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SMPS input filter question.

Hi to all. I am currently designing an input filter for a off-line switcher. The switcher is a tinyswitch 278 running at about 11W. 220Vac to 13.8Vdc , 800mA. When designing the filter , do you design for the average input current , or must you design for the peak input currents you see when the bulk input cap is "topped up" every 10mS. There will be a big difference in component (inductor) size depending on which way you go. The average current is only a few hundred mA , where as the peaks are about 1,5A. If desighning for the peak currents of 1,5A the filter components are bigger than the whole rest of the supply!!! If this is the correct way to go than that is fine , but I don't want to design for something that is not necessary.

Cheers Robin

Reply to
seegoon99
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** Standard practice is to use a " common mode choke" - which has two windings carrying the AC supply current in opposite directions. This prevents saturation with normal AC load currents.

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Even the smallest take 1 amp rms with ease.

...... Phil

Reply to
Phil Allison

Thanks for the help guys.

Reply to
seegoon99

two

Normally the CM choke has differential inductance also, so it will saturate at a given current. You need to design for the peak current, but often the CM choke is spec'ed with a RMS current and they can handle a lot more peak current. The peak current is not shown in the datasheet, so you need to test the actual peak current of your device (the derate according to temperature, tolerances, etc). Normally permeability rise with temperature

One core I just worked with 45% rise of permeability, so you need to derate current accordingly. The same CM choke had a RMS current spec of 0.5A, but a measured saturation current of 3.2A at 25 degrees

But, this is differential current. Be ware of the CM current. (caused by stray currents to ground). The same CM choke measured only 7mA of common mode saturation current. And this current running in your product is difficult to measure. The end result is that the CM and DM saturation currents add up. So if this choke is run at 3.2A peak DM current, then it will tolerate 0A CM noise current.....

Regards

Klaus

Reply to
Klaus Kragelund

"Klaus Kragelund"

"One core I just worked with 45% rise of permeability, so you need to derate current accordingly. The same CM choke had a RMS current spec of 0.5A, but a measured saturation current of 3.2A at 25 degrees"

** A peak to rms ratio ( crest factor ) of 6 or more is extraordinary.

Never be seen in any normal AC supply current draw waveform.

The usual is about 2 or 3.

The OP has nothing to worry about.

....... Phil

Reply to
Phil Allison

What about the CM current? (how can you know his application?)

About the crest factor - the project I working on now is a 50W pump with a crest factor of 6. (electronics on a low-impedance mains network). And this pump is not "extraordinary", standard RFI filter with diode bridge and large cap.. When you design you need to take the mains network into consideration, are you working on a soft net since you have so low peak currents?

Regards

Klaus

Reply to
Klaus Kragelund

"Klaus Kragelund" "Phil Allison"

** The OP described it - shame you are a blind f****it.
** Yawn ............

Fuck off - you idiotic PEDANTIC IMBECILE !!

....... Phil

Reply to
Phil Allison

Plunk ;-)

Reply to
Klaus Kragelund

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