voltage limit of ceramic capacitors

[Simon]

With electrolytic's we always leave lots of margin on the voltage rating. How about ceramic's? we you have a 48V system it feels kind of silly to put 100V ceramic capacitors in, but is 50V cutting it too fine?

[TimFox]

How much more expensive are 100 V ceramics compared to 50 V or 63 V?
Two issues:
1.  Can you guarantee the 48 V line will never, ever, exceed 50 V?  If it spikes up and takes out a capacitor, you can't blame the manufacturer.
2.  As often pointed out here, unless the capacitor is C0G/NP0, it will have a voltage co-efficient of capacitance (decreasing with increasing voltage).  The 50 V rating is a non-smoke rating, and for a Z5U (extreme case), the decrease in capacitance up to the manufacturer's rating is substantial.  A good Vishay Z5U SMT capacitor will fall to 30% of its zero-voltage capacitance at full voltage (50 V).  https://docs.rs-online.com/ce09/0900766b801f8531.pdf

[T3sl4co1l]

Safe rating only.

In practice, one might withstand 2x, 3x, even 10x or more.  But only on an individual basis; you'd have to test each one for that.  None should fail at a test voltage of, or below, the rating (or actually some times it, check the datasheet), which is really what the rating is about.

Actual value in circuit, is a characteristic not usually put into the datasheet-as-such.  It may be in the datasheet itself, it may be linked from the supplier, you may have to look it up on the manufacturer's site.  Offhand, Samsung, Taiyo Yuden, Murata, KEMET, TDK and a few others have these data available.  (Samsungs are usually linked from Digi-Key, but the links don't usually work, so...)

The C(V) curve largely depends on size, larger and thicker chips having higher C at given V.  I wouldn't recommend shopping for anything worse than X5R, and it doesn't run up to much temp so you may prefer X7R or better.

C0G and other type 1 dielectrics do not depend on voltage, and have a specified tempco.

AFAIK, voltage rating doesn't help any with respect to crack formation or the like.  If that's a problem, consider smaller chips (typically 1210 to 1812 is considered the reasonable upper limit), flexible terminations, or other mitigation (like series pairs or more, or the kind on leadframes ($$)).

Tim

[Simon]

Well it's a 48V power supply, I'm looking at using them in a filter to a motor controller so I'm trying to get the most capacitance I can for the space available, the motor controllers have a lot of ceramic capacitors on them and the manual does say that extra bypassing is unnecessary but feel free to put an electrolytic close by if the path to the power supply is long. Well having put an inductor in I have kind of made that a long path so just want to make sure that locally the motor controller is suitably buffered.

[BrianHG]

Careful when running ceramic cap to their peak voltage.  It is not that you may burn them out, it is that their capacitance drops when applying a DC voltage close to their limit.

IE: If you are using a 50v ceramic cap at 48v and need 10uf, it may only be giving you around 5uf to 6uf whereas if you used a 100v ceramic cap, at 48v dc, it's capacitance may have only dropped to around 8.5uf.

This drop in capacitance is usually associated with the cap's grade.  The higher quality data sheets usually have charts illustrating the cap's grade VS % drop in capacitance VS DC voltage.

[Simon]

Yes I am looking at the combination of what is available and if I go lower voltage but end up with more capacitance in the same space given the capacitance drop it may be worth it but I'm trying to aim higher as I'm unlikely to get 50V X7R caps in the current supply chain situation.

[wraper]

There is more problem with them losing capacitance with applied voltage. If they have high capacitance for the size, there will be barely any capacitance left under high voltage bias.

IE: If you are using a 50v ceramic cap at 48v and need 10uf, it may only be giving you around 5uf to 6uf whereas if you used a 100v ceramic cap, at 48v dc, it's capacitance may have only dropped to around 8.5uf.

More often it would be like 2uF in the first case and 5uF in second.

[tggzzz]

What current will be going through the capacitor?
What power will be dissipated in them?
What temperature will they be at?
How do those compare with what's in the datasheet?

[wraper]

Do not forget about tempco and ageing too. After years pass likely there will be only 60-70% of initial capacitance left.

[davidrichard4748]

Now for the 48V power supply. I want to use them in a motor controller filter, so I'm trying to get as much capacity as possible for the available space. However, if the supply path is long, feel free to place the chlorinator nearby. Once I put in the inductor it worked. I just want to make sure the motor controller is properly buffered.

