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At the input of each power-supply, bulk capacitors are needed for following reasons:
- Bypassing of converter-stage.
- EMC-filtering.
- 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.
Advantages:
- Simple
Disadvantages:
- 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.
Advantages:
- 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.
Disadvantages:
- 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.
