Mini-Tip: Diode Without a Voltage Drop


G16709BI 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:

mosfetdiode


Posted: 5 years 5 months ago by Jon Chandler #10696
Jon Chandler's Avatar
This is a great application for a p-channel MOSFET. It's the same circuit shown in

Power Protection Circuits and

Power Protection Circuits - A Closer Look at the P-Channel MOSFET Circuit

In this case, rather than acting to prevent reverse polarity connections, the p-channel MOSFET is acting as a diode to prevent discharging of the battery through the solar cell during periods of darkness.

If you're going to use this circuit at greater than about 12 volts, a slight modification of the circuit is needed to prevent the MOSFET gate voltage limits being exceeded.

http://digital-diy.com/forum/download/file.php?id=1897click here.
Posted: 5 years 5 months ago by majenko #10697
majenko's Avatar
That's a nice modification there. The circuit I am using has 15v maximum from the solar cells, and a GS limit of +/- 20V, so I'm ok with this one for the time being. But yes, it's good to check your limits and modify accordingly.
Posted: 5 years 5 months ago by Jon G #10724
 Jon G's Avatar
Excellent!

So, I'm wondering if I can accomplish this task: general-electronics/supplying-power-from-different-locations-t2187.html using two of these? Seems doable to me...
Posted: 5 years 5 months ago by majenko #10726
majenko's Avatar
I don't see why not - they just act like diodes as long as the potentials are of the right style and you have the ability to pull the gate down below the source.
Posted: 5 years 2 months ago by markjo #12204
markjo's Avatar
That's a generality for normal operating currents where the forward drop is typically about 0.65-0.7v, but it varies from that with current.

smt assembly

Forum Activity

Member Access