What kind of electrolyte is ethylene glycol

Water-based electrolytic capacitors: from plague to indispensable component

In the early 2000s, water-based electrolytic capacitors were often manufactured with the wrong mix of inhibitors or passivators. The result was electrolytic capacitors with an open valve, rubber stoppers pushed out or components completely destroyed by an explosion - the so-called "Elko-Plage". These problems no longer exist today. In order to understand the advantages of these capacitors and their benefits for modern electronics, a basic understanding of the components is required.

How does an electrolytic capacitor work?

Compared to other capacitor technologies, the aluminum electrolytic capacitor offers a great advantage: a high capacity in the smallest space with an attractive price / performance ratio. In addition, it is insensitive to overvoltage, which is shown in the data sheet by the surge voltage. Disadvantages are its generally higher impedance, the drying out over time, a strong increase in impedance at low temperatures and the dependence on the operating temperature. This is determined by the specified component parameters, which in turn are defined from the electrolyte used.

An electrolytic capacitor with a liquid electrolyte (e-cap) essentially consists of two strips of aluminum foil, which are separated by a separator paper. The anode foil is electrochemically roughened to enlarge the surface. When a voltage is applied (forming), a thin layer of aluminum oxide is created on the surface, which acts as a dielectric. The liquid or solid electrolyte forms the cathode, which is contacted to the outside via the second aluminum foil. Both aluminum foils are provided with contacts at the intended location (stitching) and then wound together with the separator paper and soaked in a liquid electrolyte for impregnation. Finally, a rubber stopper closes the capacitor cup with the soaked winding. During the construction of the capacitor, the stitching, the electrolyte used and the separator paper essentially determine the later ESR (Equivalent Series Resistance).

Electrolytes in comparison

Various liquid electrolytes are used in electrolytic capacitors today. Electrolytes containing ethylene glycol (EG) or boric acid are mainly used in medium to high voltage electrolytic capacitors at temperatures of up to 85 ° C. Here the water content in the electrolyte is approx. 5-20%, with inhibitors (chemical inhibitors) the aggressiveness of the water against the aluminum oxide layer is suppressed.

Organic electrolytes such as dimethylformamides (DMF), γ-butyrolactones (GBL) and dimethylacetamides (DMA) allow a wide temperature range from -55 to 150 ° C. They have stable parameters such as low leakage currents and good long-term properties and thus enable long operating times. Their water content is extremely low.

The water content of water-containing 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 leads to excellent conductivity, which is noticeable in an extremely low ESR. Conversely, significantly higher ripple currents can be achieved than with conventional, almost anhydrous electrolytes. In addition, the high water content means that the material costs for the electrolyte filling are significantly lower.

However, they also have a serious disadvantage, because water reacts by hydration when it comes into direct contact with aluminum. However, the stable aluminum oxide layer protects the aluminum. Inhibitors or passivators are added to the electrolyte in order to prevent hydration or corrosion even in the case of a damaged layer, e.g. due to production or longer storage. If this step is not carried out, a lot of heat and gas (hydrogen) can form when water and aluminum come into contact. The capacitors are severely damaged and in extreme cases can even explode.

Even today there is a note in component specifications that water-based electrolytic capacitors should not be used under any circumstances. However, this information is not specified, e.g. by the maximum permissible water content. In addition, the negative effect of adding additives no longer exists, so that the capacitors are ideally suited for applications with a long service life or high loads. In particular among the low-ESR types known today with high ripple current capacity and a service life of at least 10,000 hours at 105 ° C, the electrolytes with a higher water content are often found.

Special hybrid type with polymer

If the ultimate goal is not pure capacity, 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 partly produced on a polymer basis. The aluminum oxide layer and the opposite cathode foil are coated with a conductive polymer that is later present in the capacitor in a solid state. The high conductivity of the polymer significantly improves the contact resistance of aluminum oxide to liquid electrolyte and to the cathode foil.

The result: a very low ESR and the possibility of high ripple currents. The improved ESR reduces self-heating during operation; 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) applies as a rough guide for estimating the service life at different temperatures.

Particularly important when designing hybrid capacitors in the circuit is their behavior in terms of service life, frequency and temperature curve, which is completely different from the previous one due to the new electrolytes. While the ESR of an electrolytic capacitor increases in the negative temperature range and during its service life, it is absolutely stable with hybrid types. The strong dependence of the capacitance on the frequency is also not given with hybrid capacitors, here there is hardly any change up to 100kHz. An electrolytic capacitor, on the other hand, already breaks by a good 40% at 20kHz.

As a result, when designing a circuit with hybrid capacitors, the total capacitance can nominally be reduced significantly and its efficiency can nevertheless be improved. Miniaturization is also possible, since the hybrid technology enables higher ripple currents in a smaller design.

Solid polymer with even better properties

If you want to do without a liquid component completely, you can use solid polymer electrolytic capacitors. Here the liquid component is replaced by a solid, conductive polymer. This leads to an even better ESR and ripple current and does not offer any possibility for dehydration. The service life can be roughly specified as -20 ° C temperature = 10 times the service life.

The disadvantages are the price, a significantly higher leakage current and the sensitivity to moisture. Since the solid polymer attracts moisture, the components are delivered in a dry pack and, once opened, are subject to strict processing requirements. These types are only available with AECQ200 certification in exceptional cases. In addition, with this technology, the decision between voltage and capacitance always has to be made for the given design. A good mixture, as it is possible with the electrolytic capacitor or the hybrid type, cannot be achieved to the same extent here due to the solid electrolyte.

In addition, the residual current is more pronounced with the solid types than with the hybrid types, since there is no free oxygen for self-healing of the manufacturing-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 get completely into every pore of the roughened aluminum foil. This has a negative effect on the achievable capacity and at the same time increases the leakage current. With regard to the stability of frequency, temperature and service life, solid polymer electrolytic capacitors are on a par with hybrid capacitors.

Conclusion

With steadily increasing demands on ESR, design, long-term stability and component price, water-based electrolytic capacitors have become indispensable. Polymer types offer an alternative for those who cannot gain anything from the technology. The hybrid versions in particular represent a good compromise between performance and price and are constantly being further developed by the manufacturers. In terms of miniaturization and efficiency, they offer new options for designing the circuit.

Components are available at www.rutronik24.de.

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