Specifications

9
Characterize inrush current with
an ac power source/analyzer
Tip
8
Uncover component stresses
Check to see if a product produces
ac mains disturbances that
interact with other products
connected to the same branch
circuit
Select proper fuses and circuit
breakers
However, this can be a challenging
measurement because you have to
synchronize the current digitization
and peak current measurement with
the startup phase of the voltage.
Worst case inrush currents occur
near the voltage cycle’s peak and
when the ac input capacitor of the
DUT is fully discharged at startup.
Therefore, you must perform tests at
incremental voltage startup phases
from about 40° to 90° (Figure 1) and
let the DUT’s ac input capacitor
discharge between tests.
A traditional test setup includes an
ac source with programmable phase
capability and an output trigger port,
a digital oscilloscope, and a current
probe. However, using an advanced
ac power source/analyzer such as the
Agilent 6800 series ac power
source/analyzers is easier because
they have built-in generation, current
waveform digitization, peak current
measurement, and synchronization
capabilities that let you perform
inrush current characterization
without cabling and synchronizing
separate instruments.
On the dc side, the Agilent N6705A
dc power analyzer helps characterize
the power of a device much like the
ac power source/analyzer, except
for dc power.
Figure 1: An inrush current measurement at 40° using Agilent 6800 series ac power
source/analyzers
The inrush current characteristics of
ac-dc switch mode power supplies
vary with the turn-on phase of the
voltage cycle. Usually, these power
supplies have input capacitors that
draw high peaks of inrush current as
they charge from the rectified ac line
at turn-on. Characterizing inrush
current versus turn-on phase can
provide some important design
insights:
Output
voltage
Start up
phase of
40 degrees
Bus trigger
Peak current measurement
Inrush current
Digitized inrush
current
data points