Polymer Capacitor

A polymer capacitor, or more accurately a polymer electrolytic capacitor, is an electrolytic capacitor (e-cap) with a solid conductive polymer electrolyte. There are four different types:
* Polymer tantalum electrolytic capacitor (Polymer Ta-e-cap)
* Polymer aluminium electrolytic capacitor (Polymer Al-e-cap)
* Hybrid polymer capacitor (Hybrid polymer Al-e-cap)
* Polymer niobium electrolytic capacitors

Polymer Ta-e-caps are available in rectangular surface-mounted device ( SMD) chip style. Polymer Al-e-caps and hybrid polymer Al-e-caps are available in rectangular surface-mounted device (SMD) chip style, in cylindrical SMDs (V-chips) style or as radial leaded versions (single-ended).

Polymer electrolytic capacitors are characterized by particularly low internal equivalent series resistances (ESR) and high ripple current ratings. Their electrical parameters have similar temperature dependence, reliability and service life compared to solid tantalum capacitors, but have a much better temperature dependence and a considerably longer service life than aluminium electrolytic capacitors with non-solid electrolytes. In general polymer e-caps have a higher leakage current rating than the other solid or non-solid electrolytic capacitors.

Polymer electrolytic capacitors are also available in a hybrid construction. The hybrid polymer aluminium electrolytic capacitors combine a solid polymer electrolyte with a liquid electrolyte. These types are characterized by low ESR values but have low leakage currents and are insensitive to transients, however they have a temperature-dependent service life similar to non-solid e-caps.

Polymer electrolytic capacitors are mainly used in power supplies of integrated electronic circuits as buffer, bypass and decoupling capacitors, especially in devices with flat or compact design. Thus they compete with MLCC capacitors, but offer higher capacitance values than MLCC, and they display no microphonic effect (such as class 2 and 3 ceramic capacitors).

History

Aluminium electrolytic capacitors (Al-e-caps) with liquid electrolytes were invented in 1896 by Charles Pollak.

Tantalum electrolytic capacitors with solid manganese dioxide (MnO₂) electrolytes were invented by Bell Laboratories in the early 1950s, as a miniaturized and more reliable low-voltage support capacitor to complement the newly invented transistor, see Tantalum capacitor. The first Ta-e-caps with MnO₂ electrolytes had 10 times better conductivity and a higher ripple current load than earlier types Al-e-caps with liquid electrolyte. Additionally, unlike standard Al-e-caps, the equivalent series resistance (ESR) of Ta-caps is stable in varying temperatures.

During the 1970s, the increasing digitization of electronic circuits came with decreasing operating voltages, and increasing switching frequencies and ripple current loads. This had consequences for power supplies and their electrolytic capacitors. Capacitors with lower ESR and lower equivalent series inductance (ESL) for bypass and decoupling capacitors used in power supply lines were needed. see Role of ESR, ESL and capacitance.

A breakthrough came in 1973, with the discovery by A. Heeger and F. Wudl of an organic conductor, the charge-transfer salt TCNQ. TCNQ (7,7,8,8-tetracyanoquinodimethane or N-n-butyl isoquinolinium in combination with TTF (Tetrathiafulvalene)) is a chain molecule of almost perfect one-dimensional structure that has a 10-fold better conductivity along the chains than does MnO₂, and has a 100-fold better conductivity than non-solid electrolytes.

The first Al-e-caps to use the charge transfer salt TTF-TCNQ as a solid organic electrolyte was the OS-CON series offered in 1983 from Sanyo. These were wound, cylindrical capacitors with 10x increased electrolyte conductivity compared with MnO₂

These capacitors were used in devices for applications that required the lowest possible ESR or highest possible ripple current. One OS-CON e-cap could replace three more bulky "wet" e-caps or two Ta-caps.J. Both, "Electrolytic Capacitors from the Postwar Period to the Present", IEEE Electrical Insulation Magazine, Vol.32, Issue:2, pp.8-26, March–April 2016, ,

/ref> By 1995, the Sanyo OS-CON became the preferred decoupling capacitor for Pentium processor-based IBM personal computers.
The Sanyo OS-CON e-cap product line was sold in 2010 to Panasonic. Panasonic then replaced the TCNQ salt with a conducting polymer under the same brand.

The next step in ESR reduction was the development of conducting polymers by Alan J. Heeger, Alan MacDiarmid and Hideki Shirakawa in 1975. The conductivity of conductive polymers such as polypyrrole (PPy) or PEDOT is better than that of TCNQ by a factor of 100 to 500, and close to the conductivity of metals.

In 1988 the first polymer electrolyte e-cap, "APYCAP" with PPy polymer electrolyte, was launched by the Japanese manufacturer Nitsuko. The product was not successful, in part because it was not available in SMD versions.

