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.