We'll conclude our discussion of power diodes with several additional important topics. The first one is the trade off between the blocking voltage or, or breakdown voltage of the diode versus the forward voltage drop versus the switching speed. And this is a trade off that is inherent in any of our powered semiconductor devices. but it's interesting to see how it works in the, the p-n junction type diode. In any semiconductor device the breakdown voltage is related to the doping concentration of the silicon mat, material. Basically silicon with heavy doping has a low on resistance. But it also breaks down at a lower electric field strength than if there was light doping. This is a fundamental characteristic of avalanche breakdown in semiconductor materials. And so in a diode, we have to include a low doping concentration, high resistivity region in order to achieve a specified voltage break down. Here's a diagram that's then, a more accurate description of the diode which, where we have the p region on the left, the n region on the right and a lightly doped region in the center. Here, it's denoted in n-minus, which means low concentration of doping. Often, this is also called i for intrinsic. And this is called a p-i-n diode. so we have this low doping concentration in the middle of the diode. And under reverse bias constant conditions the depletion region extends far out into this n minus region. with electric fields that the, that are high in strength that the light or low doping concentration material can withstand. And we get our voltage breakdown from that. Now the worry is that by having such a large region that is high resistivity, this will increase the on resistance or the forward voltage drop of the device. In a, a minority carrier device though, such as the pn diode that doesn't happen. And instead what happens to the phenomena known as conductivity modulation. Basically, the holes from the p region can diffuse into the lightly doped region where they become minority carriers and they actually reduce the effective resistivity of the material. They provide charges that are able to conduct current, even though the, the doping concentration is low. And in fact, electrons from the end region can diffuse into this n minus region as well, as well, and so we get, [COUGH] a, a large concentration of minority carriers in the lightly doped region that gives us a low on resistance. So, the good news is, we can get a high voltage breakdown with a low on-resistance at the same time. But the bad news is, we have a lot more stored minority charge in the n-minus region. And therefore the switching time is longer because it takes longer to remove all of that charge to turn the device off. So then here are what wave farms we actually observe in a real para diode. So we start out with a diode in the off state, blocking negative voltage and conducting no current. With this large depletion region that is blocking the voltage. We then try to turn on the, the diode so the, whatever circuit is connected to the diode will apply positive current to the diode, and raise the voltage across the diode. Initially this positive current charges the capacitance of the depletion region and raises the voltage at some rate. Eventually the voltage on the diode is positive and the diode turns on and starts to conduct current. We may or may not observe some voltage overshoot in, as shown here. What's happening here is that if we turn the, the diode on quickly, it's possible that the we, we initially observe the voltage of the. lightly doped region without conductivity modulation. So we have a high on resistance then and we have a large forward drop that might be 10 or 20 volts. But then after a short period, we get minority charge and conductivity modulation that brings the forward drop back down to something like. 7 10ths or 1 volt, and then the diode after that conducts with the low on resistance. When it's time to turn off the diode the external circuit attempts to reverse polarity or reverse the current through the diode. And as we discussed in the previous several lectures. We get a reverse recovery transient with current flowing backwards through the diode to extract the minority charge. And the only difference here now is that we have a lot more minority charge from the conductivity modulation of the, the lightly doped region. So we observe the reverse recovery, which involves removing the recovered charge. And also while the voltage is changing, we're also charging the capacitance of the depletion region. So [UNKNOWN] as we've already discussed then this re, rev, reverse recovery can induce substantial paralize in the mosfet and during the last part of, of the reverse recovery in the diode as well. but at a minimum we see this large reverse recovery current causing a current spike in the mosfet. While the mosfet is held on with high, with voltage across it. and so we get large instantaneous power loss induced in the mosfet by the diode reverse recovery. Okay, here is the little terminology and it's traditional in the power semi-conductor field to give names to diodes according to their application. A standard recovery diode typically is one that is intended for low volt, or low frequency