Fuses and inrush current

[jolshefsky]

The specific case I have is a device that draws about 20mA at 12V during operation. I'm using a Littelfuse 0437.500WR which is rated for nominal 500mA. However, the circuit has a 10µF input filter capacitor, and when it's powered from a lowly wall-wart supply, the peak surge current is over 9A for little 200nS bursts. The Littelfuse specs only say it's rated for 350% overcurrent up to 1 second, so only 1.75A, and their graph bottoms out at 1ms (maximum 4.5A). It [em]seems[/em] to work fine, but I can't be sure it actually is okay. Is this just a normal thing that one must accept?

I was fiddling with putting 10Ω in series (figuring it would limit inrush current to 1.2A at the full 12V but only drop 0.2V at normal operation) but that causes the internal DC-DC converter to power-cycle, so I can't afford a current-limiting resistor.

[thm_w]

You could extend that graph or find the I²T value on their site: http://www.littelfuse.com/products/fuses/surface-mount-fuses/thin-film-chip-fuses/437.aspx

Nominal Melting I²T (A²sec): 0.018

See page 13 here: https://www.schurter.com/content/download/676506/13549060/version/3/file/Guide_to_Fuse_Selection.pdf

You should be well under that calculated number from what I see. May also want to check for voltage spikes, and what the limit of the input circuitry is.

[jolshefsky]

You could extend that graph or find the I²T value on their site: http://www.littelfuse.com/products/fuses/surface-mount-fuses/thin-film-chip-fuses/437.aspx

Excellent—thanks. I knew there must be some way to characterize the way fuses accumulate the energy needed to melt. Even the "simplified" I²t where you just take the peak current over the whole time isn't enough in this particular case.

[james_s]

Fuses are not precision devices, don't over-think the matter. Consider what you are trying to protect with the fuse, generally it is futile to prevent damage to a semiconductor but then again often it's the semiconductor failing that blows the fuse in the first place, and what you normally want to protect is the wiring to and in the device and the traces on the PCB. The fuse should be there to prevent a fire or other collateral damage, it doesn't have to be on the ragged edge under normal conditions.

Also if you have a high inrush current that can pose problems of its own, you might consider a thermistor or other means to soften that a bit.