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.