Aluminum electrolytic capacitor

A non-solid electrolyte covers the rough surface of the oxide layer, serving in principle as the second electrode (cathode) (-) of the capacitor.

However, it can evaporate through a temperature-dependent drying-out process, which causes electrical parameters to drift, limiting the service life time of the capacitors.

The exception is the bipolar or non-polar aluminum electrolytic capacitor, which has a back-to-back configuration of two anodes in a single case, and which can be safely used in AC applications.

The dielectric thickness of electrolytic capacitors is very thin, in the range of nanometers per volt, but the voltage strengths of these oxide layers are quite high.

An electrically insulating oxide layer Al2O3 is thereby formed on the aluminum surface by application of a current in correct polarity if it is inserted in an electrolytic bath.

[3] The disadvantage of crystalline oxide is its greater sensitivity to tensile stress, which may lead to microcracks when subjected to mechanical (winding) or thermal (soldering) stressors during the post-forming processes.

In order to reduce the contact resistance to the electrolyte and to make it difficult for oxide formation during discharging, the cathode foil is alloyed with metals such as copper, silicon, or titanium.

In case of a malfunction, overload or wrong polarity operating inside the electrolytic capacitor housing, substantial gas pressure can arise.

[11][12] The foils are fed to an automatic winder, which makes a wound section in a consecutive operation involving three sequential steps: terminal welding, winding, and length cutting.

This optically ready capacitor is then contacted at rated voltage in a high temperature post-forming device for healing all the dielectric defects resulting from the cutting and winding procedure.

Karol Pollak, a producer of accumulators, found out that the oxide layer on an aluminum anode remained stable in a neutral or alkaline electrolyte, even when the power was switched off.

The first common application of wet aluminum electrolytic capacitors was in large telephone exchanges, to reduce relay hash (noise) on the 48 volt DC power supply.

The development of AC-operated domestic radio receivers in the late 1920s created a demand for large-capacitance (for the time) and high-voltage capacitors for the valve amplifier technique, typically at least 4 microfarads and rated at around 500 volts DC.

Waxed paper and oiled silk film capacitors were available, but devices with that order of capacitance and voltage rating were bulky and prohibitively expensive.

The ancestor of the modern electrolytic capacitor was patented by Samuel Ruben in 1925,[23][24] who teamed with Philip Mallory, the founder of the battery company that is now known as Duracell International.

The period after World War II is associated with a rapid development in radio and television technology as well as in industrial applications, which had great influence on production quantities but also on styles, sizes and series diversification of electrolytic capacitors.

[31] The decades from 1970 to 1990 were marked by the development of various new professional aluminum electrolytic capacitor series with f. e. very low leakage currents or with long life characteristics or for higher temperatures up to 125 °C, which were specifically suited to certain industrial applications.

These capacitors use as solid organic conductor the charge transfer salt TTF-TCNQ (tetracyanoquinodimethane), which provided an improvement in conductivity by a factor of 10 with respect to the manganese dioxide electrolyte.

Due to their large capacitance values, aluminum electrolytic capacitors have relatively good decoupling properties in the lower frequency range up to about 1 MHz or a little more.

[42] Non-solid aluminum electrolytic capacitors that leakage current after an operation time of, for example, one hour remain on a higher level than specified.

For other conditions of applied voltage, current load, temperature, capacitance value, circuit resistance (for tantalum capacitors), mechanical influences and humidity the FIT figure can recalculated with acceleration factors standardized for industrial[49] or military[50] contexts.

In response to demands for long life, high temperature performance from automotive and green energy applications (solar microvinverters, LEDs, wind turbines, etc.

But the additional internal heat of 3 to 10 K, depending on the series, which is generated by the ripple current is usually taken into account by the manufacturer due to safety margins when interpreting the results of its endurance tests.

They use different ways achieve the specification; some provide special formulas,[55][56][57] others specify their capacitor lifetime calculation with graphs that take into account the influence of applied voltage.

With today's high levels of purity in the manufacture of electrolytic capacitors it is not to be expected that short circuits occur after the end-of-life-point with progressive evaporation combined with parameter degradation.