[gamalot]

If the working voltage is 48V, I strongly recommend choosing ceramic capacitors with a withstand voltage of 100V, because their capacitance may drop significantly after applying a bias voltage, read this:

https://www.analog.com/en/technical-articles/temperature-and-voltage-variation-ceramic-capacitor.html

[Siwastaja]

If the working voltage is 48V, I strongly recommend choosing ceramic capacitors with a withstand voltage of 100V, because their capacitance may drop significantly

I don't understand why to do this in such roundabout way.

If you need certain amount of actual C, look up the bias curves. Assuming a 100V rated part would deliver more C than a 50V rated part has statistical chances to go right, but it's still just a guess. If they are in the same package (height included) and same rated capacitance, difference in actual capacitance at 50V might be small or none.

Volumetric energy density (i.e., package size given the same C) is already better indicator than rated voltage, but even that is iffy.

Only if bias curves are not available and you can't afford buying parts from manufacturers who publish them (like TDK), then you need to guess, or measure.


In any case, Simon, concentrate on the following:
* Make sure you get enough actual C under your 48V bias. Try to find parts with this information available, no guessing.
* For large-C DC links, often it's a good idea to calculate uF_actual per dollar or per area or whatever. It may happen a part rated to smaller capacitance offers more bang for buck! Quickly done in Excel.
* Especially if parts are bigger than 0805, pay extreme care to avoid any board flex.
* Consider soft terminated parts, if you can afford them. For high reliability, it's much better investment than extra voltage rating.
* Remember, ceramics fail short, so if the upstream power supply can supply more than >10W, consider each capacitor as ignition source!
* Soft terminated parts greatly reduce this risk, but it's still not zero. Make sure there are no combustible materials nearby.
* Finally, if all these check out, 50V rated part is just fine on 48V bus. MLCCs can be used over the max voltage rating, but obviously managers do not like that, so keep within ratings. But no derating is needed.

[gamalot]

If the working voltage is 48V, I strongly recommend choosing ceramic capacitors with a withstand voltage of 100V, because their capacitance may drop significantly

I don't understand why to do this in such roundabout way.

...

Just because usually high voltage means larger package, unless the required capacitance is very small.

[jonpaul]

depending on the cost, quality and reliability tradeoff for motor driver, industrial applications I would get the best brand part and de rate 50%

We used 100k pcd Fuji 3kV parts for HV DC, never a failure

Choice of ceramic is critical eg big difference between low, hi k like NPO, vs Z5U

great brands are TDK, Céramite, Fuji,

I would avoid cheap Chinese caps.

Jon

[T3sl4co1l]

I surveyed some 1210s at that recently.  Take a look at:
Samsung CL32B225KCJSNNE
Taiyo Yuden HMK325B7225KM-P
Taiyo Yuden HMK325C7475KM-PE
TDK CNC6P1X7R2A475K250AE

Tim

[Siwastaja]

I would get the best brand part and de rate 50%

This is of little help. The only time I have had a MLCC fail catastrophically was a quality Murata part, derating was by 80% as a 50V part was used on a 10V bus. There was no board flex even, but it still failed because of poor manual soldering practices. One such mistake is enough to send an MLCC over the edge.

[coppercone2]

I would use 63V. Someone might hook it up to a meanwell set to 52.

[Jay_Diddy_B]

Hi,
This is the capacitance versus bias voltage for the TDK  CNC6P1X7R2A475K250AE suggested by Tim. This a 1210 4.7uF 100V capacitor.



At 50V the capacitance has dropped to 1.4uF

For filtering applications the impedance is more important than the capacitance. Depending on the frequency you might be on the inductive side of the impedance curve. I have added, in blue, the impedance curve with bias voltage. With a 3x reduction in C, the Fo will be 1.73 higher.



The inductive side doesn't change with bias voltage.


Regards,
Jay_Diddy_B

[vad]

How confident are you that the applied voltage will not exceed 50V under any circumstances, such as transients, noise, ringing or due to part’s tolerances?

Also, as others have mentioned, capacitance of MLCC capacitors would have to be derated depending on DC voltage. Capacitance of a typical 50V X7R would have to be derated by as much as 85%-95% under 48V DC condition. Check manufacturer’s characteristics sheets for DC derating curves.

[Simon]

We will use 100V parts.