What is a schottky diode?


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.

100 thoughts on “What is a schottky diode?

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  2. What if you put one of each in series? You get the advantage of both, fast switching and low reverse current flow.

  3. What kind of thermal camera are u using ?
    Which one would you recommend for closeups small electronic components such ass SMD ?

  4. This Confirms what I learned in electronics class in the 80's. Question: What makes electronics work? answer: Smoke, When smoke comes out of circuit it no longer works. Good class.

  5. Why is it referred to as forward flow (as in +ve to -ve) when electrons are negative and should leave the -ve terminal and flow towards the +ve? Why do -ve electrons flow to a -ve terminal?

  6. You wouldn't believe what it still takes to make a pure silicon crystal and create PN junctions.
    Also, smoking diodes smell really bad. Don't breathe that stuff. I try to get all my little diodes to give up smoking or never start. But, I caught them vaping. They aren't watching their reverse leakage either. Ohms law isn't actually a law but everybody obeys it.
    Do a vid on why it was named Schottky.

  7. In the provided information, I would emphasize that the peak inverse voltage (PIV) is always lower than 100 volts due to the Schottky diode thinner silicon layer and that is why high-voltage hi-speed switching normal PN diodes are used instead for switching power supplies working with line voltages. You know, 0.6V Vf is not much anyway at those voltages.

    On the other hand, Schottky diodes are great for low voltage applications like DC-DC voltage (buck) converters and things like rectifying or "protecting" solar panels from reverse current flows. In those cases, the very low Vf of Schottky diodes is ideal, as 0.6v Vf from normal diodes is actually the output of one single solar cell. It would be like if the cell wouldn't even be there. I have seen power Schottky diodes (600A) with a Vf of about 0.08v which are great for that purpose.

  8. These diagrams (0:09) look like they'd be used to describe rave line-dancing moves; skirt and petticoats being a requirement of rave line-dancing.

  9. No Schottky diodes were harmed in the production of this film.. Bipolar diodes weren't so lucky however…

  10. Old post, but it should be mentioned that not all Schottky diode have high reverse current ratings. Especially germanium types.

  11. What type of diode would work better for a mediumwave AM crystal radio? The old Radio Shack kits called for a 1N34.

  12. I have always understood that current flows against the "arrow" on a diode……Have I been wrong for 50-years?

  13. I ran into an interesting issue with my pipe organ, it has several hundred 12vdc valves typically used in groups of 61 or 73 for one rank of pipes. I used a standard diode on the valves to stop the coil voltage collapse from sending DC current back to the contacts which tends to damage them over time.
    I turn each rank on- with a 12vdc auto headlight relay which connects the bare copper wire negative bus to the neg terminal on the rectifier to complete the circuit for the valves.
    The problem was two things- all of the electro magnets in the 40, 60 and 90 OHM valves were acting as a sort of pseudo ground and completing the circuit without the 12vdc contact being on. In addition, I had "pairs" of 2 ranks wired to a "66 block" telco punch block to save having to run 73 wires to the relays for every single rank.
    That was when I wound up having to add another diode on every single valve's positive input wire so the groups of valves could not self ground, and to keep each rank separated despite having common positives for pairs of 2 ranks on the 66 blocks.
    I also added diodes on the 12vdc auto headlight relays too, to make sure they only work in one direction.

    That did the trick 🙂

  14. Where can I find older capacitors at? I cannot seem to find any replacements for my DOD FX35 pedal.

  15. The video is excellent, except why do we keep the populace ignorant by continuing to teach conventional flow from the 18th century? It only contradicts how electrons truely flow in any circuit, and breeds confusion.

  16. What if you put both silicon diod and schottky diode in series, will that make them more efficient when working together?

  17. You did not answer the question posed in the title. I now know what a schottky diode does, when to use one, and when not to use one; but I still do not know what a schottky diode IS.

  18. Can you use Schottky and Sillicon diodes simultaneously in serious connection to have both, low back current and better rectification on higher frequency?

  19. Great video. Thanks! I had three diodes which looked similar, but one had significantly lower bias voltage. Pretty sure this one was a schottky.

  20. quick question, i want hook 4 3v led in series.3 are flashing led and one normal, my problem is the normal one flashing too, any idea hw to hook them up, I using 12v, cheers graham

  21. I studied electronics many moons ago but for some reason Schottky diodes were never covered in the course. Not a problem with this excellent tutorial 👍

  22. Help i build boost converter i use 2 rectifier diodes but if i make sparks with out voltage the diodes get worm is that normaal pls help me

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