- Given the required attenuation for a given frequency band, how should I design the EMI filter?
You design the filter based on the required attenuation needed for the frequency bands in question. You appear to be asking for a general solution but, I'm saying that for a particular requirement, you design the filter to suit that requirement. I'm sorry that you might think this isn't helpful but, sat where I am, that's how I do it. To try and gives rules and options for an unspecified requirement is unreasonable and impractical.
For the specific filter design shown in your question, here's what it does generally: -
Breakdown each one in turn if you want to learn about them - using a simulation package to mimic the behaviour is something I highly recommend. Without a good data sheet for that filter, I wouldn't begin this process. Collect the information, understand it then simulate.
You study the requirements for the filter and figure out what topology is likely to be most successful. This may mean a fairly standard filter as per the one shown in your question or, it might mean doubling up on those filters to cover overlapping bands to obtain maximum spectrum-wide performance. Note here that I'm suggesting that a single simple EMI filter can easily run-out-of-steam in the higher spectrum but be perfectly good in the lower part of the spectrum. Sometimes, it's necessary to put different inductors in series so that one inductor (that is good for the lower spectrum) is supplemented by another series inductor that is good for the higher part of the spectrum. I'm talking about problems of self-resonant frequency and how this factor can morph an inductor into a capacitor and therefore let higher frequency interference through at ease.
- How to choose component values?
I use a simulation package (as do most pros) and that means starting off from a blank sheet and developing ideas that eventually yield component values. It's a suck-it-and-see approach. Sure, the experienced guy will have something in mind as a starting point so, as you gain experience, this process becomes easier but, for the 1st time designer, it's a daunting process that rapidly gets easier as you start to believe in yourself.
- Are there any SW packages available for filter synthesis and/or simulation to aid the design?
Absolutely yes. I use micro-cap 12 - that's a download link. Many people use LTSpice and there are others but, I'm an experienced micro-cap user and wouldn't bother with other sims. But, the biggest mistake is not understanding the real characteristics of inductors IMHO. Make you models with great care and you'll get good results. Also, you might be surprised to find how relatively easily you can model the differential current waveforms that you design produces. You don't need a great degree of precision here; if your current waveform is roughly mimicked to within 10% of actual that will be good enough for use in a simulator. You must also model your LISNs too - that's really easy so don't skimp on the simulation is my advice.
- I am comfortable with spice simulation, but without a methodical way to choose topology and component values this becomes hit and miss
Yeah, for sure it's hit and miss but, so is any design process and I'm not just talking EE designs. That's what design is all about. That's what makes design so interesting (and frustrating). Get used to it. I mentioned the "suck-it-and-see approach" approach earlier.
I am not aware of an analytical approach to multi-stage filter design
There are many, many analytical approaches; you just haven't found them or not recognized them yet. The main one that springs to mind is cascading pi filters to get massive differential-mode attenuation above certain frequencies but, as with any theoretical approach, the devil isn't in the theory but in component selection and understanding component limitations.
From comments, this arose: -
my intention was to ask how I could go about mimicking the attenuation
as plotted in the graph, using the circuit topology as shown
The data sheet says this: -
- Total Common-Mode Capacitance 2 x 9.4 nF
- Total Differential-Mode Capacitance 336 nF
This tells me that you can assume this: -
- C3 = C4 = 4.7 nF
- C1 + C2 = 332 nF (336 nF minus C3 in series with C4)
The data sheet also says this: -
- Tcase = 25ºC 230 mΩ (series resistance of filter)
- Zero Load, 115 Vrms 60 Hz 0.1 W
- Zero Load, 115 Vrms 400 Hz 0.6 W
Hence, you can start to figure out the values of series resistance especially in those inductors that I've marked L1 above. From the attenuation graph you can see that at about 1 kHz the DM attenuation is about 7 dB so you can double check the numbers and, because this is such a low frequency you can assume that the inductors are not playing a significant role in opposing currents.
The front page of the data sheet says this: -
- Differential & Common-mode Output Attenuation >40dB @ 250kHz
But, if you are choosing values, it should be more like 40 dB attenuation at 120 kHz: -
Then, just keep plugging away at it covering every little hint in the data sheet you can find. Cross-check the numbers and you're good to go. It'd probably take me 4 hours but, it might take you 2 days. That's the way it is unfortunately; experience trumps inexperience!