I'm starting to get a better feel for this. I ran a few simulations and this is what I observed:
First simulation is the response of the PI filter in the RECOM document with input and output as shown.
- Looks similar to image in post 7
- fr at about 4kHz
- ESR of C2 has very significant effect on the Q of the LC. makes we wonder if its not wise to add a small value resistor in series with Cd to allow for control of Q just in case stability issues arise?
2nd simulation is looking the other way, how much of the 370kHz noise from the converter makes it back out to the line?
- the filter seems quite effective >50dB @ 370kHz
- The value of C1 is interesting:
- if too small say 3uF the resonant peak starts to grow significantly and fr shifts up slightly
- if large the peak disappears, and the resonant peak shifts down slightly.
- so it appears a large input capacitor (C1) is both good for stability and EMC reduction
1) Does this sound correct, or am I out in left field again?
2) If I have two of these SMPS both running at about 350kHz, is there any reason I can't feed both from one filter? I'm mostly concerned with impact on stability of the SMPS's.