For the next several lectures, we're going to discuss power semiconductor devices. So we'll cover some of the most widely used devices currently in power electronics. And specifically power diodes, Power MOSFETs and the Insulated Gate Bipolar Transistor. Which is the evolution of the Bipolar Junction Transistors. some of the things we're going to talk about. first of all we're going to talk about this basic trade off that's important to power electronics. A trade off between the on resistance or forward voltage drop of the device versus it's breakdown voltage versus it's switching times. And these different devices have different trade offs between these three figures of merit. we l'll also talk about minority carrier versus majority carrier devices. And how they lead to different trade offs. And then, the other thing we really want to understand is from a power electronics standpoint is switching times of these devices. Where the switching times come from, how they work. And how they lead to switching loss. Which is one of the important loss mechanisms in power converters that we haven't yet modeled. And so another objective here is going to be to extend the equivalent circuit models. Of chapter three to include switch and loss. And we'll talk about how to do that as well. Now I don't want this to become a course in semiconductor device physics. the assumption here is that you have been introduced to semiconductor devices in an electronics class. My intention is to perhaps remind you of some of the introductory basics and then discuss at a high level. the important things that happen in these devices from a power electronic standpoint. So we want to build power switches effectively out of semiconductor devices. And what this means is that the power switch must be able to change between the off state and on state And another way to say that is the off state is an insulating state in which the device does not conduct current. And the on state is a conducting state where we have mobile charges that can conduct current. And we need to be able to switch between the two. So here's a diagram of a some kind of device where we have contacts on the two sides that are say, metal contacts to the outside world, these blue regions. And the clear region in the middle is our switching device. to be an insulator, what that really means is that there are no mobile charges inside this insulator that can move and conduct current between the two contacts. So if we connect the voltage across this device there will be no current because we have it insulating element in the path. Conversely in a conductor, we have a We have mobile charges in the conducting material that can easily jump from one atom to the next and conduct current. So for example the cla, in the classic metal material The electrons are thermally excited to higher energy states where they are easily able to Jump from one atom to the next and conduct current. So, in that case if we place a voltage across the conductor then what we see is that the we'll get a current to conduct. So, here the plus charges are Or really the charges in the the nuclei of the atoms. And the minus charges are electrons that are mobile and easily able to jump from one atom to the next. So, the plus charges stay fixed and the minus charges move. Now in this in this drawing, another thing that we can say is that there are no big fields coming out of the conductor The number of plus charges and minus charges is the same. So that overall the conductor is charged neutral. We simply have charges hopping from one. item to the next and every time an electron leaves the left side another comes in on the right side so that we're charged neutral. so if we say Q minus is the total amount of mobile electro, electron charge or the charge contained by all the minuses in this volume. And we have what we call a transit time. Which is the time it takes your average electron to, to get from one end to the other of this conducting device. Then we can say that the total current, the average current. Is the total amount of charge divided by the time it takes to move across the, the material. Okay. also again since the fixed positive charge is equal in magnitude to the fixed negative charge. Another way to say that is that the current is equal to the plus charge divided by the transit time. So these are basic ideas that we call charge control ideas of, of how a switch will conduct. Another thing to say about the conductor in a semi-conductor material the basic pure silicone is is mostly an insulator. However, we can dope the silicon with impurities in the crystal lattice of the silicon, that provide charges that can be, become mobile. So by doping the silicon to make it n-type silicon there are extra electrons in the lattice of the silicon. And at room temperature they're, they're in a thermally excited states where they are able to easily jump from one atom to the next and conduct electricity. So it's similar in that case to the conductor. And in p dope silicon we actually conduct the silicon with impurities in which there are, there is a deficit of electrons. And actually we have what are called holes that are essentially an absence of an electron that can also jump from one atom to the next. And effectively it behaves as if there's a plus charge that we call a hole that can conduct current. another case, yet another case is the minority