At the beginning of this century, water-based electrolytic capacitors were often manufactured with the wrong blend of inhibitors or passivators. The result was electrolytic capacitors with an open vent, a pushed-out rubber plug or components fully destroyed by an explosion - the so-called "capacitor plague". These problems no longer exist. Understanding the advantages of these capacitors and their benefits for modern-day electronics requires fundamental knowledge of the components.
How does an electrolytic capacitor work?
Compared to other capacitor technologies, the aluminum electrolytic capacitor offers a major advantage: an attractive value-for-money option guaranteeing high capacitance in the smallest of spaces. Moreover, it is insensitive to overvoltage, a fact underlined in the data sheet by the surge voltage. Disadvantages are its higher impedance, a tendency to dry out over time, a strong impedance increase at low temperatures, and its dependency on the operating temperature. This is determined by the stipulated component parameters, which in turn are defined by the electrolyte used.
An electrolytic capacitor with a liquid electrolyte (or e-cap) essentially consists of two strips of aluminum foil separated by a separator paper. The effective contact area of the anode foil is enlarged by electrochemical etching. When applying voltage (forming), a thin layer of aluminum oxide develops on the surface which acts as the dielectric. The liquid or solid electrolyte forms the cathode, which is contacted to the outside via the second aluminum foil. Both aluminum foils are stitched together at the intended point and then wound together with the separator paper and soaked in a liquid electrolyte for impregnation purposes. Finally, a rubber plug seals the capacitor can with the impregnated winding. When designing the capacitor, the subsequent ESR (equivalent series resistance) is determined by the stitching, the electrolyte used, and the separator paper.
Comparison of electrolytes
Various liquid electrolytes are used in electrolytic capacitors today. Electrolytes containing ethylene glycol (EG) or boric acid are used mainly in medium to high-voltage electrolytic capacitors at temperatures of up to 85°C. In this case, the water content in the electrolyte is approx. 5-20% and inhibitors (chemical inhibitors) are used to prevent the aluminum oxide layer being negatively impacted by the water.
Organic electrolytes such as Dimethylformamide (DMF), γ-Butyrolactone (GBL), and Dimethylacetamide (DMA) allow for a wide temperature range from -55 to 150°C. They have stable parameters, such as low leakage currents and good long-term properties, thus enabling long operating periods. Their water content is extremely low.
The water content of aqueous electrolytes can be up to 70%. This high concentration offers advantages: Water with a permittivity (dielectric conductivity) of ε = 81 has the excellent property of binding an extremely large number of salt ions. This results in outstanding conductivity, which is reflected in an extremely low ESR. Conversely, significantly higher ripple currents can be achieved than with conventional, almost water-free electrolytes. In addition, the material costs of the electrolyte filling are significantly lower due to the high water content.
They, nevertheless, also have a major disadvantage, as water reacts through hydration when in direct contact with aluminum. However, the robust aluminum oxide layer protects the aluminum. To prevent hydration or corrosion even in case of a damaged layer, e.g. due to a production error or prolonged storage, inhibitors or passivators are added to the electrolyte. If this step is not taken, a significant amount of heat and gas (hydrogen) can form when water and aluminum come into contact. The capacitors will be damaged considerably and can even explode in extreme cases.
Even today, component specifications still state that water-based electrolytic capacitors should never be used. However, this specification is not specifically defined, e.g. by the maximum permissible water content. In addition, the negative effect of adding additives is no longer present, making the capacitors ideal for applications with a long service life or high load factors. Electrolytes with a higher water content are frequently found in today's low ESR types with high ripple current resistance and a service life of at least 10,000h at 105°C.
Special hybrid type with polymer
If the primary goal is not simply capacitance but a very low ESR, a liquid electrolyte can be partially or completely replaced by a conductive polymer. These hybrid types are fully AECQ200 certified. They combine the liquid, anhydrous electrolyte with the high conductivity of a solid polymer. For this purpose, the liquid electrolyte is also partially polymer based. The aluminum oxide layer and the opposite cathode foil are coated with a conductive polymer, which is subsequently present in the capacitor as a solid state medium. The high conductivity of the polymer significantly improves the contact resistance of aluminum oxide to liquid electrolyte and to cathode foil.
The result: A very low ESR and the possibility of high ripple currents. The improved ESR reduces self-heating during operation, while the solid polymer reduces the proportion of liquid components that can dry out. This is why hybrid electrolytic capacitors have a significantly longer basic service life than the water-based low-ESR standard variants. As with the standard type, the Arrhenius formula (-10°C temperature = double the service life) is used as a rough guide to estimate the service life at various temperatures.
Particularly important when designing hybrid capacitors in the circuit is their behavior with regard to service life, frequency, and temperature curve, which is completely different to the previous one due to the new electrolytes. While the ESR increases with an electrolytic capacitor in the negative temperature range and during its service life, it behaves absolutely stable with hybrid types. Further, strong dependence of the capacitance on the frequency is not given with hybrid capacitors, as there is hardly a change up to 100kHz here. An electrolytic capacitor, on the other hand, breaks down by at least 40% at 20kHz.
In nominal terms, it is possible to reduce the overall capacitance significantly while still improving its efficiency when designing a circuit with hybrid capacitors. Miniaturization is also possible, as hybrid technology enables higher ripple currents in a smaller structural shape.
Solid polymer with even better properties
Solid-polymer electrolytic capacitors can be used if you want to do without a liquid part completely. In this case, the liquid component is replaced by a solid, conductive polymer. This leads to an even better ESR and ripple current while eliminating the possibility of drying out. The service life can be roughly stated as a -20°C temperature = 10 times the service life.
The disadvantages are the price, a considerably higher leakage current, and moisture sensitivity. As the solid polymer attracts moisture, the components are supplied in dry packs and are subject to strict processing requirements as soon as they are opened. These types are only available with AECQ200 certification in exceptional cases. In addition, this technology always requires a decision between voltage and capacitance in terms of the actual structural shape. A good mixture, as is possible with electrolytic capacitors or with the hybrid type, cannot be achieved to the same extent here due to the solid electrolytes.
In addition, the residual current is more pronounced in the solid types than in the hybrid ones, since free oxygen is missing for self-healing of the production-related defects in the dielectric. The liquid electrolyte of the hybrid type contains oxygen, which enables self-healing and keeps the residual current at the level of standard electrolytic capacitors. In addition, the solid electrolyte does not completely penetrate every pore of the etched aluminum foil. This has a negative effect on the achievable capacitance and at the same time increases the leakage current. In terms of stability of frequency, temperature, and service life, solid-polymer electrolytic capacitors are on a par with hybrid capacitors.
With increasing requirements being placed on ESR, structural shape, long-term stability, and component price, water-based electrolytic capacitors have become indispensable. If the technology fails to meet your needs, polymer capacitors offer you an alternative. The hybrid variants, in particular, represent an excellent compromise between performance and price and are subject to constant further development by suppliers. In terms of miniaturization and efficiency, they offer new options for designing the circuit.
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