EMI choke

well , with a common-mode-choke, you will know that you do not need to worry about it saturating due to the normal (differntial mode)current that is going into your smps...............cuzz common-mode-chokes dont saturate from diff mode current. It is only common mode currents that could saturate your common-mode-choke.......and if you have done a reasonably good job of your power supply layout (in fact, at 30W, even if you have done a poor layout job) you will only get tiny tiddley common-mode-currents which will not saturate your common-mode-choke...............................so your common-mode-choke can be small and be made of a nice soft ferrite...............................but as with anything that you insert into the line.......you have to make sure that the wire of the common-mode-choke is thick enough to be able to handle the rms current that is being drawn by your smps............so find your RMS current input, and check the common-mode-choke datasheet so you know it can handle that rms current without its wires getting far too hot........................so as you have rightly i believe insinuated......yuo can use a small common-mode-choke for a 30 Watts SMPS.................BUT............"STOP" i hear you say..........you are going to tell me about inrush current into SMPS's when they are first turned ON?..............................common-mode-chokes fortunately dont bring about inrush current themselves.........because the fields of the go and return current cancel each other and the common-mode-choke just looks like a piece of wire to the current that rushes into the input electrolytics at start-up................the thing is, the wire of the common-mode-choke should be able to withstand the inrush surge..and in normal cases this should be no problem.......................................it is the dreaded differential mode chokes that will need to be the big-boys....and they WILL bring about extra inrush current......................as you know well that when the SMPS is turned on the current will start flowing into the diff mode chokes........and what happens when current first goes into the coils of an inductor?..........it gets increased as the greedy domains of the inductor's core start screaming out to be magnetised and greedily pull loads of current to give them the field that they need to get magnetised up..........................but you need not worry about the common-mode-choke.................................common-mode-noise currents are usually above 1MHz and small in magnitude. -Remember the stray reactances that bring about common-mode-currents are tiny......so just use your tiny common-mode-choke

Is there a AppNote for designing the EMI filter and common-mode?

Thanks

Please see AN 15
http://www.powerint.com/sites/default/files/product-docs/an15.pdf

About your comment about the size of common mode choke. Pi Expert merely makes a suggestion... and based on the components available in the component library it selestc the smallest available component.
In reality you will need a wire that will see a current equal to IAVG as calculated by the software.