intreXis AG Aktive Einschaltstrombegrenzung

At the input of each power-supply, bulk capacitors are needed for following reasons:

- Bypassing of converter-stage.


Holdup time: Short interruptions of the input voltage supply do not cause any equipment failure (class S2 according to EN50155).

At power-supply turn-on, the bulk capacitors of the power-supply are completely discharged. The equivalent series resistance of the bulk-capacitors is way below 1 Ohm. This combination results in huge charging currents at power-supply turn-on.

These inrush-currents can reach hundreds of amperes, especially at higher input voltages (e.g. 110V-battery). The high inrush current can trigger a circuit breaker or burn a fuse unless you take precautions. 

Classic solution to limit the inrush current by using NTC

Most power supply manufactorers use NTC's to limit the inrush-current. At the first power-supply turn-on, the NTC is cold and has a high resistance, which limits the current effectively. After a relatively short time, the NTC heats up due to its own internal dissipation and becomes low-resistant. Therefore, the power-dissipation during normal operation is reduced.


-       Simple


-       The effectiveness of this solution is very dependent on the ambient temperature: At high ambient temperatures, the inrush-current might not be limited sufficiently and at low temperatures, startup of the power-supply is not guaranteed because the NTC limits the current excessively (example: see table below with a 5 Ohm-NTC).

-       At short interruption of the input-voltage (couple of 100 ms) the input capacitor is discharged but the NTC remains still hot and does not limit the current.

-       The NTC dissipates and reduces the efficiency.

-       During normal operation of the power-supply, the NTC heats up until it reaches steady state (approx. 110 °C). Therefore, the NTC heats up adjacent circuitry and the power-supply unnecessarily.


Ambient Temperature

NTC-resistance at turn-on

Inrush-current @ 110 VDC

-40 °C

68 Ohm

1.6 A

-20 °C

27 Ohm

4.1 A

0 °C

12 Ohm

9.2 A

+25 °C

5.0 Ohm

22 A

+50 °C

2.4 Ohm

46 A

+70 °C

1.4 Ohm

79 A


intreXis AG active inrush current limiter technology

intreXis uses a fixed resistor to limit the inrush-current which can handle safely the high energy during the  charging of the input capacitors. After the input capacitors have been charged, the resistor is bridged with a very low-resistant active component. When an interruption of the input-voltage discharges the input-capacitors, the active bridging component is deactivated and the inrush-current is limited again by the fixed resistor.


-       The effectiveness of the solution is independent of the ambient temperature. It avoids false circuit breaker tripping and guarantees proper start-up of the power-supply.

-       At short interruption of the input-voltage (couple of 100 ms) the input capacitor is discharged, the bridging component deactivated and the inrush-current is limited again by the fixed resistor.

-       The active bridging component dissipates much less than the NTC and does not heat up the power-supply. The efficiency of the converter remains high.


-       More complex.

The advantages of the inrush-limiter technology by intreXis outweigh the higher complexity of the circuit. The effectiveness and robustness of the circuit have been proven by field-experience and severe tests in the EMC-laboratory.


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