# Ceramic output capacitor selection for ripple current (SEPIC)

I'm designing a SEPIC converter based on a wide input voltage range SMPS controller (LTC3758). The requirement is for a wide input voltage (10-60V) with a fixed output of 28V @ 1.5A. I'm trying to determine Cout selection which per the data sheet, is the same for a SEPIC converter as it is a Boost converter.

I've calculated an output ripple current of around 2.4A which appears difficult to achieve using electrolytics in a small form factor and given I require an output capacitance of 17uF I would like to use ceramics.

Electrolytic's often have a ripple current rating specification in their data sheet however, ceramics seem to not. Is this due to their inherently low ESR, and if so, how does one determine if a ceramic capacitor of an appropriate voltage rating and capacitance is suitable?

For example I've found a particular X7R 10uF 35V ceramic cap - how can I determine the ripple currents effect on it's performance?

• Frequency and capacitance influence voltage ripple as the other equations in the datasheet suggest. The capacitor RMS current only depends on the output current and duty cycle, so i think the datasheet is correct. Why do you think 2.4A is high and problematic for SMD caps? Commented Aug 1, 2021 at 7:07
• Thanks @LarsHankeln, I've updated my question to better represent the problem I'm trying to solve. Ultimately what I don't understand is why ceramic caps don't have a ripple current rating and what the limits are. Say, if I had a circuit with 100A of ripple current and a requirement for 10uF, surely a 1210 would fail. How does one determine when this happens (and why)? Commented Aug 3, 2021 at 11:07
• As a first estimate the thermal dissipation of ESR x I^2 / 8 will be one limit for ripple current. If you assume half a Watt for a 1210 and an ESR of 10 mohm you can guess 20 A of peak to peak ripple current. Commented Aug 3, 2021 at 11:47
• @AlexTurner I posted about ripple current for ceramics some time ago: electronics.stackexchange.com/questions/542464/… Commented Aug 3, 2021 at 12:37