How is the capacitance of a decoupling capacitor calculated in SEPIC?

Question

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I chose the topic of the sepic converter for a small abstract (abstract)
I found the material - https://cyberleninka.ru/article/n/razrabotka-preob...

It is not clear to me how a separating capacitor is selected.
1) Where is its capacity considered. It seemed to me one of the main parameters. I could not understand where 10 microfarads came from
2) What does the current have to do with it. The RMS current is calculated. But I could not find in any datasheet on the capacitor that there was something said about the current that the capacitor can withstand. Why is it calculated. Why this formula at all

On the last pages there is even a calculation, but there is nothing there about the capacitance of the separating capacitor.

Answer 1

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Answer 2

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Answer 3

1) The capacitance of the isolation capacitor is chosen so that the voltage ripple on it is small compared to Vin (<10% - normal)
2) The allowable ripple current (Ripple current) is an important characteristic, and it should be indicated in a good datasheet. If it is exceeded, the capacitor will overheat and degrade. Here I took the first datasheet that came across for Muratov's ceramics: https://search.murata.co.jp/Ceramy/image/img/A01X/... - there is even a heating graph depending on the current.
PS: The article you found seems to be a translation of the TI appnote https://www.ti.com/lit/an/snva168e/snva168e.pdf

Answer 4

1. The basic parameter for calculating the capacitance of an isolation capacitor is not the current through it, but the resistance of the parts of the circuit that it separates - Rout of the source and Rin of the receiver (at the signal frequency). As you know, the capacitor has a reactance Xc, which attenuates the signal passed through it. In order for the signal not to be weakened too much, this resistance must be much smaller than the above-mentioned input and output resistances (for definiteness, let's set it to ten times less, i.e., an order of magnitude). According to the selected Xc, it is already possible to calculate the capacitance at the lowest operating frequency. Well, you probably know the formulas for Xs.
On the Internet, I found a picture that illustrates the above well:

2. Nevertheless, sometimes there are cases when you have to deal with the current through the capacitor. This happens when a really large current passes through the capacitor, leading to its heating due to the so-called. equivalent loss resistance (this parameter must be in the datasheet, especially for electrolytic ones, but it can be measured on a real capacitor), but due to excessive heating and failure. But I don't think this is your case.