156

Circuit Impedance

All solid tantalum capacitors require current limiting

resistance to protect the dielectric from surges. A series

resistor is recommended for this purpose. A lower circuit

impedance may cause an increase in failure rate, especially

at temperatures higher than 20°C. An inductive low imped-

ance circuit may apply voltage surges to the capacitor and

similarly a non-inductive circuit may apply current surges

to the capacitor, causing localized over-heating and failure.

The recommended impedance is 1Ω per volt. Where this is

not feasible, equivalent voltage derating should be used

(See MIL HANDBOOK 217E). Table I shows the correction

Table I: Circuit Impedance

Correction factor to failure rate F for series resistance R

con. level).

Example calculation

Consider a 12 volt power line. The designer needs about

10μF of capacitance to act as a decoupling capacitor near a

video bandwidth amplifier. Thus the circuit impedance will be

limited only by the output impedance of the boards power

unit and the track resistance. Let us assume it to be about

2 Ohms minimum, i.e., 0.167 Ohms/Volt. The operating

temperature range is -25°C to +85°C. If a 10μF 16 Volt

capacitor was designed-in, the operating failure rate would

be as follows:

If the capacitor was changed for a 20 volt capacitor, the

operating failure rate will change as shown.

3.2 DYNAMIC

As stated in Section 1.2.4 (page 151), the solid Tantalum

capacitor has a limited ability to withstand voltage and current

surges. Such current surges can cause a capacitor to fail.

The expected failure rate cannot be calculated by a simple

formula as in the case of steady-state reliability. The two

parameters under the control of the circuit design engineer

known to reduce the incidence of failures are derating and

series resistance.The table below summarizes the results of

trials carried out at AVX with a piece of equipment which has

very low series resistance and applied no derating. So that

the capacitor was tested at its rated voltage.

Results of production scale derating experiment

As can clearly be seen from the results of this experiment,

the more derating applied by the user, the less likely the

probability of a surge failure occurring.

It must be remembered that these results were derived from

a highly accelerated surge test machine, and failure rates in

the low ppm are more likely with the end customer.

Circuit Resistance ohms/volt

FR

3.0

0.07

2.0

0.1

1.0

0.2

0.8

0.3

0.6

0.4

0.4

0.6

0.2

0.8

0.1

1.0

Capacitance and Number of units 50% derating No derating

Voltage

tested

applied

applied

47μF 16V

1,547,587

0.03%

1.1%

100μF 10V

632,876

0.01%

0.5%

22μF 25V

2,256,258

0.05%

0.3%

TAP/TEP Technical Summary and

Application Guidelines