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This question already has an answer here:

In the power supply design, there are always smaller value capacitors placed at the output.What is the main role played by these capacitors in the module? Check the screenshot attached and I am referring to C7.

schematic

simulate this circuit – Schematic created using CircuitLab

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marked as duplicate by Edgar Brown, RoyC, JRE, Finbarr, StainlessSteelRat Mar 11 at 21:29

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    \$\begingroup\$ With smaller capacitors, you can create a mechanically-smaller loop, and reduce the inductance (perhaps greatly).This reduces the stored energy in a parasitic path, and reduces the amplitude of parasitic ringing voltage. \$\endgroup\$ – analogsystemsrf Feb 1 at 15:13
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    \$\begingroup\$ overly-short explanation: one is built for speed (little one), the other is for power (big one). \$\endgroup\$ – dandavis Feb 1 at 20:24
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That is done because the 100 uF capacitor isn't very "good" / doesn't work very well at higher frequencies. The 100 nF is much better at those frequencies, but has a smaller value so does not work for lower frequencies. The 100 uF capacitor takes care of those lower frequencies.

By using these two in parallel we create a "better" capacitor that works at both low and high frequencies.

It is explained in more detail by Dave from the EEVBlog in this video.

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