Aluminum electrolytic capacitors

You may need to parallel several aluminum electrolytics to lower the ESR sufficiently, and you may also have to substantially increase the capacitance just to stay within the total 1% limit somehow. Also remember that the ESR of aluminum electrolytic capacitors gets significantly worse over time. So if you have a customer return after several months in the field, it may well be because of the aging of the electrolytic bulk capacitor Try replacing the capacitor and then recheck. 

Except for some exotic surface-mount technology (SMT/SMD) aluminum electrolytic capacitor types with solid electrolyte systems, in general, an aluminum electrolytic capacitor contains a wound capacitor element (the coil), impregnated with liquid electrolyte, connected to terminals, and sealed in a can (with a rubber plug at the end). The aluminum in the name,... 

Figure 4-3 Impedance of Aluminum Electrolytic Capacitors (Frequency and Temperature)... 

The second blog entry is from a senior defense engineer in South Africa. My impression is that he or she has very aptly expressed a lot of the finer aspects of selecting aluminum electrolytic capacitors. I suggest you read that very closely. It s folks like these (no hidden agendas, just helpfulness) that make it worthwhile. 

As shown in Fig. 12.4, aluminum electrolytic capacitors usually consist of an aluminum foil with a thin film of anodically-formed aluminum oxide (dielectric), an aluminum foil, an electrolyte solution, and a separator. The whole sandwich is compactly rolled and packed in a container. The electrolytic capacitors are in wide use, because of their small sizes, high capacitances, and low prices. However, the characteristics of electrolytic capacitors are apt to deteriorate with time. Recently,... 

Takaaki M, Yoshishige I, Noburo K. Nichicon Corporation. Aluminum electrolytic capacitor. US patent /6307733. 

Capacitors can be polarized or non-polarized, depending on the - dielectric. Non-polarized devices have dielectrics consisting of ceramics or polymers (such as polystyrene, polyester, or polypropylene). They are normally box-shaped and their capacity is usually in the range from pF to pF, the maximum voltage up to 1000 V. Polarized capacitors are electrochemical devices the dielectric is an anodic oxide of A1 (pF to 100 mF, potentials up to 1000 V), Ta (capacities pF to 100 pF, potentials up to 20 V), or Nb (- electrolytic capacitor) or a double layer (- supercapacitor, capacities up to some 10 F and potentials up to 2.5 V or 5 V). Aluminum electrolytic capacitors are normally of cylindrical shape with radial or axial leads. Tantalum capacitors are of spherical shape and super capacitors form flat cylinders. 

Typical labeling schemes for common capacitors, (a) and (h) are ceramic capacitors of 150 and 10 pF values, respectively. Tolerances are often indicated with letters, with lower values meaning less uncertainty, e.g.,J = 5%, K = 10%. (c) and (< are tantalum and aluminum electrolytic capacitors of values 2.2 and 22 /xF, respectively. Polarity is irrelevant for ceramic capacitors but is indicated and must be maintained for electrolytic capacitors. 

In attempting to generally reduce parasitics and their associated losses, we may notice that these are often dependent on various external factors — temperature for one. Some losses increase with temperature — for example the conduction loss in a mosfet. And some may decrease — for example the conduction loss in a bjt (when operated with low currents). Another example of the latter type is the ESR-related loss of a typical aluminum electrolytic capacitor, which also decreases with temperature. On the other hand, some losses may have rather strange shapes. For example, we could have an inverted bell-shaped curve — representing an optimum operating point somewhere between the two extremes. This is what the core loss term of many modern ferrite materials (used for inductor cores) looks like — it is at its minimum at around 80 to 90°C, increasing on either side. 

A common example of lifetime considerations in a commercial power supply design comes from its use of aluminum electrolytic capacitors. Despite their great affordability and respectable performance in many applications, such capacitors are a victim of wearout due to the steady evaporation of their enclosed electrolyte over time. Extensive calculations are needed to predict their internal temperature ( core temperature ) and thereby estimate the true rate of evaporation and thereby extend the capacitor s useful life. The rule recommended for doing this life calculation is — the useful life of an aluminum electrolytic capacitor halves every 10°C rise in temperature. We can see that this relatively hard-and-fast rule is uncannily similar to the rule-of-thumb of failure rate. But that again is just a coincidence, since life and failure rate are really two different issues altogether. 

Data Handbook, Aluminum Electrolytic Capacitors , PA01-A, 1993 N.A. Edition Philips Components... 

Understanding Aluminum Electrolytic Capacitors , 2nd Edition, 1995 United Chemi-Con Inc. 

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