How resilient are X7R ceramic capacitors to short term overvoltages?

[Flyback]

How resilient are X7R ceramic capacitors to short term overvoltages.

In other words, say i have a 16V, X7R, 22uF sepic capacitor, and every time i switch on the sepic, i get a 20V spike across the 16V ceramic for about 30us......is that going to be ok?

How long will such a capacitor last.?

 

[crutschow]

You should never exceed the maximum voltage rating of a capacitor, even for a short period. It may not immediately fail but its life is indeterminate under such conditions. You either need some method of suppressing the spike or use a higher voltage capacitor.

 

[languer]

i get a 20V spike across the 16V ceramic for about 30us

You know, this is actually not that uncommon. That is one of the reasons you have the cap in there to begin with; to absorb (filter) spikes (noise) in power supplies.

What you have to look for. For a 16V cap (as an example) your normal operating voltage should be at or below 11-12V. That is about 70-75% below the rated voltage and is such that you ensure safe operation over temperature, humidity, possible over-voltage conditions, etc.

As for the 30us overvoltage condition at 20V; that is 25% above the rated voltage for a very short time. Good manufacturers (read - AVX, Kemet, ATC, Panasonic) rate their caps so they can operate at 300% of their rated voltage for a few seconds without dielectric breakdown. They are not saying to use them above their rated voltage, they are just measuring and ensuring the capacitor is built to sustain some abuse. So if you are operating the capacitor within its rated voltage (and preferably with some derating), and this 30us pulses have a very low duty cycle (i.e. less than 20%), then you are probably ok. Oh, and of course you are buying from reputable manufacturers.

If you're still concerned; of if you're using this in a critical application; then yes a TVS (for spike suppression) and/or higher rated capacitors may be better.

 

[Mr RB]

Sorry Languer but I'm with Crutschow on this one!

Gently "floating" a 16v cap gently at 30v for a couple of seconds may not be harmful to it.

But transient spikes etc have a lot of energy in a very short time, and spikes that up the voltage far above max are likely to punch through the dialectric.

It's like the difference between gently pushing on a wall at force above its max load, compared to hitting the wall with a wrecking ball at forces above its max load.

 

[languer]

I hear you, and I'm not going to say I completely disagree with you. You are correct; spikes can have different amounts of energy. But a 30us can equally carry 10's of joules as well as 100's of joules; you can't really tell. So yes, the generalization I made is perhaps too "general". But in the industry I work we life test components (capacitors and protection diodes in particular) to quite severe lightning waveforms (the ones which are very short in duration, quite repetitive, and can carry a lot of energy). We work very closely with the reputable capacitor manufacturers as well, and my statement was not just a gut feel, but based on measurements and facts which we have compiled for several decades. In particular you will find that ceramic capacitors are extremely tolerant to dielectric breakdown.

Having said all that, if you do not know the source of the spike; then yes good design practice says to clamp or dampen the spike before it hits the capacitor.

 

[Mr RB]

Thanks for the info.