Aluminum Electrolytics

In this particular chapter, we will focus a great deal on ceramic capacitors since these have become extremely popular today. However, in commercial AC-DC power supplies, the aluminum electrolytic (or elko) is still king, so we will talk about that component too. Unfortunately, we will have to pretend none of the others even exist. We just don t have the space for all of them here. 

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, coimected to terminals, and sealed in a can (with a rubber plug at the end). The aluminum in the name, 

why not use an elko OK, it has a higher ESR. It gives higher output ripple. Granted  

See Eigure 4-2 for a can t miss polarity indicator of a typical SMD elko. In through-hole designs, there is invariably a long bar (or arrows) running alongside the negative terminal. 

In addition, the negative terminal lead is also typically the shorter of the two. Always be very careful, and double-check the polarity after soldering and before powering on. 

---

It is easy to reduce anhydrous rare-earth hatides to the metal by reaction of mote electropositive metals such as calcium, lithium, sodium, potassium, and aluminum. Electrolytic reduction is an alternative in the production of the light lanthanide metals, including didymium, a Nd—Pt mixture. The rare-earth metals have a great affinity for oxygen, sulfur, nitrogen, carbon, silicon, boron, phosphoms, and hydrogen at elevated temperature and remove these elements from most other metals. 

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. 

Most of the evaluation boards of such ESR-sensitive parts are shipped out to customers with only aluminum electrolytic or tantalum capacitors at their outputs. But what really happens is that the customer happily connects the eval board (rather expectantly) into his or her system, and completely forgets there are a bunch of ceramic capacitors all over the system board (for local decoupling at different points). In effect, the switcher can lose that valuable zero in its control loop and break into oscillations (see Figure 3-5). More so if the connecting leads are short. 

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

Note that in the Buck and the Buck-Boost, the input capacitor is included in the critical path. That implies we need very good input decoupling in these topologies (for the power section). So, besides the necessary bulk capacitor for the power stage (typically a tantalum or aluminum electrolytic of large capacitance), we should also place a small ceramic capacitor (about 0.1 to IpF) directly between the quiet end of the switch (i.e., at the supply side) and the ground—and also as close as possible to the switch. 

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,... 

Coagulants and coagulant aids iron and aluminum sulfates, activated silicates, alginates, synthetic polyelectrolytes and their impurities, silica, and aluminum electrolyte monomers. 

Fig. 1. Aluminum electrolytic reduction cell of the pre-bake type. The anodes are constructed of separate blocks of carbon which have been pre-haked. There is a lead to the main bus bar from each block. (Reynolds Metals Company)...

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. 

The double-layer capacitor is one of the electrochemical capacitors showing intermediate performances between conventional capacitors and rechargeable batteries from the viewpoint of energy and power densities. Although the terms supercapacitor and ultracapacitor are often used for double-layer capacitors, in a sense that they have higher capacitance than conventional capacitors (ceramic, film, aluminum electrolytic, or tantalum electrolytic capacitors), these terms are not to be used because they are the trademarks of certain companies products. 

Electrolytic manufacture of sodium and chlorine. Electrolytic manufacture of aluminum. Electrolytic refining of copper. 

Primary cells the dry cell. The lead storage cell. The aluminum electrolytic rectifier. 

Of all aluminum electrolyte systems, to our knowledge type 3 is today the technically most accepted. This type of electrolyte was discovered and intensely studied by Ziegler and Lehmkuhl [118, 217, 218, 221]. The companies Siemens AG, HGA, MBB, SEDEC, Interatom, and ALU 2000 have developed the industrial scale process (Sigal Process = Siemens galvanoaluminum). The used electrolytes consist of aluminumalkyls as well as alkali metal halides or quaternary onium salts, which are dissolved in aromatics (i.e., toluene). Electrolysis is carried out at temperatures around 90-100°C. Electrolytes of this kind have demonstrated their high produc-... 

In 1954, Lehmkuhl in his dissertation discussed two alternative mechanisms for the cathodic deposition and anodic dissolution of aluminum from the new organo-aluminum electrolytes [118]. One possibility is the preliminary deposition of alkali metals as a result of electrolytic dissociation of the 1 2 complex, resulting in alkali metal cations, and subsequent chemical reaction of this alkali metal with free or coordinated aluminum trialkyl, yielding aluminum metal and alkali metal tetraalkyl aluminate (see Scheme 2). 

---