If you're working with switch mode power supplies, you're likely trying to improve
efficiency, increased power density, and reliability, comply with EMC regulations,
improve power rail integrity, and decrease thermals. This is a lot to think
about and all this design optimization can increase test time. Learn how to
easily test your switch mode power supplies and save time in the lab.
Hi, I'm Melissa at Keysight technologies and in this video you'll learn how to test your
switch mode power supplies with an oscilloscope. The primary purpose of a
power supply is to efficiently produce well regulated and low noise DC power
from an input power rail. Depending on the part of the design you're working on
you might be looking at the switch mode power supply holistically or only be
concerned about one portion of the power supply: the input, the switching, or the
output. The input side converts and filters your input voltage that's
typically an AC line voltage of a 110 volts in the U.S. or 220
in other regions. If you're primarily concerned with the input side of your
supply you'll likely be focused on power quality, current harmonics, and inrush
current. After the input your signal goes into the main part of the supply which
is the switching transistor. This regulates the voltage with the duty
cycle or the amount of time it's on vs. off. If this is the part of the power
supply you're focused on you'll be concerned about power losses, modulation
analysis, slew rate, and save operating area. After the switching transistor at
the signal gets filtered again and rectified so you get a stepped DC output
which is then used to run power through the rest of your device. If you're
focused on the output side of the power supply you'll want to focus on output
ripple, turn on and turn off time, transient response, power supply
rejection ratio, and efficiency. An oscilloscope is the most common tool for
making power supply measurements since you can hook up both a voltage probe and
a current probe to calculate power which as you know
is voltage times current. Another reason why oscilloscopes are a great tool for
characterizing power supplies are the analysis applications that can run on
them, making testing much more efficient. Let's set up some tests so you can see
how some of these measurements are made. If you have an Infiniivision
oscilloscope and want to follow along with the steps you can download a free
trial of the power application with the link on screen or in the description.
First let's start by looking at power quality. I'll clear any existing settings
from previous tests by selecting default setup, then I'll press analyze and select
the power application.
You can now see the full list of the power applications supported by the
Infiniivision power application bundle. I have a differential probe on the input
side of my power supply and I connected that to channel 1. I also have a current
probe on the input side of my power supply hooked up in the direction of
current flow and that's on channel 2. Since we'll calculate power from our
current and voltage measurements I'll double-check that my channels are
assigned on the scope to match this. I choose the signals menu and the default
on the scope is that channel 1 is voltage and channel 2 is current. There's
also this handy diagram that pops up to show you where to probe your power
supply for this test. I'm also going to look at 5 cycles. Now I can simply select
Auto setup. Since I'm using a power application, the scope automatically
scales the signal properly to take advantage of the full bits on the
oscilloscope for accurate measurements and sets up the waveform math. In this
case of course it's V times I to get instantaneous power. Real power is the
mean or average power of n cycles. That's the value you're measuring directly.
The application will also calculate the other power parameters such as apparent
power which is voltage RMS times current RMS over a number of cycles. The reactive
power is computed by the square root of the apparent power squared minus the
real power squared. Power factor is real power over apparent power and the phase
angle is the inverse cosine of the power factor or real power over apparent power.
We can dive into the details of power quality results here by pressing apply.
So the primary advantage of having an automated power measurement application
is that these are all calculated for you with the press of a button, you don't
waste time working out the math with pencil and paper or having to extract
the voltage and current measurement results to calculate power quality on a
separate computer. Another important characteristic to look at on your input
line is the current harmonics. In a switchable power supply your current is
pulsed which can inject harmonics back into the mains which can disturb
other devices on your power network. If your customer plugs your device into
their power outlet at home you don't want the harmonics disrupting the
performance of something else they have running on the same source like their
lights or TV. To prevent this IEC has specifications that you'll have to pass
up to the 40th harmonic to make sure you won't be causing disruption to test.
I turn on the current harmonics measurement. We haven't changed our probe
setup but if this was the first test you were setting up you can adjust those
with the signal menus as before. In the settings I can choose the frequency of
the signal and the IEC standard that I need to test to.
My design is qualified as a Class A. Now I'll hit apply to begin the current
harmonics measurement. The scope performs an FFT on the current waveform and the
results are listed in the top of the screen. It also measures total harmonic
distortion and tells me if I've passed or failed the test. To show you an
example of what happens when we fail I'll change the standard that I have to
comply with. Now you can see where the design would not pass these regulations
and by what margin. Let's switch modes and look at the
switching portion of the supply. You'll lose energy primarily during the
switching phases of the transistor when it turns on and off and during the
conduction phase when the voltage is at the transistor saturated minimum
and current flows. To check if the losses are acceptable I'll choose the switching
loss measurement. I also have to change my probe setup to probe on the switching
portion of my power supply. When I run the application we can see two switching
cycles on the top view and a zoomed in view of one cycle on the bottom view.
We also get all the power loss measurements displayed automatically. You can zoom in
and look at each phase: the conduction, the switching,
and the non conduction phase. The purple waveform is the instantaneous power.
Let's look at one more measurement: slew rate. This determines how fast your
device turns on and off. The faster you can turn on and off the lower losses
you'll have. We'll follow similar setup steps as before.
The purple trace is the derivative of your voltage over time so the peak of this is the maximum slew rate.
In addition to the measurements I cover today you can also use the
Infiniivision power application to test inrush current, effective resistance of
your switching transistor, modulation, and output ripple. Output ripple or parallel
integrity measurements are particularly important if you're driving a high-speed
digital device. The noise on the power rail could cause jitter and timing
uncertainty, and affect the validity of your digital signal transmission. You can
also measure transient response, efficiency, power supply rejection ratio,
and control loop response. You can use these measurements to quantify and
optimize your designs on the input side, your switching, and the output side of
your supply. If you want to learn more about measuring switch mode power
supplies check out the links in the description and download a free trial of
the application to try it out. Don't forget to follow us on Facebook
Instagram and YouTube for more how-to videos. Thanks for watching!