Linear PSU with modern components, will it matter ?

[Kleinstein]

Fast switching and high PWM frequency would cause more RF "noise". However the higher frequency also makes filtering more efficient. It takes more care with the layout but filtering can do the job.  For the final performance the frequency of the SMPS stage does not have so much influence. It takes larger inductors and capacitors at lower frequency - so the higher frequency can be attractive for economic reasons.

In principle the lower frequency SMPS could be higher efficiency, but at a high price. So in the practical implementation a higher frequency SMPS with modern switches can be higher efficiency.

For high power modern parts make a multi phase SMPS circuit more practical. This can be more important than the frequency.

[exe]

So what are you saying is the old switcher that work at sub 100K Hz switching actually are better in our case here ?

Not really  as
1) modern designs can have less noise just because SMPS techology also advanced last years (e.g., smaller components, often integrated, better layouts => smaller loop area, less noise).
2) old design might not optimized for noise performance.

Ultimately, the only way to evaluate a design is to measure it (imho). And it's often not trivial, esp. if you are interested in the whole spectrum, not just lower 20MHz noise. That's because noise goes well into hundreds of MHz, a typical scope with passive probes simply not good-enough to do such measurements. Because of this most noise measurements done by mere mortlas are garbage. I've seen even one dude on youtube showing off his aliexpress SMPS. He limited scope bandwidth to 100kHz or something.

Anyway, as a base for a new design, I'd take a relevant app note or demo board. I've seen a few, even those claiming noise well below 1mV (but for limited power output and not sure if it was p-p or rms noise). BTW, look what I found: https://www.analog.com/media/en/technical-documentation/application-notes/an101f.pdf . An app note discussing post filters. Only looked at plots, seems to be relevant.

Also, most applications are not noise-sensite (as long as noise is within 10-30mV p-p). Although, for my lab I built a linear one just to be sure it's suitable for all the possible case I might have.

But if efficiency is not a problem, why not take a linear PSU blackdog's pre-regulator? It's proved design, work well for me. BTW, even with linear PSU it's possible to screw the noise. Like, it can get injected through mains or something.

PS one more thing, it seems a high-power and/or high voltage LDOs for postfiltering don't exists. That's my conclusion after reading hundreds of datasheets and many month of searching for components when I was designing a lab psu with buck converter + ldo post-filtering. More beefy regulators have less ripple rejection in 10kHz+ region. I'd say an LDO does nothing to filter noise that's above its control loop frequency. All the filtering about 100-200kHz is because of input/output caps, imho. Some datasheets even mention this explicitly and show how PSRR is affected by different output caps (different size / esr (esl?)).

PS for some ideas how to design a low-noise PSU one can have a look at modern high-freq scopes. They have tens of local regulators. Like, R&S. Just keep in mind right component selection and proper layout a critical for success. Just taking the same schematic will probably not work.

PPS at freq above few MHz "normal" 0805 capacitors become inefficient due to ESL. This can be seen in a datasheet showing impedance vs freq plot (murata, avx and kemet provide this). Paralleling caps of different size will make it even worse due to, uhm, parasitic resonanse? I can find a video explaining this, if needed.