Electronic Basics #31: Schottky Diode & Zener Diode

As you might know the common diode is a very useful electrical component. By applying a positive voltage to its anode and a negative voltage to its cathode, current will flow through it and simultaneously create the characteristic forward voltage drop across the diode which is always mentioned in the datasheet. But if we reverse the voltage potential at the cathode and anode sides, no current will flow through the diode This makes it perfect for reverse voltage protection, or to rectify AC voltage to DC voltage. But you should already be familiar with those concepts, since I talked about it during a previous basics video The thing is though that there are two more diode types which are pretty popular: The so-called Schottky diodes and the so-called zener diodes. So in this video let's find out how those two diode types are different and when it makes sense to utilise them in a circuit. Let's get started! Let's start off with the schottky diode, whose symbol looks like this. As a practical example, I will be using the 1N5819 Schottky diode, whose advantages can be found in its data sheet, which is for one it's small conduction losses. As I mentioned earlier, a diode will always create a forward voltage drop when current passes through it. With the 1N4007 as an example, I passed 1 amp of current through it which creates about the drop of 0.87 volts and thus a power loss of obviously 0.87 watts; which is noticeable by the increased temperature of the diode. If I recreate the same circuit with the Schottky diode however the voltage drop is only around 0.45 volts which thus creates a power loss of only 0.45 watts and reaches a temperature of only 50ºC. So the key advantage of Schottky diodes are their low forward voltage drop, which more or less goes hand-in-hand with the small conduction losses. That makes them very useful for reverse voltage protection in order to increase the overall efficiency of the circuits. But there's a second big advantage which the datasheet once again divides into two, which are extremely fast switching speeds and thus high frequency operations. To get a more practical feeling for these aspects, we can have a look at a boost converter Its job is to boost a low DC voltage to a higher DC voltage using the simplified schematic consisting of coil, a misfit, a diode and a capacitor. Now the coil basically builds up energy when the mosfet is closed and then releases it when the mosfet is open to create a higher voltage at the outputs. The minimum required inductance of the coil can be approximately calculated with this formula. Of course you want your coil or basically any energy storage component in your circuit to be as small as possible so you have to choose a high frequency to achieve that and there starts the problem. If we try to use a common diode to rectify the sine voltage from my frequency generator, then we can see that it does its job very well for frequencies underneath one kilohertz. But once we reach around 5 kilohertz, the diode starts conducting noticeably during the negative half wave, which is not recommended for a switch-mode power supply, like our boost converter. If we try the same setup with a Schottky diode however we can see there rectifies without a problem up to 100 kilohertz and beyond, which is mandatory for our boost converter since it uses a frequency of up to 1.2 Megahertz. So, in conclusion Schottky diodes have a low forward voltage drop and can switch very fast. The only negative aspects about them are that their blocking voltage can be rather low in comparison to common diodes and that their reverse leakage current can reach a relatively high value. But if that does not matter for your circuits, then Schottky diodes can be very useful and often even mandatory. Next we've got the zener diodes, also known as the 'Z' diode, whose symbol looks like this. Once again, we could simply apply a positive voltage to its anode and a negative voltage to its cathode and use it like common diode with a comparable high voltage drop, but no one actually uses a zener diode like this. Instead we can add a resistor in front of it and connect it reverse biased, which means that the cathode has the positive potential and the Anode the negative potential. Before we can do anything useful with the circuit though, we need the two most important specifications of a zener diode: its zener voltage and its power dissipation. If we, for example, use a 5.1 volt zener diodes in combination with a 1kΩ resistor we can see that by applying a voltage above the zener voltage, the zener diode becomes conductive and creates a forward voltage drop equivalent to its zener voltage, well at least close to it. This way we can create a crude voltage regulator or a voltage reference. Here's a simple calculation example: we know that our zener Diode will create a 5.1 volt voltage drop and can handle 500 Milliwatts. That means that the maximum current through it is around 98 milliamps which therefore means the series resistor equals the input voltage minus the zener voltage divided by the maximum current which is around 70 Ohms. I went with the closest available value of 100 Ohms, which means that the circuit can provide a maximum of around 69 milliamps to the load. If we try to draw more current on the output of the regulator though the voltage will collapse. But overall this is a rather crude stabilised voltage, since the zener voltage depends a lot on the input voltage/current, and the temperature of the diode itself. That is why it is better suited to clip a voltage to certain value. Here for example, I have a MOSFET with a maximum gate-to-drain voltage of 16 volts. So I can use a 15 volt zener diode on its gate which will basically clip off any over voltage and thus protect the gate from destruction. Another way of utilising them is by adding two of them in series and applying an AC signal. This way the positive and negative peaks are once again clipped to certain voltage value. So as you might have noticed, common diodes are great but zener diodes and schottky diodes definitely earned the right to be so popular. I hope you enjoyed this small, but hopefully effective dose of Diode knowledge. If so don't forget to like, share and subscribe — that would be awesome! Stay creative and I will see you next time!

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Keywords: schottky, zener, z-diode, diode, z diode, tutorial, explaination, beginner, beginners, guide, when, to, use, compare, comparison, forward, voltage, drop, switch, switching, speed, high, frequency, operation, calculation, example, calculate, regulator, reference, clip, protect, protection, electronics, greatscott, great scott, basics, ac, dc, rectifier, reverse

Id: GtH8lAzQf2A

Length: 7min 31sec (451 seconds)

Published: Sun Aug 20 2017