Aluminum Capacitors
Aluminum capacitors are a family of devices that fall under the umbrella of “electrolytic” capacitors. As such, they offer high capacitance values in small packages at relatively low cost. In trade for these desirable qualities, their electrical properties and service lives tend to be relatively dismal. Though ill-suited for all but the most barbaric of signal-related applications, aluminum capacitors are a staple for DC power-related functions. Three distinct types are available; the standard aluminum electrolytic capacitor, a bipolar variant on that theme, and a newer type which incorporates a conductive polymer electrode. Referring to the family as “aluminum capacitors” rather than “aluminum electrolytic capacitors” is a hat-tip to this latter device type which doesn’t contain a traditional liquid electrolyte.
Device Construction
Standard aluminum electrolytic capacitors consist of two sheets of high-purity aluminum foil, interleaved and separated by a spacer material such as paper that is saturated with an electrolyte solution. These foil sheets are usually etched on a microscopic level to increase their effective surface area, by as much as a few hundred times what it would be were the foil left smooth.
On one of the foil sheets (in standard Aluminum electrolytic capacitors) a layer of aluminum oxide is formed that serves as the capacitor’s dielectric material, by applying a voltage to the foil through an oxygen-bearing electrolyte solution. So doing causes oxygen from the electrolyte to bind to the aluminum foil’s surface, forming an oxide layer with a thickness proportional to the voltage applied during the formation process, and determined by the intended working voltage of the capacitors to be produced. Typically, the thickness of this oxide layer is on the order of 1 micrometer, or 0.00004 inches.
The underlying, non-oxidized metal forms one of the electrodes of the aluminum electrolytic capacitor. The other electrode is not the second sheet of foil, but rather the electrolyte solution. In standard aluminum electrolytic capacitors, the second foil sheet has no deliberately formed oxide layer and is simply used to make electrical contact with the electrolyte, because it’s kind of hard to solder a liquid to a circuit board… In bi-polar capacitors, an oxide layer is formed on both aluminum sheets, resulting in a device that’s effectively two capacitors connected in inverse series.
Because the electrolyte is a fluid (except in the case of aluminum polymer capacitors, where it’s a conductive polymer material) it is able to conform to the microstructure of the etched & oxidized foil sheet, resulting in a large area between the two electrodes of the capacitor. Since the dielectric material (aluminum oxide) is quite thin, the end result is a capacitor with a high value; per the basic capacitor equation, capacitance increases in proportion to electrode area and in inverse proportion to electrode separation distance/dielectric thickness. Lead wires are attached to the foil sheets, the assembly is wound, folded, or otherwise formed to fit in a container (usually also made of aluminum) and the assembly is sealed using a rubber sealing plug.
Because fault conditions can result in a buildup of internal pressure, most aluminum capacitors also include provisions for venting such pressure in a relatively safe manner. A dedicated mechanism is usually used for this purpose in larger devices, whereas smaller devices achieve the protective venting function through careful design of the rubber sealing plug and/or scoring of the container so that it ruptures in a relatively controlled fashion should excess internal pressure develop.
Range of Available Capacitances & Voltages
The chart below illustrates the range of voltage/capacitance ratings for aluminum capacitors available through Digi-Key at the time of writing. Standard, bi-polar, and polymer types are included.
Application Strengths & Weaknesses
The primary strength of aluminum capacitors is their ability to provide a large capacitance value in a small package, and do so for a relatively low cost. Additionally, they tend to have good self-healing characteristics; when a localized weak spot in the aluminum oxide dielectric layer develops, the increased leakage current flow through the weak point in the dielectric causes a chemical reaction similar to that used during the initial formation of the dielectric layer, resulting in a thickening of the dielectric at the weak point, and a consequent reduction in leakage current.
The shortcomings of aluminum capacitors are mostly related to (a) the chemically-reactive nature of the materials used in their construction, (b) the conductive properties of the electrolyte solutions, and (c) the volatility of liquid electrolytes.
The chemically reactive nature of the materials used in aluminum capacitors is problematic on two points; the stability of the dielectric layer and the long-term mechanical integrity of the device. Since the aluminum oxide dielectric layer in these devices is formed through an electrochemical process, it can also be eroded by an electrochemical process simply by reversing the applied voltage. This is why most aluminum capacitors are polarized; application of voltage with the wrong polarity causes rapid erosion & thinning of the dielectric, resulting in high leakage current and excessive internal heating.
From a mechanical integrity standpoint, mixing a highly reactive metal (aluminum) with a corrosive electrolyte solution is a delicate proposition; errors in electrolyte composition can result in premature failure, as evidenced by the “capacitor plague” of the early 2000’s.
