EMI filter questions

[BobM3]

I'm playing with a program from Power Integrations that designs simple flyback power supplies. I'd like to understand how it comes up with the EMI filter (between the 115/230 VAC input and the primary leg of the transformer). As a general question, what do you use for the impedance of the incoming 115/230 VAC line? I'm guessing a small (<1 ohm) resistor and maybe a small inductance?

How about on the other side of the filter with the transformer and FET transistor? Would you typically take as its impedance the primary winding inductance and resistance?

Finally, given the large difference in impedance between the line and the transformer I assume you don't worry much about matching impedances (line impedance, filter impedance and load impedance). Is this true?

 

[VEBill]

Google "LISN".

That's probably what you'll be tested against.

 

[VEBill]

"...matching impedances..."

You only match impedances if you wish to avail yourself of the maximum theoretically possible power. When connected to the power grid, that would be a very bad idea.

 

[BobM3]

Don't you also match impedances to critically damp resonances?

 

[VEBill]

That's pretty far outside the context of your question, "I assume you don't worry much about matching impedances (line impedance, filter impedance and load impedance)."