Electronics > Beginners

Switching frequencies and noise in DC-DC converters

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T3sl4co1l:
Ehh, RFI is typically "fairly periodic and constant", too.  Maybe a more practical definition: an undesirable signal.  It's not exactly white noise if it's a modulated carrier buzzing at 120Hz (as so many PSUs create), but it's not helping your radio reception, or your high speed ADC, or whatever, in terms of reading some other signal that is desired.

Thinking about "noise", there's another aspects to RFI, that's noisy (random), but in a different way: structurally, rather than temporally.

It's difficult to predict the response of an arbitrary circuit.  Even in a design structured to constrain the RFI output, there is uncertainty in just how much will be present at any given point, and how much will conduct out the connecting cables, or radiate off the circuit itself.  The errors lie in uncertainty over component parasitics, circuit layout, phasing between different regions of the board, etc.

In the same way that you might guard-band an intentional radiator for some minimum SNR, so too, you want to guard-band the design of an unintentional radiator, to ensure you're some minimum factor below whatever constraint your design has (usually regulatory, but also functional, say for that sensitive high speed ADC).

Tim

Ice-Tea:
This is a classic example of ripple as you would get in a rectifier. You will find something comparable in a buck DC/DC.



This is switching noise superimposed on that ripple. This is stuff you can get rid of with series resistors in the gate drive, decreasing parasitics, whatnot. Two different things.

Kjelt:
With high frequencies beeds can be a lifesaver, low ohmage for DC , high ohmage for high frequencies.
Saved me a couple of times, but hard to simulate or does someone have a tip for that?

analogo:

--- Quote from: Ice-Tea on July 23, 2018, 08:56:51 pm ---This is switching noise superimposed on that ripple. This is stuff you can get rid of with series resistors in the gate drive, decreasing parasitics, whatnot. Two different things.

--- End quote ---

IIUC, higher switching frequency means less ripple but more noise. However the noise can be filtered out more easily at higher frequencies (says Tim/T3sl4co1l).

Did I get it right?

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