UM2027
Inrush-current
limitation
DocID029048 Rev 1
19/37
6
Inrush-current limitation
IEC 61000-3-3 standard reminder
The IEC 61000-3-3 standard gives the limitation of voltage changes and fluctuations for
equipment with rated RMS current lower than 16 A connected to a public low-voltage grid.
These voltage fluctuations are indeed caused by the equipment if too high a current is sunk
from the grid. A voltage drop is then caused due to the line impedance.
The mains voltage fluctuation causes an undesirable variation in the brightness of lamps
and displays; namely, the flicker phenomenon. This is why designers must keep the inrush
current sunk by their equipment to below specific limits.
The following equation explains the link between the line current variation
I
input
(due to the
equipment operation) and the relative mains voltage variation (
U), which must be lower
than the maximum allowed value (d
max
given in %).
U =
I
input
x Z
ref
/ U x 100 < d
max
where Z
ref
is the normalized line impedance (0.6
with 796 µH in series for a single-phase
grid) and U is the nominal RMS line voltage.
The d
max
level should not exceed 4%. A 6% or 7% limit is also allowed according to the way
the equipment is switched (manually or automatically, delayed or not, etc.) or for specific
appliances.
If the
U variation exceeds 3.3% during a single voltage change, this should not last more
than 500 ms.
Table 3: "Maximum input RMS current variation for 230 V single-phase grid according to
IEC 61000-3-3"
gives the associated maximum input current variation related to these
different d
max
levels. To simplify the analysis, it could be said that an appliance will fulfill
with the IEC 61000-3-3 limit at startup if its RMS current remains below 16.1 A. The relative
variation is thus lower than 3.3% and compliance is ensured even if startup lasts more than
500 ms. It is clear that this is a restricted case for simplification purposes, higher current
variations may also still allow compliance with this standard.
Table 3: Maximum input RMS current variation for 230 V single-phase grid according to IEC
61000-3-3
d
max
(%)
DU (V)
DI
input
(A)
3.3
7.6
16.1
4
9.2
19.5
6
13.8
29.3
7
16.1
34.1
STEVAL-IHT008V1 compliance with the IEC 61000-3-3 limit
Currently, one of the most used solutions to limit inrush current consists of adding a resistor
(refer to RLIM in
Figure 5: "Solution using relays to limit inrush current and standby losses"
)
in series with the DC capacitor (refer to C). This resistor must then be bypassed to limit
power losses during steady-state operation. Usually, a relay or a Triac (S1) is used for this
purpose. To disconnect the DC bus during standby mode, a second switch (S2) is required.
To avoid the use of the RLIM resistor, a different startup procedure can be implemented.
With the Triac T_ICL placed in series with the diode bridge, the capacitor can be charged
smoothly through progressive phase control. As long as the Triac is not triggered, the
bridge does not conduct any current, and the DC bus capacitor is not charged. To start
