Tweet

Posted on 01 May 2019

Polycarbonate Capacitors Yesterday, Today and Tomorrow

Free Bodo's Power Magazines!

 

 

 

They are a strong candidate for applications requiring a long projected life

With a strong market need for high precision, high temperature, high reliability and long life film capacitors, a viable business still exists to produce the industry accepted Polycarbonate film. Military grade capacitors are especially in high demand. However, many producers of Polycarbonate film capacitors have discontinued production. Today’s challenge is to continue processing the raw film.

By Sal Cesario, Electronic Concepts, Inc.

 

Electronic Concepts manufactures military (Qualified Product List, QPL) and non-military grade polycarbonate capacitors, built using Bayer or Electronic Concepts manufactured film. From the 1960s to the 1980s, Electronic Concepts used Peter Schweitzer (a Division of Kimberly Clark) manufactured film, the only United States supplier of material, by license agreement with Bayer. In 1984, Electronic Concepts acquired the Peter Schweitzer film division, terminating the license, allowing Bayer to market the film in the United States. For economic considerations, Electronic Concepts started manufacturing capacitors using a balance of Bayer and Electronic Concepts film.

Basis of Producing Good Film

In 1990, the conclusion of a polycarbonate film capacitor paper[1] stated, “Both the orientation and crystal structure of PC (polycarbonate) film affects its mechanical properties and electrical dissipation factor”. The paper was a cooperative investigation by the Jet Propulsion Laboratory and Electronic Concepts’ film manufacturing division, Capfilm. Solvent casting Polycarbonate material will produce high quality, capacitor grade film. However, stretching the film is one of the most critical processes for the final production. Other higher volume production methods are used to produce Polycarbonate film, although, film thickness and electrical properties prohibit the materials use for capacitor manufacturing application.

The Casting Process

Casting is a chemical process, by mixing Polycarbonate into a liquid state, and reforming the composition into sheets or log rolls. The liquid material flows onto a matte finished drum. The drum provides a critical component to successfully process the film. The matte finish on the casting drum prevents “blocking” or the elimination of the fusing of layers of plastic under wound tension. Another benefit, of the surface texture, is increased instantaneous peak current (Ipk). Ipk is rated on a linear per inch basis of the wound dielectric edge length. Increased Ipk corresponds to an increase in the capacitor’s voltage rate of change. The relationship of base peak current is Ipk = C(dv/dt). By example, Polycarbonate film would have around 1A per inch of wound film length, compared to Polypropylene experiencing around ¼A per inch of wound length.

By comparison, Polyphenylene Sulfide (PPS) or Polyester (PET) are processed by heat extrusion and requires additives, which provide the surface texture, to prevent blocking. Particulate additives used in heat extruded dielectrics like PPS, PEN and PET although increasing Ipk are foreign particles to the base dielectric material. The particle distribution size, in the materials, is often thicker than the final stretched film thickness. Therefore, the dielectric properties and mechanical strength around the particles is compromised compared to a pure polymer dielectric. Decreases in voltage strength and insulation resistance are typical affects of particulate additives.

Polycarbonate produced specifically for dielectric applications, by an ultra-filtered solution casting, retains the polymer’s intrinsic properties without the affects of the heat required in melt extrusion. The surface roughness of Polycarbonate is achieved through the finish of the casting drum without particulate additives or other contaminants. The homogenous nature of the mix slurry in solvent cast polycarbonate allows for ultra-filtration, a process not possible in films requiring particulate additives. The properties and characteristics of polycarbonate dielectric produced by solvent casting approaches the pure resin. Cast film, when correctly processed, will yield very high voltage strength and insulation resistance.

Stretching Solvent Cast Film, the Key

One of the most important processes in film production is controlling thickness and uniformity of the film during the stretching phase. The sheet film has three critical dimensions: thickness, length (or the machine direction (MD)), and, width (or the transverse direction (TD)). The MD orientation correlates to the diameter of the capacitor and the TD orientation correlates to the length of capacitor. Stretching the film aids in the manufacturing process to: align the polymers or crystalline structure, increase machine direction shrinkage, nullifying the transverse direction shrinkage, and decrease film thickness.

The alignment of the crystal structure is important for increasing the mechanical strength of material (especially in the MD), increasing the voltage strength, and adding increased thermal stability. Controlling shrinkage of the material during the manufacturing process of the capacitor is critical for maintaining good capacitor tolerances and high (dv/dt). During the capacitor winding process, shrinkage in the winding direction will yield a tighter section with tighter tolerance drift.

Conversely, a wound section undergoes an end spray, where, shrinkage in the TD (length of capacitor) can cause a lower (dv/dt) and raise the ESR (equivalent series resistance). Effectively controlling the stretch process of a cast polymer proves critical in producing mechanically and electrically strong capacitors.

Thermal Characteristics of Polycarbonate Film

Differential Scanning Calorimeter measurements are used to characterize the Polycarbonate film for melt point, glass transition and crystallinity. Melt point analysis as shown in the Figure 1 occurs at 242°C (typical range is 230°C to 260°C).

Melting Point Analysis

Glass transition is shown in Figure 2 at 160°C. Notice the smooth curves with no other events evident over the temperature range of 200°C to 300°C indicating purity of the polymer. Foreign materials in the film would appear as additional “bumps” due to the different melting characteristics.

Glass Transition Analysis

To better understand the effects of stable thermal break down, tests were executed to characterize Polycarbonate film capacitors for DF against varying temperatures and frequencies. Additionally, measurements for capacitance fluctuations against varying temperatures were recorded.