Another shortcoming of aluminum electrolytic capacitors is the fact that the electrolytes used aren’t particularly efficient conductors, because conduction in electrolyte solutions is achieved through ionic, rather than electronic conduction; instead of loose electrons moving between atoms serving as the charge carriers, ions (atoms or small groups thereof that have a charge due to a surplus or deficit of electrons) are moving about through the solution. Since ions are more bulky than electrons, they don’t move as easily and hence ionic conduction generally tends to be a higher-resistance proposition than electronic conduction. The extent to which this is the case is influenced significantly by temperature; the lower the temperature, the more difficult it is for ions in an electrolyte solution to move about through the solution, which translates into a higher resistance. Thus, electrolytic capacitors tend to have a relatively high ESR that exhibits a strong inverse correlation with temperature.
The third major downside to aluminum capacitors (with the exception of the solid polymer types) is that the liquid electrolyte solutions tend to evaporate over time, eventually being lost to the atmosphere by diffusion through the rubber sealing plug, leaks in safety vent structures, or similar phenomena.
Common uses and applications
Aluminum capacitors are primarily used in DC power applications calling for a relatively large value, low-cost capacitor, when AC performance and parameter stability over time are not particularly critical. Such applications include bulk filtering of rectified AC line voltage in power supply applications, output filtering in low-frequency switching power supplies, etc. Due to the time constant formed by their relatively high ESR in series with their large nominal capacitance, aluminum capacitors as a class tend to loose their appeal quickly as ripple frequencies approach about 100 kHz. Device optimizations vary widely though, and the useful frequency limit for any given device might be as low as a few kHz to 1MHz.
Aluminum electrolytic capacitors are generally not suitable for applications where high losses and wide variability of device parameters with environmental and operating conditions is undesirable, which includes most analog signal paths.
Common failure mechanisms/critical design considerations
Electrolyte Loss
The liquid electrolyte found in most aluminum capacitors is subject to evaporation over time, leading to an increase in ESR and reduction in capacitance. This is a wear mechanism which is typically the limiting factor for the service life of an aluminum electrolytic capacitor. The clock starts immediately upon manufacture of a device and does not stop, although application and storage conditions influence the rate at which the hands move.
Temperature is the principle factor in determining the rate of electrolyte loss, and is well-described by the Arrhenius equation, which predicts roughly a factor-of-two change in process rate for every 10°C change in temperature. Stated differently, reducing the temperature of an electrolytic capacitor by 10°C roughly doubles its expected service life, all other factors being equal.
Electrolyte loss is also influenced by atmospheric pressure, with lower pressure resulting in accelerated electrolyte loss. Extreme low-pressure environments may cause devices not designed for such environments to experience case rupture or opening of the safety vent, resulting in failure much sooner than would occur at higher ambient pressures.
When estimating capacitor lifetime on the basis of the Arrhenius relationship and the manufacturer’s stated lifetime specification, self-heating due to ripple current must be taken into account; the internal temperature of the capacitor is the quantity of interest, not simply the application’s ambient temperature.
For high-altitude or low-pressure operations, consult manufacturers’ specifications, as de-rating of stated lifetime will be required, down to zero at an ambient pressure at which the difference between the electrolyte’s vapor pressure and the outside ambient pressure will cause the capacitor’s safety vent to open. Note that vapor pressure generally increases with temperature, resulting in a tradeoff between operating temperature and maximum permissible operating altitude.
Faulty Electrolyte
Improper electrolyte formulation can cause rapid corrosion of internal components and/or buildup of gas pressure in an aluminum capacitor, resulting in premature failure. This mechanism is reportedly responsible for widespread premature failures of aluminum electrolytic capacitors in many consumer electronic devices in the early 2000’s.
Aside from independent testing and evaluation, the best way to avoid this problem (which has proven –extremely- expensive for many companies) is to purchase product only from reputable manufacturers, either direct or through a manufacturer-authorized distributor. Buying cheap electronic components from questionable sources is a lot like buying pharmaceuticals in plastic baggies from a stranger on a street corner in the bad part of town at 2AM… Don’t do it.
Voltage Overstress
As the voltage applied across an aluminum electrolytic capacitor exceeds prescribed limits, the leakage current through the aluminum oxide dielectric layer increases rapidly, beginning at localized “thin” spots within the dielectric material. This increase in leakage current results in increased localized heating within the device. If the leakage current is not limited, the increased localized heating can cause further damage to the dielectric layer, resulting in a cascading failure of the dielectric material and destruction of the capacitor.
