In this video demonstration today I'm going to show how to perform a control
loop response measurement on a switch mode power supply using an
InfiniiVision X-Series oscilloscope. A control loop response measurement is
basically a Bode gain and phase measurement you typically use either a
network analyzer or frequency response analyzer to perform this type of
measurement. It's a very critical measurement to characterize the
stability of your power supplies but it can also be performed with an
InfiniiVision X-Series oscilloscope if it has the power app option.
Today what I'm going to be testing is a switch mode power supply off of a demo board from
Picotest.
I'm probing right here across the shunt resistor. I'm also going to be using
and it's required in isolation transformers because what we're going to
do is inject a low-level signal across that shunt resistor that's in the
feedback path and we can't directly connect the output of the scope's
waveform generator to that shunt resistor, it needs to be isolated. The scope's
generator is grounded, so we can do it with an isolation transformer. What I'm
using, also from Picotest, is called the J2101 injection transformer.
So what I have set up here is first of all you can see the BNC connected to the
WaveGen output and this is the output of the generator connected across the shunt
resistor, which is built into this board. It's only 5 ohms. The feedback path
voltage resisted divider network I believe is in the hundreds of K ohm
range so 5 ohms doesn't affect it at all.
It's built-in you might consider doing the same on your power supplies and just
leave it there
it's really great for test purposes. So I have two probes connected across the
shunt resistor to measure what's V in and V out. What I'm using is one-to-one
passive probes, not the standard ten-to-one passive probes
to come with the scope. Ten-to-one passive probes are going to divide the
signal down by a factor of 10. We're really looking at low level signals, so 1:1 probes
work a whole lot better. So let's go ahead and get started with the demo.
I'm going to begin with the default set up so we can set everything up from scratch.
Next I'll go into the Analyze menu and select the power application.
Under the analysis key are all the various power measurements that you can perform with
the power options such as power quality, current harmonic,s RDS on, switching loss
and so on. Now we have videos on all of these so I encourage you to go watch
those if you're interested. Now down here at the bottom are two frequency response
measurements: power supply rejection ratio and control loop response or bode
plot. I'm going to select control loop response.
Next I'll go into the setup menu. First, just to show you, the
question mark over here is the help screen, and this brings up a
connections diagram that shows you how to connect everything. So you can see
we have our WaveGen connected to the input of the one-to-one transformer and
there's our 5 ohm shunt resistor. This is the feedback path of the power supply so
we're probing V out up here with channel 2 and V in which is the top of the
feedback path below the shunt resistor with the channel one that's V in.
Now let's go back and set everything up and see what our options are there are two
basic modes: sweep mode, which is the normal mode that you would use for
performing this type of measurement. I'll show single later. There's a start
frequency and a stop frequency, the start frequency can be as low as 20 Hertz, the stop
frequency max is 20 megahertz, I'm only going to test up to 10 megahertz for this
particular device. You can select points per decade up to 50 points per decade
you can define which of the scope's inputs
are V-in, in this case channel 1 and V out is channel 2. We'll use the defaults.
Then the most critical setup is your test amplitude. The default is 200
millivolts, most frequency response analyzers specify it in either dbm or RMS
I'm going to set this to 100 millivolts. We're going to start with a low-level
signal, and we're going to do the test a couple of times. So this is equivalent to
about thirty-five millivolts in 50 ohms. So everything is set up, we just
press apply. Now one advantage of the oscilloscope solutions is you can see the
waveforms during the test. One thing you can see, it looks like
very noisy waveform. What we're seeing is all the switching or output ripple of
this power supply. What you can be monitoring during the test is looking
for distortion that's something you can't do with a network analyzer
frequency response analyzer. So there's our first test, now I'd like to point out
a couple of things, first of all it immediately shows you PM. which stands
for phase margin, 54.82 degrees and approximately 60 kilohertz.
That's this point that it's marketing right here and it also measures gain
margin at approximately a 141 kilohertz. Now we can move the
cursors independently, there's two independent cursors, the phase margin and
gain margin measurements stay there. Now you'll notice up here at the
lower frequencies we have what I call chatter going on. The problem there is we
have our signal to noise ratio too low, and we have similar phenomena going on
here at the upper frequencies. Things are pretty good here in the mid-range around
the 0 DB crossover point. So how do we fix these to get a higher resolution
measurement? One method is to simply pump up the test amplitude. So let's do
that now. I'm going to go to the other extreme and put in a really big signal,
2 volts peak to peak.
