230509 TI- Engineer It - How to test power supplies - Measuring Stability

Hi, my name is Bob Hanrahan, Application Engineering at Texas Instruments,
and this is a series on measuring performance of power supplies.
Here we'll be measuring stability of a power supply.
Now a power supply is an amplifier, obviously a DC amplifier, but it's also an AC amplifier.
If it didn't have any AC component, it wouldn't be able to react to load changes.
And like any control system,
we have to maintain a proper margin between the feedback of the output into the error amplifier to make sure that the output doesn't go in phase with the input.
And that amount of phase is the actual phase margin.
Today what we'll be utilizing is a traditional stability measurement tool called a frequency response analyzer or a network analyzer,
a low frequency network analyzer designed for this measurement.
And that requires that there be some method of injecting an error signal into the feedback path of the control loop.
As shown on the graphic here, we provide that point by adding a resistor in the feedback loop.
This is above the top side network in almost every regulator, every power supply.
And this is in the order of 10 to 50 ohms.
For the testing, the equipment will have an output that will be driven into an isolation transformer.
The isolation transformer is there so that there's no DC bias point that could either disturb the measurement itself or cause damage.
And then two points that come back up, receiver points, that come back up into the equipment.
Okay, let's come over to the equipment itself.
In this case, we'll be using an evaluation board.  It's the TITPS56221.
Now this is a 25-amp buck regulator with built-in pass devices.
We're feeding it with 12 volts from power supply here, and it's delivering 1 volt up into a load box.
Now when measuring stability, it's important that you make measurements at all different operating conditions that you can expect.
What I mean by that is at minimum current, output current, and maximum output current.
In this case, we'll run all the way down to zero output current and all the way up to 25 amps.
You also want to change your input voltage from nominal to your maximum that you expect and the minimal that you expect.
And after you run with a load box, I highly recommend you connect your system,
because output is going to be different, and run the stability one more time to get good results.
OK, so let's now show you a couple of things.
First off, calibration of a network analyzer is very important.
The technique for calibrating varies from system to system, so I'm not going to get into that here now.
But we're using an AP200, which is up here.
Again, probe's coming down to the receiver point, and there's a connection to the isolation transformer,
which is being injected into two connections that go down to a resistor that's on our evaluation board.
Again, all of our evaluation boards have that resistor just for this reason.
By the way, good idea to put that resistor maybe even onto your PC board, otherwise you'll have to wire it up to get access to that feedback loop.
Now we already calibrated, and one method of verifying calibration with any system is to take your receivers,
connect them together to the output, and verify that your Bode plot becomes a straight line.
Now I'll bring your attention to the GUI that is drawing a Bode plot right now.
And as you can see, our phase and gain measurement is exactly horizontal. So that's good news.
You know that everything is calibrated properly.
So now we go back, and now we'll run some measurements.
I'll be running the plot at minimum current as well as 25 amps.
So we'll turn on our power supply, and now we see the Bode plot being drawn.
Now the blue line is your magnitude, the actual gain of the feedback loop.
The red line is your phase.
And what we're interested in looking at here is the point where your magnitude passes zero,
where my pointer is now, move up to the phase, and that delta is the actual phase margin.
And this tool measures it for us, and you can see down in the lower left,
the phase margin in this case with no load is 51 degrees.
The gain margin is where the phase pass is zero here. It's a negative gain.
You can see it's there, and it measured minus 15.9 dB.
So you log that, capture the screen for your log, and then now I will turn the load box on for full 25 amps.
You'll see another plot coming across.  It'll be very similar.
You can see there it's modifying the plot.
But because of the stability of this circuit, it's almost identical.
As a matter of fact, it's within a few degrees.
It's now 48 degree phase margin and minus 16 dB gain margin.
So in summary, measuring stability, gain, and phase margin, not complicated, doesn't
take long, and it is important for almost every system.
So for more information on this or other videos on other power supply tests, you can visit the following websites.