carrier. Where if we have some in dope silicon next to say p dope silicon the, in the in dope silicon the electrons can diffuse into the p dope silicon. Where they are electrons at a high energy state in the p dope silicon and these are called minority carrier electrons. And they can also conduct current likewise with we can have minority carrier holes. Okay lets discuss the MOSFET now. In MOSFET we have a Source and a Drain that are dope silicon. And there are conducts to the, the source and the drain to the outside world. We have a channel in between the source and drain. That is where we can turn the device on and off by putting charge in and out of the channel to provide a conducting path between the source and the drain. The gate in a power MOSFET is polysilicon. It's basically a conducting material. that is a control terminal at the MOSFET. And separating the gate from the channel is what we call an Oxide layer. It's Silicon dioxide that is glass. And it is an insulator. And so basically the Gate and Gate oxide built on top of the Channel form a capacitor. And so the gate and the channel are the two plates of the capacitor and the oxide layer is the dielectric of the capacitor. So right now I've illustrated the MOSFET in the off state. There is no charge in the channel to conduct current between the source and the drain. So if we apply a voltage between the drain and the source, this is an N-channel MOSFET I've, I've shown So when we put positive voltage on the drain with respect to the source, we get no drain current because the channel is turned off. Okay, to turn the MOSFET on, we apply positive voltage to the gate. And effectively we charge up the capacitance between the gate and the channel. So what's happening here is you can see the plus charges actually come in from the gate. Minus charges come in from the source. And charges up this capacitance of the gate to the channel. And the minus charges now are in the channel, inhabit the channel and they're able to conduct between the source and the drain. [COUGH] Okay. What I should, should mention that what, what you just saw with the charges coming is was the turn on switching time. The time it takes the gate driver to supply this charge through the gate And establish charge in the channel is the turn on switching time. and during that time we have a current coming in from the gate driver, a gate current that is supplying this charge. So, it's supplying the plus charges to the gate. And we get minus charges equal number of minus charges from the source. And there's a current then that is the, the change equal to the change in charge on this capacitance. Okay. Once the device is turned on, we don't need to supply any more current. We have a capacitor, it's turned on, and if its voltage doesn't change any further. Then we don't need to have any gate current and the gate current can go to 0. So now with the capacitor charged up. These electrons can conduct current between the source and the drain. So with a voltage applied to the drain, you'll get electrons flowing between source and drain. And the drain current then we can say is equal to the total amount of minus charge in the channel. Divided by the Transit time. Which is the time it takes an average charge to, to move across the channel. Okay. since the plus charge on the gait equals the minus charge in the channel. Another way to write the drain current is that it is equal to the plus charge on the gait divided by the transit time. So the drain current is effectively controlled by the charge that we put on the control terminal, or on the gate. 'Kay, now let's turn it off, so here I've shorted out the gate. To, to the source which shorts out the capacitance and it shorts the charge back out of the gate So that was the turn off switching time that you just saw and the turn off switching transition. so we have gate current now that is extracting the, the charge, the plus charges out of the gate. So, it's a negative current. The channel charges went to the drain, and the drain current continued to conduct until all of the charge was gone. And after that, then the drain current goes to 0. So the turn-off switching time is the time that it takes to remove all of this charge. Okay. The bipolar junction transistor has a similar-looking structure with an emitter and a collector. That are in, for an npn transistor, they're n dope silicon, there's connected through contacts to the outside world. And instead of a channel we have a base region that is p type silicon. There's a contact there, that's the control terminal and the base terminal of the BJT. Here, effectively, the base emitter junction which is a diode junction is controlled by the base terminal. And in this way, we can control the charge and the base and control the current. So here there is no charge in the base and BJT is initially off. We apply a current to the collector and nothing happens. We're in the off state. Okay, to turn the BJT on we forward bias the base emitter junction. So we apply a positive voltage to the base that is sufficient to forward bias this diode junction. So say plus 0.7 volts or so. When we do that electrons from the emitter, the n region of the emitter will diffuse into the base and become minority carriers. likewise positive charges or majority carrier holes will come in from the external base driver circuit can come into the base region. Okay, so during