applications such as 60 cycle or 50 cycle rectifiers. Often these, for these diodes the reverse recovery time is not specified. But they wont work in a, 100 kilohertz switching converter because they're reverse recovery times are too long. Fast recovery or, ultra-fast recovery, or more recently, other adjatives like super or hyper fast. refer to diodes where the reverse recovery time is specified and they are intended for switching converter applications. So these will have reverse recovery times for fast recovery diodes that might be between 100 nanoseconds and a microsecond. Or ultrafast might be a hundred nanoseconds or below. And nowadays we can get, 600 volt pn junction rectifiers with recovery times as low as 25 or 35 nanoseconds. a Schottky diode we have to mention also. Schot, a Schottky diode has a metal semi-conductor junction instead of a pn junction. It is inherently a majority carrier device. Does, does not work on minority carriers as we've been discussing so far. as a result the Schottky diode for silicon rectifiers doesn't have conductivity modulation, nor does it have long reverse recovery times. So that's good news and bad news also. Schottky diodes are limited, for silicon devices, to low voltage applications. Generally a hundred volts and below, and they're very good at say, 45 volts and below. they switch very fast, although, they do have significant junction capacitance. that, that can affect the circuit and cause switching loss. Recently, we have Schottky diodes built with wide band gap semi-conductors such as silicon carbide or even more recently gallium nitride. these wide band gap devices can attain high high breakdown voltages of 600 or 1200 volts. in a shocky diode and still have a majority carrier device. They have larger forward drops. Or maybe one and a half or two volts instead of something a little below one volt. However they're very fast switching, and very often in, in a power converter you will find that if you add a, or if you replace your pn diode with a, a schottky barrier silicon carbide diode. Even though the conduction loss is higher the switching loss is much lower, and overall the efficiency is reduced. so these have found there, there are commercial devices that have found some good industry acceptance. The major problem today with them is that they, is the high cost. A silicon carbide 600 volt diode might cost five or 10 times as much. As a comparable silicon diode, but there definitely is a measurable reduction in, in switching loss. Okay, let's discuss paralleling diodes. With this minority carrier, or pn junction diode classically we. We're not supposed to parallel diodes. So for example, suppose we needed a10 amp diode, but all we were able to buy, or all we had in the lab were 5 amp diodes. So you might say, well I'll, I'll take 2, 5 amp diodes and put them in parallel. And in the process we're hoping that the 10 amps will divide evenly between the two diodes so we get 5 amps through each diode. Generally this doesn't work. And the reason for this is the temperature dependence of the equilibrium ID characteristic of the diode. Basically, if when a diode gets hotter, its voltage goes down for the same current or conversely for the same voltage, the current would go up, and this in a minority carrier, say, pn junction diode. So here's what happens, you, you put the two diodes in parallel, we apply 10 amps. The, there's some small difference between the diodes which makes one of the diodes take a little more current than the other and it gets a little hotter. And so say that this diode one is hotter, runs a little hotter than diode 2, that makes it take more current. So instead of 5 amps it might take 6 amps. And diode two then will take 4 amps. Okay? That makes the first diode get hotter yet, the second diode get cooler. And even more, the current goes through the first diode and less through the second. And so we have thermo instability where one diode hogs all the current. And what will happen is, that diode will hog the current. It will overheat and it will in fact exceed it's rating. And so it will fail and you know once it fails, if fails as on open circuit and that makes all 10 amps go through the other diode and it fails also. So, in general with minority carrier devices we aren't able to parallel. And with majority carrier devices it the forward drop goes up as the temperature goes up and it may be possible to parallel. So if you want to get minority carrier diodes to, to share current. We have to do what I would consider heroic measures. So we can sort devices and select match devices. We can package them on a common thermal substrate so they run at the same temperature. and in fact if you want to build a high current module of diodes that's exactly what's done. Or we could try to build some ex, external circuitry that forces the currents to balance. Usually that circuitry takes up a lot of work to make work and generally what we would rather do is find a, a large enough diode and just use one. One last short topic. We talked about. How diode reverse recovery induces switching loss in the mosfet in a, say a buck converter. Sometimes we have circuits where the diodes is not directly connected to a transistor. And here's