Current Overstress
Aluminum electrolytic capacitors often have a comparably large ESR value, mostly due to the resistivity of the electrolyte solution. AC currents flowing through this resistance result in ohmic heating, which contributes to electrolyte loss and increases the risk of a dielectric breakdown event. It should be noted that the apparent capacitance of an aluminum electrolytic capacitor is frequency dependent. Consequently, the ripple current specification provided by the manufacturer should be interpreted in light of the ripple frequencies present in the application. Maximum ripple current figures for aluminum electrolytic capacitors are commonly quoted at 120Hz and 100kHz, so one should take care not only to observe the quoted ripple current value when selecting the device, but also the test frequency for which the figure is quoted.
Voltage Overstress Due to Aging
Due to the electrochemical nature of the dielectric formation process, storage at zero applied voltage for extended periods of time results in degradation of the aluminum oxide dielectric layer. With the dielectric weakened, voltage overstress conditions can occur even though an applied voltage may be within a device’s rated limits. In mild cases, the only symptoms may be an increased leakage current and elevated device temperatures for a time, until the device self-heals. In severe cases where maximum rated voltage is applied though a low source impedance across a badly-degraded dielectric, a device may fail short-circuit and rupture in spectacular fashion. While developments in electrolyte formulations to address the issue have been and continue to be made, storage stability varies significantly among different products, with some experiencing measurable degradation after only 1 to 3 years of storage in a discharged state.
In designing applications that may lie dormant for extended periods of time, moderate voltage de-rating of devices is recommended in order to provide an improved margin of safety against this effect. Use of products specifically designed to be robust against degradation in storage is also suggested.
In repair/recommisioning situations, the generally prescribed treatment for atrophied aluminum electrolytic capacitors is to apply the system voltage gradually, over a period of 4-8 hours. Before so doing, verify that the equipment will not be damaged by a prolonged period of operation at below-spec supply voltages.
Device features, Options, & Targeted applications
Audio
Aluminum electrolytic capacitors marketed for audio applications are commonly low-ESR types, and design compromises in their construction may be skewed in favor of electrical performance and parameter stability at the expense of things such as size and cost.
It should be noted however, that the audio field is riddled with subjectivism and marketing designed to separate fools from their money, and that this effect permeates even down to the component level. If capacitor A has a prettier label and costs ten times more than capacitor B, then obviously capacitor A is better, right? Not necessarily. Check the specifications, know which ones are important for the application at hand, and pick whatever device best meets the application requirements. Unless you’re building something to sell to the sort of person who is willing to spend hundreds or thousands of dollars on “directional” speaker cables. In that case, pick whatever offers you the most bling for the buck…
Automotive
Devices highlighted for automotive applications are typically designed for long life and operation over an extended temperature range, extending at least to 105°C. Most are qualified to AEC (Automotive Electronics Council) standards.
Bi-Polar
Bi-polar electrolytic capacitors are designed to operate without damage when subject to voltages that change polarity, by forming an oxide film on both of the foil sheets used in a standard aluminum electrolytic capacitor, rather than just one. Because of the high ESR of such devices they’re generally considered unsuitable for operations with a continuously-applied AC voltage, and for this reason are occasionally referred to as “non-polar DC capacitors” to emphasize the point. Their use is typically limited to DC applications where the polarity to be applied is uncertain, may occasionally reverse on a transient basis, or where the current flow through the device can be limited to values that do not result in excess self-heating.
General Purpose
“General purpose” is a catch-all designation for devices that are not expressly designed to address a particular application category, and have no major distinguishing characteristics in their construction.
High Temp Reflow
Devices designated as “high temperature reflow” types are designed and qualified for use in applications where higher process temperatures are encountered during manufacturing, as is commonly encountered in lead-free/RoHS-compliant reflow soldering operations.
Motor Run
Aluminum Electrolytic capacitors with this designation are designed for continuous-duty, high-ripple applications such as variable-speed motor drives and inverter applications.
Motor Start
Aluminum Electrolytic capacitors with this designation are generally designed for use in AC motor starting applications. Typically they’re bi-polar, rated for several hundred volts, and have values between a few tens to a few thousand uF.
Polymer
This designation is associated with aluminum electrolytic capacitors which use a solid conductive polymer as an electrolyte material, rather than a liquid electrolyte. Typically they exhibit better stability, lower ESR, and better lifetimes at elevated temperature than comparable liquid electrolyte devices, though availability is constrained to relatively low capacitance and voltage ratings and the cost of devices for a given capacitance and voltage rating is significantly higher than a similar liquid electrolyte type.
Stainless Steel Case
Devices with this designation are designed with ruggedized, stainless steel cases that are capable of withstanding higher-than-typical pressure differentials between the inside and outside of the capacitor. This allows operation at lower atmospheric pressures than most other devices, and permits a longer expected operating life due to the ability to mitigate electrolyte loss. Typically, these devices are also quite costly.