Polymer capacitor

Polymer electrolytic capacitors are characterized by particularly low internal equivalent series resistances (ESR) and high ripple current ratings.

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.

[8] 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.

[18] 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.

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.

If the circuit (e.g. a microprocessor) has a sudden power demand, the supply voltage drops by ESL, ESR and capacitance charge loss.

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.

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.

The conductivity of a polymer is obtained by conjugated double bonds which permit free movement of charge carriers in the doped state.

[33] For this purpose, ionic dopants are added to the basic substances of the polymer, forming a conductive surface layer on the dielectric during the first impregnation.

For capacitors with high capacitance values with high-roughened aluminium anode foils or fine-grained tantalum powders, dispersions having very small particle sizes are offered.

The liquid electrolyte is soaked in the separator (spacer) and achieves with its ion conductivity the electrical contact between the two polymer layers covering the dielectric and on the cathode foil.

The advantage of the multi-anode technology in addition to the very low ESR values is the lower inductance ESL, whereby the capacitors are suitable for higher frequencies.

Advantage of this construction is that the liquid electrolyte in operation delivers the oxygen which is necessary for self-healing of the dielectric layer in the presence of any small defects.

In hybrid polymer capacitors liquid can flow to the defect, delivering oxygen and healing the dielectric by generating new oxides, decreasing the leakage current.

The curves show the progressively lower impedance and ESR values of "wet" Al-e-caps and MnO2 Ta-e-caps, Al/TCNQ and tantalum polymer e-caps.

This dissipation power loss PL is caused by ESR and is the squared value of the effective (RMS) ripple current IR.

The ripple current for polymer e-caps is specified as a maximum effective (RMS) value at 100 kHz at upper rated temperature.

Non-sinusoidal ripple currents have to be analyzed and separated into their individual single frequencies by means of Fourier analysis and summarized by squared addition to calculate a RMS value.

[53][54] Polymer Ta-e-caps which are exposed to surge, peak or pulse currents, for example, in highly inductive circuits, require a voltage derating.

[67] Datasheet leakage current specification is given by multiplication of the rated capacitance value CR with the value of the rated voltage UR together with an added figure, measured after 2 or 5 minutes, for example a formula for non-solid Al-e-caps: Leakage current in solid polymer e-caps generally drops very fast but then remain on the reached level.

The value depends on the voltage applied, temperature, measuring time and influence of moisture caused by case sealing conditions.

This leakage current cannot be reduced by "healing" in the sense of generating new oxide, because under normal conditions polymer electrolytes cannot deliver oxygen for forming processes.

The failure rates "λ" and "FTa" depend on operational conditions including temperature, voltage applied, and various environmental factors such as humidity, shocks or vibrations and of the capacitance value of the capacitor.

Solid Ta-e-caps and "wet" Al-e-caps failure rates can be recalculated with acceleration factors standardized for industrial[69] or military[70] contexts.

These reliability levels within the calculated lifetime are comparable with other electronic components and achieve safe operation for decades under normal conditions.

The lifetime specification for polymer capacitors is specified in similar terms to non-solid Al-e-caps with a time in hours at maximum voltage and temperature, for example: 2000h/105 °C.

This value can be used for an estimation of an operational life time at individual conditions by a formula called "20-degree-rule":[72][73][74] This rule characterizes the change of thermic polymer reactions speed within the specified degradation limits.

According to this formula the theoretical expected service life of a 2000 h/105 °C polymer capacitor, which is operated at 65 °C, can be calculated (better estimated) with about 200,000 hours or approximately 20 years.

Many capacitor manufacturers compose these crucial arguments of their technologies against the competition in presentations and articles,[87] f. e.: Radial, SMD, Axial As of July 2016

Rectangular shaped polymer aluminium (black) and tantalum (brown) electrolytic chip capacitors
Cylindrical (wound) polymer aluminium electrolytic capacitors
Conductivities of some electrolytes
OS-CON capacitors with solid TCNQ electrolyte had a typical lilac insulation sleeve
For a sudden power demand of a subsequent circuit, the supply voltage drops by ESL, ESR and capacitance charge loss
Basic principle of anodic oxidation (forming), 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 of d .
Structural formula of TCNQ
Structural formula of polypyrrole , doped with p-Toluenesulfonic acid
Pyrrole can be polymerized electrochemically to control the rate of polymerizion. [ 10 ]
Structural formula of PEDOT
Structural formula of PEDOT:PSS
The multi-anode construction has several sintered tantalum anodes which are connected in parallel, whereby both ESR and ESL will be reduced.
In the "face-down" construction of tantalum chip capacitors, the internal current path is constructively reduced, which reduce the parasitic impedance (ESL) with the result, that the resonance shifts to higher frequencies. Simply said, the capacitor gets "faster"
Cross-sectional view of the capacitive cell of a hybrid polymer aluminum capacitor, polymer electrolyte in the pores of the aluminum foils and liquid electrolyte as the electrical connection between the polymer layers.
Series-equivalent circuit model of an electrolytic capacitor
Typical capacitance capacitor as a function of temperature for a polymer Al-e-cap and two non-solid Al-e-caps
Relation between rated voltage U R and category voltage U C and rated temperature T R and category temperature T C
Typical impedance characteristics over the frequency for 100 μF e-caps with different electrolytes compared with a 100 μF class-2 MLCC ceramic capacitor .
Typical curve of the as a function of temperature for polymer capacitors ( ) and "wet" Al-e-caps ( )
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 .
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
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
After application of a voltage at weakened spots in the oxide of the capacitor a localized higher leakage current is formed, which leads to a local heating of the polymer, whereby the polymer either oxidized and becomes highly resistive or evaporates.