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Film

 

    Film Capacitors

          Film capacitors are mostly used in high-performance applications.  Polycarbonate, polyester, and polypropylene have been the "big  three" of film capacitors.  They are the only ones that most film capacitor makers make, although polyphenylene sulphide (PPS) is on the rise.  For the lower volume capacitors, I have listed known manufacturers.  This does not mean they actually stock parts, but at least they have the capability to make them if they want to. 

 

          Polycarbonate:
          Polycarbonate has a fairly low temperature drift (lower than most films), dissipation factor, and dielectric absorption.  It can be used in timing circuits, although C0G ceramics are a better choice for small sizes.  It is suitable for some pulse applications, and for some precision analog applications, especially if you need its good temperature stability and relatively high temperature rating.  Moisture absorption is high compared to most other film dielectrics, a problem for some critical applications.  Good heat resistance, to 125C, but not good enough to be found in surface-mount packages.  Some manufacturers recommend it for automotive applications. 

          Unfortunately, polycarbonate is about to go the way of polystyrene. The last manufacturer of capacitor-grade film, Bayer AG, says they will cease production at the end of 2000, but may have a substitute material.  A number of manufacturers are making recommendations for polycarbonate replacements (some of which should not be taken seriously).  Evox-Rifa for one, recommends polyphenylene sulfide, and this is probably the best choice for most applications.  Polycarbonate has a slight edge in high-temperature leakage and is better at self-healing, but PPS is as good or better in most other parameters.  PPS´s only weaknesses are a higher high-temperature temperature drift and higher DF at high temperature, >100C.  Its K is about the same as PC, so PPS caps will be about the same size.  Polypropylene may also be considered for applications where very low leakage is important, but high temperature is not a issue, although its temperature drift is not as good as PC or PPS.  Polystyrene has had availability long after the film ceased production, but this may not happen with polycarbonate.  Production is expected to cease no later than 2006.  Unlike polystyrene, polycarbonate has only a few significant advantages over replacement dielectrics.  PPS's main problems are that the capacitor-grade film is single sourced (Toray in Japan), expensive, and more difficult to process into capacitors than polycarbonate. 
 

>>Update 4/10/05<<
          Electronic Concepts Inc. (
http://www.ecicaps.com/ ) has announced that they are now making their own capacitor-grade polycarbonate film and are selling the finished capacitors.  ECI specializes in upscale parts for aerospace, medical, and similar industries.  We may not see cheap, dipped, polycarbonate parts. http://www.ecicaps.com/pub/Polycarbonate_Status.pdf

 

          Polyester:
          Polyester is probably the most popular of the film capacitors, at least for board-level applications.  Actually, polyester is a generic term for a class of similar polymers, the one used in polyester capacitors being polyethylene terephthalate.  Dupont's trade name is Mylar, some people call it PET, PETE, or PETP just to be confusing (and that doesn´t even include the various European trade names for it).  It doesn't really do anything particularly well but low cost, small size and the ability to do many things well enough makes it a good choice for many noncritical applications.  High dissipation factor, especially at increasing frequency, means it is best used in DC or relatively low-frequency/low-current pulse and AC power applications.  Poor temperature drift, dielectric absorption, and leakage relegate it to non-critical analog circuit applications.  Typical applications would be where you want a capacitor larger than a C0G but better electrical properties than an X7R.  Polyester capacitors can typically be found in values from 0.01 uF through at least 10 uF and beyond.  Polyester has a high temperature drift but can be found layered with polypropylene to flatten the temperature curve (the two go in opposite directions).  Polyester capacitors are available to 125C.  Good heat resistance allows polyester capacitors to be made in surface-mount styles.

 

          Polypropylene:
          Polypropylene (PP) capacitors have a lot going for them.  They are available in a wide range of sizes and voltages, and are used in a wide variety of circuits.  PP has a very low dissipation factor over it´s entire temperature range and over a wide frequency range.   This makes polypropylene capacitors popular for high-frequency, high-current applications like switching power supplies.  Large film, film-oil, and paper-oil-film types are found in power-line applications like power-factor correction.  These can have operating-voltage ratinhv film2gs >400 kV AC.  Polypropylene motor-starter, motor-run, and SCR snubbers are replacing older electrolytic and paper-oil types which all have much higher dissipation factors.  Sizes for these run into the 10s of uF and >500 VAC.
 
          While PPs very low dissipation factor has made it the only viable material for many high-power AC applications, its self-healing properties, critical for reliable high voltage operation, are only fair.  When an arc-through of the dielectric occurs, PP tends to leave more carbon at the site than polyester.  Also, the arc is more quickly extinguished if the gas pressure at the failure site is as high as possible.  This requires that the dielectric is as strong and heat resistant as possible.  Polyester is better in this regard as well.  Oil impregnation helps make up for these deficiencies however.

hv film4
 

          The small through-hole styles are available in the usual range of sizes, roughly 100 pF-10 uF.  Low leakage and low dielectric absorption make small polypropylene capacitors suitable for integrators and sample-and-hold circuits.  Moisture absorption is negligible.  Only its higher temperature drift makes it inferior to polystyrene.  Polypropylene has limited heat resistance (to 105°C), and is not found in surface mount.


 


          Polystyrene:
          Polystyrene (PS), (the Europeans often call it "styroflex" or "styrol") has long been the material of choice for critical analog circuits.  Low leakage, low dielectric absorption and a shallow, flat temperature curve makes these capacitors suitable for timing circuits, filters, integrators, and sample-and-hold circuits.  Moisture absorption is very low.  Size, cost, availability, and temperature range limitations make polystyrene unsuitable for most other applications.  Heat resistance is limited to about 85C, so forget surface mount.  They can be damaged by soldering and by chlorinated board cleaning solvents.  I don´t believe I have seen them in metallized film, only in film-foil. 

