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In power electronics supplies when a common mode choke is used at the supply or before a converter (say a synchronous rectifier) , does it mean the choke is used to avoid common noise coming from the rectifier side to supply or is it implemented to control the common mode currents or noise coming from the source or supply itself ?

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Typically a common mode choke will be used to prevent common-mode noise from any sort of switching power device from going upstream.

Notice however that the presence of a common mode choke will represent a high impedance for common-mode signals for a given loop, so in practice it's preventing noisy current from flowing on this entire loop, which may include many more elements.

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It works both ways and can be used both ways simultaneously. But if it's before a switching converter then the primary intention is to probably stop the regulator from injecting noise back onto its supply lines.

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  • \$\begingroup\$ This is correct. But it should be noted that the CM impedance of L1,L2 must be high relative. To the shunt C on either side at the noise frequency and harmonics. For SMPS it is the rise time spectrum that exceeds 10MHz that is attenuated mostly and not the fundamental f. BW-3dB=0.35/Tr. thus L/R and Tr are considered for dI/dt=V/L and V=IR so dI/dt =IR/L so choose lowest DCR for efficiency and highest L/DCR for attenuation. Also lowest cap ESR=tau=ESR*C such as plastic is better than ceramic. \$\endgroup\$ – Tony Stewart Sunnyskyguy EE75 Jun 20 '19 at 17:51

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