this turn on switching time of the BJT. We have positive current flowing into the base that is supplying the holes in the base region. we likewise have equal number of electrons diffusing across the, the diode junction to become minority carrier electrons in the base. Okay, so we now have our electrons in the base and they're able to conduct current between the collector and the emitter. [COUGH] what I should say though is once we have supplied these minority carriers. The first order, we don't need any more base current. Just like the MOSFET, we've supplied the charge to the base to provide conducting er, carriers that can conduct current. And so the first order the base order can go to 0 at this point. The second order, we have an effect with minority carriers called recombination. where these electrons are in an a higher energy state in the base and they can actually decay out of this energy state. and that's called recombination. So effectively when they decay, the minority carrier electron combines with a majority carrier hole. And effectively the charges go away. So if we want to maintain the, our minority carriers in the base region, we have to supply base current. To basically supply more carriers to and counteract the recombination. So, the second order, we can say that we need a current supplied from the base that is equal to the amount of charge divided by the lifetime. Where the lifetime is the average time that it takes minority carrier to decay and go away. So, this is a maintenance current but it's smaller than the current that we initially had to turn the device on. Okay. Having said that, if we apply a voltage to the collector now, then the BJT will conduct. And so we have our electrons that are able to conduct a, across the base into the collector and provide current. So just as in the previous examples the, the current that we get is equal to the total amount of electron or minus charge in the base. Divided by the transit time which is the time it takes to get across the base. And since the minority carrier electrons are equal in magnitude to the majority carrier holes, We can say that there's that, that the current is equal to the. Total amount of holes or plus charge that came in from the base terminal and is still there in the base divided by the transit time. Okay. Let's turn the BJT off, now. So, to turn the BJT off, one thing we can do is. Simply remove the base current and wait for the electrons to go away or recombine. But that may take a long time and lead to a long switching time. So a better way to do it and I'll show it to you again. Is to reverse the direction of voltage on the base and actively remove the charge. So if we actively pull the poles or these plus charges out of these we can turn the bipolar transistor off faster. So here we put a negative voltage on the base and actively remove the charge. [COUGH] And that was the turn off switching time or the time it took to remove the charge between the base and collector. Again, this is a charge control device like the other devices. By pulling the plus charges out of the base we can get the minus charges to go away also. Aand then the device stops conducting. So, now we're back into the off state or the insulating state. Where we have no minority charge in the base available to conduct current between the collector and emitter. So, I've introduced the notions that we have a charge-controlled powered semiconductor devices. And in the time that it takes to insert or remove the charge necessary to make the device conduct or insulate is a switching time so it takes time to do that. Now switching time leads to switching loss because during these switching transitions the semi-conductor devices. Are not all the way on or all the way off. But they actually have voltage in across them and current through them that leads to high instantaneous power. So let's look at that. Here is a, a first attempt at explaining switching loss. And we're going to refine this several times in the upcoming lectures. But this is a good starting point. So lets suppose we have a buck converter built here with a transistor that has some switching times and we have an ideal diode, okay? Now our real diode is not ideal. But after we talk about diodes we'll talk about how the diodes effect the switching time. But for now let's call it ideal. And we have a transistor that has, is driven by some gate driver circuit. And it takes time for the gate driver to turn the transistor on and off. So here's an example of the turn-off transition of, of the MOSFET. So we start out with the MOSFET on, right here. Okay. So by being on the voltage across the MOSFET is essentially 0. And the current through the MOSFET is the inductor current, iL. Okay. What, during this time, what's happening on the diode, is that the diode is off, it is reverse biased. And the diode voltage is minus Vg, like this. And the diode current is 0 which is shown here. Okay. The bottom wave form here is the instantaneous power in the transistor or in the MOSFET. So it's the product of the MOSFET voltage and current. I'm not modeling forward voltage drops or conduction losses at the MOSFET right here. So initially with the MOSFET all the way on we have 0 power loss in the MOSFET. Okay, at this point right here. The gate driver starts to turn off the MOSFET. Okay, so it starts, when the gate driver voltage goes low or goes negative. And