Relationship between SMPS input filter frequency and F(sw) and crossover frequency?

[treez]

Hello

What is the relationship between SMPS input filter frequency and F(sw) and also, crossover frequency?

is it that the crossover freq must be at least half as much as as the filter resonant frequency?

(the input filter resonant frequency as "seen" from the SMPS input)

 

[mtwieg]

Having a crossover frequency lower than the input filter resonance frequency can help ensure that the system is stable, but it also means that there will be greater feedthrough from input to output in the band between the crossover frequency and the input filter cutoff frequency. A well-designed supply will have the input filter resonant frequency below the crossover frequency of the converter. Stability is maintained by damping the input filter, often by using bulk capacitors with some nominal amount of ESR.

Here's a good document on the subject.
https://www.venable.biz/tp-12.pdf