Let's continue our discussion of switch applications to talk about voltage bidirectional two quadrant switches, and four quadrant switches. So, a, another possible function a switch must perform is the ability to block both polarities of voltage when the switch is turned off. And so this is a voltage-bidirectional function for a switch. We can build a voltage bidirectional switch by putting two single quadrant switches in series, one of which blocks positive voltage when the switches are off, and the other which blocks negative voltage. So here is a, one well known way to do it, in which we put a diode and a transistor in series. So, if the voltage is negative and we want the switch to be turned off. So if we have negative to positive. In this case, the transistor isn't designed to be able to block any significant amount of negative voltage. However, the diode in series will be reversed biased. When the voltage is in this direction and so the diode will turn off. And even if the transistor breaks down from having too much voltage across it the diode will still be off. And it will prevent any current from flowing. So effectively the diode performs the function of blocking the voltage when the voltage is in this polarity. With positive voltage across the switch network, we can block this voltage by turning off the transistor, and then it operates in the normal fashion when it's turned off to block the voltage. So the diode, in this case, will be forward biased, but still no current will flow because the transistor will block. On the other hand, as far as conducting current, you can see that the switch really only is designed to work with positive current, when it's on. If the current reverses polarity, of course the diode will become reverse biased and turn off, and with the reverse polarity, the transistor in general isn't designed to conduct reverse current, either. So, we can only have positive current, and so this is a two-quadrant switch in which we can conduct either polarity, I mean conduct positive current only, but block either polarity of voltage. Another well known device that can function in this way is the silicon controlled rectifier, or scr, which is able to block reverse voltage, or positive voltage if it's not triggered. But it can conduct current in only one direction So then here's our ideal switch,uh, the IV curve of the composite connection of semiconductors is this where we can be anywhere on the horizontal axis when the transistor is off and we can be on the positive vertical axis when the transistor is on. So, this, switch will work in, quadrant one and in quadrant two. And, of course, if you would, if you need a voltage bi-directional two quadrant switch for quadrants three and four. You just need to connect the, the transistor and diode in the opposite direction. to get it to conduct current in the other direction. there are examples of the need for, voltage by directional switches. There aren't as, as varied as the current by directional case. Here is one, but is, it's a, DC to three phase AC inverter that is based on the buck-boost converter. And the left-hand side looks like a buck-boost converter, where the inductor is connected to vg during the first interval by turning this transistor on. during what we normally call the D prime interval with the DCDC version of a converter, here it's actually divided into multiple intervals. And what you do is, you take the energy that was stored in the inductor during the first interval, and you release it to the output by connecting different combinations of these switches or turning them on. So, for example, if you turn this one on and this one on, then the inductor current will flow this way, it'll charge this capacitor, and flow that way. [COUGH] By controlling the relative amounts of time that the, these different switches are on on the sec, the output side, we can control the amount of current that we send to each of the three phases, and synthesize output sinusoidal wave forms in that manner. I'm not going to go into that in great detail, it can get complicated. but basically we can build a DC to three phase AC inverter. That, that synthesizes output side current wave forms that are sinusoidal. A, perhaps more well-known converter, that is similar to this one, is called the current source inverter, and, we can view that one as being derived from the boost converter. So basically instead of this part of the circuit, we replace it simply with a DC source and an inductor. And connect it here this, this inductor has a, a current with small ripple. and that current is again, switched to the different three phase outputs to synthesize output's sinusoidal current wave forms. So, that's based on the boost, and it's called the current source inverter. It's a well-known inverter circuit, and it requires these voltage bi-directional switches as well. [COUGH] the re, reason we need voltage-bidirectional switches is that these switches have to block the output line to line voltages and since the output voltages are AC, they have to block either polarity of voltage. On the other hand if power only flows in one direction, then the inductor current is always of one polarity. And that current is what the switches have to conduct. so a uni-directional current flow types which is appropriate in that case. Okay, the most general kind of switch is the four quadrant switch. And here we have a switch that's capable of blocking either polarity of voltage in conducting either polarity of current so it can work at all four currents, in all four quadrants. this essentially has to be a, an active switch that is controlled by a control terminal. There are some exceptions, but in the general case it is active. And you have to be careful what you wish for, because it can be complicated control, to control the switch, to turn on and off at exactly the right time. And we don't have a diode that will turn on or off when, in coordination with some other switch in the circuit. here are some ways to realize a four quadrant switch. this one, this first one involves taking two voltage byte directional switches that are two-quadrant and putting them in parallel. So each of these switches here can block either polarity of voltage, and the, the left side switch conducts positive current and the right side switch conducts negative current. So, together, they can conduct either polarity of current. The the second case here, involves two current bidirectional switches that are put in series, so each of these can conduct either