Datasheet

Application Note
www.tektronix.com/power10
Figure 16. Modulation Analysis on an IGBT’s Gate Drive During Power-up.
Output Analysis
Ideally, the output of a DC power supply should not
have any switching harmonics or other non-ideal noise
components. Realistically, that is not possible. Output analysis
measurements are essential to determine the effects of
variations in input voltage or load on the output voltage. These
measurements include:
Modulation Analysis
Ripple
Modulation Analysis
The digital phosphor acquisition technology of the MDO/MSO/
DPO4000 and MDO/MSO/DPO3000 Series offers unique
advantages when troubleshooting designs, especially when
identifying excessive modulation effects in a switching power
supply. These oscilloscopes have greater than 50,000 wfm/s
maximum waveform capture rate, which is many times higher
than that of a typical digital storage oscilloscope (DSO). This
provides two advantages when investigating modulation
effects. First, the oscilloscope is active more of the time, and
less time is spent processing waveforms for display. Thus
the oscilloscope has significantly more chances to capture
the modulation. Second, the digital phosphor display makes
it easier to see the modulated waveforms in real time. The
display intensifies the areas where the signal trace crosses
most frequently, much like an analog scope. The modulation
is dimmer than the main waveform that repeats continuously,
making it easier to see.
Measuring modulation effects with a Tektronix oscilloscope is
also easy. Figure 16 shows the modulated signal controlling
the output of a current mode control loop on a power supply.
Modulation is important in a feedback system to control the
loop. However, too much modulation can cause the loop
to become unstable. Notice that the waveform is dimmer
in regions where the modulation is less frequent. The red
waveform is a math waveform, showing the trend in cycle-
to-cycle pulse width measurements made on an IGBT gate
drive signal as the power supply’s oscillator starts up. Since
the math waveform represents pulse width measurement
values (with units of time), variations in pulse widths may
be measured using cursors. The math values represent the
trends in the selected modulation measurement across the
acquired waveform. In this case, it represents the response
of the oscillator’s control loop during startup. This modulation
analysis could also be used to measure the response of the
power supply’s control loop to a change in input voltage (“line
regulation”) or a change in load (“load regulation”).