Understanding Schottky diodes (with bench tests)

welcome back to the bench today we're going to look at Schottky diodes Schottky diodes are a special type of diode normal diodes have an N and P type semiconductor Junction Schottky diodes have a semiconductor to metal type Junction and that gives it some interesting characteristics so I went through my parts drawer of diodes and pulled out some Schottky diodes and a few regular diodes to compare it with and lay them out here this is the part number under that is the voltage and current rating so I'm going to perform some tests on these diodes and see what the results are before I get on with the test I want to talk about the advantages and disadvantages of the Schottky diode first the advantages first advantage is a low forward voltage drop if you take an ordinary PN Junction silicon diode apply a forward bias to it it will not start conducting until you reach around 700 millivolts give or take depending on the current and that could be a source of power loss for example let's say you have 3 amps current passing through the diode and it has the 700 millivolt drop across it well the diet is going to dissipate a little over 2 watts so you know that's a significant loss of power right there a Schottky diode on the other hand has a lower forward voltage drop it'll be around 3 or 400 millivolts give or take again depending on the current and the type of diode so you will save about 50% of losses right there so that's an important advantage next advantage is fast switching because of the design of the diode it can switch at a higher frequency than ordinary silicon diodes ordinary die have a reverse recovery mechanism where Schottky diodes don't really have that issue so they can switch fast you probably heard of fast switching diodes or fast recovery diodes Schottky diodes are even faster than those now on to the disadvantages one disadvantage is they're relatively more expensive you know diodes are not that expensive they're not a real big deal to the weekend hobbyist but if you're designing a product and you know using a Schottky diode is going to cost a few more pennies more than an ordinary diode you might be able to purchase in quantity diodes at like five cents apiece but a Schottky diode might set you back fifteen cents apiece in quantity and if you're making several million copies of a product that adds up in the cost so yeah that there is some expense to consider another issue is the reverse leakage Schottky diodes tend to leak a little bit of current through them and it's usually not an issue in most cases but there are situations where it can be a problem and in some cases as the diode heats up there could be a reverse conduction thermal runaway issue as it heats up it conducts a little more and that causes more current to flow and it heats up even more and you can see it just becomes a thermal runaway issue so these are just some basic advantages and disadvantages there's a little more detail to talk about this stuff when I actually run the tests not everything is so cut and dry in electronics and we'll take a look at that here all right let's move on with the tests okay I'm going to start with a switching speed test one megahertz what I have is a microcontroller and I programmed its pulse with modulated output to give me a one megahertz signal or square wave on the output and we'll use that to test the speed of these various diodes and see how they perform one thing I found it's very important is to set up your scope properly you have to use the probes and 10x mode and I have to keep these wires short I first I tried using alligator clips and the length of those leads had extra inductance in it it was causing a lot of ringing in the on the oscilloscope screen so I just using this little lead here and I'll just have to handhold the diode and get a reading off the scope okay I'll point the camera at the scope while I'm doing this and announce what device I'm testing here's a schematic of my test setup I have my 1 megahertz signal source here passes through a 220 ohm resistor into the diode under test and the scope leads will connect here this is the ground side you have to use both grounds because if you leave one lead off you'll get a lot of extra inductance and get ringing one channel will connect across the diode under test the other across the signal source so you can see that original source as a reference by the way this is the schematic symbol of a Schottky diode ok so I have the waveform up on the screen those microcontrollers can give you a nice waveform it does have a little bit of overshoot you have the channels on top of one another but the direct output of the microcontroller does have a little bit of overshoot that's not my scope probe it is actually that way because when I move the lead to the same place I don't get that anymore so channel 1 the yellow channel is going to be the actual device under test you might see the blue channel or the microcontroller output drop a little bit that's just because we're loading it down so first I'll start with a 1 n 5 4 0 4 that's just an ordinary 3 amp rectifier diode and let's take a look what happens here ok forward bias ok so what's happening here is and when the output turns on the diode turns on and you can see this is about 0.7 volts because this is our baseline and we're volt per division but when the output of the microcontroller goes low the diode doesn't change it stays on it's not fast enough you can see the result there you know these diodes are not very fast it just meant for like around 60 Hertz they can do a little faster than that next we'll take a look at a fast recovery diode as a P it's an NP Junction fast recovery diode it's not a Schottky it's fr 3:06 if you're wondering so let's see what happens when we try that well it's still not fast enough you can see there is a little bit let me turn this up you can see there is a little bit of change in the voltage level but it's not turn all the way off so this particular diode is not very fast at least for one megahertz okay now I'll try a Schottky diode this is an SB three-30 it's a 30 volt 3 amp diode okay and you can see the forward voltage drop is quite a bit lower and it's hard to tell so let me turn the channel up so we're at 200 volts per division so it's a little over 200 millivolts when it's turned on and you can see it is dropping down so this is kind of weak for such a high frequency but you do see it is switching a lot faster now just for comparison turn that back down I'm going to try a ordinary silicon diode one of these small signal type diodes switching diodes they call them this should be pretty fast and now look at that and clearly see it is switching off let me turn that up so we're around 600 700 millivolts and it does turn off pretty quickly a slight little curve here a little bit of delay you can see a little turn on spike they're not instantaneous and I'll try did I say it's a 1 in 914 I didn't mention it before now I'll try a Schottky diode it's a signal