In 1991 Panasonic launched its polymer Al-e-cap series "SP-Cap", These e-caps used PPy polymer electrolyte and reached ESR values that were directly comparable to Ceramic capacitor, ceramic multilayer capacitors (MLCCs). They were still less expensive than tantalum capacitors and with their flat design useful in compact devices such as laptops and cell phones they competed with tantalum chip capacitors as well.

Tantalum electrolytic capacitors with PPy polymer electrolyte cathode followed three years later. In 1993 NEC introduced its SMD polymer Ta-e-caps called "NeoCap". In 1997 Sanyo followed with the "POSCAP" polymer tantalum chips.

A new conductive polymer for tantalum polymer capacitors was presented by Kemet at the "1999 Carts" conference. This capacitor used the newly developed organic conductive polymer PEDT (Poly(3,4-ethylenedioxythiophene)), also known as PEDOT (trade name Baytron®).

Two years later at the 2001 APEC Conference, Kemet introduced PEDOT polymer aluminium e-caps to the market. PEDOT polymer has a higher temperature stability, and as PEDOT:PSS solution this electrolyte could be inserted only by dipping instead of in-situ polymerization like for PPy which makes the production faster and cheaper. Its AO-Cap series included SMD capacitors with stacked anode in "D" size with heights from 1.0 to 4.0 mm, in competition to the Panasonic SP-Caps using PPy at that time.

Around the turn of the millennium hybrid polymer capacitors were developed, which have in addition to the solid polymer electrolyte a liquid electrolyte connecting the polymer layers covering the dielectric layer on the anode and the cathode foil. The non-solid electrolyte provide oxygen for self-healing purposes to reduce the leakage current. In 2001, NIC Components, NIC launched a hybrid polymer e-cap to replace a polymer type at lower price and with lower leakage current. As of 2016 hybrid polymer capacitors are available from multiple manufacturers.

Application basics

Role of ESR, ESL and capacitance

The predominant application of all electrolytic capacitors is in power supplies. They are used in input and output smoothing capacitors, as decoupling capacitors to circulate the harmonic current in a short loop, as bypass capacitors to shunt AC noise to the ground by bypassing the power supply lines, as backup capacitors to mitigate the drop in line voltage during sudden power demand or as filter capacitor in low-pass filter to reduce switching noises. In these applications, in addition to the size, are the capacitance, the impedance ''Z'', the ESR, and the inductance ESL important electrical characteristics for the functionality of these capacitors in the circuits.

The change to digital electronic equipment led to the development of switching power supplies with higher frequencies and "on-board" DC/DC converter, lower supply voltages and higher supply currents. Capacitors for this applications needed lower ESR values, which at that time with Al-e-caps could only be realized with larger case sizes or by replacement with much more expensive solid Ta-caps.

The reason how the ESR influences the functionality of an integrated circuit is simple. If the circuit, f. e. a microprocessor, has a sudden power demand, the supply voltage drops by ESL, ESR and capacitance charge loss. Because in case of a sudden current demand the voltage of the power line drops:
:Δ''U'' = ESR × ''I''.

For example:

Given a supply voltage of 3 V, with a tolerance of 10% (300 mV) and supply current of a maximum of 10 A, a sudden power demand drops the voltage by

: ESR = ''U''/''I'' = (0.3 V)/(10 A) = 30 mΩ.

This means that the ESR in a CPU power supply must be less than 30 mΩ, otherwise the circuit malfunctions.
Similar rules are valid for capacitance and ESL. The specific capacitance could be increased over the years by higher etched anode foils respectively by smaller and finer tantalum powder grains by a factor of 10 to 15 and could follow the trend of miniaturizing. The ESL challenge has led to the stacked foil versions of polymer Al e-caps. However, for lowering the ESR only the development of new, solid conductive materials, first TCNQ, after that the conductive polymers, which led to the development of the polymer electrolyte capacitors with their very low ESR values, the ESR challenge of digitization of electronic circuits could be accepted.

Electrolytic capacitors – basics

Anodic oxidation

Electrolytic capacitors use a chemical feature of some special metals, earlier called "valve metals", that by anodic oxidation form an insulating oxide layer. By applying a positive voltage to the anode (+) material in an electrolytic bath an oxide barrier layer with a thickness corresponding to the applied voltage can be formed. This oxide layer acts as the dielectric in an e-cap. To increase the capacitors capacitance the anode surface is roughened and so the oxide layer surface also is roughened. To complete a capacitor a counter electrode has to match the rough insulating oxide surface. This is accomplished by the electrolyte, which acts as the cathode (-) electrode of an electrolytic capacitor.
The main difference between the polymer capacitors is the anode material and its oxide used as the dielectric:

* Polymer tantalum electrolytic capacitors use high purity sintered tantalum powder as an anode with tantalum pentoxide (Ta₂O₅) as a dielectric and
* Polymer aluminium electrolytic capacitors use a high purity and electrochemically etched (roughened) aluminium foil as an anode with aluminium oxide (Al₂O₃) as the dielectric

The properties of the aluminium oxide layer compared with tantalum pentoxide dielectric layer are given in the following table:

Every e-cap in principle forms a "plate capacitor" whose capacitance is an increasing function of the electrode area A, the permittivity ε of the dielectric material and the thickness of the dielectric (d).
:$C = \varepsilon \cdot \frac$
Capacitance is proportional to the product of the area of one plate multiplied by the permittivity and divided by the dielectric thickness.

