Mini-Tip: Diode Without a Voltage Drop

I often find that the 0.7v voltage drop across a diode can be somewhat annoying.

Yes, you can use a Schottky diode to reduce that to 0.3v or so, but still, in a project where every volt counts, 0.3v is a lot of voltage to waste.

Luckily there is a trick you can use in some situations whereby you can have pretty much no voltage drop at all.

The trick is to replace the diode with a P-channel MOSFET in reverse.

The parasitic diode inside the MOSFET will, under normal circumstances, act just like a normal diode.  You'll get the normal 0.7v drop.  Now, using that normal 0.7v dropped voltage to power the rest of your circuit, you are in a position to pull the gate of the MOSFET down below the source voltage.  Being a P-channel, that turns the MOSFET on, and allows current to pass through the main semiconductor layer.  That effectively short-circuits the parasitic diode, giving you a very low resistance path.  You typically go from say 0.7v dropped down to 0.05v dropped.  Quite a difference!

I use this as a diode in a solar charger where every volt counts: