Automation Canada - Machine Safety - 20

Another disadvantage of NTC thermistors is the inadequate inrush
current limitation following short AC voltage interruptions: As soon as
the AC voltage is interrupted the electrolytic capacitors are discharged.
But the NTC thermistor remains at low impedance because it is already
heated up. Therefore, the thermistor is practically ineffective when the
AC is restored. This effect can damage the capacitors.
NTC thermistors also generate power losses due to its functionality.
This can lower the overall efficiency of the power supply.
2. INRUSH CURRENT LIMITATION USING FIXED RESISTORS
The inrush current can also be limited with a fixed power resistor. Aſter
the electrolytic capacitors on the input of the power supply are
charged, the resistor is bypassed.
There are several different components that can be used to bypass the
resistor such as relays, triacs or IGBTs.
This method of inrush current limitation is significantly more complex
and expensive than the usage of NTC thermistors. Therefore, it is
primarily used in power supplies with larger wattage designs (250W
and above). The advantages are that fixed resistors work independently
from the ambient temperature and cause significantly less power losses.
3. INRUSH CURRENT LIMITATION USING TRAILING EDGE PHASE
DIMMING
Trailing edge phase dimming works similar to the solution with fixed
resistors. The limiting path is bridged with a relay aſter charging the
capacitors. What makes this solution special is the limiting part itself.
An electronic system measures the instantaneous value of the AC
voltage and compares it with the voltage of the partly charged
capacitors. If the difference is less than a set threshold of e.g. 30V, a
MOSFET closes. If the difference is bigger than 30V, the MOSFET
reopens.
The on-resistance of the MOSFET thereby limits the peak charging
current. For example, if the on-resistance has a value of 4 ohms, the
current is limited to 7.5A (30V /4 ohms). A gentle start for all input
CANADIAN AUTOMATION
voltages is therefore guaranteed. If the input capacitor is fully charged
the inrush current limiting circuit is bridged to avoid power losses.
PULS uses this technique in many of its DIMENSION C series devices.
4. INRUSH CURRENT LIMITATION USING PULSE CHARGING OF
THE INPUT CAPACITOR
Pulse charging is a smart and efficient way of soſtly controlling how
much energy is allowed to charge the input capacitor. To achieve this, a
little switched-mode power supply is integrated and used as a charging
circuit. This enables a very efficient charging of the input capacitor with
no losses.
This method leads to a couple of advantages:
* The inrush current is only slightly higher than the normal operating
current.
* The peak current can be defined more accurately, as there are no
variables involved.
* A delay on the charging is also incorporated into the design. This
means when power is initiated or restored, the supply does not
contribute to unnecessary inrush as neighboring equipment is also
energized.
* Fuses or circuit breakers can be sized for operating current and not
peak current.
*The inrush current limitation is independent of input voltage or
temperature
The pulse charging is highly efficient and is used in many DIN rail
power supplies of PULS DIMENSION Q series.
SUMMARY
There are various options to limit the AC inrush current to a point
where it is nearly on the same level as the operational current. Each
method has its benefits and disadvantages, like temperature sensitivity
or a higher price point. Which one is the right for you depends on your
application and in which environment it is used. You can also consult
our team of skilled application engineers to find the best solution.
VOLUME 4, ISSUE 2
20
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