In-rush current - how much before we need to take prevention

Regarding In-rush current - how high is too high and what duration is too long that it would needs to be current limited? Does anyone know how to relate the magnitude and duration to possible long term reliability concern for the PCB and/or components e.g. power mosfet etc i.e. what can we calculate to show that it is still alright or otherwise....?

Thanks in advance for any help and advice.

Re: In-rush current - how much before we need to take preven

jsmith,
For capacitors, you must observe both the peak current rating and the RMS current rating. Manufacturers of capacitors intended for power applications give both of these ratings. See, for eaxample:
.
h**p://www.americancapacitor.com/PDF/VData.pdf
.
For solid state devices such as power Fets, You must observe the maximum current ratings. In addition, you must use the Transient Thermal Impedance ratings to ensure that for the duration of the inrush current pulse, the maximum allowable junction temperature is not exceeded. Transient Thermal Resistance takes into account the pulse width and duty cycle of the current (or Power) pulse. Using the TTR allows you to calculate the maximum junction temparature during the inrush current event. This parameter is specified via a family of curves.
Regards,
Kral

Transient Thermal Impedance