Input filter design. So we'll be looking into the setup where we have our switching converter, operating close loop, regulating, for example, output voltage. And in that setup, we will be adding an input filter on the input side of the converter. So the input filter design refers to the situation where an input filter is added on the input side of the converter. And there are really two top-level questions to address here. One is, why are we doing this? What is the purpose of the input filter to start with? And then the second, once we understand that there are many situations where we have no choice but to add the input filter, we would like to see, what is the effect of that input filter on the closed-loop performance of the converter that we have already designed. So how does the addition of an input filter affect performance of the closed-loop converter? To give you some background of why we need to add the input filter, we need to do a little bit of a review of what is called electromagnetic compatibility. So, electromagnetic compatibility is a broad area. It refers generally to the ability of the device, such as a power supply, to function in a proper manner, satisfactorily, in an electromagnetic environment that may have other components or systems present. So that's being capable of operating in the presence of the electromagnetic environment around, that can be affected by other subsystems in the surrounding of the unit itself. That particular aspect is referred to as susceptibility. So electromagnetic susceptibility is the ability to function satisfactorily in the electromagnetic environment that's surrounding the system itself. So that's one branch of the electromagnetic compatibility. The other one that actually will be the one that we are more concerned with, with respect to the input filter design, relates to the unit itself creating electromagnetic interference to the environment in two different forms. One is radiated, and the other one is conducted. Okay, so if you look at the power supply as a product, you put it in the environment. It's supplying loads on one side and it's tied to the AC grid on the other side. It is in the operation of that switching power converter is producing radiated emission. Because you have currents flowing through the loops in the converter. And there is radiation associated with these loops serving as unintended antennas with respect to the environment. And then second, in the process of switching, we will be producing all kinds of conducted noise, in particular, conducted noise on the input terminals of the device or the system. And those input terminal conducted emissions are regulated, or are have to be constrained in two different ways. One is in terms of what is called harmonics, the harmonics of the input current. And we'll refer to those when we get to discussion of the electrifier part of the system. And the other one is the electromagnetic interference, and that's the one that I will be briefly summarizing today. So again, electromagnetic compatibility refers to both the system being able to operate in the electromagnetic environment, that's called susceptibility. And then the other one is limiting the emissions of that system with respect to the environment in the radiated or conducted form. So you will see that the purpose really of placing the input filter in the design of a switching power supply is to deal with conducted emissions. And so we refer to those filters on the input side as EMI, electromagnetic interference filters. So here is a diagram that shows a typical power supply that's connected to an AC line source. So AC line source is the grid, that's what you would plug into the wall. Going from there, you have the AC rectifier connected on the front side. Followed by, for example, a DC/DC converter that's regulating the voltage across the load. So this would be a fairly typical setup for a power supply that's connected to the AC grid. Now there are some elements to this that we haven't really talked about at all so far. One is, how do we actually design this rectifier in the first place? Just note that we have a switching power converter through a rectifier connected to, ultimately, the AC line source on the input side. And you see in that chain from the AC line source, we have two components that I'd like to talk a little bit about. One is called the EMI filter. This is really the input filter that we will be referring to in the input filter design part of the class. And then there is a device that's called the LISN. And I'll explain what that is in just a second. And it's related to how we actually verify whether our system is complying to the EMI requirements or not. You can see that we are talking about requirements that are subject to regulations. Where is this really coming from? It's coming from the fact that you have to obey certain rules, right, if you want to have a product that is then used in the real world. And those rules are set up by regulations and they're typically government enforced, right? Without any overriding regulations outside of the companies involved in producing these products, you'd have a chaotic electromagnetic environment where nothing would work perfectly. Different parts of the world have different regulations. One that is very commonly applied worldwide, but is European in origin is this Euro-Norm 55022. We aren't going to go to that in any level of detail. But just to be aware of the fact that these regulations do exist and are actually very, very important parts of the design. You simply cannot market a product without being able to meet those regulations. And meeting those regulations can be actually quite tough. Because in the presence of the switching noise generated by the converter, meeting those regulations inevitably is going to require some amount of EMI filtering on the input side. Now a little bit about this LISN block. LISN stands for Line Impedance Stabilization Network. And the one particular version of that LISN is shown right here. These terminals right here are terminals for the device under test is connected. And on the left-hand side, you have two terminals where you connect to the AC line. The LISN is a standardize device. These values inside are not arbitrary. They are decided so that the measured noise is done always in exactly the same manner. So the purpose of the LISN element, and you see that is actually LISN that's connected right here between the AC line source and the input side of the converters. This is where you connect your converter to. This is your design right here. This is the grid side aiming between the grid and the device under test or your system under test, you connect that LISN box. And that LISN box has standardized impedances, the purpose of which is to standardize the way the noise is measured. So we are basically connecting the device that allows us to measure the noise levels across these two resistors that are called RN. The series inductors here are providing high impedance with respect to the AC line. And so the noise from the converter, this would be the input current of the converter, is diverted into this path right here. And this is where the measurement device is connected in order to find out how large these noise levels really are. So the measurement device operates as a spectrum analyzer. It measures the noise in frequency domain and checks where that noise generated by the switching converter attached to the LISN is larger or smaller than the prescribed limit. How do these limits in fact look like? Here is an example for that European Norm Class B limits. So you see, this is the frequency axis on the horizontal axis. And on the vertical axis, we have the measured level of noise in dB microvolts. Where is that noise measured? It's measured as voltage across the RN resistance with the spectrum analyzer in two different setups of that spectrum analyzer. They're called quasi-peak and average. Again, we won't spend time on the details of how exactly these setups are done for the the spectrum analyzer. But the measurement bandwidth that is used in the process is 9kHz. So you're basically sweeping a bandwidth of 9kHz across frequencies, and you're finding out what your level of noise is. And let's say in the average mode, you get something like this. And you say, all right, good, this device is actually meeting that particular regulation. So see this gives the maximum value of noise for any given frequency in the spectrum. The regulation starts typically from 150kHz. So the low limit right here is 150kHz, and they extend up to 30MHz. So this last point right here is 30MHz. That, by the way, is the range of frequencies where we typically have switching frequency of the switching converter. Hundreds of kilohertz are fairly commonly done, and that makes it potentially difficult to meet those regulations. Plus, even if your switching frequency is less than 150kHz, you're not automatically guaranteed to meet these regulations because of harmonics. Switching at 100 kilohertz is going to generate large amounts of harmonics at multiples of the switching frequency. And this why we are going to likely have the spikes in the spectrum that we measure right there. And we want to make sure that those spikes are below what is actually allowed.
