
SYS POWER SUPPLY UM E
3.14
Starting Behavior of the Power Supply Unit
Active inrush current
limiting instead of NTC
Particular attention should be paid to the starting behavior of the power supply units in
warm operating state. The cost-effective circuit version protects the power supply unit
inputs from high input currents by means of an ohmic resistor and an NTC thermistor
connected in series. The disadvantages of these circuits for powerful devices are that
electrical power is always converted to heat loss across the ohmic resistor, and that the
NTC has a very low resistance when switching warm devices on again. With powerful
devices (P>250 W), low-resistance inputs allow inrush currents, which may become so
high that the primary fuses blow unintentionally.
When using active current limiting the power supply unit input is also protected against high
inrush currents by means of an ohmic resistor. However, this resistor is bypassed in normal
operating mode, i.e., there are no losses. When using active inrush current limiting, starting
up warm devices is no problem.
R
NTC
R
Passive current limiting
t
Active current limiting
5598D028
Figure 3-25
Current limiting
All QUINT POWER power supply units from Phoenix Contact are provided with active
inrush current limiting.
This ensures safe starting up of warm devices and provides high efficiency.
Due to their low output power, MINI POWER and STEP POWER are provided with passive
inrush current limiting.
3-38
PHOENIX CONTACT
5598_en_03