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Common Mode choke external

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Winsu

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Hi All,

We have been testing a product to pass Class B FCC. It is passing with a common mode choke connected externally to the main. The problem is that it is huge and there is no way to fit it on the board as it has a chassis which constraints the room to fit such a component. The idea is to fit the common mode choke in a case ( or something ) to fit it externally to the product through the main cable.

Any idea? .


Cheers
 

Saying the test could be passed with a certain CM choke tells nothing about minimal required choke and it's parameters. It doesn't even prove that CM interferences are the dominant problem, although it's quite likely.
 

Hi,

some cables come with a molded CM choke. But what do we know? You don´t show it, you don´t give a part number, you dont give specifications.
So all we can do is guess.

I guess there somewhere is a suitable choke that can be implemented onto your PCB.

Klaus
 

Sorry,

I haven't given too many details. The product has already passed a pre scan with the common mode choke connected to the mains. The choke used was this one:

View attachment 145058
 

Hi,

3.3mH.. this is huge. Are you sure you need this high impedance?
And what current (max) does your application draw?

I assume your first test without the choke did fail. At what frequecy? Do you have the frequency plot?


Klaus
 

The large CM choke is only appropriate if your EMC violations are at the lower end of the "conducted" frequency range, is it so?
 

Yes, It is....Unfortunately :)
 

Did you prove that CM interference is the problem, not DM?
 

The interference is seen on the screen of an Ethernet cable. When we disconnect the "earth" from the Ethernet cable the disturbances disappears. The interference is making its way up from the earth of the Power Supply to the device through the common ground. That's way we are filtering on the mains. I guess this prove is a common mode interference.
 

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