Inrush Currents & External Fusing on Power Supplies

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Most power provides have some type of an internal inrush present limiting circuit.  This avoids a big present being drawn when AC is first utilized, inflicting a circuit breaker to journey or an external fuse to blow.

The power provide inrush circuit often consists of a thermistor in collection with the AC line.  This thermistor has a high resistance when chilly, however as soon as the power provide has turned on; its self heating impact drops the resistance to cut back losses (rising the power provide effectivity).

See my earlier put up at http://power-topics.blogspot.com/2010/04/cold-temperature-start-up-of-low-cost.html

 A typical inrush present plot for 115VAC enter appears to be like like this:

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You may see the AC is utilized on the peak of the AC enter voltage to measure worse case situations.  The height inrush present is 25.65A for a interval of 2-3ms with what we name a chilly begin, in that the inrush thermistor is initially at room temperature (and in a high resistance state).

If we had been to develop the time scale, on high of that peak could be a bigger spike of present with a pulse width of lower than 200μs, generated by the “X capacitors” charging up.  X capacitors are fitted throughout the enter to cut back electrical high frequency noise from exiting the power provide.  As it is a low power spike, most power provide producers exclude it from the inrush present specification.  The power drawn is so small it is not going to journey a circuit breaker, or blow a fuse.

Many purchasers are confused with the external fuse ranking suggestion discovered within the set up guide.  They see from the power provide datasheet that the inrush present is say 30A, however then learn from the application note that the really helpful external fuse is simply 4A (which corresponds to a gradual state enter present draw by the power provide of round 2A).

This prompts a name to our technical assist group saying that they imagine there may be an error within the application note.

Most of these application notes specify using a time delay, or "slo-blo" fuse.

Taking a look at Littlefuse®'s datasheet for such a 4A fuse we are able to see from the graph that the typical time for the fuse to open varies with the size of time the present passes by the fuse.

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Going again to the power provide analysis information, one can see that the inrush present is a most of 25.65A and that the time for that pulse is say 3ms (worst case).  From the above graph, even at 10ms (0.01s) the present must be some 70A for the 4A fuse to blow, giving an satisfactory design margin.

If a quick performing fuse (sort F) had been chosen, the heart beat present for the fuse to open could be roughly 30A, which is why we advocate that slo-blo (sort T) fuses be used.