I should mention that the input supply filtering to the DC-DC converter
should have good attenuation at the switching frequency, but not at low
frequencies, so there's no need to get carried away with the size of the
filtering at the converter input. Too much filtering, especially
inductance, can be detrimental, depending on the type of converter used.
If the converter is a simple chopper type without regulation, it will
put out in proportion to input, so line rejection will be poor. If line
ripple is an issue, then common solutions are brute force more filtering
with capacitance, or linear regulation at the input or output.
If a self-regulating type DC-DC converter is used, and uses PWM, it will
usually have pretty good line rejection, so eases the low frequency
filtering requirement. However, since its input resistance is negative
at low frequencies, it affects the choice of input filtering. Avoid
overdoing it with L-C input filters that cut at too low a frequency or
have high Q - it can result in oscillation. Lots of C is usually OK, but
not too much L.
Ed
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