The growing complexity of active electronic components and their applications has resulted in the use of different types of passive components, especially capacitors. The advances in conductive polymers now offer a universe of capacitors for embedded systems applications and others.
Some advanced capacitors use conductive polymers for their electrolyte. Others such as hybrid capacitors use the conductive polymers in conjunction with a liquid electrolyte. Both these polymer-based capacitors offer improved characteristics over conventional ceramic and electrolytic capacitors, namely, life cycle, safety, longevity, reliability, stability, ESR or Equivalent Series Resistance and voltage rating. These special hybrid and polymer capacitors show distinct performance advantages in terms of ideal voltages, environmental conditions, and frequency characteristics.
Polymer Capacitors
Layered Polymer Aluminum Capacitors: These use conductive polymer as the electrolyte with an aluminum cathode. They operate within a voltage range of 2-25 VDC and manufacturers make them in capacities of 2.2-500 µF. Packaged in molded resin as low profile SMDs, they offer very low ESR.
Wound Polymer Aluminum Capacitors: Although they use conductive polymers and aluminum, they are constructed with a wound foil structure. They operate over a wider voltage range of 2.5-100 VDC and their capacities range from 3.3-2700 µF. With low ESR values, the capacitors are packaged as SMD, although layered capacitors are more compact in comparison.
Polymer Tantalum Capacitors: They use a tantalum cathode and conductive polymers as electrolyte. They are available in capacitance values of 2.7-680 µF and their operating voltage ranges from 1.8-35 VDC. Among the most compact capacitors on the market, polymer tantalum capacitors are available in SMD packages.
Hybrid Polymer Aluminum Capacitors: These use a combination of conductive and liquid polymers as electrolyte along with an aluminum electrode. The polymer offers low ESR as well as high conductivity. The liquid electrolyte offers higher capacitance ratings as it has a larger surface area, while being able to withstand high voltages. These capacitors come in a capacitance range of 10-330 µF with voltage range of 25-80 VDC. Although compared to other types, the ESR value for hybrid capacitors are on the higher side, the values are far lower than what conventional capacitors offer.
Advantages of Polymer Capacitors
Although different in material and construction, the four types of capacitors share common desirable electrical properties.
Superior Frequency Characteristics: As polymer capacitors have very low ESR, the impedance at their resonance point is also very low, resulting in reduced ripple in power circuits by nearly five times when compared to that produced by conventional tantalum capacitors.
Capacitance Stability: Ceramic capacitors tend to drift in response to DC bias and temperature. Polymer capacitors are devoid of such problems. This stability is of importance in automotive and industrial applications, where the operating temperatures vary broadly. Even under common operating conditions such as high temperatures and high frequencies, where ceramic capacitors show an effective capacitance loss of over 90%, polymer capacitors remain stable.
Enhanced Safety: Conventional capacitors can short circuit and fail, and these are causes for safety issues. Mechanical stresses or electrical overload can create discontinuities or defects in the oxide films that forms the dielectric leading to safety failures. The self-healing capability of the polymer capacitors eliminates such failure modes.