diode as well and a kiddy came in though it's voltages definitely higher it's like 1 volt it's probably not meant for the current I'm putting through it 20-some milliamps but let's see here turn that up it's switching about the same it's maybe a little better it definitely turns on better there's not a huge turn-on spike so you can see their comparison of those two types of diodes now I'll show you another interesting aspect of the diodes here I'm going to connect them in reverse bias so they're not actually going to turn on but you still will see in effect see what's happening man that is caused by capacitance this is the one end 50 404 again there is some capacitance whenever you have differential and voltage at a very close distance you're going to have capacitance effects across that so that was the one in 50 404 let's try the fast recovery diode and not the same now the Schottky diode the SB 3 3 0 again see that it has a lot more Junction capacitance now let me try a 10 amp Schottky diode this is the to-220 case see what its capacitance looks like yeah you're seeing the charge and discharge curve of the capacitance and I think has a little bit more capacitance and it's a bigger is the beefier 10 amp diode so see what's going on there so there is more capacitance in the Schottky diodes that I'm seeing here so that can be detrimental depending on your design especially in a very high frequency switching ok that was interesting let's move on to the forward voltage drop test okay so now I'll use my power supply and current limit mode and I'll test that different current intervals of the voltage drop across the diode so at 100 milliamps across the one in 50 404 getting 0.72 volts and I'll go through several different current levels and they'll test all of these diodes I'm not going to sit here and do it all on camera but a log in the data and come back with the results okay here's the results for the one in 54 oh for ordinary silicon diode and started at 720 millivolts forward voltage drop at 100 milliamps and I went to 200 500 1 amp to lamp and 3 you can see it started to rise but then it started to drop again the reason for that is it gets really warm at higher currents and higher currents the forward voltage drop actually starts to decrease again because of the heat okay so now I switched over to the SB 330 and you'll see how that performs only 293 millivolts so you can see how much lower the forward voltage drop is so I'll log that in and continue testing and here is the results of the SB three-30 notice the much lower voltage drop of the forward voltage I did notice it was a lot more temperature sensitive I had to let it stabilize as it heats up the forward voltage did drop but you can see across the voltage range it did change quite a bit but at its maximum level it's still quite a bit less than the silicon diode you might say well this is a 400 volt diode versus a 30 volt Schottky diode is that a fair test well if I had a 50 full of this I would get similar results I just don't have a 50 volt version on hand so I had to use this diode okay so anyway I'm going to continue testing all these other diodes and come back with results and here is the forward voltage drop test results as you can see here all the Schottky diodes had a lower forward voltage drop than the standard silicon diode by a pretty significant amount now of all these Schottky diodes the STP s5l 60 60 volt 5 amp diode had the lowest forward voltage drop of all of them in fact you see here here's a 100 volt 10 amp Schottky diode at 3 amps it was 520 millivolts or as the SCP s5l 60 was only 372 and it's better than the other ones of course this one's only a one amp diet so I didn't test it beyond one amp and yes this little STMicroelectronics diode the STP s5l 60 is my favorite diode it has a very low forward voltage drop so it's going to provide the lowest losses of all these others if you're using it as a blocking diode for example for like a solar panel one thing you notice about Schottky diodes is their blocking voltage rating is pretty low I mean you have twenty volts thirty sixty forty five a hundred and there's a reason for that to get this very low figure of the forward voltage drop you have to select a blocking voltage that's pretty low and you have to be careful when you're designing your circuit that you know the maximum voltage that the diode blocks doesn't exceed its rating or you could possibly damage it now in recent years they've come up with higher voltage Schottky diodes I guess they're based on silicon carbide and they can operate up above a thousand volts however their forward voltage drop is not as good as these lower voltage ones but they still might be better than a standard silicon diode in fact they are pretty close actually it's just mainly the fast switching speed benefits you get from both so now I'm doing a reverse bias leak test to see how much current these leak first we'll start with the one in 50 404 and I'm passing the current through a green LED apologies I do have to hold hand hold the camera but nothing no leakage at all okay now I have the one end fifty-eight seventeen and you can see the LED is glowing and it's passing about 2.6 micro amps not a lot but yeah there is some reverse conduction the voltage is twelve point six one minus the LED drop around ten volts or so across the diode okay we'll try one more this is the STP s5l 60 and you can see the LED is glowing brighter because it's passing about 12 micro amps now let's see what happens I have this resistor here it's has five volts across it and it's 25 ohms so you get exactly one watt dissipation and you can still touch it it's not real hot so I'm going to put this diode on there and watch how the current increases okay so I let it go a minute and it's more than tripled it's leakage current just from the heat and it's still nothing serious now 40 micro amps but in some situations where the diode can get really hot it the reverse conduction can become significant and thermally runaway actually but you know the diode would have to get really hot to do that it'd be a poor thermal design for that to happen Schottky diodes are commonly used in power supplies and switch mode regulators they utilize the energy in the collapsing magnetic field of the coil and they're often called fly back freewheeling or even efficiency diode the designer of the circuit has to weigh all the properties you know the forward voltage drop the switching speed and the extra capacitance to see if it's the best type of diode to use in their circuit in this example here this is a little switch mode regulator it has an SS 54 flyback diode right here and it turns out to be a Schottky diode so there you have it little delve into Schottky diodes appreciate everybody watching and we'll see you next time

Channel: JohnAudioTech

Views: 43,814

Rating: 4.9208522 out of 5

Keywords: schottky diode, diode operation, forward voltage, reverse leakage

Id: kLX4ZFzzFDg

Length: 21min 49sec (1309 seconds)

Published: Thu Aug 03 2017