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I am trying to understand which of the two (buck or boost) in general more/less noisy. For example I've got a buck converter with Vin=16,8 V and Vout=4,5 V and a boost converter with Vin=4,2 V and Vout=5 V.

  1. Let's imagine they power the same device consuming e.g. 1 A, this means that the consumption is respectively
  • 4.5 W for buck converter
  • 5 W for boost converter

This means that the input current is

  • roughly 4,5/16,8=0,26 A for buck converter
  • roughly 5/4,2=1,2 A for boost converter

As far as I understand (as an amateur) higher input current effectively means higher voltage spikes at the MOSFET switch time.

  1. Another reason for this speculation is switching frequency. AFAIK it's higher for buck converters which means it's better smoothed by the output capacitor.

So the question is whether my assumption is true and if not, what should I take into account when speculating about noise level?

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A buck converter is generally designed so the inductor current is continuous into the output capacitor, albeit with some ripple current.

A boost converter operates with intermittent current, with its current either being increased when the transistor is on, or going into the output when the transistor is off.

So it's the difference in the current into the output capacitor being continuous for a buck converter, and intermittent for a boost converter, that can make the output ripple voltage of a boost converter higher.

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