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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

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    \$\begingroup\$ (1.5 ohms) (1 A) =1.5 V across the MOSFET. Why not use a power Schottky diode instead? You can probably find one with a forward voltage drop of about 0.5 V or less at 1 Amp forward current. \$\endgroup\$ – Jim Fischer Jun 7 '17 at 7:57
  • \$\begingroup\$ Thanks Jim, I didn't know there is such a drop, I don't understand why some call the mosfet a perfect diode. I will use the schottky diode even if planning to use a nimh 2.4v battery the solar panel will loose all its power to charge. Thanks anyway for your help \$\endgroup\$ – Andrea Jun 7 '17 at 8:14
  • \$\begingroup\$ I would imagine that they call it a perfect diode because MOSFETS typically have zero voltage loss accross them when they are in active mode, we're with a diode you usually lose about .7 seven V sometimes more. \$\endgroup\$ – Michael Sims Jun 7 '17 at 8:36
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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:-

enter image description here

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.

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  • \$\begingroup\$ Thank you very much Bruce, after several searches I have found this one that seems matching your recommendation. Thanks again Andrea. . STMicroelectronics STN5PF02VP P-Channel 20V 4.2A (Tc) 2.5W (Tc) Surface Mount SOT-223 \$\endgroup\$ – Andrea Jun 7 '17 at 16:25
  • \$\begingroup\$ Looks like a good choice. BTW you could use an N-channel FET if you put it the -ve lead. \$\endgroup\$ – Bruce Abbott Jun 7 '17 at 18:46
  • \$\begingroup\$ At the end I bought the below one that is matching your requirement. Once I receive it I will test it and let you know the results. Thanks again for your help Bruce. fairchildsemi.com/pf/FD/FDN302P.html \$\endgroup\$ – Andrea Jun 9 '17 at 7:41

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