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the Boost converter figure is like this

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  • \$\begingroup\$ Hi, it is always nicer to add a few sentences with "please'' and "thank you", especially when posting for the first time. Check out this link cbasso.pagesperso-orange.fr/Downloads/PPTs/… and apply the technique for the boost converter. The input capacitor plays no role as long as the source impedance is close to 0 ohms: \$V_{in}\$ is always considered constant in small-signal analysis except when you insert a front-end filter. \$\endgroup\$ – Verbal Kint Dec 22 '18 at 12:28
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    \$\begingroup\$ @VerbalKint, "please" and "thank you" aren't required here. But a few sentences saying what solution OP already tried and why the results aren't satisfactory would be helpful. \$\endgroup\$ – The Photon Dec 22 '18 at 17:21
  • \$\begingroup\$ @The Photon, I am probably too old fashioned to believe these simple words still have a meaning when you start a discussion in a forum. No problem with me. \$\endgroup\$ – Verbal Kint Dec 23 '18 at 8:49
  • \$\begingroup\$ @VerbalKint thank you Sir for your explanation It's the first time that I ask in a discussion.I 'm working in controlling a photovoltaic system and I have a lot of difficulty with the boost converter so I asked quickly. thank you Sir again. \$\endgroup\$ – Asma Dec 23 '18 at 17:45
  • \$\begingroup\$ Hi Asma, hey, no big deal! : ) Please check my other APEC seminars in the given page (2013 and 2014). I describe the PWM Switch model introduced by V. Vorpérian in 1990 and it is the best tool to study switching converters small-signal responses. It is, in my opinion, simpler to manipulate than state-space averaging technique or SSA. Besides, it predicts that the three basic dc-dc converters remain heavily damped second-order systems when operated in the discontinuous conduction mode or DCM what SSA originally failed to demonstrate. Happy Holiday Season! \$\endgroup\$ – Verbal Kint Dec 23 '18 at 18:05
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If the input voltage V is assumed constant, then there is no current through C1, and it has no effect on the circuit.

In the real world, whatever source supplies V won't be ideal, it will have an equivalent output resistance, so its output will drop depending how much current is drawn from it.

There will also be resistance and inductance in the connection from the actual power source to the input of the boost converter, which will cause V to vary as the drawn current varies.

In those cases, the input capacitor smoothes out the variation in current that needs to be drawn from the actual source, and reduces the voltage ripple at the converter's input.

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