In this video I'm going to teach you about schottky diodes. They are very similar to
regular silicon diodes, but with some important differences.
For starters, they have a different circuit symbol.
Notice how similar they look to other diodes - make sure you don't get them
confused because they behave very differently! Okay, before I talk about what is special
about schottky diodes I want to remind you of some basic
diode concepts. In my previous video about diodes we talked about how they only let current
flow in one direction, and when current is flowing
through the diode, there is a voltage drop across the diode
called the forward voltage drop... or "Vf". Since you have a drop in voltage across a
device, and there's current flowing through it, you end up with some
heat being generated in the diode. And here's the equation for that - Vf multiplied by the
current gives the power in watts.
One of the main schottky diode advantages is that they have a lower Vf than silicon
diodes. This results in less heat being generated. Let
me show you an example. Here I have a regular 1N4007 silicon diode
with 500mA flowing through it. If I measure the voltage drop
across the diode it's 0.832 volts. 0.5 Amps multiplied by 0.832 Volts gives 416 mW of
heat. And that's causing the diode
to have a temperature of 54 degrees. Now let's try the same experiment with a 1N5817
schottky diode. We've got the same 500mA flowing through it, but the forward voltage drop is
only 0.345 volts instead of 0.832 volts! 0.5 amps multiplied
by 0.345 volts gives 173 mW of heat instead of the 416 mW we were getting with the silicon
diode. This results in a lower temperature of 38
degrees instead of 54 degrees. So basically schottky diodes are a more efficient
way to block the reverse flow of current. You can always find out the Vf of a schottky
diode from the datasheet. Make sure you check out the
graph of Vf versus current, because the forward voltage is going to change depending on
the current.
The temperature affects it too! Ok, are there any other advantages of schottkys?
Well, they tend to have very fast switching speeds, so you can use them at higher frequencies.
I have a demo set up here where I am generating a
60Hz sine wave, and I am feeding it into two different
types of diodes - a 1N4007 silicon diode and a 1N5817 schottky diode. These diodes are
very common and I'm just using a couple of resistors for
loads.
Okay, let me explain what you are seeing here. In yellow, we have the input sine wave. It's
not a perfect sine wave because I'm putting an unusual
load on my waveform generator with the multiple diodes and resistors. In green, the silicon
diode is blocking off the negative half of the sine wave. We
are successfully doing half wave rectification, which gives us these positive voltage bumps.
In blue, the schottky diode is also doing a great job,
and as you would expect, there's less of a voltage drop. All of
this is happening at 60Hz, which is a frequency that both diodes are designed to be used with.
So what happens if we increase the frequency of the input sine wave to 300kHz? That's a
frequency you'd expect to see in a switch mode power
supply. Woah! What's the matter? It's like the schottky
diode is on steroids and the silicon diode has been
pushing too many pencils. The schottky diode has no trouble with the
higher frequency, and successfully prevents the reverse
flow of current. But the silicon diode is doing a terrible job of rectification. In
every cycle, it's spending a lot of time allowing current to flow backwards,
before finally blocking it off. Every diode takes a
certain amount of time to switch from allowing forward current, to blocking reverse current.
Schottky diodes tend to be very quick, so that's why
they are often used in medium to high frequency applications.
If you want to learn more about this behavior and how to accurately measure the recovery
time of a diode, enable annotations and check out Alan's
excellent video on the subject. Okay, so if schottky diodes are quick and
efficient, why doesn't everyone use them all the time? Why
would you ever use a silicon diode? To answer that, I have to talk about another
property of diodes, called the reverse leakage current.
You know how diodes block the reverse flow of direct current? Well... that's not 100%
true. There's a small leak. Check this out. I have a power
supply set to 19 volts, and that's connected to a silicon diode
that is backwards. It's in series with my multimeter, so I am measuring the amount of
current that is flowing backwards through the diode. As you
can see, the reverse leakage current is almost unmeasurably small. That's what you want to
see for a perfect diode. Now let's try the same
experiment with the schottky diode. You can see that with -19V across it, there's
almost 20 microamps of reverse current flow. That's a LOT
more than the silicon diode. Now you might be thinking that 20 microamps is not a big
deal, and if you're using a diode for reverse voltage protection,
it's not a big deal. But if you are using a diode as
part of something like a peak detector circuit, that 20uA could be significant. And across
the whole temperature range of the diode, the leakage
current can reach well into the milliamps! So you can't
just blindly use schottkys everywhere. Now there's one last thing I want you to know
about diodes, and not many people realize this. The
forward voltage drop tends to correlate with the maximum voltage rating on the diode.
When searching for diodes you might be tempted to go out and buy the diode with the highest
voltage rating possible because you'd have a larger safety margin. Well, you can do that,
but you'd be sacrificing efficiency. Try figure out what
your peak reverse voltage is, and pick a diode that's rated for
about 10 volts more than that. But make sure you figure it out accurately, otherwise...
Thank you for watching! Make sure you check out my other videos about electronics.