Nuisance tripping of mains fuse woth non-PFC SMPS?

[grigson]

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)………

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

Anyway. Here is the mains input current waveform……

http://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….the initial higher peak current is inrush at start-up )

(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?

 

[VEBill]

Well, guitar amplifiers are *never* abused in the field. Right? ;-)

I'll bet that Littelfuse has Applications Engineers that could give you the definitive word.

Is that a solder-in fuse? If so, then it's more than a nuisance. High field failure rate, with all the associated warranty claims and returns, could bankrupt (+/-) the manufacturer.

Why 6.3A? The mantra we use is that circuit breakers (and fuses, if we used them) are to protect the wiring, not to protect the hardware. If the correctly-sized fuse blows, then the hardware needs fixing anyway. In other words, there's little to be gained by putting the fuse rating close to the actual current. Size it to protect the wiring from flames and fire.

E.g. your 15A household circuit breaker protect your 14-ga household wiring. They're not concerned about limiting the damage in your TV that draws less than 3A.

Good luck.

 

[Skogsgurra]

I see no problem with the 6.3 A fuse and that inrush. Two half-cycles at a lot less than 1000% RMS current followed by RMS current well below rated current will not be a problem since the fuse can withstand 1000% (63A)for at least 20 ms and possibly up to 150 ms.
I would be more concerned with the limited breaking capacity.

Gunnar Englund

http://www.gke.org

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