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Correct voltage rate selection

JohnsonMiller , 01-05-2017, 11:11 AM

For multi layer ceramic capacitor, how do you select the proper voltage rating? For example application is 1.8V power supply decoupling.
From basic course, by 20% tolerance I may select 4V > (1 + .2) * 1.8V = 2.16; however when I see the reference design from major companies they prefer to use 16V or even 25V!
Do I need to consider so much over design in this case on ceramic capacitor (with X7R or similar dielectrics)?

Other issue related to this capacitor is their hear able noise under high voltage (max 35V), what is reason for that? One suggestion say that it is due to piezoelectric nature of dielectric material, do you agree?

robertferanec , 01-06-2017, 11:29 AM
- For cheap capacitors (under 1uF) I almost always go for 50V. The price is low, but I can use them over multiple projects and I only keep one real in stock (dont have to buy reals for different voltages).
- For bigger capacitors (e.g. 100uF) - this depends. In some applications you may want to use double voltage (e.g. in some switching applications you can get double voltage peaks). Also, you may want to have a look at characteristics showing capacity vs temperature. These may differ between capacitors for different voltages. But definitely, you may want to use capacitors on higher voltage than is the voltage of the signal where you are connecting it.
- is not this an inductor noise? Maybe some old big aluminium (or other kind of capacitors) can make noise, but I am not sure about ceramic / tantal / etc. From my experience the noise is usually generated by an inductor - but I mostly design digital boards. Maybe in other kind of boards there may be other sources of noise.
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