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Regarding the common mode choke

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malli_1729

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Hi,

I have a basic doubt regarding Commonmode choke.
As far as i know the use of commom mode choke in SMPS before Rectifier is to stop high frequency common mode currents.

Common mode currents are the currents which flows in the same direction in both lines (I have Phase and neutral. So, these are the currents which flow in Phase and neutral in same direction). Please correct me if i am wrong.

To stop these high frequency common mode currents, 2 inductors in 2 lines series with each line will do the same job. each inductor will stop high freq Common mode currents in each line.

Then what is the significance of it common mode choke when compared 2 inductors.

i am attaching the figure also regarding this.
 

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Two uncoupled inductors will choke both differential and common mode currents, while a common mode choke will only choke common mode currents. What kind of choking you want should depend on your application.
 

Common mode currents are the currents which flows in the same direction in both lines (I have Phase and neutral. So, these are the currents which flow in Phase and neutral in same direction). Please correct me if i am wrong.
Click to expand...

The winding on the common mode inductor is such that the resulting magnetic field within the core from line current and the return to neutral cancel each other.

It means you can realise a much higher 'common mode' inductance in one device implemented on the same core, avoiding saturation, as opposed to trying to do the same on separate cores.

The leftovers are how hard you can really drive things before any inherent mismatch takes the device out of specification.

The other side of the story is that the mismatch gives you 'differential' inductance which you can use to filter Line/Ground and Neutral/Ground noise.

It is as much as I know.

Genome.
 
malli_1729 said:
To stop these high frequency common mode currents, 2 inductors in 2 lines series with each line will do the same job. each inductor will stop high freq Common mode currents in each line.

Then what is the significance of it common mode choke when compared 2 inductors.
Click to expand...

Yes, beside differential mode reactance, two inductors introduce common mode reactance also.

On the other side, common mode choke (ideally) introduces only common mode reactance (due to wounding manner). So that is one difference.

Don't forget that circuit should not have reactance for useful (power) "signal" so no differential mode reactance is beneficial (eg useful power will not saturate choke).

Marko
 

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