Conducted Emissions

Posted by: mike_mccrady on 02/03/2010

I have 2 power supply designs that are in for FCC testing. They both have failed the Conducted Emissions test.

What can I do to decrease these emissions?

I have the documentation in a ZIP file which I cannot attach. If you would reply with an email, I will forward the files.

Thanks,
Mike

Comments

EMI can be broken down into differential mode and common mode.
What region of the frequency range are you failing in?
What is your output power?
Are you using any EMI filters (X caps, Y caps common-mode chokes, beads etc)
Can you send me your schematic so that I can make suggestions?
Also take a look at applicaton note AN 15 for EMI techniques...

I have attached the report and a schematic for you to look at.

I think I need to add 14mH inductors to the AC inputs with a .1uF cap accross the inputs.

Thanks,
Mike

You have very little or no EMI filtering...
You will need to use X capacitor across the AC input to reduce diff EMI around 150 to 1 Mhz.
You will need a Y capacitor across the transformer to reduce Common mode EMI at higher frequencies. PLease read AN 15 or see suggested values of components in PI Expert for the equivalent design.

I have installed 10mH inductors on each of the AC inputs with a .1uF cap across the inputs. Also a 1500pF from the positive pin of the primary to the negative pin on the secondary. This was done on both power supplies with the end result of passing the conducted emissions test. Thanks for the help.

Mike