          Because of the poor heat resistance, polystyrene has largely been replaced by polypropylene and C0G ceramics, and the capacitor-grade film is no longer being made.  There is several year´s supply still available, and they are still being sold, but be careful using them in new applications.  Some manufacturers have noted that polystyrene caps are "not for new designs".  One company, ITW Paktron,  makes polypropylene capacitors with a guaranteed temperature drift similar to (if not quite as good as) polystyrene.  PS's other electrical properties are mostly very similar to PP. 

          Whether polystyrene capacitors will really go away any time soon is not certain.  I have been warned of polystyrene´s demise "in a few years" for over a decade now, and yet it is still available.  There almost seems to be vast supplys of the film stashed away in manufacturers back rooms.  It may also be that declining usage will extend available stocks for many years to come.  I imagine that many of polystyrene´s traditional applications are fading away as newer technologies take over. 

          Companies that advertise polystyrene capacitors include:
http://www.americancapacitor.com/
http://www.centechusa.com/welcome.htm
http://www.tecategroup.com/ti/ti2.htm
http://www.vitelelectronics.com/asc.htm
http://www.filmcapacitors.com/
http://www.richeycap.com/
http://www.southernelectronics.com/
http://www.wescocap.com/
http://www.crcfilm.com/
http://www.seacorinc.com/
http://www.lcr-inc.com/products/polystyrene.htm
http://www.eci-capacitors.com/  I believe these guys make their own film
http://www.evox-rifa.com/
http://www.electrocube.com/
http://www.seasonshk.com/ 
http://www.am-21.com  Centalab Hong Kong !?!
http://www.susco.com 
http://www.rtie.com 
http://www.tscgroup.com/
http://www.eurofarad.com
http://www.capacitors.com.hk  Suntan

 

          Companies that advertise "polystyrene-replacement" capacitors include:
http://www.electrocube.com/
http://www.paktron.com/
http://www.sbelectronics.com/sbe.htm 

 

          Polysulfone:
          A few people spell it polysulphone.  Now rare, it was once considered to be the dielectric of the future.  Not even sure the capacitor-grade film is still in production.  Very good heat resistance, to 150C.  Dissipation factor fairly good, and it remains good at relatively high temperature and high frequency.  Moisture absoption is high however, similar to polycarbonate.  Temperature drift is about the lowest among film capacitors.  Its major advantage is good high-temperature leakage.  I have never seen it in SMD. 

          Companies that advertise polysulfone capacitors include:
http://www.americancapacitor.com/
http://www.eci-capacitors.com/
http://www.rti-corp.com/Electronics/imb.html

 

          Polyethylene naphthalate:
          Polyethylene naphthalate (PEN), another form of polyester,  is a relatively new material to the capacitor world.  It has very good heat resistance, but is otherwise much like polyester.  It is available in larger sizes than C0G ceramic,  lower temperature drift than polyester, and lower leakage than X7R.  PEN capacitors are available to 125C.  It is commonly found in SMD capacitors, including large values (>1 uF).

 

          Polyphenylene sulfide:
          Polyphenylene sulfide (PPS) is another "newcomer", although it has long been used as an engineering plastic.  It is found mostly in SMD but also in through-hole packages.  It has many attractions, including low dissipation factor and very good heat resistance, a combination not found in other common capacitors.  Dielectric absorption is fairly low.  Moisture absorption is very low.  One manufacturer says it has C0G-like electrical properties.  This is a bit of an exaggeration but it is probably your best choice if you need a C0G-like capacitor >0.05 uF in SMD.  PPS caps are commonly available to 150C.  PPS is now being recommended as the main replacement for polycarbonate capacitors which will be going out of production.  PPS's only serious drawback comparied to polycarbonate is an increasingly high dissipation factor as the temperature goes above 100C.  PPS has about the lowest temperature drift of film capacitors that still have good availability. 

 

          Teflon:
          Teflon TFE is DuPont´s trade name for polytetrafluoroethylene or PTFE.  Teflon is actually a blanket trademark for wide variety of DuPont fluorocarbon polymers.  PTFE is also sold under a number of other trademarks such as Fluoroplast-4 and Fluon PTPE.   PTFE has very low leakage, very low dielectric absorption (probably the lowest in both cases), very low dissipation factor, a wide temperature range (to 200C for some), low temperature drift, negligible moisture absorption, and very good stability.  It is about the best film capacitor for critical analog applications.  Expensive, however, which is why you don't see them every day and few companies still make them.  The film has exceptionally poor mechanical properties and inconsistent thickness, and manufacturers find it difficult to work with.  Available from 0.001 to at least 2 uF.  Have not seen them in SMD.  Well almost.  CDE now makes a few Teflon SMD parts in very low values, <10 pF.

 

          Companies that advertise Teflon capacitors include:
http://www.americancapacitor.com/
http://www.polyflon.com/ also non-magnetic
http://www.customelec.com/
http://www.dei2000.com/
http://www.crcfilm.com/
http://www.condenser.com/
http://www.eci-capacitors.com/
http://www.semco-usa.com  SEMCO is really a mica specialist, but apparently makes some Teflons in small values
http://www.v-cap.com  big audio caps
http://www.cde.com/catalogs/MCM-MIN.pdf  SMD

 

 

 

          Acrylic
          Acrylic is a real newcomer.  The main attraction seems is size.  C-D suggests it as an X7R replacement.  Dissipation factor is high relative to other films, 0.5-1.5% through 20 kHz, temperature drift is only fair, and useful frequency range is limited to well under 1 MHz.  Although the upper temperature limit is only 85C, they are only available in SMD. 

          Companies that advertise acrylic capacitors include:
http://www.cornell-dubilier.com/
http://www.tecategroup.com/   Has graphs.