Datasheet
www.tektronix.com/power 7
Power Supply Measurement and Analysis with Bench Oscilloscopes
Figure 11. Harmonics Analysis. Figure 12. Power Quality Measurements.
Harmonics
Switching power supplies tend to generate predominantly
odd-order harmonics, which can find their way back into the
power grid. The effect is cumulative, and as more and more
switching supplies are connected to the grid (for example, as
an office adds more desktop computers), the total percentage
of harmonic distortion returned to the grid can rise. Since this
distortion causes heat buildup in the cabling and transformers
of the power grid, it’s necessary to minimize harmonics.
Regulatory standards such as IEC61000-3-2 are in place to
oversee power quality from a particular non-linear load.
Determining the effects of these distortions is an important
part of power engineering, and the benefits of using an
oscilloscope rather than a multimeter are significant. The
measurement system must be able to capture harmonic
components up to the 50th harmonic of the fundamental.
Power line frequency is usually 50 Hz or 60 Hz; though for
some military and avionics applications, the line frequency
may be 400 Hz. It should also be noted that signal aberrations
may contain spectral components with even higher frequency
components. With the high sampling rate of modern
oscilloscopes, fast-changing events may be captured with
great detail (resolution). In contrast, conventional power
meters can overlook signal details due to their relatively slow
response time.
Performing harmonics analysis is as easy as taking an ordinary
waveform measurement. Since the signal in this case is a
repeating periodic waveform, it’s a simple matter to trigger and
display it. At least five cycles should be displayed to ensure
good frequency resolution, and the vertical scale should be set
such that the signal occupies as many vertical divisions on the
display as possible for best measurement accuracy.
Figure 11 shows the result of a harmonic analysis on a power
supply’s load current. In the Display menu, measurements on
a specific harmonic may be selected. In this example, the fifth
harmonic was selected. Users may choose to view the results
as a table or a graph and can select whether to view “All,
Odd, or Even” harmonics. Harmonics data may be saved as a
CSV file to a USB storage device. Total Harmonics Distortion
(THD) values relative to the fundamental and RMS value are
also displayed. These measurements are useful in analyzing
compliance to standards such as IEC61000-3-2 and MIL-
STD-1399 that are included in the power application software.
Power Quality
Power quality does not depend on the electricity producer
alone. It also depends on the power supply and the end-user’s
load. The power quality characteristics at the power supply
define the “health” of the power supply and determine the
effects of distortions caused by non-linear loads. As shown in
Figure 12, the power application software provides a results
table with the following automatic measurements: V
RMS
and
I
RMS
, Voltage and Current Crest Factors, True Power, Reactive
Power, Apparent Power, and Power Factor.