Hello.
My name is Frank De Stasi, and I'm an applications engineer
here at Texas Instruments.
And today, we're going to be looking
at the EMI of a nonsynchronous buck converter.
Now, many engineers believe that it's almost impossible
to get the same good EMI performance
from a nonsynchronous regulator as it is
from a synchronous converter.
And today, we're going to show you
how to get just as good performance
from a nonsynchronous regulator as you can
from a synchronous converter.
So first, let's take a look at the differences
between a synchronous and a nonsynchronous converter.
We see on this slide on the left-hand side
a typical synchronous buck converter.
Now, we can see that the power MOSFETs
M1 and M2 are inside the IC.
So therefore, they're in very close proximity.
The input capacitor is also very, very close
to the IC, because it's physically
smaller than the IC itself.
Now, the critical loop that we have
to deal with when dealing with EMI
is between the input capacitors and the two MOSFET transistors
that you see here.
The reason that this current loop is critical
is that the currents in the loop are changing very fast.
They're turning on and off at a very rapid rate.
And that's basically what causes both
conducted and radiated EMI.
So the whole key to minimizing your EMI
is to keep that critical loop area very, very small,
or at least as small as possible.
Basically, what happens is that small loop between the input
capacitors and the power MOSFETs creates a small antenna
that radiates EMI.
Now, if we look over here on the right-hand side,
we see a typical nonsynchronous converter.
In this case, we have a Schottky diode
that takes the place of transistor M2.
And that's the key difference between these two power
supplies.
Because the Schottky diode is external and physically larger
than typically the input capacitor or the IC sometimes,
that necessitates the current loop
to be actually a little bit larger than it
is in a synchronous converter for the same current rating
and the same voltage and power ratings.
So you can see here, basically with the nonsynchronous
converter, you're almost forced to have
a slightly larger critical loop area than you
do for a synchronous converter.
Now, here's a typical example of a layout
for a synchronous converter.
On the right-hand side, you see the IC itself
with the two power MOSFETs inside,
and you see the input capacitor right next to it,
physically very close.
So you can see by moving the input capacitor as close as
possible to the IC that current loop
area is going to be very small.
In this case, the dotted red line
signifies the critical loop area that we're trying to minimize.
In the nonsynchronous converter, it's
easy to see that we have the Schottky diode designated
as D1, which is actually a little bit
larger than the IC itself and quite a bit larger
than the input capacitor.
So it makes it a little bit difficult
to get all the components as close as possible-- a little
bit more difficult than it is in a synchronous converter,
but it's not impossible to get a good, tight loop.
But you can see here from the dotted box that
represents the critical loop area that it's always
going to be a little bit more than for a synchronous
converter would be.
So the question is how do we get good EMI performance
from a nonsynchronous buck converter.
Well, there are three basic principles
you have to keep in mind.
One, as we just mentioned, is to keep the critical loop
area as physically small as possible.
Second, use both a top side and a bottom side ground plane
on your PC board.
Those ground planes will help to manage the loop currents
and keep them where you want them to flow.
And finally, make sure that you use both common mode
and differential mode filters on the input to your power supply.
Those filters will help to filter out the fast rising
edges of the currents in those critical current loops
we just talked about.
And in fact, the filters will help with both radiated
and conducted EMI.
So we have an example to show you here today.
We have an evaluation board that was designed by TI engineers
specifically to minimize both conducted and radiated EMI.
And essentially, the board has a very tight loop area,
as we mentioned.
It has both top side and bottom side ground planes,
and it has both differential and common mode input filters.
This particular board is based on the LMR 16030
simple switcher nonsynchronous book converter.
So this is what we'll use as our example
today in our lab to see what kind of EMR performance
we can get.
So the next step is we'll take this board into the EMI chamber
and go ahead and run an EMI scan.
So here we are inside of our EMI chamber.
And we have the PC board we saw earlier
set up and ready to go to run an EMI scan.
The input power supply to our PC board
is a pair of 12-volt batteries and series
that gives us 24 volts for our input supply.
And it's very important to use batteries
whenever you're doing any kind of EMI testing,
because they provide a very low noise source for our regulator.
The output of this particular regulator
is designed to provide 5 volts with a load current of 3 amps.
And again, we also need some very low noise load.
So what we choose to use for our load
is essentially a set of power resistors
that are noninductive.
And if you look underneath the turntable here,
you will see that our load resistors are sitting
on that heat sink, out of the way
so they don't interfere with anything else.
On the opposite side of the corner to the PC board,
we have our EMI receiving antenna.
And essentially, what will happen
is that the EMI coming off of our PC board
will be picked up by the antenna, sent
through the coaxial cable, and into the receiver that's
in the lab.
So now we're all set to go back in the lab and run a scan.
Well, we have our PC board inside the EMI chamber.
And it's all powered up, and we're ready to run an EMI scan.
So let's go ahead and run the receiver.
So now the scan is completed.
We just ran a radiated EMI scan over the frequency range
of 30 megahertz to 1 gigahertz.
And it's easy to see that our PC board has passed the CISPR 22
class B limits.
One thing to keep in mind is that the chamber
we have here at TI is what's known as a three meter chamber.
That means that the antenna is approximately three meters away
from the PC board regulator that we're trying to test.
Typically, however, in industry, the antenna
will be approximately 10 meters away from the board under test.
That means that typically the EMI results
that you get at the antenna will be a little bit less
than what we see here in our three meter chamber.
So even though this EMI scan seems
like it's coming very close to the CISPR limit, in reality,
it will actually be a little bit further away.
This slide shows us a comparison between a typical synchronous
converter and the nonsynchronous converter
that we just scanned, essentially.
These scans were taken with an industrial 10-meter chamber.
The red graph is the nonsynchronous,
and the black graph is the synchronous.
It's easy to see that both converters easily
passed the CISPR 22 class B requirement.
It's also easy to see that the nonsynchronous converter
at some frequency ranges will have
a little bit more EMI than a comparable synchronous
converter.
And that's what you would expect,
because it's a little bit more difficult to get
the critical loop area as small as it would
be for a synchronous converter.
So in summary, to achieve good EMI results
from a nonsynchronous converter, just keep in mind
these three points.
Keep the critical PC board loop area as small as possible.
Use generous ground planes to keep the currents flowing
in small loops, and use both common mode and differential
mode input filters to help filter
out some of the EMI noise generated by the switch.
So there you have it.
We've showed you today how you can minimize
the amount of both radiated and conducted
EMI in a nonsynchronous design by following
a few simple rules.
So for more information, please see the links
at the end of this video.
And thank you for watching.