FILM CAPACITORS
Film includes a variety of polymers, such as polyester, polycarbonate, Teflon, polypropylene, and polystyrene. Traditional film capacitors were only available in modest sizes, <10 uF. In recent years, film capacitors have sought to leverage their superior longevity compared to electrolytics, to move into some applications that call for much larger parts, even to thousands of uF. Film capacitors come in two broad categories, film-foil, and metallized film. Film-foil capacitors are made of alternating layers of plastic film and metal foil, while metallized film capacitors have the metal vacuum deposited directly on the film. In general, film-foil is better at handling high current, while metallized film caps are much better at self-healing. Various hybrid types can also be found.
Pros: The film capacitors mostly have reasonably well behaved electrical properties and offer many tradeoffs of performance and cost for people with precise requirements. The main parameters of interest include capacitance vs. temperature, dissipation factor, and dielectric absorption. Their main virtues include low leakage and low aging.
Cons: The main drawback of film capacitors is their low dielectric constants (K), which is only partly offset by their relatively good breakdown voltages. That means that film capacitors are physically large for their capacitance. Their Ks vary from a low of about 2.2 for Teflon to about 8 for PVDF (rarely used). Unfortunately, the rule-of-thumb is that the higher the K (and therefore the smaller the size), the worse the electrical properties tend to be. Film capacitors have not made an entirely graceful transition into the age of surface mounting. While some film dielectrics are suitable for surface mounting, most can’t withstand the heat of soldering. Even polyester, the toughest of the traditional films, is barely good enough. However, capacitor makers have responded by developing several new dielectrics. SMD film capacitors are not as widely second-sourced as other capacitors however.
CERAMIC CAPACITORS
Ceramic capacitors offer a broad range of size vs. performance tradeoffs and are easily the most popular in numbers sold. Ceramic capacitors are available from < 1 pF to 1000s of uF.
Pros: The main virtue of ceramic capacitors are their relatively high dielectric constants. This can vary from C0G with a K of up to 60, which has excellent electrical properties but is relatively large and expensive, to ceramics with Ks in the tens of thousands but with very poor electrical properties. Large- value ceramics can replace electrolytic capacitors in high-frequency applications like switch-mode power supplies because of their lower ESR. Ceramic capacitors are especially suitable for surface mounting due to their heat resistance, mechanical integrity, and the ability to make them in very small packages at low cost, for portable equipment. This has greatly added to their usage. To some extent ceramics are slowly displacing other types of capacitors.
Cons: Low breakdown voltage means that the low-K ceramics (Class 1), the ones with the good electrical properties, have poor volumetric efficiency, and are usually found only in small values. High-K ceramics (Class 2 and higher) have poor electrical properties, which are highly dependent on temperature, voltage, and frequency, plus a significant aging rate. Unlike many other capacitors, ceramics have no self-healing mechanism. This means that manufacturers must maintain a high level of quality control over the dielectric. Ceramics are most cost affective in small sizes at present. Very large ceramics are a bit of a challenge, especially in SMD.
ELECTROLYTIC CAPACITORS
“Electrolytic” means any capacitor that requires a conductive layer between the dielectric and one electrode. In the original electrolytic capacitor, the layer was an actual electrolyte, a conductive salt in a solvent. Some electrolytic capacitors today don´t actually use an electrolyte, but the word is still commonly used, to the annoyance of some. Electrolytic capacitors are made by growing a oxide film, the dielectric, on a metal, the anode, by electrochemical means. The films are very thin with fairly high Ks (roughly 10-25) which make for a lot of capacitance in a small package. The resulting devices pass current much better in one direction than the other, making a rectifier of a sort. Because of this, the metals are sometimes called “valve” metals. The metals presently used are aluminum, tantalum, and niobium.
Pros: Electrolytic capacitors are best used when you need a lot of capacitance in a small space and at a reasonable price, such as power supply filtering, or energy storage. They are available in sizes far beyond that of other capacitors. Aluminum electrolytics are presently available from 0.1 uF to several F. I have no idea why someone would use a 0.1 uF electrolytic capacitor however. Tantalum electrolytics are available from 0.1 uF to a few thousand uF.
Cons: Marginal electrical properties means that these capacitors must be applied with care. The parameters to be watched include leakage, service life vs. temperature, ESR, ESL, and low-temperature performance. Unlike other capacitors, electrolytic capacitors are not inherently non-polar, but non-polar types are available. Electrolytics are widely available in SMD packages, at least in moderate sizes, but users complain of more reliability problems than with through-hole styles.
MISCELLANEOUS DIELECTRICS
Miscellaneous capacitors include materials like glass, mica, porcelain, and even gas and vacuum. A few exotic dielectrics like silicon dioxide and sapphire are used in niche applications like microwave capacitors and trimmers. Some are available in surface-mount packages.
Pros: The electrical properties of the miscellaneous capacitors are generally most similar to film capacitors. However, they all have electrical properties that make them useful in some special applications.
Cons: These materials also have Ks similar to plastic films so they have no advantage in size. Except for mica, these capacitors are commonly available only in small sizes, <1 uF. They also tend to be more expensive than other capacitors of similar size.
However, since it’s after the regulator, something would have to go seriously wrong to get the voltage up to 25V I’m guessing. That said, if I can get the same filtering ability (or better) using other caps, I’m find with that. Tantalum seemed to sort of kill two birds with one stone.