn the FET fully on. If you only have 3V available then look for a FET that is specified for 2.5V Gate drive.
Finally, ensure that the VGS and VDS ratings are at least 20% higher than the maximum voltages they may receive in your circuit.
The other thing to ignore is Gate threshold voltage. This is the voltage required to just start turning the FET on, and is poorly controlled so it may not turn your FET on at all!
To get an idea of required Gate voltage and expected voltage drop, look at the graph of ID vs VDS in the datasheet. Here's an example:-
You can see that at 3V (4th curve from bottom) it should drop 0.1V at ~2A. Note that as temperature increases so does resistance, so if the FET gets hot it will drop more voltage (and get hotter, and...). Therefore it pays to use a FET which is rated well above the actual current it is expected to pass.