What is an NTC resistor?

A negative temperature coefficient or NTC resistor has a relatively high resistance when it is cold, but almost no resistance at high temperatures.

NTCs can be used as input inrush current limiters. When the power supply is switched on, the resistance is high and this limits the charging current of the input electrolytic capacitor. The flow of current through the NTC heats it up and the resistance reduces. This lowers the losses in the NTC to <1 W.

However, using NTCs as input inrush current limiters also has disadvantages. 

Dependency on the ambient temperature:
If it is too cold (e.g. below 0°C), there can be start-up problems, because the thermistor will not heat up quickly enough. By contrast, if it is too hot, the NTC thermistors may not function correctly as input inrush current limiters.

Inadequate input inrush current limiting after short interruptions to the input voltage: When the input voltage is interrupted, the electrolytic capacitors are discharged. But the NTC thermistor still has low impedance because it has already been heated up. As a result, the thermistor has almost no effect when the voltage is restored. This can lead to damage to the capacitors.

No-load losses:
Because of the way they function, NTC thermistors also produce no-load losses. This can have a negative impact on the overall efficiency of the power supply.

Therefore, PULS uses trailing edge phase control and pulse charging in many of its power supplies to limit the input inrush current.