polarity of current. But the top one can only block positive voltage, while the bottom one can only block negative voltage. But when you put them in series they can block either polarity of voltage. And then the third case is not derived from any of the previous configurations. basically, these diodes connect the transistor up in either direction as appropriate to conduct current in the right direction. And when the transistor's off, the, the switch network will block either polarity of voltage. The most commonly used version here, when we need four quadrant switches, is the first one. And the real reason for that has to do with how we coordinate the switching. Turning off one four quadrant, switching, turning on the next one. it's easier to coordinate that switching with this network than with the other two. But that's beyond the scope of what we're going to talk about. But just to give you an example, here is a well known converter that uses four quadrant switches. This is called a matrix converter, and it converts three phase AC into three phase AC of a different frequency and voltage. and basically it takes nine four quadrant switches. And these switches are able to produce output voltage wave forms. These, these three nodes here that are different combinations of the input voltage, the three phase input voltages. And by pulse with modulating, are quickly switching these devices. we can actually make the average values of these output voltage wave forms, be controlled. And, and so we can actually synthesize, switched wave form here whose low frequency components have a different frequency and, and amplitude than the input. these require four quadrant switches because the switches have to block the line to line AC input voltage, which is AC. And they have to conduct the three phase AC output current, which is also AC. So we need the most general type of four quadrant switch. This is a pretty interesting converter, but it takes some extended time to understand how to control these switches and synthesize the wave forms, and it can, it can get pretty complicated. Okay, I want to talk also just for a minute about synchronous rectifiers. [COUGH] we talked about the idea that a MOSFET is inherently a current bidirectional switch and this property can be used in what is called a synchronous rectifier application where we replace a diode with a MOSFET. and take advantage of this, reverse current capability. So, if we need a single quadrant switch that turns out to be a diode type characteristic, what we can do is, take the diode in this polarity and replace it with a MOSFET here in which the current goes, what is normally considered backwards, from source to drain instead of from drain to source. so with the MOSFET connected backwards it's ID instantaneous characteristic looks like this. Basically I've just flipped the voltage axis because in fact I flipped both axes because we've connected the, the MOSFET up backwards. And what we do is we take advantage of this part, and this part of the instantaneous IV characteristic to get a characteristic that works like a diode. Now to get this to happen, you have to have some control circuit that drives the gate as needed to make the MOSFET turn on and off, when the diode would have turned on and off. But, we can build a circuit to do that [COUGH] and get the MOSFET to work like a diode. Now, why would you want to do that? Well the celebrated application of this that is the original application I'm aware of was in low voltage computer power supplies and the trend for decades in computer power supplies has been that the voltage goes down and the current goes up as time passes. And we now have processor chips with gate lengths of tens of nanometers, and the, what happens when the, the gate lengths are scaled down is that the voltage of the power supply has to scale down as well. At the same time, we're increasing the number of transistors on the chip, which makes the current go up. And so, what's happened in the computer business is that we need power supplies that are a fraction of a volt in tens of amps or hundreds of amps, even, depending on the, the size of the, the, system. So how do we build a converter that can operate with high efficiency and produce such low voltages and high currents? Well, in the Chapter three, we looked at modeling these converters and what we found was that the diode forward voltage drop appeared in our model and was a source of conduction loss. And, so if we have a diode with a forward voltage drop that's, just to pick a round number, one volt, and our output voltage is less than a volt, then we're going to have a lot of power loss in the diode, and nowadays the diode power loss or its conduction loss may be greater than the power that goes to the load. So we don't have a high efficiency system and the four voltage drop is something that is hard to scale. We can just buy a larger diode chip and get the voltage to go down. There are shocky diodes, mobile lead shocky diodes may have a lower drop of say, 0.4 volts instead of 0.7 or one volt, but even then, we, we want to get the, the voltage drop of our switches to be below a 10th of a volt to get good efficiency. So the solution is to put a MOSFET here where the forward voltage drop is dependent simply on the on resistance and the current. And so, if you buy a bigger MOSFET, you, you can get a lower on resistance. And if you're willing to pay, you can buy as big a, of a MOSFET as you want. And get as low of an on resistance as you want and make the forward voltage drop be low so that you can get high efficiency and so that's what's done. So we have our main MOSFET that is the normal MOSFET of the buck converter here and then we replace the diode of the buck converter with a synchronous rectifier. And we basically drive the synchronous rectifier with the gate drive signal that is the compliment of the gate drive signal of the main FET, so that when the main fet turns off, the, the synchronous FET turns on, and vice versa. There's actually a little bit of dead time there, where we make sure that we don't have the case where both FETs are on momentarily at the same time, and shorting out VG. [COUGH] so this is useful in low voltage, high current applications, and we find nowadays that even at applications of, tens of volts, you know, of below 100 volts. I would say, the synchronous rectifier will lead to higher efficiency. And so we find it used quite a bit in applications that below a 100 volts where we're concerned with the efficiency.
