Selecting An Inrush Current LimiterWhen 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 = 528VVp= 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 |
