Electrostatic capacitors dominates the market among the other capacitor technologies. The article provides introduction into construction of electrostatic capacitors, such as ceramic, film, paper technologies. Assembly styles, termination techniques or metallization processes are explained including impact to the basic paramters. Specifically described is film capacitors’ self-healing process and how this can be impacted by selection of materials and manufacturing process.
Lets start with a general consideration of electrostatic capacitor designs. There are some design solutions and “tricks” to reinforce capacitor features to meet specific application requirements such as higher power, higher safety robustness.
The search for capacitance in small volumes has created a number of different solutions, each with its advantages and drawbacks. We shall try in this chapter to describe both methods and problems of achieving large C/V ratios. Further details may be added in specific material sections.
The latter part of this chapter deals with some general construction questions.
Suppose we have 6 capacitor elements consisting of 2×6 electrodes organized as in Figure 1. We understand that spread in one plane and connected together they will result in a total capacitance 6C. Generally, N electrodes give the capacitance N/2 C.
Instead, if we stack them according to Figure 2. capacitance will be developed between every electrode with different polarity which here results in 12-1 = 11 elements connected in parallel. Ctot = 11C. In general N electrodes give Ctot = (N-1)C. The higher N the closer to a doubling of the one-plane capacitance with the stacked capacitor type.
If we spread two metal foils separated by two insulating foils as shown in Figure C3. (a) and wind the foil package to a winding (b) the capacitance is doubled in the same way as with the stacked type. The electrodes will be capacitive in two directions (c).
The termination to the electrodes is performed either with one or several so called inserting tabs (for example tinned copper foils) which connect the electrodes with the terminals). Or one can let the electrode foils extend from the winding and bend them against the winding end (extended foil construction) where they are soldered or welded to the terminal leads (Figure 4). A remark concerning inserted tabs. Should the working voltage be low and the capacitor oil impregnated the tabs have to be welded to the electrode foils if we shall avoid open circuit risks. In electrolytics aluminum tabs have to be welded – preferably cold welded – to the foils because of the corrosion risk.
Minimized electrode spacing
In accordance with the formula C = ε x A/d we improve capacitance by the degree to which we decrease the electrode spacing, d. Improved technology has permitted manufacture of certain plastic films down to less than 0.5 μm (0.02 mils) in thickness. The so called wet method used in ceramic manufacturing has lead to thicknesses less than 15 μm (0.6 mils) and a developed dry method based on a plastic film conveyor or carrier has reduced the dielectric thickness even further, to less than 5 μm (0.2 mils).
To the thinnest dielectric of different material groups corresponds the lowest rated voltage capability. We have to comply with that thickness irrespective of the working voltage. If we, for example, need 6 V DC the nearest working voltage might be 10 V. But only in ceramic. Plastic films stop at 16 V, porcelain and paper at 100 V, mica at 125 and glass at 250 V DC.
The dielectric withstanding voltage
puts another limit to the thinness of a dielectric. The higher the working voltage, the thicker the dielectric must be. But if the thickness increases 5 times, the capacitance decreases to 1/5 or for the same capacitance, the volume of the part must be 5 times greater. Hence high voltage capacitors always will be bulky.
Higher dielectric constant, εr
Plastic and paper foils have a relative dielectric constant around 3. If we change to ceramics it’s possible to increase the εr by several thousands. But in different ways we loose the quality characteristics significant for other materials, such as, polymers. In the different dielectric material groups we will account for the typical characteristics.