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Ceramic capacitor troubles

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Jason C.

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Hello,

I've got a little problem with a multi-layer ceramic capacitor that I'm trying to use. I'm using some as filter capacitors on a buck regulator circuit. The noise is filtered just fine, but it seems that too much current is conducting through them during their charge cycle (1st half of the arch on half sine wave).

If I change the capacitor to an electrolytic, the current moving through it is attenuated significantly. I believe this is due to the larger ESR on the electrolytic, but when I physically add series resistance to the MLCC, the problem is only partially alleviated.

Why don't I use the electrolytic? Because the application doesn't not call for components that will freeze (or become too hot and explode for that matter). I'm trying to build everything without the electrolytic, period. There doesn't need to be a real nice voltage on the line, hence I don't need bulk capacitors. The line does need to be filtered though, and that's where the MLCCs come in.

My question is this: Is there a way to make my MLCC act more like an electrolytic besides adding a little ESR to it? Is there a way to make my MLCC polarized (add simple components in series, parallel etc...), such that current only travels one way through it?

This is my first time here, so I really don't know what to expect for responses. Any help would be greatly appreciated though. I will be happy to explain more if there's a need to. Thank you very much in advance.

Jason
 

Hi
you didn t give enough information about the current in the capacitor to allow us to get adequate information about ur problem; but without knowing the currents you have no problem what you are saying is expected and normal if you want to reduce the current you can reduce the value of mlcc and I don t recommend that beside what you are saying is not a problem since there is no power losses nearly inside the mlcc ( very low ESR) .I believe ur problem lies in big amount of harmonics which generate the noise u are suffering from therefore reduce the harmnics by increasing the value of the smoothing capacitor. I wanted to pick up ur attention when choosing electrolytic capacitor choose 105 C instead of 85 C°
and i preffere using the capacitor written on it the RMS current because that is a good indication for a respectful manufacturer.
Regards
Mokhtar
 
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    ghaly

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I wanted to add to the previous answer that the RMS capacitor current is the ripple RMS current.and I almost agree to the previous answer .
 

Why does JasonC thinks high current through the MLCC is a problem?
Strange.
But I am wondering how he measured the current. It is not an easy task.
What is the switching frequency and how much current are you worried?
Regards,
 

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