Surge Current Limiting NTC Thermistors in Switching Power Supplies

　　The current surge problem in switch mode power supplies is caused by a large filter capacitor, which is used to smooth the ripple in the 60 Hz rectifier current before high-frequency chopping.

　　Ideally, during the process of turning on the power supply, the resistance value of the series NTC thermistor in the switching power supply should be very large and should be reduced to zero after the power supply is in operation. The NTC thermistor series is very suitable for this application, as it acts as a power resistor to limit surge current. When heated by the current flowing through it, the power resistor will decrease from high cold resistance to low thermal resistance. When designing NTC thermistors as surge current limiters, the following factors should be considered:

　　Maximum allowable surge current when turned on

　　Match the size of the thermistor with the filter capacitor

　　Maximum steady-state current

　　• Maximum ambient temperature

　　• Expected lifespan of power supply

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