Buck Converter Output Capacitor Voltage Rating

[Thewafflication]

I am working on a project with a micro controller that has an integrated DC to DC converter.  It appears to be a buck converter.  The output capacitor values are listed in the reference schematic, but the voltage ratings are not.
The output of the buck converter will range from 1.62V-3.30V.
The current capacitors I selected are X5R and X7R types.  I chose a voltage rating of 25V, but I fear the rating is too high and will result in an excessively high ESR.

 Can I get away with using a 5V caps?

[james_s]

5V caps should be fine, but I wouldn't worry about ESR with ceramic caps, they tend to have extremely low ESR. One thing you do need to keep in mind is that many of them lose a great deal of their capacitance when used with DC across them, that's a trait that has bitten me before and seems to not be nearly as well known as it should be. Also some regulators actually need capacitors with a bit more ESR in order to be stable.

[Zero999]

Yes, ceramic capacitors are notorious for losing their capacitance, as the voltage is increased. 25V seems like a sensible voltage rating, even if it's only going to be used at 3.3V.

[rachaelp]

For clarification on the DC bias effect on capacitance take a look at the following links:

http://www.kemet.com/Lists/TechnicalArticles/Attachments/191/Why%2047%20uF%20capacitor%20drops%20to%2037%20uF-%2030%20uF-%20or%20lower.pdf
https://www.murata.com/en-eu/support/faqs/products/capacitor/mlcc/char/0005

As a practical example take a look at the following datasheet for a 10uF 4V X5R ceramic capacitor:

http://ds.yuden.co.jp/TYCOMPAS/ut/detail.do?productNo=AMK107ABJ106MAHT&fileName=AMK107ABJ106MAHT_SS&mode=specSheetDownload

On page 2 you can see there is a graph for the DC bias showing how the capacitance drops off based on the applied voltage. You can see from this that for your 3.3V rail this capacitor would lose about 55% of its capacitance. If you were using this for a 1.1V rail for something like a CPU or FPGA core then it would only lose about 10% of its rated capacitance.

Now looking at a similar part with a higher voltage rating, 10uF 50V X5R ceramic capacitor:

https://media.digikey.com/pdf/Data%20Sheets/Taiyo%20Yuden%20PDFs%20URL%20links/UMK325BJ106MM-T_Spec.pdf

On page 2, again on the graph for the DC bias you can see that for your 3.3V you'd actually be seeing a slight increase in the rated capacitance of a few percent.

Of course, I am not saying either of these capacitors are right/wrong for your application, this just illustrates the effect of DC bias on the rated capacitance. You could use lower voltage rated capacitors if you increased the capacitance value to compensate for the DC bias effect. Personally I'd probably just choose to go with a higher voltage rating and be done with it or maybe use a different type of capacitor which doesn't exhibit this DC bias effect.

** Value increases/decreases quoted above are for DC bias effect only. You'd need to factor in capacitor tolerance, etc into your capacitor choice too.

Best Regards,

Rachael

[David Hess]

The current capacitors I selected are X5R and X7R types.  I chose a voltage rating of 25V, but I fear the rating is too high and will result in an excessively high ESR.

The packaging has more effect on the ESR than the voltage rating.  For practical purposes, the ESR of the ceramic capacitors can be ignored unless the application involves very high currents.

Can I get away with using a 5V caps?

For 3.3 volts I would probably derate to at least 6.3 or 10 volt capacitors for greater reliability but 5 volt parts should work fine also.  Higher voltage parts have a lower voltage coefficient of capacitance which may or may not matter but why take chances?

[Faranight]

Hello. I know this is a bit dated thread, but I found it over google, and I thought I'd just ask here rather than starting a new thread.

I am also working on a DC/DC buck converter, and I was wondering about the voltage ratings of output caps. The texts and application notes I've read usually state that the buck input capacitor should have its voltage rating about twice the maximum input voltage. For example, if I'm using 24V as input, I should select 50V input filter caps. This is to be on the safe side due to possible voltage spikes during the mosfet switching phase. But none of the texts that I've read mention anything about the voltage rating of the output caps. So I'm wondering in confusion what's the rule here. Should I go for output caps with a voltage rating twice the max input voltage? Should I go for a voltage rating twice the maximum output voltage? Or should I go for other value??? Is there any relation between max input voltage and the output cap voltage rating?

[David Hess]

Output capacitors may have a larger voltage rating than strictly needed to get a higher ripple current rating.

[james_s]

I typically choose a capacitor with a voltage rating about 50% higher than the voltage I expect it to see. There is nothing wrong with using a higher voltage part if you have space though. Sometimes I use 25V caps on both the input and the output just for simplicity. If you're building thousands of something then it pays to shave pennies but for a hobby project the savings are usually negligible.

[Faranight]

There's also a problem of availability. The bigger the capacity, the fewer caps are available at higher voltages.

[james_s]

Then use a lower voltage, as long as the capacitor is rated for at least the expected voltage plus maybe 10% it will usually be fine.