Principles of SMPS
Technical Information
Inrush Current
Inrush Current
Definition
I i /I inr p
The inrush current is the waveform of the input current I i,
measured immediately after the input voltage is connected.
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Characteristics
The inrush current depends on the external input voltage
U i, the source and supply line impedance R s ext and L s ext,
the internal input inductance L i, the internal input resistance
R i as well as capacitance and ESR of the internal input capacitor C i of the power supply. The peak value I inr p of the
inrush current can be estimated as follows:
Ui
I inr p < –––––––––
R s ext + R i
0.5
t inr h
0.3
0.1
0
t
t inr r
Fig. 1
Inrush current
The rise time tinr r is given by:
L s ext + L i
tinr r ≈ –––––––––
R s ext + R i
The time to half value t inr h is given by:
t inr h ≈ 0.7 • (R s ext + R i) • C i + t inr r
The inrush current determines the rating of input switches,
relays, fuses, connectors and cables.
The integration ∫ I 2 dt of the inrush current can be calculated as follows:
∫ I 2 dt ≈ I inr p2 • (0.72 • t inr h – 0.39 • t inr r)
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R s ext Ls ext
Ui
Ii
Ri
Li
Ci
Fig. 2
Input circuitry
Limitation of the Inrush Current
The peak value of the inrush current can be limited by
means of an NTC resistor or by an active limitation circuitry
(resistor, being short-circuited after the switch-on by a relay,
thyristor or transistor; or option E, available with some of
the Power-One converters).
Edition 5/6.2000
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