Ron Demcko, Senior Fellow, KYOCERA AVX
A growing number of applications, ranging from solar power converters all the way to miniature power supplies for highly complex processing cores, are starting to capitalize on the benefits of aluminum electrolytic capacitors. Aluminum electrolytics are also increasingly used to satisfy miniaturization demands in complex power tree applications like FPGAs powered by multiple voltages.
And while it’s true that aluminum electrolytics also come with disadvantages, several of the more well-known ones are no longer true, and other perceived disadvantages can be reduced or eliminated by selecting newer components engineered to overcome some of the technology’s traditional shortcomings.
Technology Performance and Options
Electrolytics are named after their anode material, which can include aluminum, tantalum, tantalum polymer, and niobium oxide. Since aluminum electrolytics exhibit a relatively low dielectric permittivity compared to tantalum devises (about 9 vs. 30), it would be reasonable to assume that they have the worst capacitance density of all electrolytics. However, aluminum electrolytics have highly etched aluminum electrodes that significantly increase the surface area of the capacitor and more than offset their low dielectric permittivity.
Some newer aluminum electrolytics also leverage one of the most notable SMT advances in recent years: the vertical chip package. The construction of these devices is simple, consisting of a radial can electrolytic mounted to a back plate to become an SMT device, as illustrated by the cross section in Figure 1. Essentially, two deeply etched aluminum foil electrodes are separated by a paper and wound into a cylinder that is taped, placed in a sleeve, and inserted into a metal can with a rubber seal on the bottom.
The simple structure of miniature, wound, wet SMT aluminum electrolytic capacitors tend to limit long-term reliability, high-frequency efficacy, and temperature stability. But component manufacturers have noted these shortcomings and, in recent years, developed several material systems engineered to improve their reliability and performance, including wet, polymer, and hybrid aluminum electrolytic capacitor technologies.
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Figure 2: Modern miniature SMT aluminum electrolytic capacitors are available with wet, polymer, and hybrid electrolytic material systems
Traditional wet aluminum electrolytic capacitors use a liquid electrolyte to make electrical contact with the wound aluminum electrode foils. The electrolyte is sealed in an aluminum can with a rubber gasket that is tightly filled and then crimp-fit into place.
This material system has two primary limitations in addition to the standard limitations imposed by the simple structure of these miniature wound aluminum electrolytic capacitors. The chemical composition of the liquid electrolyte is a major contributor to capacitor performance as it relates to temperature, pressure, electrical stress, time, low-temperature capacitance, and ESR stability, and it can vary between suppliers. And a liquid electrolyte has the potential to leak or evaporate over time, which could cause the capacitors to fail.
However, users can employ several end user derating rules to increase capacitor reliability. For instance, derating the temperature by 10°C roughly doubles the component lifetime, and derating the applied voltage to rated voltage ratio also yields notable reliability improvements.
So, on balance, wet aluminum electrolytics offer the broadest range of values and reasonable reliability at the lowest possible cost.
Conductive Polymer Electrolytics
The replacement of a wet electrolyte with a conductive polymer electrolyte eliminates the possibility of liquid electrolyte leakage between the seal case and leads along with long-term aging (evaporation) concerns. Conductive polymer electrolytics also exhibit nearly two times less ESR, two to three times greater RMS current capability, and about three times more stability with temperature, and they’re more reliable than wet electrolytics with a similar case size, value, and voltage rating. In addition, if temperature derating effects are taken into account, a 20°C derating of each technology shows that the life expectancy of a wet aluminum capacitor increases by a factor of four, while a conductive polymer aluminum capacitor increases by a factor of roughly 10.
Limitations of this material technology include increased DC leakage, a higher price point, and sensitivity to high shock and vibration environments. The DC leakage of these capacitors increases from about 0.01CV or 3µA for wet aluminum electrolytics to approximately 0.2CV or 300 to 500µA. But it’s a manageable increase early in the design process, and especially so considering that conductive polymer material systems more than double the reliability of wet electrolytics while simultaneously reducing ESR and increasing RMS current.
Hybrid electrolytics were developed to reduce the DC leakage effects of conductive polymer electrolytics and reduce the ESR of wet electrolytics, thereby improving on the reliability and performance characteristics of both materials. Hybrid electrolytics also perform exceptionally well in high humidity environments. As such, they also have a higher price point than wet and conductive polymer material systems. And while they do have some CV limitations, those challenges are actively being overcome with additional materials research and process efforts.
Layered vs. Wound Conductive Polymer Electrolytics
It is important to note that conductive polymer aluminum electrolytic capacitors can be manufactured in either layered or wound styles. Layered devices have an aluminum anode and cathode stack with layers of conductive polymer in between each electrode and are typically finished in a chip SMT package encapsulated in resin compound and equipped with J leads.
Layered aluminum polymers tend to offer reduced inductance compared to wound aluminum polymers in order to extend the frequency response, which is beneficial in a number of applications, including power conversion and noise filtering. They also have significantly reduced height profiles relative to wound aluminum polymers, which results in better shock and vibration performance and makes them easier to implement in height-constrained designs.
Common drawbacks of layered aluminum polymer capacitors include increased cost, non-optimized ESR/RMS current performance, and a reduced value range.
Wound aluminum polymers are more cost competitive and have a larger capacitance range than layered aluminum polymers.
Regardless of the manufacturing method and design, the increased stability and reduced ESR characteristics of conductive polymer electrolytic capacitors is highly beneficial in applications including power supply output filters and control loops.
A Comparison of Stability and Case Inductance
Another important point of comparison for miniature, SMT wet, conductive polymer, and hybrid aluminum electrolytic capacitor technologies is ESR across temperature. The ESR stability of these three electrolytics measured at 120Hz and plotted across temperature is graphed in Figure 3.
As expected, the stability of the conductive polymer aluminum electrolytics is outstanding compared to the wet and hybrid electrolytics. And that’s important because ESR stability at temperature has an enormous impact on the ease with which power supply control loops are designed. For example, power supply control loops in current control mode are often set by the ESR of the output capacitor, which means that a large change in the ESR of the capacitor will negatively affect the transient response of the power supply. Similarly, if the output impedance of an input filter exhibits unstable ESR across the supply’s operating temperature, the ESR instability will affect the source impedance and can cause the power supply to oscillate.
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Figure 3: The comparative ESR stability of wet, conductive polymer, and hybrid aluminum electrolytics vs. temperature
Miniature SMT aluminum electrolytic capacitors are experiencing steady growth in electronic designs due to their attractive cost and high energy density storage capabilities. The relatively recent introduction of conductive polymer and hybrid electrolytic material systems allows designers to capitalize on these benefits while mitigating or even eliminating previous limitations of aluminum electrolytic technologies.
Conductive polymer and hybrid aluminum electrolytics exhibit reduced ESR, increased RMS current capacity, better parametric stability, improved endurance, and longer lifetimes compared to traditional wet aluminum electrolytics. Conductive polymer and hybrid electrolytic solutions can also offer smaller case sizes with higher ripple current and inrush current capabilities and exceed the temperature ratings of standard wet aluminum electrolytic solutions — and all while maintaining attractive charge storage and cost characteristics.
The vertical SMT aluminum capacitor offering is also expanding in terms of package sizes and capacitance, voltage, and ESR values and package sizes. These capacitor families provide high-CV performance in small packages and are compatible with lead-free and RoHS requirements. Vertical SMT aluminum electrolytics — regardless of the material system — offer a wide breadth of solutions ideally suited for satisfying the growing need for bulk capacitors on robust supply rails, in innovative energy harvesting applications, and in the new modules that are replacing historically non-electronic processes.