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[SOLVED] Leakage inductance of common mode choke?

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Well, the common mode choke then, is the one magnetic component which does not have any magnetising current....hang on though, what "magnetizes" the ferrite if there is no magnetising current?
Also, interestingly, many common mode chokes for offline smps are several millihenries...isn't that odd?, common mode effects are usually above 30MHz, so why do you need high inductances like several millihenries to sort them out?
 

Excitation current is DM while CM currents cancel thus raising impedance to CM noise to prevent exgress and ingress that gets shunted to caps to gnd.

Whats the confusion. It is broadband up to 30MHz only conducted noise not what u assumed.
 

a 2mH CM choke such as yours, has a high impedance up to about 2-3MHz, where it is parallel resonant with its own capacitance, maybe 6k ohms, above this it is capacitive (a few pF).

At the risk of repeating some of the above, there is no magnetising current and hence no flux in the core, if the send and return currents are equal.
 
Of course specs or tests should indicate SRF, but my point was your thinking was inverted around 30MHz.

for example Bandwidth of molded CM choke on all VGA cables is suitable up to maximum pixel rate at 60Hz x 2 to 8 Megpixel over 75 Ohms
 
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a 2mH CM choke such as yours, has a high impedance up to about 2-3MHz, where it is parallel resonant with its own capacitance, maybe 6k ohms, above this it is capacitive (a few pF).
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....thanks, so for common mode noise above 2-3MHz, this common mode choke isn't very useful...because it will not see an inductance, but will see a "short circuit" through the interwinding capacitance?
 

No, it depends on how the choke's capacitance compares to the other paths for CM currents. For example the interwinding capacitance of a transformer can be hundreds of pF, so ten pF from the choke would still block much of it.
 
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CM...101

cy2Kd.gif
 
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The parallel resonance effect decreases as you go above 2-3 MHz... gradually, you end up with a capacitive voltage divider, cap of 2mH choke and Y caps to earth. This is why 2 CM chokes usually required for higher power converters, one for the higher freqs...

The Y caps to earth need to be on the mains side of the choke to get the attenuation....
 
Orson Cart said:
The parallel resonance effect decreases as you go above 2-3 MHz... gradually, you end up with a capacitive voltage divider, cap of 2mH choke and Y caps to earth. This is why 2 CM chokes usually required for higher power converters, one for the higher freqs...

The Y caps to earth need to be on the mains side of the choke to get the attenuation....
Click to expand...

Agreed Orson,
Different mu ratings for ferrite , low mu CM for UHF and high mu for ULF to HF.
Since Pi filter is bi-directional for ingress and egress, Cx for DM ( differential mode ) and Cy for CM, such as Power line transient (PLT) ingress is for as shown and reverse for egress from SMPS and both sides for Cy (Y rated cap) for bidirectional immunity improvements.

EMI suppression is bidirectional for a good EMC design.
 

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