Use an NTC Power Thermistor

Inrush Current

Switching Power Supplies (SPS) have low impedance capacitors that expose the diode-bridge-rectifier to an excessively high inrush current at turn-on.

Protection of Switching Power Supplies

This application note provides some design considerations for inrush current protection of the AC current applicable to Switching Power Supplies (SPS). The solution suggested in this application note uses an NTC Power Thermistor in separate legs of the power supply circuit (see Figure 1). This NTC Power Thermistor offers high resistance at the beginning of switching and limits the inrush current. After a short time, the NTC Power Thermistor resistance decreases to a low value due to self heating and does not affect normal operation.

NTC Selection Criteria: Steady State Current

First determine the 100% load of the switching power supply (SPS). 100% load:

Assuming, the SPS has wattage of 1000 W and efficiency 90%

NTC Selection Criteria: Minimum Resistance at 25

NTC Selection Criteria: Minimum Resistance at 25

Minimum resistance should limit the one-cycle maximum current rating of the diode-bridge rectifier, for example, to 50% of its rating for adequate surge protection.

If the desired maximum input rush current is: 100 A (when peak current rating is) And Peak voltage: 375 Volt (for 265 V AC)



Note: Use the Resistance Temperature Conversion Table - to find the resistance - if the operating temperature is other than 25.

NTC Selection Criteria: Energy

NTC Selection Criteria:

Select a Thermistor that can handle 13 A of current and has a capacity of at least 126 Joules.