The dielectric thickness is in the range of nanometers per volt. On the other hand, the breakdown voltage of these oxide layers is quite high. Using etched or sintered anodes, with their much higher surface area compared to a smooth surface of the same size or volume, e-caps can achieve a high volumetric capacitance. The latest developments in high etched or sintered anodes increases the capacitance value, depending on the rated voltage, by a factor of up to 200 for Al-e-caps or Ta-e-caps compared with smooth anodes.

Because the forming voltage defines the oxide thickness, the desired voltage tolerance can be easily produced. Therefore, the volume of a capacitor is defined by the product of capacitance and voltage, the so-called "CV product".

Comparing the dielectric constants of tantalum and aluminium oxides, Ta₂O₅ has permittivity approximately 3-fold higher than Al₂O₃. Ta-caps therefore theoretically can be smaller than Al-caps with the same capacitance and rated voltage.
For real tantalum electrolytic capacitors, the oxide layer thicknesses are much thicker than the rated voltage of the capacitor actually requires. This is done for safety reasons to avoid shorts coming from field crystallization. For this reason the real differences of sizes that derive from the different permittivities, are partially ineffective.

Electrolytes

The most important electrical property of an electrolyte in an electrolytic capacitor is its electrical conductivity. The electrolyte forms the counter electrode, of the e-cap, the cathode. The roughened structures of the anode surface continue in the structure of the oxide layer, the dielectric, the cathode must adapt precisely to the roughened structure. With a liquid, as in the conventional "wet" e-caps that is easy to achieve. In polymer e-caps in which a solid conductive polymer forms the electrolyte, this is much more difficult to achieve, because its conductivity comes by a chemical process of polymerization. However, the benefits of a solid polymer electrolyte, the significantly lower ESR of the capacitor and the low temperature dependence of the electrical parameters, in many cases justify the additional production steps as well as higher costs.

Conducting salt TCNQ electrolyte

Electrolytic capacitors with the charge transfer salt tetracyanoquinodimethane TCNQ as electrolyte, formerly produced by Sanyo with the trade name "OS-CON", in the true sense of the term "polymer" were not "polymer capacitors". TCNQ electrolytic capacitors are mentioned here to point out the danger of confusion with 'real' polymer capacitors, which are sold nowadays under the same trade name OS-CON. The original OS-CON capacitors with TCNQ electrolyte sold by the former manufacturer Sanyo has been discontinued with the integration of Sanyo capacitor businesses by Panasonic 2010. Panasonic keep the trade name OS-CON but change the TCNQ electrolyte into a conductive polymer electrolyte (PPy).

Electrolytic capacitors with TCNQ electrolyte are not available anymore.

Polymer electrolyte

Polymers are formed by a chemical reaction, polymerization. In this reaction monomers are continuously attached to a growing polymer strand.Clayden, J., Greeves, N. and Warren, S. (2000)
''Organic chemistry''
Oxford University Press pp. 1450–1466 Usually polymers are electrically insulators, at best, semiconductors. For use as an electrolyte in e-caps, electrical conductive polymers are employed. The conductivity of a polymer is obtained by conjugated double bonds which permit free movement of charge carriers in the doped state. As charge carriers serve electron holes. That means, the conductivity of conducting polymers, which is nearly comparable with metallic conductors, only starts when the polymers are doped oxidatively or reductively.

A polymer electrolyte must be able to penetrate the anode's finest pores to form a complete, homogeneous layer, because only anode oxide sections covered by the electrolyte contribute to the capacitance. For this the precursors of the polymer has to consist of very small base materials that can penetrate even the smallest pores. The size of this precursors are the limiting factor in the size of the pores in the etched aluminium anode foils or of the size of tantalum powder. The rate of polymerization must be controlled for capacitor manufacturing. Too rapid polymerization does not lead to a complete anode coverage, while too slow polymerization increases production costs. Neither the precursors nor the polymer or its residues may attack the anodes oxide chemically or mechanically. The polymer electrolyte must have high stability over a wide temperature range over long times. The polymer film is not only the counter electrode of the e-cap it also protects the dielectric even against external influences such as the direct contact of graphite in this capacitors, which are provided with a cathode contact via graphite and silver.

Polymer e-caps employ either polypyrrole (PPy) or polythiophene (PEDOT or PEDT)

Polypyrrole PPy