# Selecting capacitor for SEPIC converter

I'm looking for an advice how to select capacitors for my SEPIC converter. I have done calculations, but I haven't find Cin, Cs and Cout yet. What seems like a problem is that there are few work papers that recommends to use capacitor "capable to endure calculated current ripple". How can I be sure that the selected capacitor would endure calculated current ripple? I know that ESR is what produces heat in capacitor, but how should I know if some ESR won't produce enough heat to overheat my capacitor? For example, I got that RMS current for an input capacitor is 2.2A, and some voltage ripple condition tells me that the capacitance of that input capacitor must be greater than 180uF. So I'm having a problem how to choose those current capable capacitors on my own. Can someone helps me?

• You mean Electronic Series Resistance (ESR). – Voltage Spike Aug 26 at 16:48
• Yes, I edited that mistake – Marc Paul Aug 26 at 17:20

## 1 Answer

How can I be sure that the selected capacitor would endure calculated current ripple?

Many capacitor vendors will specify the maximum ripple in their datasheets.

For example, in this datasheet from Kemet, the maximum ripple current (for 105 C ambient at 100 kHz) is given in the "RC" column:

If you have a ripple voltage requirement, you will have to determine the ripple current from the capacitance value and a little calculus ($$\I=C\frac{dV}{dt}\$$).

Instead of choosing a badly specified part from EBay, choose one with a proper datasheet, so that you can be sure the specifications meet your requirements.

I know that ERS is what produces heat in capacitor, but how should I know if some ERS won't produce enough heat to overheat my capacitor?

This will depend on the ambient temperature around the capacitor, how it's mounted to the PCB, etc.

So be sure you understand what conditions the vendor used to specify maximum ripple, and if your specifications are more strenuous (higher ambient temperature, poorer heat sinking), de-rate the part appropriately.

• Well I need 200uF capacitor that can endure ripple current of 2.2A at 100kHz, but I couldn't find any capacitor like that – Marc Paul Aug 26 at 17:31
• @Marc, you might need to use several smaller caps in parallel. 10 22-uF caps, with 220 mA ripple current capability, for example. – The Photon Aug 26 at 17:33