CSR91B565KB datasheet(79/84 Pages) KEMET

79 / 84 page

Dissipated power must not exceed the limits

specified for the Series.

The positive peak AC voltage plus the DC voltage

must not exceed the maximum working voltage permitted

at the ambient temperature.

The negative peak AC voltage, in combination

with the DC voltage, must not exceed the permissible

reverse voltage at the ambient temperature.

The rms ripple voltage limitation imposed by power

dissipation is given by:

P= I2R=

where: I = rms ripple current (amperes)

E = rms ripple voltage (volts)

P = power (watts)

Z = impedance at specified frequency

(ohms)

R = equivalent series resistance at

specified frequency (ohms)

Maximum allowable rms ripple voltage may be

determined as follows:

E(max) @

25°C=Z

E(max)

85°C=0.9 E(max) @ 25°C

E(max) @

125°C=0.4 E(max) @ 25°C

P(max)

maximum watts shown on

Performance Characteristic pages 5,

42, 49, 58 and 61.

Permissible AC ripple current can be determined by

the following:

I rms =

If two polar capacitors are connected back-to-back,

(1) the pair may be operated on AC without need for DC

bias. The first two criteria above must be observed. If DC

is applied, the sum of DC and peak AC must not exceed,

in either direction, the maximum working voltage specified

for the ambient temperature.

(1) Some KEMET Series provide convenient assemblies of

non-polar pairs. The two negative terminals are connected

internally. It is also permissible to connect the two positive

terminals to form a non-polar pair.

14. LONG-TERM STABILITY

Within the general class of electrolytic capacitors,

solid tantalum capacitors offer unusual stability of the three

important parameters: capacitance, dissipation factor, and

leakage current. These solid-state devices are not subject

to the effects of electrolysis, deforming or drying-out asso-

ciated with liquid-electrolyte capacitors.

When stabilized for measurement at standard condi-

tions, capacitance will typically change less than ±3% dur-

ing a 10,000 hour life test +85° C. The same comparative

change has been observed in shelf tests at +25° C extend-

ing for 50,000 hours. (Some of this change may stem from

instrument or fixture error.)

Dissipation factor exhibits no typical trend. Data from

10,000 hour life tests at +85° C show that initial limits (at

standard conditions) are not exceeded at the conclusion of

these tests.

Leakage current is more variable than capacitance or

DF; in fact, leakage current typically exhibits a logarithmic

dependence in several respects. MIL-C-39003/1 permits

leakage current (measured at standard conditions) to rise

by a factor of four over 10,000 hour life tests. Typical

behavior shows a lower rate of change, which may be

negative or positive. Initial leakage currents are frequently

so low (less than 0.1 nanoampere in the smallest CV

capacitors, to about 10 microampere in the largest CV

types) that changes of several orders of magnitude have

no discernable effect on the usual circuit designs.

15. FAILURE MODE

Capacitor failure may be induced by exceeding the

rated conditions of forward DC voltage, reverse DC volt-

age, surge voltage, surge current, power dissipation, or

temperature. As with any practical device, these capaci-

tors also possess an inherent, although low, failure rate

when operated within the rated condition.

The dominant failure mode is by short-circuit. Minor

parametric drifts (see Section 14 “Long-Term Stability”) are

of no consequence in circuits suitable for solid tantalum

capacitors. Catastrophic failure occurs as an avalanche in

DC leakage current over a short (millisecond) time span.

The failed capacitor, while called “short-circuited”, may

exhibit a DC resistance of 10 to 104 ohm.

If a failed capacitor is in an unprotected low-imped-

ance circuit, continued flow of current through the capaci-

tor may obviously produce severe overheating. This heat

may melt the internal solder (all Series) and the sealing

solder used in hermetic Series. The short-circuit failure

may thereby be converted to an open-circuit failure. If the

circuit does not open promptly, the over-heated capacitor

may damage the circuit board or nearby components.

Protection against such occurrence is obtained by current-

limiting devices or fuses provided by the circuit design.

Fortunately, the inherent failure rate of KEMET solid

tantalum capacitors is low, and this failure rate may be fur-

ther improved by circuit design. Statistical failure rates are

provided for those capacitors with characters other than

“A” in the next-to-last position of the part number. Relating

circuit conditions to failure rate is aided by the guides in

the section following.

16. RELIABILITY PREDICTION

Three important application conditions largely control

failure rate: DC voltage, temperature, and circuit imped-

ance. Estimates of the respective effects are provided by

the nomograph in Figure 12 and Table 3 following. The

nomograph related failure rate to voltage and temperature

while the table relates failure rate to impedance. These

estimates apply to steady-state DC conditions, and they

assume usage within all other rated conditions.

Standard conditions, which produce a unity failure

rate factor, are rated voltage, +85° C, and 0.1 ohm-per-volt

circuit impedance. While voltage and temperature are

straightforward there is sometimes difficulty in determining

impedance. What is required is the circuit impedance seen

by the capacitor. If several capacitors are connected in

parallel, the impedance seen by each is lowered by the

source of energy stored in the other capacitors. Energy is

similarly stored in series inductors.

Failure rate is conventionally expressed in units of

percent per thousand hours. As a sample calculation, sup-

pose a particular batch of capacitors has a failure rate of

0.5% Khr under standard conditions. What would be pre-

dicted failure rate at 0.7 times rated voltage, +60° C and

0.8

Ω/V? The nomograph gives a factor of 7 x 10-4, and the

table gives a factor of 0.3. The failure rate estimate is then:

0.5 x 7 x10-4 x 0.3 = 1.05 x 10-4, or 0.0001% Khr

P(max)

E2R

KEMET

APPLICATION NOTES FOR TANTALUM CAPACITORS

KEMET Electronics Corporation, P.O. Box 5928, Greenville, S.C. 29606 (864) 963-6300

P(max)

√

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