Input ripple

A little later, after we have better understood the current waveforms in the input capacitors, we will do a calculation to correctly compute the amount of bulk capacitance based on the 1% input ripple criterion mentioned above. 

Figure 2-11 Input Ripple of a Buck Converter Showing the Effect of a Non-ideal Bulk Capacitor...

The rule-of-thumb is to pick an output capacitor with a ripple current rating equal to or greater than the worst-case RMS capacitor current calculated above. Its voltage rating is usually picked to be at least 20 to 50% higher than what it will see in the application (i.e. Vin.max for all topologies). The input voltage ripple of the converter is also usually a concern because a small part of it does get transmitted to the output. There can also be EMI considerations involved. In addition, every control IC has a certain (usually unspecified) amount of input noise and ripple rejection, and it may misbehave if the ripple is too much. Typically, the input ripple needs to be kept down to less than 5% to 10% of the input... 

Ripple rejection A measure of how well the regulator attenuates the input ripple, usually specified in... 

Zero-Input-Ripple Boost Converter followed by an Isolated Multilevel Inverter and a Forced Cycloconverter... 

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