Physically Large vs Small Capacitors

Question

Suppose you had two capacitors with the same capacitance values, C. Are there any important differences in how the capacitors behave if one is physically larger by a significant amount?

I've read that there are likely differences in ESR and ESL that could have an effect, especially at high frequencies. This is based on the non-ideal model of the capacitor.

However, are there any other effects to consider? What is the dominant effect to watch out for in the charging/discharging profile at various frequencies?

Edit: Image from iequalscdvdt.com webpage on capacitor models

Edit: The question was not necessarily aimed at any one type of capacitor.

Answer 1

Capacitance is given as ε(A/d) where A is the area of the plates and d is the separation between them. ε is the permittivity of the dielectric. Minimizing physical size for a given capacitance means minimizing both A and d, so ε must be large. Dielectric materials strive to maximize ε but there are tradeoffs:

• A low breakdown voltage means d must be larger to prevent conduction between plates, so A must grow to compensate.
• A high ε material may not have a constant value with increasing voltage (common in ceramic caps), so the working voltage of the capacitor drops.
• The material may not have a constant value over the required temperature range e.g. C0G vs Z5U.
• The material may be prohibitively costly and/or difficult to form, handle, or produce.
• The material or construction may require that the capacitor only be operated in a specific polarization

In addition, the physical size of the capacitor partially determines its inductance and resistance through the contribution of its leads and internal connections.

Answer 2

Suppose you had two capacitors with the same capacitance values, C. Are there any important differences in how the capacitors behave if one is physically larger by a significant amount?

A big factor that affects size/volume (if the capacitance is held constant) is the voltage rating. So, if both capacitors (small and large) have the same capacitance then one will (more than likely) work up to a larger voltage.

A capacitor that is polarized (e.g. electrolytic dielectric) can be physically smaller compared to a capacitor with a better (lower loss) dielectric and this is also a significant trade-off.

What is the dominant effect to watch out for in the charging/discharging profile at various frequencies?

You need to examine (in the data sheets) the specifications and, in my personal opinion, polypropylene dielectric capacitors produce the widest range of decent valued capacitance with very high peak current rating and high withstand voltage. It's often disguised in the data sheet as dv/dt (i.e. rate of change of voltage that can be safely applied without deterioration of the component). However, I notice that C0G dielectric capacitors are starting to invade the polypropylene territory.

Both can have very high peak current ratings (several hundreds of amps). This means they must have low ESR and ESL.

Answer 3

Ceramic capacitors of the same dielectric type and voltage rating that are physically larger will typically have less voltage coefficient (except C0G/NP0 type, which have hardly any voltage coefficient). They typically have more ESL. The voltage coefficient can lead to extreme drop in capacitance, to the point where they mostly disappear at rated voltage. You have to dig into the detailed information from manufacturers to get that information. You should not assume that smaller is always better.

With other capacitor types such as electrolytic and film, for the same capacitance, dielectric type, and voltage rating, the size is perhaps more indicative of technology level than of any systemic difference in performance.

But some capacitors with different dielectrics with (say) extremely good DA performance may be physically much larger and have other unpleasant characteristics (thinking of polystyrene here, which cannot withstand the SMT process and barely tolerates hand soldering). Similarly PP capacitors tend to be somewhat better and larger than polyester (Mylar). There are newer SMT-compatible (high temperature) film dielectrics such as PPS that are middling good.

That's just an overview of a few types, and a fat book could easily be written just on the various types of capacitors and their various non-ideal characteristics and trade-offs.