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Radiated EMI from SMPS Cables

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eem2am

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Hello Good People,

I am speaking about the radiation of EMI from the cables which connect to the input and output of an SMPS.

Please, can you state how radiation would be emitted from a cable?.......because in a cable we have the "Go" and "Return" conductor, and the currents in these conductors are equal and opposite, -and this in turn means that there fields cancel.............

....so, regarding such cancellation, how is it that EMI can radiate from these cables?

(I understand that common mode currents would create a component of current whose field was not cancelled , and so this could cause radiation, but surely this will be cancelled out by the common-mode-filter?)
 

Allan1942

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It's a good idea to wind the cable around a ferrite ring to help reduce radiation. You see this practice in CRT monitors and inside computer cases.
 

eem2am

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thankyou so much Allan,

but how can there possibly be radiation from a 2-wire cable ?.......the fields of each conductor cancel ?
 

cdh7

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Hi,

Twisting the cable will cancel the magnetic field, but I think you can still have capacitive coupling through the electric field. The common mode filter should largely reduce this though.

Regards,
Chris
 

dick_freebird

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The far-field will be first-order cancelled, but can you get
"far enough" in the real system?

Any common-mode activity (such as ground bounce) will
-not- be nulled by twisting. Killing the high frequency common
mode before it gets outside the case, is what you wind the
twisted pair through the toroid for.
 

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