- Published: Saturday, 26 May 2012
- Written by Majenko
- Hits: 15434
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: