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I understand (at least at a high level) how to use both capacitors and inductors to reduce noise but I don't understand the advantages/disadvantages of each approach or why one is chosen over the other.

For example every LDO recommends a capacitor on the output to reduce the ripple noise and I have experimented with uC PWM with a capacitor to make a smooth output voltage.

In every buck converter I had purchased they always use a inductor in series to reduce the ripple (with other components.)

Is there something obvious I am missing why for example a switch mode regulator/power supply couldn't reduce ripple using just well matched capacitors?

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    \$\begingroup\$ You need an inductor and a capacitor. The best way to explain their function depends on the topology in question, though. \$\endgroup\$
    – Hearth
    Commented Jun 29, 2022 at 0:21
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    \$\begingroup\$ The obvious thing you are missing is that an ideal capacitor does not act as a filter to an ideal voltage source unless there is a series element between the voltage source and capacitor. If that series element is an inductor, you have the conventional arrangement. \$\endgroup\$
    – user57037
    Commented Jun 29, 2022 at 3:02

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In a switching regulator (buck, boost, buck-boost etc.) the inductor isn’t there to smooth the output. It’s there to store and release energy in a controlled way. The input switch will ‘charge’ the inductor (build flux), then change the circuit to let the inductor ‘discharge’ (collapse flux) into the output.

Switching regulators also have a filter capacitor to help with the ripple that happens with this store-and-release process.

It’s also possible to use capacitors for energy transfer. Some topologies (like Ćuk or SEPIC) use them alongside inductors to achieve this. Capacitors are used by themselves in flying-cap step-up converters and in voltage multipliers.

Meanwhile, it is of course possible to use inductors as filters, even as pre- and post-filters for switching regulators.

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    \$\begingroup\$ I knew that was the case for Boost converters but was under the impression Buck was essentially just PWM with smoothing... \$\endgroup\$
    – AndrewT
    Commented Jun 29, 2022 at 1:27
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    \$\begingroup\$ The inductor also controls the rate of change of current. For a given pulse period, this limits the total change in current during each phase of a pulse. The result is to control current ripple. Keeping this within limits can reduce power dissipation. \$\endgroup\$
    – Charles B. Cameron
    Commented Jun 29, 2022 at 1:32
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    \$\begingroup\$ The PWM example you mention uses an RC filter. You could of course make that an LC filter, which at first glance kind of looks like a synchronous buck converter. But the goals are somewhat different: the buck uses feedback to modulate the PWM to achieve a target DC value regardless of the load and input voltage. \$\endgroup\$
    – hacktastical
    Commented Jun 29, 2022 at 1:37
  • \$\begingroup\$ Armed with helpful clarifications here I found information online that confirms this. Thank you all for you input. \$\endgroup\$
    – AndrewT
    Commented Jun 29, 2022 at 1:45

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