So let's run the test again. With the higher test amplitude, our signal to noise
ratio has improved a lot, but notice the distortion on the yellow waveform.That's
V in coming off channel one. That distortion is eventually going to
transfer over to V out as well, the green wave form.
So there's our final measurement.
You can see the chatter is gone, but
notice the phase margin measurement of 67.8 degrees at about
46 kilohertz. If you remember the first measurement we performed measures
about 60 kilohertz, so things have changed and the problem is we fixed our
signal noise problem but now we have distortion and inaccurate measurements.
Now let me show you how to optimize and find what is the correct amplitude to test
that that. We can go back and test at a single frequency. I could go to the setup menu
and remember where I showed single? Another alternative is go to the table view, this
shows all of the measurements, showing the frequency, amplitude, gain, and phase.
Let's first test at the low end, 100 Hertz, and now it's going to set up a
single measurement at that frequency and show us the the gain and phase at that
frequency and there we can look at the waveform that it's testing. Let me turn
the intensity up. Here you can see the green is V out, that looks good, looks
sinusoidal, and the yellow is V in. Now there's a lot of noise on that signal it's not
noise its output ripple. Also look at these spikes here, this is distortion.
Llet's go into the WaveGen's menu, select amplitude and let's begin reducing the
amplitude until we see those noise spikes go away.
That's down around 800 millivolts.
Now if you look closely you can see what looks like an envelope riding on top of
that switching noise. That's what we're measuring, we're measuring the
fundamental within all that switching noise. Now let's go to a different
frequency and this time we'll use this menu and just go directly
to 60 kilohertz, which is the approximate 0db crossover point. That is
actually the most critical point.
Take a look at the waveforms, so there you can see V in.
Things are flattening on the sine wave and V out, the green waveform, is
beginning to look triangular so this is distorted. And that was what was inducing
the error on that phase margin measurement where it measured 47
kilohertz instead of 60 kilohertz. Again we could go back into the WaveGen menu
and start reducing the amplitude and find the optimum amplitude at this frequency,
let's just go back and let's test at the high end, let's go up to 1 megahertz.
So there it is at 1 megahertz. This is with the 2 volt peak-to-peak input if you look
at the envelope here it looks like a sine wave it's not distorting so we can
see at the high-end you can easily stand a very high voltage. If I press stop, you
can see the switching noise this is switchers running at about 3 megahertz
but what the measurement does is it pulls out the test signal fundamental.
Which in this case is at one megahertz. So you go through this process test at a
couple points, find out what the optimum amplitude is and probably what you're
going to find is there's not a single amplitude that's perfect.
So for that we have to go back to the sweep mode.
We have amplitude profile.
This is where you can set amplitudes for different frequency ranges. So I've
already gone through the process I'm going to start at about 800
millivolts at 100 Hertz I'm going to keep it constant through that
decade up to 1 kilohertz, then at 10 kilohertz I'm going to ramp down to
about a half a volt. Then at 100 kilohertz I'm going to ramp all the way
down to 100 millivolts. Now I'm going to ramp back up.
Let's go 1 volt at 1 megahertz and 2 volts at 10 megahertz. So it's going to
stay constant from a 100 Hertz to 1k, it's going to move down from 800
millivolts to 500 millivolts at 10k, ramp down further to 100k, ramp up and ramp up.
So let's go.
So there you can see we don't have the signal-to-noise ratio problem. The
waveforms are not distorted, they look sinusoidal if you can filter out the
switching noise in your in your eyes. We'll probably get a very accurate
measurement of the phase margin, it should be back to about 60 kilohertz.
So again it measures 59.48 kilohertz so there's a good measurement using the
amplitude profile capability.
As you've seen in this video you can now perform
frequency response measurements using a Keysight oscilloscope. Now these are the
only scopes on the market that can perform these. There are some real advantages
of the measurement capabilities of this scope compared to other solutions. First
of all you can see the time domain waveform and see if there's distortion.
You have one instrument that can perform a broad range of power-related measurements that
you may need to characterize and it's also a fairly inexpensive solution
compared to a network analyzer.
If you'd like to watch the other videos on power
testing or download an application note on the application I showed
today go to the URL on your screen and thanks for watching!