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 then choose and 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 worst case total resistance @ 85°C = 3(thermistors)*17.16Ω *0.75 = 38.61

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