[BLANK_AUDIO] In this lecture we'll talk about current bidirectional switches. So here is a realization one way to realize a current bidirectional switch. So this is a single pole, single throw switch that is actually a transistor connected with what we call an anti-parallel diode. The combination of the two is able to conduct current in either direction. So if we have our I of T switch turned as positive, it can flow through the transistor. And if it's negative it can flow through the diode. And of course we can use a bipolar transistor here, or we could use a mossphet or IGBT or other single quadrant switch. So the combination can conduct current in either direction. however it can only block voltage in one direction. So when we want the switch to be off, if it blocks the voltage that is positive, so plus to minus in this direction, both devices can be off. But if we reverse the direction of the voltage you can see that the diode will turn on and short out the voltage source. So this combination is capable only of blocking one polarity of voltage and so it's what we call a two-quadrant switch that is current bidirectional. [COUGH] ' Kay, so formally here is the single quadrant switch with the voltage and current directions defined. The combination of transistor and diode have this composite IV characteristic. And as far as the plane of off-state voltage versus on-state current is concerned We can operate anywhere in the first quadrant or the fourth quadrant. and of course also if we want to build a current bidirectional switch operates in only the second and third quadrants, all we have to do is connect the, are transistor and diode up backwards. Just, define things going in the opposite direction and we'll get the second and third quadrant. the MOSFET, I mentioned in the last lecture is in fact a current bidirectional switch so the MOSFET channel itself can conduct current in either direction. And in addition it has this built in body diode that can conduct a negative current. And so the MOSFET by itself can be a current bidirectional two quadrant switch. I also mentioned that the body diode of practical MOSFETs may not be a very good diode. So. If we have a slow body diode we might not want to let it conduct and have to switch it off because of its slow switching time. So what one can do if you really have to is to put an external diode in series with a MOSFET that only lets current flow in the positive direction. And then put in a externally an antiparallel diode to to get a current bidirectional switch. We generally don't like to have to do this and we look, if we're in that situation we really have two other choices. One is to buy a MOS fit that has a very fast and good body diode. And especially at low voltages this is a good solution. or at high voltages the other option that we have is to use a different kind of device such as an IGBT. And nowadays you can get good IGBTs that are co-packaged with fast anti parallel diodes built in. Here's an example of two quadrant switch application. So this is a simple inverter. We have split DC input voltage. So there's, we have plus Vg coming into our converter here. This is ground or 0 volts and we have minus vg there. And net this is a DC input to the converter and then we have a load and we want to produce AC across this load. And this no De here is connected to this ground. So each of these is a two quadrant current bidirectional switch. If we succeed in producing an AC output, then the current in our load is AC and can go either direction. And that will coincide with the inductor current. So our inductor current can go either direction as well. [COUGH] That means that our switches when they're on and conducting inductor current, must be able to conduct either polarity current. So have to conduct the AC output current. On the other hand the switches block the DC input voltage. So for example, if the top switch is on and the bottom switch is off then this node gets pulled up to plus Vg and the voltage across the lower switch will be plus Vg minus minus Vg or 2Vg. But it's DC. And since our DC input voltage doesn't change polarity, we only have to block positive voltages with the switch. So the current-bidirectional two-quadrant switch will work and in fact current-bidirectional two-quadrant switches are commonly used in inverter applications. [COUGH] let's work out exactly how this converter works. So, let's sketch the voltage at the switch norm. So when the operator switch is turned on, Q1 is on, then this now gets pulled up to plus Vg. And we leave it on for some duty cycle or some first interval length. Then we switch the upper device, devices off, turn the lower devices on. And that pulls this node down to here which is minus Vg. And we'll leave them in lower position for the remainder of the switching period, for D prime TS, then we repeat. They're supposed to be straight vertical lines. Okay? So the average value of this wave form, or DC component or at least the low frequency component. Is equal to D times the voltage during the first interval, plus D prime times the voltage during the second interval. And you can work that out then, it's equal to Vg times D minus D prime. D prime is 1 minus D, so we can also write this as Vg times 2 D minus 1. [BLANK_AUDIO] Which is what's shown here. [COUGH] So this this low frequency component of this Waveform gets filtered out by the LC filter and that's what's applied across our output load. Here's a plot of that function. You can see that if D is a half, the function goes to 0. So I have 0 net output voltage at duty cycle of a half. And if you increase the duty cycle above a half we get positive voltage and when we decrease the duty cycle below a half we get negative voltage. And the overall conversion ratio function looks like this. So what you can do is you can vary the duty cycle about a half so you vary it sinusoidally like this. Okay? And if you do that, what happens to the output voltage is that the output voltage will vary sinusoidally about 0. So if you plug this function into here you can find that the output voltage is sinusoidal. So we vary the duty cycle sinusoidally if we wanted to make say a 60 hz output we could make this this modulation frequency of the duty cycle to be 60. And we would get a 60 hertz AC output. the inductor current. You know, which is the load current will also be will also vary sinusoidally in the same way. So again it's positive or negative depending on which part of the sine wave we're on and so we need current bidirectional switches. Here's a three phase version of the same circuit. Each of the three phases here is one of those circuits we just talked about. So it has our current bidirectional switches and an inductor driving one phase of the three phase output. Here's the three phase AC load. So we vary each of these phases or vary their duty cycles, sinusoidally in a three phase manner. So there're 120 and 240 degrees shifted in phase to get a three phase AC output. And with this, we can get You know, by varying all three phases in this way you get, you know, generate a three phase AC. And drive an AC load such as say, a motor, an AC motor or even build an inverter to, to connect to the utility here. So, again the switches have to block the DC input voltage. And they have to conduct the AC outlet current. So this is fundamentally a two quadrant current bidirectional application. Here's a dc-dc application of the same thing. This is basically a buck converter where our input voltage here vg is. Some bus voltage and one example is a spacecraft power bus. and in this case the load resistor is replaced with a battery. And there may be a filler capacitor here or not. But this is basically a butt converter except that what we were calling the resistive load before now is a battery that can both. Store energy or supply energy. So when we charge the battery we have power going this way. So it comes out of the source we, this actually works like a Buck converter and if you just draw transistor Q1 and Diode D2 here and ignore the other two devices. You would recognize this as a buck converter that we've already talked about. in this case, you don't have to turn Q2 on but in fact, it doesn't hurt to turn Q2 on, any time Q1 is off. So we can actually drive Q2 with a complement of the gate, of the base drive of Q1. If we want to supply energy out of the battery back to the bus we reverse the direction of current. And in that case Q2 and D1 are the ones that conduct. And again we can turn on Q1 whenever Q2 is off. And just continue to drive Q1 with the complement of the Q2 drive signal. together operated together we simply have current bidirectional switches that can conduct current in either direction depending on whether we are charging or discharging of battery. This is one socket that is popular on space some spacecraft they have say, dc power supplied by solar panels. But when the spacecraft is eclipsed, say in the earth orbit it, it's eclipsed by the earth and we don't have sun. Then we need to supply power to the spacecraft from the batteries and so we need to both charge and discharge the batteries. So we can build current bidirectional switches. Basically It has the parallel combination of a transistor and a diode guard and anti-parallel diode. This is basically taking two single quadrant switches and putting them in parallel so that they can conduct current in either direction.
