Selection of a Common Mode Choke

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I am designing a smart plug. In which is used a power IC- TinySwitch III. Attached is the screenshot of the design used.

Now I need to place a common mode choke in the design.

I am planning on placing the common mode choke before the bridge rectifier section where the green circle is in the screenshot.

Please help with the design of a common mode choke. What should the impedance and frequency and the inductance of the choke be? if the input current is 20mA.

What kind of core would be efficient in reducing noise?

Thank you,

Thank you for choosing Power Integrations.

Common mode choke selection would largely depend on the EMI characteristics of your power supply. Normally, you would choose the inductance value based on the amount of EMI that you would like to attenuate. The higher the impedance, the higher the attenuation. The core selection would then depend on how much you would like the inductance to allow to drop when current flows through it. Ferrite cores typically have a higher saturation flux density compared to powdered iron core for example. The same is true with the current rating, the rated current in the datasheet usually refers to the amount of current allowed for an acceptable inductance drop.

For more information, you could refer to an application note on EMI and safety.

Hope this helps.

Best Regards,