an example of one, this often happens in transformer isolated converters. Or in some other converter circuits that have effectively an inductor that provides current to the diode. So in this, this example I'm going to model the rest of the converter circuit with some switched voltage source that's shown here. It's initially a positive voltage and then at some time it switches to a negative voltage. And what, what's shown here is an inductor. And then a diode with it's junction capacitance, here as seen, it's shown explicitly, this is the capacitance of the depletion region. so initially we have positive voltage applied, that positive voltage makes positive current flow to the inductor, and that current flows through the dial. Turning the diode on, okay? So, the diode is on, and basically this positive input voltage V1 appears across the inductor, and the inductor current increases with the slope given by the voltage divided by the inductants, and rises to some positive current. And at this time where we switch the input, this voltage negative. at this point. So we had negative voltage applied across the inductor. The inductor current is still positive initially so, this positive current continues to flow through the diode, which keeps it on. And so the input voltage appears across the inductor. And the inductor current. Then ramps it down from a negative applied voltage, like this. Eventually it reaches zero, and what happens then? Well, just because the inductor current is zero doesn't mean the diode turns off. We have to first remove its stored charge and that takes some time. So during the reverse recovery time of the diode, the diode stays on. It maintains its same forward voltage drop. And so we have the same negative voltage across the inductor, which continues to charge the inductor current negative. Okay? So we get some negative current in the inductor. And then at some point right here. Finally the diode turns off because it's, it's stored charge has been removed by the negative current and at that point the diode switches off. And what happens to the inducter current? We built up some negative current in the inductor, we're storing energy in the inductor. One half LI squared, and the inductor current has to go somewhere. Well, the only place it can go is through the capacitance now. So the junction capacitance of the reverse-biased diode will, will form a resonance circuit in connection with the inductor. That resonance circuit has some initial condition on current, and so we get the inductor and capacitor forming a resonance circuit that rings like this. And we get diode voltage that rings as well. Well, eventually that ringing decays to zero. By parasitic loses in the circuit. So we have resistance of the inductor winding, we have coral loss and other losses around the circuit and something damps out the ringing and makes it the K to 0. Whatever that is, whatever damps it, dissipates the power and so it will get hot. And this is in fact a form of switching loss. The diode induces energy storage in the inductor which is then lost after the diode turns off. Okay. However much energy loss that is, I'm going to call it W off right now, is the energy lost during the diode turn off train, or stored in the energy during the diode turn off transition and then lost. That energy multiplied by the switching frequency is another form of switching loss. And so in our power converters, when we see ringing like this that decays, that ringing is an additional component of switching loss. In this particular case we can actually calculate how much energy there is. The recovered charge which is the area under the curve is defined as the integral of the current during this, this time. As usual. So then the recovered charge is something that is a property of the diode. You can also calculate the energy stored in the inductor during this interval. And basically if we integrate the voltage on the inductor, it's negative voltage, times the inductor current. That's the integral of the power which is the total energy stored in the inductor from here to here. Okay? Well, the voltage is this voltage minus V2. And the current is this current wave form. But we can, we can plug V2 into here, and since V2 is a constant, the, the integral of the power is actually just the constant V2 times the integral of the current, and the integral of the current is the recovered charge at the diode. So what we get is that the, the, during the diode reverse recovery time, energy is stored in the inductor equal to the applied voltage V2, times the diode stored charge or recovered charge. Okay? That energy becomes stored in the inductor, so it's equal to one half LI squared at this point right here at the peak of the current. And that energy rings between the inductor and the capacitor during these oscillations and eventually gets dissipated by whatever damps the ringing. So this is the amount of energy we loose every-time this diode switches off. We multiply it by the switching frequency to get the switching loss. [SOUND]. And so the diode can induce switching loss, and things the size of mosfet. this recov, reverse recovery will induce ringing that, that is another form of switching loss. [BLANK_AUDIO].