CDI Blog For Engineers

Inrush Current Limiting Thermistors In Switching Power Supplies

CDI and GE

The problem of current surges in switch-mode power supplies is caused by the large filter capacitors used to smooth the ripple in the rectified 60 Hz current prior to being chopped at a high frequency.

Ideally, during turn-on the series Thermistor by Thermometrics/Amphenol in a switch power supply should be very large, and after the supply is operating, should be reduced to zero. The NTC CL Series family of  thermistors is ideally suited for this application. It limits surge current by functioning as a power resistor which drops from a high cold resistance to a low hot resistance when heated by the current flowing through it. Some of the factors to consider when designing NTC thermistor as an inrush current limiter are:

  • Maximum permissible surge current at turn-on
  • Matching the thermistor to the size of the filter capacitors
  • Maximum value of steady state current
  • Maximum ambient temperature
  • Expected life of the power supply
CL Series

The main purpose of limiting inrush current is to prevent components in series with the input to the DC/DC converter from being damaged. Typically, inrush protection prevents nuisance blowing of fuses or breakers as well as welding of switch contacts. Since most thermistor materials are very nearly ohmic at any given temperature, the minimum no-load resistance of the thermistor is calculated by dividing the peak input voltage by the maximum permissible surge current in the power supply (Vpeak/Imax surge).

CDI website has a selection of the CL-Series of thermistors available to avoid the long lead times often associated with them. In addition, CDI has many other types of thermistors available from GE.