LinkSwitch-TN Input Stage calculations

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Does anyone have any knowledge of the input stage component selection in AN-37 for an LNK302DN


I have a design that mirrored table 9's 0.25-1W input suggestion.  It originally utilized a 6.8ohm 1W fusible resistor for RF1 and is reliable but the component was not available for the next production run and 10 ohm was selected and briefly tested but in testing a larger batch we're noticing failures of the fusible resistor only.  Bypass it and the unit works pretty reliably.  The normal operating current is very low It would appear that the inrush current is high enough to pop the 10ohm fuse but not the 6.8.  I'd like to know the method behind selecting these components.


Thank you. 

Its the same as fuse selection. The resistor I2t  must be greater than the circuit input circuit i2t. For fusible resistors, check fusing characteristics on the datasheet and make sure the value is not exceeded during normal start-up.