UM2076
Inrush-current
limitation
DocID029457 Rev 1
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potentiometer is turned clockwise from point 1, the Step_Phase_Control increases
roughly linearly from around 50 µs (position 1) to 600 µs (position 6).
When the SCR Triac turn-on delay is lower than 3 ms, SCR1 and SCR2 are triggered
by a DC gate current according to the AC line voltage polarity.
Figure 4: "Inrush current at STEVAL-ISF003V1 start-up on 230 V
line (1 mF output DC capacitor)"
) shows a maximum inrush peak current around 22 A. The
RMS current is thus far below the 16.1 A limit. This means that the relative variation is
below 3.3% and so checking the duration of the accumulated time of deviation exceeding
3.3 % is not required. The output capacitor is charged in 900 ms and compliance with the
IEC 61000-3-3 standard is fulfilled.
The peak current during output capacitor charging is not constant; indeed, only the step
reduction of the SCR turn-on delay is constant. Hence, according to the time this SCR
turns on, the peak current can vary slightly from one period to another.
Note that we have limited the inrush peak current to below 22 A, but the IEC 61000-3-3
limit applies to the RMS current. As the SCRs conduct a few hundred microseconds at
each half-cycle, the RMS current is much lower than the peak value. The figure below
shows the Triac current for the highest peak current event measured in
"STEVAL-ISF003V1 compliance with the IEC 61000-3-3 limit"
. The Triac conduction lasts
700 µs; its RMS current equals then 2.8 A, which is much lower than the measured 22 A
peak current. The output DC voltage increases by 12 V during this single Triac conduction.
Figure 18: Triac current for the highest peak current during start-up
