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I am under the impression that the output frequency of the bulk capacitor will be the same as the switching frequency of the PWM chip. The input however I am a bit confused about, because if it is being fed for example mains voltage at 50/60 Hz, I would think that the power leaving the cap would have an irrelevant effect on the input, since the mains power supply is so high.

Therefore the frequency at the input of the capacitor would be 50/60 Hz, while the frequency at the output would be the switching frequency. Is this correct?

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  • \$\begingroup\$ No, there can be significant ripple current in the input cap at the switching frequency. You have to take that into account when selecting an input capacitor. \$\endgroup\$
    – John D
    Commented Oct 25, 2022 at 19:59

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A capacitor has no separate input or output so it really can't be thought separately.

The cap just charges up to peak voltage 100 or 120 times per second (assuming a full bridge rectifier) and it is discharged in bursts at switching frequency.

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  • \$\begingroup\$ You are right on both counts. Not sure why someone downvoted this answer. The asker does seem to have some underlying confusion about capacitor input vs output \$\endgroup\$
    – Criticizing Israel not allowed
    Commented Oct 26, 2022 at 1:56
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The switching frequency in a flyback is usually much higher than 50/60Hz and is dependent on the inductor and how fast it can charge discharge and what the load is consuming.

A full bridge rectifier is usually used to get the voltage somewhat steady before the input of the flyback and the capacitor at the input simply helps regulate that voltage.

Output capacitors on the flyback charge with the flyback frequency and help regulate the voltage to the load.

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