I need to choose a ceramic capacitor (ESR<10mOhms, 5VDC) for my buck converter application, whose value should be around 300uF. The reason for going for a ceramic capacitor is the low ESR they have. Also my converter would be operating at 100kHz. Off course these capacitors can be placed in parallel (for eg three 100uF caps in parallel) to give equivalent capacitance as well as low total ESR.
Now the confusion is that the capacitors data sheets are very very vague. Vague in a sense that all of them don't give the ESR directly. While searching for it on the Google, I read a post on this website where this issue has been discussed that some caps data sheets give DF instead of direct ESR, and from that D.F we can calculate the ESR knowing the frequency at which the capacitor is operated.
Also some data sheets give the max ripple current and mostly do not. And this is the most confusing for me. How can i select the capacitor for my buck converter application (max 40A current)? I mean is this current ripple rating of the capacitor of my concern or not? As my thought earlier about selecting a capacitor was just keeping the capacitor value, the voltage rating and the ESR value in mind, but I'm not sure about that current ripple thing (which is not mentioned in most of the ceramic capacitors data sheet), if it is to be considered or not? Its been almost 3 hours that I am trying to find something suitable for my application, but unexpectedly I haven't found anything. Either the price is too high (more than 5$ for e.g), or the data sheet isn't providing enough information (some don't give ESR as well as D.F) and now I am confused about current rating (as this parameter also, is not mentioned in most of the data sheets).
Please help me out, you comments would be appreciated.
Thank you!
Best Answer
The ripple current in the output caps is a significant compared to the output current in a buck convertor running in current mode with a not too large output inductor In fact the peak to peak ripple current could be say 30% of the output current in this orthodox setup SO at 40 amps dc you need caps to handle almost 7 amps peak now the approx. current waveform is triangle so you are up for multiple caps in parallel Use big smd packages and remember that your control loop will change so you may get instability which mill mean changing compensation component values OR leaving the output electro in parallel with your new ceramic caps