Here's one last brief note about switching loss and Its affect on switching frequency and efficiency. So we've talked about different mechanisms now for switching loss from the different devices and other mechanisms around the converter where we found that we got energy loss every time we switch. and so we have different mechanisms where we could calculate the total loss during the switching transition, which might come from the reverse recovery, or from the switching times, or from ringing and energy stored in parasitic capacitors and inductors in the converter. And in each case we can find some total amount of energy that was lost during the switching transition. So what we can do now for whatever our converter is is we can add up all of those losses and get a total amount of energy that is lost. due to the difference switching mechanisms during each switching period. So then the total switching loss would be that total energy loss multiplied by the switching frequency. Okay? So, this is, what, this loss scales linearly with frequency. And as we turn up the switching frequency, we turn up the switching loss. Now, what is the affect of this on the overall efficiency? Well, we have loss mechanisms that come from different sources, back in chapter three we talked about conduction losses and how to model those. Those losses depend on, say the output current. But they don't depend on switching frequency. So if we say or interested in the, the efficiency and the total loss maybe at full load. Or at some critical load power or operating point. We can calculate the conduction loss at that point. and its a given value. We also have what are called fixed losses that don't depend on switching frequency, they don't depend on load power, their just a fixed lost that were stuck with. An example of that is the power that it takes to run the controller circuitry for our power converter. And then finally we have this switching loss that again scales linearly with switching frequency. So the total loss then is a sum of all of these three things.' Kay? Now given this we can calculate the efficiency. so the efficiency will be the output power over the input power. Or the output power divided by the output power plus this loss, and we can plot that versus switching frequency. And what we find, is that, the efficiency goes down, of course, as the frequency goes up. [COUGH] here I've plotted the efficiency for some typical values. And I have switching frequency varied on a logarithmetic scale on the horizontal axis. And so what happens is at low switching frequency we find that the conduction lost and the fix lost dominate and the switching frequency is relatively low. Now if we're in this situation we may as well raise a switching frequency. It's true that that will make the efficiency go down a little but not by very much because the total loss is dominated by other things. And the size of our reactive components, such as our inductors and capacitors, you know filters and the converter, and if we have a transformer in our converter, the power transformer also, these things depend on the frequency. If you raise the frequency, the reactive elements get smaller. So if we're down at this point, we may as well raise the frequency because it makes our reactive elements get a lot smaller. So we have a smaller and less expensive converter. And there's very little penalty in efficiency when we do that. On the other hand, if we're at high frequency, the switching loss term dominates the other terms. And then our total loss is very sensitive to switching frequency. So we probably don't want to raise the frequency too much or our, our switching loss will become large and our efficiency will really suffer. So there's some good sweet spot of switching frequencies in the middle where the switching loss is starting to hurt. But not too much yet. and that's then represents a compromise between the switching loss and its effect on efficiency, and the reactive element size. So then here you can even solve this formula for the critical frequency. Where the switching loss is equal to the other losses. at that point, the, the total loss is twice the loss you would get at zero switching frequency. and thats gives an estimate for a given technology of what kind of frequencies we can run at. You may choose to run at a little higher or a little lower frequency, depending on the details of the application, but we'll probably run somewhere in this range. Okay, so we have a fundamental trade-off then between efficiency and switching loss versus the reactive element size, And the the engineer has to choose a switching frequency accordingly. So it depends on the application. At, high powers and high voltages we tend to run at low frequencies, 10kHz or 1kHz, depending on the voltage of the IGBT. At high voltage, I mean high, high frequency, low voltage applications with MOSFETs supplying say 1 volt computer processor chips, might run at several megahertz in this f crit maybe way up here, you know higher, much higher frequency.