it starts to pull charge out of the gate of the MOSFET, out of its capacitance. And discharge the gate capacitance to to remove charge from the MOSFET. And that takes some time. The gate driver has some Thevenin equivalent impedance. So there's some resistance. We get RC circuits effectively and it takes time to discharge the gate. Okay. So, we start to turn off the MOSFET. Now the thing about turning off the MOSFET here, is that the current in the MOSFET can't change until the diode turns on. If you look at this, this is called a clamped inductive load. we have an inductor that's large and has small ripples, it's current doesn't change very quickly. And the current, the inductor is connected to this node, and its current has to come somewhere. And if the diode is off, the only place that it can come from is the mosh pit. So what happens, is that initially the voltage on the mosh pit rises like this. The the gate driver is actually discharging the gate to drain capacitance of the MOSFET during this time. And we'll talk about that more in a upcoming lecture. but as long as the voltage across the MOSFET is less then Vg, the diode is reverse biased. Because the diode voltage Vg is the MOSFET voltage minus Vg. And so the diode voltage is starting out at minus Vg. [COUGH] And as the MOSFET voltage increases, the diode voltage increases. And finally at this point, where the MOSFET voltage equals Vg. We finally get, the, the diode voltage starts to exceed 0 volts and the diode can turn on. So that's what happens right here. Okay, so, the diode starts to turn on right there. At that point, the current from the inductor can shift from flowing through the MOSFET to flowing through the diode. And during this time, the MOSFET current will fall. And how fast it, it falls depends on how fast the gate driver can remove charge from the channel. And the gate capacitance to the MOSFET, so the gate to source capacitance. And so, that takes some time. And, during that time, then the diode current increases and the MOSFET current decreases. Finally, right here, the MOSFET is all the way off with no current and blocking the full input voltage Vg. And so after that, we have zero current in the MOSFET and the diode conducts all of the current. Okay. So, here is a plot of the instantaneous power in the MOSFET. And, at this point, it starts to rise above zero because we have a voltage across the MOSFET and current through the MOSFET. And it reaches a peak right here. Where the MOSFET voltage is maximum and equal to Vg. And the MOSFET current is still equal to the inductor current. So we get a peak, instantaneous power equal to Vg times the output current or the inductor current. Now, that's a considerable amount of power in the MOSFET. After that the current goes down again until at this point the power gets to 0. so we have some high instantaneous power loss. And in fact the energy that's lost is the integral of the power. So if you find the area under this curve, you get the energy in joules that is lost during the turn off switching transition. As drawn here with straight lines that are idealized, we could say that that energy is the area of this triangle. So it's 1 2 the base times the height of the triangle. The height is the Vg times the inductor current. Here's the 1 2 and base is the switching times. The total time from here to here. Okay? So every time we turn the MOSFET off, we get that much switching loss. Now, we multiply that by the number of times per second that we do this and we get an average power loss. we also can apply the same arguments at the other transition where we turn on the transistor. And really, the same thing happens in reverse and we get a similar kind expression for an energy lost during the turn-on transition. So we get a total switching loss according to this model. That's equal to the switching frequency times the energy lost during the turn off transition plus the energy lost during the turn on transition. Okay? This can be considerable. In fact, often this is larger than all the losses combined from conduction and forward voltage drops that we model in chapter three. In many converters, this is the large, the largest loss in the converter. So we have to account for this. And one of the things we're going to do soon is refine our models from the previous chapter to include terms that account for the switching loss. The other thing I'd say is that this is highly idealized. And, in fact, it's not really right. It's a good start but it doesn't account for what happens in the real devices. So, back when I was a graduate student this was what everybody taught. And it led us all to try to do things to reduce switching loss and improve efficiency. And what we did was develop what were called zero current switching converters that would, change the order of this switching so that the current would go to zero first. And then the bullet would rise and you would think the loss would go away. But people built those converters and found that they got the same exact efficiency and the same switching loss as before. So, in fact, while this is a good start. If, we we need to refine the models of the semiconductor devices to understand what's, what the switching times are really happening. What's going on and how to refine this model to correctly predict the actual switching loss. And so we're going to do that in the next several lectures.
