Mitigating switching noise in a boost converter. Raise or lower switching freq?

[Pack34]

I'm working with an LTC3787 boost converter and I'm trying to reduce the amount of switching noise making its way around the board. I know that there's a handful of layout changes I can make, but I'm trying to find as many tweaks with the circuit as possible.

The general tradeoff for switching frequency is generally the greater the switching frequency, the greater the efficiency, but the output is generally noisier, correct?

[T3sl4co1l]

Noisier in what ways?  Higher Fsw is easier to filter the ripple (fundamental and low harmonics), at the expense of greater switching loss (and somewhat higher average HF noise).  Commutation noise (that associated with the switching event itself) depends only on the layout and filtering (the peak value of the noise, that is).

We typically call the former "EMI", and the latter "RFI".

An EMI choke (common mode) typically has little to no effect on RFI, partly because it is hardly effective at such frequencies (most chokes are effectively small capacitors at that frequency), but also because the equivalent circuit coupling into the conducted or radiated emissions is different at those frequencies.

EMI is easily filtered, through DM and CM filtering and conventional components; RFI is harder to filter, requiring a more carefully constructed filter with a compact star grounding topology and shielding if necessary, or can be treated at the source by dampening the switching edges or improving layout.

Tim

[Pack34]

I'm seeing a lot of high-frequency noise in my lower voltage supplies on the board. I know that with the existing design the low voltage regulator goes under the high voltage boost and is causing issues. Through initial testing of increasing-and-reducing the switching frequency of the boost converter, the high-frequency noise is lessened on the 3v3 digital rail and the 2v5 analog.

The issue is mainly the noise being measuring on some ADCs, with the artifact being removed once the switching frequency of the boost converter was reduced.

[T3sl4co1l]

Circuit?

Tim

[bob91343]

Changing switching frequency won't do what you want.  The transistion speed will remain the same.  To change the spectrum of the noise significantly requires a different topology or much faster (or slower) semiconductors.