Technical Attributes of Polycarbonate Film

Through good manufacturing processes and base chemical composition of Polycarbonate material, good electrical characteristics are achieved. Polycarbonate material, used in a capacitor, will yield key advantages, like, a continuously high reliable operation up to +125°C, low loss or stable dissipation factor, high current capability and stable capacitance changes over fluctuating temperatures.

Capacitor’s capability of controlling low loss or dissipation factor (DF) is effected by two key conditions: change in the application circuit’s operating frequency and temperature.

A Polycarbonate capacitor will only vary by about 2.5% over frequency sweeps from 10Hz to 250,000Hz, running at an elevated operating temperature of 125°C. Actual test data can be shown in Figure 3, highlighting the stability of a Polycarbonate film capacitor.

Dissipation Factor vs Frequency

Equally important, in controlling low loss, is to stabilize the effects of temperature variations when producing a capacitor. As illustrated in Figure 4, test data showing how a Polycarbonate film capacitor’s DF changes against varying temperature ranges. The DF will only fluctuate by around 0.5%, with temperatures ranging from -55°C to +125°C, at a fix frequency of 1kHz.

Dissipation Factor vs Temperature

As a side note, increasing temperature decreases DF. When the capacitor runs under power, heat is generated as reactive heating losses in AC or I2ESR heating losses in DC. As the internal temperature of the capacitor increases, the DF of the capacitor decreases and increased efficiency is realized.

A further look at the DF verses Temperature graph shows an extremely stable DF drift over normal (25°C) to elevated (125°C) temperatures would be around 0.5%.

One final measure of a good temperature property, found in a Polycarbonate film capacitor, is the measured constancy of capacitance value changes. As demonstrated in Figure 5, the capacitance value will experience around a 2.3% change over the full temperature range of -55°C to +125°C.

Capacitance Change with Temperature

Furthermore, over normal (25°C) to elevated (125°C) temperatures, the capacitor will only change around 0.5%.

Stable DF against temperatures and frequencies, and stable capacitance changes against temperature, provides a solid foundation to have long lasting, reliable capacitors. Actual life and reliability can be measured and calculated by applying test data.

Long Life, Low Failure Rate

Polycarbonate capacitors have maintained an excellent reliability rating. Failure rates can be expressed by a Failures In Time (FIT) calculation using accumulated test data. The formula:

FIT = (n/N)(1/t)

FIT = 1 failure in 109 component hours, and
n = number of failures
N = number of components tested
t = duration of testing, in hours

FIT Rate Example for MIL-PRF-55514

Appling test data accumulated against the FIT calculation, over a varying temperature range, shows a highly reliable part.

In fact, less than twenty five units would fail running over one billon hours (25x10-9), at fifty (50) volts, even at an elevated temperature of around one hundred degrees Celsius (100°C).

Electronic Concepts accumulated almost five hundred million hours of testing military grade Polycarbonate capacitors; and, currently meet established reliability failure rate level “R.”

Polycarbonate capacitors are a strong candidate for applications requiring a long projected life. Using a projected life calculation, derived at Electronic Concepts, using base calculation from information published in “Selection and Application of Capacitors[2]”, can be expressed as follows:

L2 = {L1/[(E2/E1){2.6087[(E2/E1)+0.5167]}]}{2[T1-T2)/n]}

L1 = Test time in hours
L2 = Projected life expectancy in hours
T1 = Test temperature applied
T2 = Temperature at which the life is projected
E1 = Test voltage applied
E2 = Voltage at which the life is projected
n = °C rule for the temperature stress
(n = 10 for organic materials)

By way of an example, if capacitors accumulated one million test hours at 100°C and 50VDC rated voltage. Then, an application calling for a part to operate at 125°C at rated voltage, the projected life would be 177 thousand hours or over 20 years.

Key Established Applications

Polycarbonate film capacitors have been produced for decades, lending to an established product. Electronic Concepts have been producing military grade capacitors, especially MIL-PRF-55514, MILPRF- 83421 and MIL-PRF-39022, for over thirty (30) years with great success. The forecast for the product shows a strong market demand.

Supply of High Temperature Film

Electronic Concepts remains a viable supplier of Polycarbonate film and capacitors, and will continue to produce given a strong market demand.

Other Solvent Cast Pure Polymers

Polycarbonate meets the market need for a reliable, stable (tight tolerance), high temperature material. Solvent casting process can produce Polysulfone at 150°C, Polyetherimide (PEI) at 200°C and aromatic Fluorene Polyester (FPE) at 250°C or greater.

Conclusion

Several manufacturers of Polycarbonate capacitors have exited the business. However, product specifications, tangible and intangible key features and customer benefits, have created a product demand resulting in Electronic Concepts’ long term plan to continually support the production of Polycarbonate film and Polycarbonate film capacitors.

The reliability, longevity, and military specification conformance solidifies the Polycarbonate capacitor as a viable product for today and tomorrow.

Acknowledgment

The original paper was written and presented at the CARTS 2007 symposium held in Albuquerque, New Mexico.

 

References:

1) S. P. S. Yen and C. R. Lewis, “Effect of structure and morphology on thermal and electrical properties of polycarbonate film capacitors,” Published in IEEE, 1990.
2) John D. Moynihan, described in “Selection and Application of Capacitors”, Second Edition, Copyright 1987 by Components Technology Institute Inc.

 

 

VN:F [1.9.17_1161]
Rating: 0.0/6 (0 votes cast)

This post was written by:

- who has written 791 posts on PowerGuru - Power Electronics Information Portal.


Contact the author

Leave a Response

You must be logged in to post a comment.