This leads in turn to a previously unnoticed water driven corrosion, which weakens the stable dielectric oxide layer during storage or disuse.

Their advantages, among other things were lower leakage currents and nearly unlimited shelf life,[70] but this led to another problem: the growing mass production with automatic insertion machines requires a washing of the PCB's after soldering; these cleaning solutions contained chloroalkane (CFC) agents.

In the meantime electrolytic systems have been developed with additives to inhibit the reaction between anodic aluminum oxide and water, which solve most of the high leakage current problems after storage.

Nearly all today's series of capacitors fulfill the 1000 hours shelf life test, which is equivalent to a minimum five years of storage at room temperature.

Some applications like AC/AC converters with DC-link for frequency controls in three-phase grids need higher voltages than electrolytic capacitors usually offer.

Aluminum electrolytic capacitors with non-solid electrolyte have a wide range of styles, sizes and series
Basic principle of anodic oxidation, in which, by applying a voltage with a current source, an oxide layer is formed on a metallic anode
A dielectric material is placed between two conducting plates (electrodes), each of area A , and with a separation d .
Surface of an etched low voltage anode foil
The cross-section view of etched 10 V low voltage and 400 V high voltage anode foils shows the different etching structure
Ultra-thin-cross-section of an etched pore in a low voltage anode foil, 100,000-fold magnification, light grey: aluminum, dark grey: amorphous aluminum oxide, white: pore in which the electrolyte is active
The thickness of the effective dielectric is proportional to the forming voltage
Anode and cathode foils are manufactured as so called "mother rolls", from which the widths and lengths are cut off, as required for capacitor production
View of three different imprinted predetermined breaking points (pressure relief vents) on the bottom of cases of radial electrolytic capacitors
Process flow diagram for production of radial aluminum electrolytic capacitors with non-solid electrolyte
The first published electrolytic capacitor from 1914. It had a capacitance of around 2 microfarads.
View of the anode of a "wet" aluminum electrolytic capacitor, Bell System Technique 1929
Some various forms of historical anode structures. For all of these anodes the outer metallic container serves as the cathode
A "dry" 100 μF electrolytic capacitor rated for 150 VDC
Miniaturization of aluminum electrolytic capacitors from 1960 to 2005 in case 10x16mm up to factor ten
Conductivity of non-solid and solid electrolytes
Series-equivalent circuit model of an electrolytic capacitor
Typical capacitance as a function of temperature
Relation between rated and category voltage and rated and category temperature
An exploded electrolytic capacitor on a PCB
Simplified series-equivalent circuit of a capacitor for higher frequencies (above); vector diagram with electrical reactances X ESL and X C and resistance ESR and for illustration the impedance Z and dissipation factor tan δ
Typical impedance curves for different capacitance values as a function of frequency showing the typical form with decreasing impedance values below resonance and increasing values above resonance. The higher the capacitance, the lower the resonance frequency.
The high ripple current across the smoothing capacitor C1 in a power supply with half-wave rectification causes significant internal heat generation corresponding to the capacitor's ESR
Ripple current causes internal heat, which has to be dissipated to the ambient environment
During discharging the current flow direction in the capacitor changes, the cathode (-) gets an anode (+), two internal voltages with opposite polarity arise. The capacitor construction rule – C K >> C A – ensures no post-forming of the cathode foil during discharging.
general leakage behavior of electrolytic capacitors: leakage current as a function of time for different kinds of electrolytes
non solid, high water content
non solid, organic
solid, polymer
Typical leakage current curve of an industrial long-life electrolytic capacitor with non-solid electrolyte
Bathtub curve with times of "early failures", "random failures", and "wear-out failures". The time of random failures is the time of constant failure rate and corresponds with the lifetime of non-solid electrolytic capacitors.
The electrical values of electrolytic capacitors with non-solid electrolytes change over time due to evaporation of the electrolyte. Reaching specified limits of the electrical parameters, the capacitors counts as "wear out failure". The graph shows this behavior in a 2000 h endurance test at 105 °C.
Several aluminum electrolytic capacitors having burst due to the usage of an improper electrolyte
Polarity marking on a SMD-V-chip capacitor