TAP/TEP Technical Summary and
Application Guidelines
Circuit Impedance
3.2 DYNAMIC
As stated in Section 1.2.4 (page 151), the solid Tantalum
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
factor, FR, for increasing series resistance.
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
Table I: Circuit Impedance
Capacitance and Number of units 50% derating No derating
Correction factor to failure rate F for series resistance R
on basic failure rate FB for a typical component (60%
con. level).
Voltage
47μF 16V
100μF 10V
22μF 25V
tested
1,547,587
632,876
applied
0.03%
0.01%
0.05%
applied
1.1%
0.5%
Circuit Resistance ohms/volt
FR
2,256,258
0.3%
3.0
2.0
1.0
0.8
0.6
0.4
0.2
0.1
0.07
0.1
0.2
0.3
0.4
0.6
0.8
1.0
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.
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:
a) FT = 0.8 @ 85°C
b) FR = 0.7 @ 0.167 Ohms/Volt
c) FU = 0.17 @ applied voltage/rated voltage = 75%
Thus FB = 0.8 x 0.7 x 0.17 x 1 = 0.0952%/1000 Hours
If the capacitor was changed for a 20 volt capacitor, the
operating failure rate will change as shown.
FU = 0.05 @ applied voltage/rated voltage = 60%
FB = 0.8 x 0.7 x 0.05 x 1 = 0.028%/1000 Hours
156 ■ MAY 2013
KYOCERA AVX [ KYOCERA AVX ]
