Noise Filtration of DC / DC Converters

1. The small value ceramics are better at decoupling high frequency noise. The electrolytics work better at lower frequencies. Thus the combination of the two is better than either on it's own.
2. Experience, trial and error, looking at manufacturers data sheets, or calculation and subsequent adjustment by trial and error.
3. It's not just what you put in but where...see #2. Layout, especially of earth/ground tracks is critical.
Del

BTW... If you read the instructions on posting questions you wouldn't have posted twice. This could be indicative of your design problems....

_{Claws gradually retract. Cat, naps.}

Nice job europium, I'd give you a GA but you're well past the GA threshold.

One thing though that makes designing switching supplies interesting. The accurate equations you cite imply that increasing the switching frequency will permit smaller value filter components to obtain the same ripple response. This is true as long as the circuit remains a lumped element design for the higher switching frequency and its harmonics. In other words as the switching frequency increases, the parasitic inductors, capacitors and resistances may no longer be trivial components. Add to that the complication from possible coupling to free space, at higher frequencies short wires become antennas, and you'll see that a low noise switching supply can seem like an oxymoron.

This detail is getting beyond the original question, but you did such a superb job europium that I just wanted to dot a few of the remaining i's and cross a few t's.

Again, nicely done.

Thanks.

Btw, I'm building a nanoswitcher that uses optical waveguides exclusively. To hell with microstriplines. Gawd, what a bother!

Seriously, though, I didn't want to bog student1 down with details he/she won't encounter for a few years yet. Best that student1 take a couple of courses in RF circuit design before engaging in discussions about distributed impedance and all that other crap you need to know about just to design a simple motherboard these days (remember the S100 bus? I hope not). By then student1 will probably be a CR4 Mage answering our questions!

"... and electrolytic ..."

student1, if you are actually planning to build this thing, and if your switching frequency is in the 100s of kHz (or more), it is highly recommended that you use tantalum capacitors instead of electrolytic ones. Tantalum capacitors do a much better job of filtering at high frequencies than do their sluggish, electrolytic cousins. You will still need the ceramic cap for the reasons Del* gave earlier.

^{_{(*Happy now? )}}

Regards

"

1: why ceramic and electrolytic both sort of capacitors are usaully placed in parallel in the filter circuits of dc to dc converter?"

electrolytics are Hi capacity storage & ceramics are "Low-Series-Resistance" to pass Hi freq specially pulses [containing hi-harmonices]