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Film capacitor design:

The internal design of of film capacitors is a compromise of cost and size versus voltage versus performance in various pulse

type applications.

This is a sample of types of common capacitor constructions. There are others. Very large high-voltage capacitors may be

made of a number of discrete capacitors in one box, into the dozens in some cases.

Metallized:

Usage: General purpose. Probably the most common metallized capacitor. Usually X and Y capacitors are made this

way, as well as very large capacitors that substitute for electrolytics (called DC-link). This cross section can be found in both

wound and stacked types.

Pro: Best size, lowest cost, good self-healing in many materials.

Con: Limited voltage ratings, probably no more than 1000 volts.

Metallized, series:

Usage: General purpose, higher voltage. some Y capacitors. This construction can be found in a few ceramic

capacitors as well.

Pro: Usable for higher voltages, it's like having two capacitors in series, with more sections possible for higher voltages

yet. Good self-healing. Double metallized types made for pulse applications like GTO and IGBT snubber circuits, .

Con:

Foil:

Usage: General purpose.

Pro: Lower ESR than metallized, better current handling.

Con: Limited voltage ratings in this form, does not self-heal so thicker film may be used for reliability. Foil is a more

expensive construction. Larger than metallized film. The foil is typically 0.0002" while metallization is more like 0.000,001"

Hybrid: metallized + foil:

Usage: A number of variations for various pulse applications and various voltage ratings. This includes double metallized

for better pulse performance.

Pro: Maintains good self-healing with good ESR.

Cons: Larger than metallized film.

Other features:

To improve reliability of high-voltage capacitors (but under 500 VAC for some parts) it is common to break the metallization

into segments that are connected by "fuses". If a short occurs in one segment, its fuse blows and isolates the shorted segment

from the rest of the capacitor. The picture on the right shows only one of a number of segment shapes in use. Segmenting the

metallization does reduce the capacitance a little, and increases the ESR. Over time, the capacitance will drift down as the

capacitor ages and more segments are isolated.

The shape of the capacitor is important for high-current, high frequency applications. The "Better" geometry (see below) has a

lower ESR and lower ESL and therefore, better performance, a lower temperature rise and longer life.

Metallization:

To improve the surge-current capacity of a film capacitor (and make it work more like a more expensive foil capacitor) the

metallization that bonds to the metal contact layer can to made extra thick (optimized). There are a number of variations on this

theme, such as extra thickness on both edges. See http://en.smec-korea.co.kr/product/atap.asp for many pictures.

Both aluminum and zinc are used, and each has its advantages and disadvantages. Aluminum has better resistance to corrosion

while zinc self-heals better. Al-Zn alloys are also used. Another variation is to put down aluminum first and zinc over it.

Many of images on this page are copyright by WIMA (WIMA Spezialvertrieb Elektronischer Bauelemente GmbH) of Bauelemente, Germany and used with their permission.

Film capacitor packaging:

Capacitors are packaged a number of ways, depending on cost versus reliability. A common way is "wrap and fill". The assembly is wound in a cylinder, wrapped with several layers of film and the ends are filled with epoxy. Some are made with an oval cross section for a lower profile against the circuit board. There are some variations on this. A few capacitors are put in a plastic tube or even a metal tube. This is done with high-voltage capacitors. Big high-voltage, high-current capacitors are generally made of many capacitors in series-parallel assembles.

Another common package is an epoxy dip. This is cheap and is also done with tantalum, ceramic and mica capacitors.

A third way is the boxed capacitor. The Europeans have made a specialty of this. The molded plastic box provides better environmental and mechanical protection than anything short of a metal can or a hermetic package makings it suitable for X capacitors. The box provides better protection from humidity and support for the leads. There is also a sort of hybrid package that is like a boxed capacitor but without a box. The capacitor is potted in a mold instead of being dipped. Metallized paper X and Y capacitors are made this way. The advantage of epoxy-paper construction is that it can't be made to burn. A box probably not as fireproof as the epoxy. Some epoxy-paper parts have had long term reliability problems, apparently due to deterioration of the epoxy. See Reliability 2.

The next up is the metal can. This package is often used in power factor correction, snubber, motor run and motor start capacitors.

High voltage capacitors:

These capacitors are often built of multiple individual capacitors using various series/parallel schemes, sometime with multiple internal fuses. Parts are available to at least 2,000,000 volts from some companies. HV capacitors are used in a wide variety of applications like lasers, x-ray machines, and even rail guns. The most important applications are in power distribution for power factor correction, harmonic filtering, and surge filtering. The link below is to an excellent pdf on power distribution capacitor construction and applications. There is more good stuff on the site.

Some companies that make very high voltage capacitors:

There are a variety of film capacitors who's construction is tweaked for other specialized applications. These include "high rf current", "pulse", "GTO spike dampening" (a kind of snubber), and others.

The edge of the metallized film can be wave-cut. This makes for a better connection to the metal end-spray. Slits can be added. In case the end-spray breaks loose from the film in one spot, the slits help isolate the damage. There are a number of variations on this.

DC link capacitors:

A special kind of capacitor is the DC link capacitor. These capacitors are used as filter (energy storage might be a better phrase) capacitors for things like high-power inverters, variable frequency drives and phase converters (single phase to three phase or three phase to single phase). In general, you have AC power from the power line ---> filtered DC ---> AC or DC output of some sort. The output circuitry may generate huge current spikes that the capacitors have to filter. They are rated from a few hundred volts to over 1500 volts, and values from a few microfarad to 1000s of uF. They must have very low ESR and ESL. Aluminum electrolytics (which may also have special winding) have been commonly used, but metallized polypropylene are more and more common. It's easier to get very high capacitance at a reasonable cost and space with aluminums, but it's easier to get very low ESR and ESL with film capacitors. Also, two or three aluminums have to be put in series for some high-voltage applications with resisters to equalize the voltages. The two capacitor types can be mixed to get the best of both. More and more there are large ceramic capacitors for DC link applications. These may have newer high-K ceramics with better stability in high-voltage applications.

They usually have a special construction to minimize their ESR and ESL. This can include four or more pins, large screw or tab connections, or special shapes. They might also be configured as two or more independent capacitors in the same package. I've seen at least six in one box. The ultimate in special shapes may be the patented SBE Power Ring film capacitor that reduces ESR and ESL about as low as they can go.

Amateur audio people have discovered DC link capacitors for use in power-amplifier power supplies.