The Circuit Designer's Companion - diagramas.diagram...

Passive components 93of two. Tan δ is around 0.1−0.3 at 100Hz and 20˚C but worsens dramatically withlower temperature and increasing frequency, higher voltage ratings having lower tan δ.Temperature ranges are typically −40˚C to 85˚C, with some types being rated forextended temperature of –55˚C to 105˚C or 125˚C. Lifetime is an issue withelectrolytics, in two respects. Non-solid electrolytics suffer from eventual drying-out ofthe electrolyte, which is a function of operating temperature and the integrity of thecomponent seal. In general, the life of these types can be doubled for each 10˚C dropin operating temperature. Solid electrolytic types do not show this failure mechanism.The second problem is that of shelf life. Non-solid aluminium electrolytics are oneof the few types of electronic component that degrade when not in use. The dielectricAl 2O 3film can deteriorate, leading to increased leakage, if the component is maintainedfor long periods without a polarising voltage. The effect is dependent on temperatureand shelf life is usually measured in years at 25˚C. Capacitors that have suffered thistype of degradation can be “re-formed” by applying the forming voltage across themthrough a current limiting resistor, if this is found to be necessary. At the same time, itis inadvisable to run this type of electrolytic in circuit configurations where it is notnormally exposed to a polarising voltage. Products which are likely to have been storedfor more than one or two years before being switched on should be designed to toleratehigh leakage currents in the first few minutes of their life.Size and weightOne disadvantage of aluminium electrolytics is that they are often the largest andheaviest components in the circuit, with a few exceptions such as transfomers. Thismeans that they are a weak point when the assembly is vibrated. Some care should betaken in choosing the right part not only for its electrical characteristics but also for themechanical strength of its terminals, if these are the only means of attachment; or inproviding alternative means of mounting.3.3.5 Solid tantalumSolid tantalum electrolytics are generally used when the various performance,construction and reliability limitations of aluminium electrolytics cannot be tolerated.The construction is similar to the solid aluminium type, with a manganese dioxideelectrolyte and sintered tantalum powder for the anode. They can be supplied with atemperature range of −55˚C to 85˚C or up to +125˚C, and have a very much greaterreliability than aluminium, and so are favoured for military use.Leakage current is around 0.01 CV µA which is comparable to the better aluminiumtypes, and tan δ is between 0.04 and 0.1, about twice as good as aluminium.Capacitance change with temperature can vary from ±15% to as good as ±3% acrossthe working temperature range. Some proportion of the working voltage can betolerated in the reverse direction, which relaxes the application constraints. The resindippedbead tantalum construction offers usually the best trade-off between price,performance and size in a given application, and tantalum beads are available from awide range of manufacturers.Tantalum chip capacitorsA major advantage of tantalum capacitors is that they can be packaged in much smallersizes than aluminium electrolytics. This means that they are better suited to surfacemount production and indeed the majority of small SM electrolytics are tantalum types.Capacitance values from 0.1µF to 470µF are available.