Inrush-current
limitation
UM2076
22/43
DocID029457 Rev 1
5
Inrush-current limitation
5.1
IEC 61000-3-3 overview
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 voltages fluctuations are caused by the equipment when the current sunk from the
grid is too high, resulting in a voltage drop due to the line impedance.
The mains voltage fluctuation causes undesired brightness variation in lamps and displays,
known as “flicker”. This is why designers must keep the inrush current sunk by their
equipment down to specific limits.
The following equation explains the link between the line current variation δI
input
(due to the
equipment operation) and the relative mains voltage varia
tion (δU) which must drop to a
maximum allowed value (d
max
, given in %).
Where Z
ref
is the normalized line impedance (0.6 Ω with 796 µH in series for a single-phase
grid) and U is the nominal RMS line voltage
The d
max
level shall 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.
A δU variation exceeding 3.3 % during a single voltage change should not last more than
500 ms.
The table below gives the associated maximum input current variation related to these
different d
max
levels. To simplify the analysis, we can say that an appliance fulfils the IEC
61000-3-3 limit at start-up if its RMS current remains below 16.1 A. The relative variation is
then lower than 3.3% and so the compliance is ensured even if the start-up lasts more than
500 ms. This is clearly a restricted case for simplification purposes; higher current
variations may also allow compliance with this standard.
Table 4: Maximum input RMS current variation for 230 V single-phase grid according to IEC
61000-3-3
d
max
(%)
δU (V)
δI
input
(A)
3.3
7.6
16.1
4
9.2
19.5
6
13.8
29.3
7
16.1
341
5.2
STEVAL-ISF003V1 compliance with the IEC 61000-3-3 limit
One of the most common solutions to limit inrush current involves adding a resistor (like
R
LIM
in
Figure 5: "Solution using relays to limit the inrush current and standby losses"
) in
series with the DC capacitor (C in the same figure). This resistor must then be bypassed to
limit power losses during steady-state operation, usually with a relay or a Triac (S1). To
disconnect the DC bus during standby mode, a second switch (S2) is required.
To avoid using an R
LIM
resistor, a different start-up procedure can be implemented. With
the mixed SCR/diode rectifier bridge, the capacitor can be smoothly charged with
progressive phase control. The bridge does not conduct any current and the DC bus
capacitor is not charged while the SCRs are not triggered. To start charging the DC
