Common Mode Choke Value

Thread Starter

jacopo1919

Joined Apr 12, 2020
43
Hello!
The circuit layout attached is a recommended layout for a Meanwell SMPS
the table shows a value of 325uH for the inductor but I was wondering if I could use other values...

I don't know how to calculate a practical value for a CM choke inductor on EMI filters, therefore I can't understand what would be a range of values that would still suit the application.

Is there a way to calculate a good value or how would you choose a different value?
 

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DickCappels

Joined Aug 21, 2008
7,746
I've been through this a few times with different products, often with the help of EMI experts. You really don't know which one will help your product pass the tests. You have to choose one based on experience and then try it.
 

Ian0

Joined Aug 7, 2020
3,281
Conducted EMC tests apply to frequencies up to 30MHz. Iinductors tend to have self-resonant frequencies not much above 1MHz. So at 30MHz, your inductor is a capacitor and its rated inductance value is meaningless.
 

Thread Starter

jacopo1919

Joined Apr 12, 2020
43
In the initial question i forgot to made clear in my application, i'm not using a AC/DC SMPS but a DC/DC SMPS
aren't EMI filtering and X Y capacitors, common chokes used in design where AC is present?
In my application I have a DC/DC (9to18VDC to +-12V) converter which I will feed with a 9 to 18VDC. I understand the use of a capacitor between the positive and GND but I'm not positive if all the rest is very useful
 

Lumenosity

Joined Mar 1, 2017
624
In the initial question i forgot to made clear in my application, i'm not using a AC/DC SMPS but a DC/DC SMPS
aren't EMI filtering and X Y capacitors, common chokes used in design where AC is present?
In my application I have a DC/DC (9to18VDC to +-12V) converter which I will feed with a 9 to 18VDC. I understand the use of a capacitor between the positive and GND but I'm not positive if all the rest is very useful
As far as i know yes. AC to DC SMPS are always divided into the mains side and the output side.
The Common Mode Filter (choke) and XY capacitors are always on the AC Mains side.

I do know that when the AC is rectified the voltage and frequency are stepped up significantly and thus the need for the choke.

I don't know about Dc to DC power supplies but I'm not sure why they would need a common mode filter.
 

Ian0

Joined Aug 7, 2020
3,281
In my application I have a DC/DC (9to18VDC to +-12V) converter which I will feed with a 9 to 18VDC. I understand the use of a capacitor between the positive and GND but I'm not positive if all the rest is very useful
Do your input and output share a common ground?
An ordinary mains-supplied SMPS is still a DC/DC converter, it is just supplied with rectified AC, but it has two supply lines (Live and neutral), neither of which is ground, so common mode interference is generated equally on each line. (Differential mode interference is generated as well, and also needs to be filtered out, but that is generally achieved by the leakage inductance of the common mode choke)
You will see differential mode filtering on the output of either type of switched mode supply, especially if the output 0V is ground. If the input is V+ and ground, that is what will be required on the input as well, such as a π-section filter.
If your DC-DC supply is supplied from a battery of which neither side is earthed, then you will need common-mode filtering, just the same as a mains-supplied SMPS.
 
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