Selecting An Inrush Current Limiter

When selecting an inrush current limiter for your application some of the things that should be considered are the energy rating, the max inrush current and the resistance.

Ex: Consider the pre charge circuit below

Given:   Operating temp = 40°C-85°C
            Input Voltage - 480 Vac
            Bridge Rating = one cycle @85A @ 25°C
                  T>50°C I<50Amp
            Current rating = 30Amp max
            Filter Cap =2350µf

Calculations:

A) Calculate the minimum energy needed
      Energy = 1/2 C*V²
        +/-10% worst case due to voltage line fluxuation = 480V *1.1 = 528V
      Vp= Peak Voltage= 528V*1.414 = 746.59V
       \E=1/2 * (2350*10^-6 F) * (736.59)^2 = 655.00 Joules

(B) To protect the bridge and relay, the max inrush current must be less that 30 amp. (See Given)

Minimum resistance required = Vp/(inrush) max = 246.59V/30A = 25

C) Since the NTC is temperature dependent
    choose an NTC with at least 25.0Ω / 655.00 Joules @ 85°C
    worst case. At this time 3 things must be considered.
          1) No one NTC can handle the energy and have that high resistance.
          2) Cost becomes an issue (do I use 2 mega surge size 35 or 3 mega surge 22 or 32).
          3) Real estate on the board, is the third thing to consider.

   So worse case total resistance @ 85°C = 3(thermistors)*17.16Ω *0.75 = 38.61

This satisfies the condition wherein the thermistor package must have a minimum of 25Ω resistance. In addition each thermistor is rated for 250.00 joules for a total of 750.00 joules