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Polycarbonate Capacitors Are No Longer Available… What Are Our Options?

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​ By Tom Colella, Engineering Manager

It’s been years since Bayer Corporation announced it was discontinuing the manufacture of polycarbonate dielectric. Capacitor manufacturers stocked up with thousands of pounds of material to ward off the pending polycarbonate demise. Since 2008, most inventories of polycarbonate capacitor dielectric are completely gone. Capacitor manufacturers have been searching worldwide to find any remaining polycarbonate dielectric to meet their customers’ needs. Many OEMs are scrambling to purchase any remaining polycarbonate capacitors to fill their requirements. Now that this material is no longer available, we need to look at all the options and find new solutions.

History

Polycarbonate was invented by Bayer Corporation in 1953, and production was started in 1958. The dielectric material was determined to have an operating temperature of -55ºC to +125ºC without derating. The insulation resistance and dissipation factor were reasonable and the size was relatively small in comparison to other dielectrics. Primarily because of its temperature range, dielectric material became the primary choice for most military applications. The military specification was based on the wide temperature of -55ºC to +125ºC.

Options

Many military Quality Products List (QPL) specifications are required to use polycarbonate. However, some military equipment OEMs are purchasing commercial off-the-shelf (COTS) devices. They have found that the QPL is not their only option for some applications. While the military specifications are their only control measure for purchased components, it does not take into consideration actual application, operating temperature, and cost. Many OEMs can replace devices at their discretion. To do so requires paperwork, justification, specification revisions, and testing. Therefore, there are other capacitor dielectrics available that can be a viable replacement for polycarbonate capacitors.

Polyphenylene Sulfide (PPS)

The closest dielectric to polycarbonate is polyphenylene sulfide (PPS). The temperature range, electrical parameters, and sizes are similar to polycarbonate.

Polypropylene (PP)

The polypropylene dielectric is electrically superior to polycarbonate. The dissipation factor is low at 0.1%. The insulation resistance is high at 100K MegOhm/uf. The major drawbacks are physical size and temperature range.

For size comparison, a 1uf/100V wrap and fill capacitor:

Metallized Polycarbonate Metallized Polypropylene
.29 x .39 x .78 .21 x .38 x 1.15

The temperature range is -55ºC to +85ºC, with derating to 50% at +105ºC. Polypropylene can replace polycarbonate very successfully as long as the application is within the temperature limitations. Some typical applications where polypropylene is being used as the dielectric of choice are coupling/decoupling, by-pass, power factor correction, snubber, filtering and energy storage capacitors.

Polyester (Mylar®)

The polyester dielectric (Mylar® is DuPont’s trademark name) has been the workhorse of the industry for many years. The electrical parameters are somewhat lower than polycarbonate. The dissipation factor is 1.0% and the insulation resistance is 20K MegOhm/uf. The temperature range can achieve +125ºC as long as the derating is met. The size is relatively smaller than polycarbonate.

For size comparison, a 1uf/100V wrap and fill capacitor:

Metallized Polycarbonate Metallized Polyester
.29 x .39 x .78 .20 x .29 x .65 (100V)
.32 x .41 x .78 (200V)*

*200V will achieve 100V at +125ºC

Polyester capacitors are used successfully in coupling/decoupling, by-pass and filtering applications.

Summary

The discontinuance of polycarbonate capacitors forced the manufacturing industry to rethink its options for its many applications. Depending on the application, environment and specifications, there are viable – even better – alternatives for replacing these dielectrics. Many customers have already successfully made the move. Contact Electrocube’s engineering department to discuss particular applications and find new solutions.

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