Tantalum Capacitor Selection: Why Derating Decides
A tantalum capacitor packs a large capacitance into a small case, which is why it appears on power rails where a ceramic would need several parts in parallel. It is also less forgiving than a ceramic, and most of the failures attributed to the component are actually failures of the way it was specified. Voltage derating, in particular, is not a suggestion.
This article covers the properties that matter when choosing one, the rules that keep it reliable, and the applications where it is still the right answer.
The moulded chip package with a J lead is the familiar form, and the family covers a wide range of capacitance and voltage in a small number of case sizes.
What The Family Offers
The useful range runs from a fraction of a microfarad up to several hundred, at voltages from a few volts to around fifty. The case sizes are standardised, which means a part from one supplier will fit the footprint of another, and the code identifies both the size and the capacitance and voltage within it. Larger cases offer more capacitance and lower resistance; smaller ones save area at the cost of both.
Electrically, the part behaves as a capacitance in series with a resistance, and the equivalent series resistance, the ESR, is quoted with the part because it is significant. Values in the hundreds of milliohms are typical for the moulded chip family. The capacitance also falls with frequency and with temperature, and the leakage current rises with temperature, so a part that looks ample at room temperature may be marginal in a hot enclosure. The layout around a switching converter is where most of these parts are used, and where their behaviour matters most.
Voltage Derating Is The Rule That Matters
A tantalum capacitor should not be operated close to its rated voltage. The accepted practice is to use it at no more than half, and often a third, of the rated voltage on a low impedance supply, with the margin increasing as the temperature rises. The reason is the failure mode: a tantalum that breaks down does not simply go open, it fails in a way that can draw enough current to ignite the material and damage the board around it.
The derating requirement is therefore a safety rule as much as a reliability one. It applies to the steady state working voltage including any tolerance on the supply, and it applies more strictly above eighty five degrees, where the derating curve in the specification becomes the reference rather than the headline figure. A part chosen at its rated voltage will appear to work, and its life will be short and unpredictable.
ESR And Why Lower Is Not Always Better
The ESR is what makes the part useful in some circuits and unsuitable in others. In a bulk decoupling position the ESR damps the resonance between the capacitor and the inductance of the supply network, which is a benefit, and a part with an unusually low ESR can remove that damping and allow a peak in the impedance. That is one reason a low ESR part is not automatically an improvement.
The ESR also sets the ripple current the part can carry without overheating. The ripple current multiplied by the resistance is the power dissipated in the component, and it has to stay within what the case can dissipate. Where a supply has a large ripple, that calculation sets a minimum case size, because the larger case spreads the same loss over more surface. The way the supply rails are planned therefore has a direct effect on the capacitor specification.
Inrush Current And Surge Failure
A second failure mode is associated with the current that flows when the supply is switched on or when the part is first connected. A large capacitance charging through a low impedance produces a current pulse that stresses the dielectric, and a part that has been operated for some time can fail at that moment. This is why a tantalum is sometimes specified with a series resistance, or replaced by a polymer part in circuits where the inrush cannot be limited.
The practical measures are to limit the inrush with a small series resistance where the circuit allows it, to avoid an unnecessarily large capacitance in a position where the charging current is high, and to consider a polymer tantalum or a ceramic for the positions where the current pulse is severe. The current the trace can carry without excessive heating is the other side of the same calculation, because the pulse also passes through the board.
Case Size, Capacitance And What Fits
Case size is where the electrical requirements meet the layout. A larger case offers more capacitance, lower ESR and better heat dissipation, and it occupies more area. A smaller case does the opposite. The trade is usually made in one direction only, because the capacitance required by the circuit and the ripple current it has to handle set a floor on the size, and the layout can only accommodate a ceiling.
The tolerance on the capacitance should be checked at the same time. A part with a wide tolerance may be smaller than the circuit needs at the bottom of its range, which matters in a circuit where the capacitance is part of a timing or a filter calculation. For hold up applications the tolerance matters less than the energy stored, which is a function of the capacitance and the square of the allowable voltage swing.
Where A Tantalum Still Wins
Ceramics have taken over many positions because they have very low resistance, no polarity and no derating requirement of the same kind. Where the capacitance needed is large and the space is small, however, a bank of ceramics becomes expensive and bulky, and a single tantalum does the job. Where the damping provided by a moderate ESR is useful, a tantalum is also the natural choice.
Polymer tantalum parts sit between the two, with much lower ESR and no inrush limitation of the same severity, at a higher price. The choice among the three is usually made on the impedance the rail needs across the frequency range rather than on capacitance alone, and the best rails normally combine a ceramic bank for the high frequencies with one bulk part for the low ones. gopcb lays out power circuits and can review the placement and the derating of the bulk capacitance as part of the design, since a part that meets its specification on paper can still fail if the position and the rating do not match the circuit it serves.
FAQ
Why derate a tantalum to half its rated voltage? Because its failure mode is violent and the risk rises sharply as the applied voltage approaches the rating, especially at elevated temperature.
Is a lower ESR always better? No. A very low ESR can remove the damping the rail needs and allow a resonance, and the ESR also determines the ripple current the part can handle.
Can a tantalum be used without polarity? No. It is a polarised part, and reverse voltage damages it. The polarity marking on the package and the footprint must both be correct.
