Nuisance tripping of mains fuse for non-PFC SMPS of 270W?

Hello,

We are worried about nuisance tripping of our mains input fuse in our Offline 270W (at the output) SMPS which supplies a guitar amplifier

Vin (min) = 90VAC and uses a voltage doubler mains rectifier.

There is no Power Factor Correction stage, its just a half-bridge SMPS.


The fuse is the 6.3Amp Littelfuse 383 series one (383-1630)………

https://www.littelfuse.com/data/en/Data_Sheets/Littelfuse_Fuse_383.pdf

Anyway. Here is the mains input current waveform……

https://i54.tinypic.com/rh4rjn.jpg

(Blue is mains input current and green is the post-rectifier DC-Bus, at 90VAC input [with voltage doubler] and max load )

(SMPS don’t require power factor correction for supplying guitar amplifiers…because the average power is so low.)

Anyway, the RMS input current from the mains is 5.5 Amps.

Though the input current (on max load and 90VAC) is comprised of 19 Amp peak pulses at 100Hz.

Do you think that we could see nuisance tripping of the above mentioned fuse?

Hello,
It is suggested as a thumb rule that the fuse continuous current rating should be 150% of Irms_input maximum (measured at min input and max load) for longer fuse life. The manufacturer's rated fuse current is for limited service time and thus the need for a larger current margin. Your selection of a time-lag type fuse is appropriate to sustain the inrush current.
Regards

You can either refer to the melting integral specification or the time-current curves. Both indicate sufficient margin for the 6.3A fuse in my opinion.

thanks,

I am always utterly amazed that it is possible to cheaply make a mere length of wire such as a fuse, with such precision in the extrusion of that wire during its manufacture, that it has a certain, precise blowing characteristic .......and to be able to state it as accurately as in this datasheet.

Surely the process cannot be that accurate?....surely many of the fuses will be inaccurately made, and will nuisance trip due to the high peak currents in my circuit.?

After all, the mains input current pulses are about 20 Amps peak at 100Hz.

Fuse datasheets also never say whether a fuse trips due to average current, or due to rms current.

If its heat that trips them then it surely must be rms.?


If fuses don't "nuisance trip" on short peak currents....then i wonder where the term "nuisance-trip" comes from
.....i mean, if if never happens....where did the term come from....one hears it iin every electrical company one goes to.


If its so easy to make fuses to such precision...then howcome inductors and capacitors off-the-shelf are typically only +-10% accurate?

These fuse datasheet characteristics just seem to good and precise top be true......for a cheap bit of wire.

In my view, fuses are far from being exactly specified, for the 383 series, this can be e.g. viewn from the minium 20 ms to maximum 150 ms trip time at 10*In. My statement about sufficient margin was already related to the variation range, as specified in the datasheet. So you shouldn't fear "nuisance tripping", I think. A way to achieve it though is switching the power fastly on and off.

Some fuse datasheets also have explicite mimimum-maximum time-current curves.

If you look at the 383 time-current curve, you derive that below about 100 ms, the fuse behaviour is mainly ruled by the thermal capacity of the fuse wire and melting integral ∫I²dt calculations have to be applied for current waveforms.

Clasical fuses are even worse, if you want a tightly specified guaranteed tripping at a certain overcurrent, allowing full rated current at the same time, e.g. for cable or transformer protection. This is almost impossible and usually demands for thermal circuit breakers or electronical fuses.