Specifications

The inrush current characteristics
of ac-dc switch mode power sup-
plies vary with the turn-on phase
of the voltage cycle. Usually, these
power supplies have input capaci-
tors that draw high peaks of inrush
current as they charge from the
rectified ac line at turn-on.
Characterizing inrush current ver-
sus turn-on phase can provide
some important design insights:
Uncover component stresses
Check to see if a product pro-
duces 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 digitiza-
tion and peak current measure-
ment with the startup phase of the
voltage. Worst case inrush cur-
rents occur near the voltage
cycle’s peak and when the ac input
capacitor of the DUT is fully dis-
charged at startup. Therefore, you
must perform tests at incremental
voltage startup phases from
around 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
HP 6800 series ac power
source/analyzers is easier because
they have built-in generation, cur-
rent waveform digitization, peak
current measurement and synchro-
nization capabilities that let you
perform inrush current characteri-
zation without cabling and syn-
chronizing separate instruments.
Characterize Inrush
Current with an AC Power
Source/Analyzer
Tip
9
Output
Voltage
Start up
phase of
40 degrees
Bus Trigger
Inrush Current
Digitized Inrush
Current
Data Points
Peak Current Measurement
Figure 1: An inrush current measurement at 40° using HP 6800 series
ac power source/analyzers.
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