Common mode EMI emissions in handheld devices (mains isolated)

[luisprata]

Hi,

When a product uses an external SMPS it's common to use a clamp ferrite to reduce EMI without redesign.

My question is: In a handheld product is a clamp ferrite effective ?  In other words, could a handheld device, not connected to mains at all, produce EMI common mode noise ?
                         
Handheld device ASCII diagram:

Battery  ======  switching boost ======= Processor

Do a system like this generate common mode noise?

Is a ferrite clamp effective anyway to reduce EMI ? Like this:

                                                          Clamp Ferrite
Battery  ======  switching boost =====()()()()==== Processor


Thank you.
Luis A P Barbosa

[jbb]

Remember there are conducted and radiated emissions.

If the product is totally unplugged, there's no path for conducted emissions, but radiated emissions must still be considered.  Common mode vs differential mode will be relevant only in the way that they drive accidental antennas which radiate emissions.

If the product plugs in (e.g. to charge via a plug pack adapter), then there is a path for conducted emissions again.  Note that some products are designed (in the firmware, I guess) so that they don't work while plugged in to charge.  This could be for functional, safety or EMC reasons.

Is a ferrite clamp effective anyway to reduce EMI ? Like this:

                                                          Clamp Ferrite
Battery  ======  switching boost =====()()()()==== Processor

EMC is a really tricky subject because, as the frequencies go up, everything matters.  What stage of development are you at and (if you're in testing) what are you having trouble with?  If possible, some diagrams (or photos) will get you much better help on a forum.