Capacitors should be derated between 30 and 60% depending on the
material's maximum ambient temperature.
Ceramic capacitors should have a voltage derating factor of 0.60 [110C
max].
Glass Dielectric, a voltage derating factor of 0.50 [110C max].
Plastic Film Dielectric, a voltage derating factor of 0.60 [85C
max].
Tantalum Foil a voltage derating factor of 0.50 [70C max].
Tantalum Solid a voltage derating factor of 0.30 to 0.50 [70C max / 110C
max].
Derate via Device [Voltage rating] / derate factor = maximum applied
voltage ~ Voltage applies to sum of AC ripple and DC voltages.
Tantalum capacitors should be de-rated as much as possible, more so when
used in low-impedance circuits [by-pass]. When Tantalum capacitors are
used in charging/discharging or switching current circuits, the derate
factor should be 30% [or more], and a series resistor should be used to limit
the current.
The reliability [Failure Rate] of Tantalum capacitors is given by the
following equation: F = FU x FT x FR x
FB
FU = Applied Voltage / Rated Voltage.
FT = Operating Temperature
FR = Series Resistance. FB = Basic
Failure rate
The capacitor manufacture would need to be consulted to obtain the
correction factors. How ever, the correction factors increase with
increasing Temperature, Applied Voltage, and failure rate, but decrease
as the series resistance is increased. The correction factor
[FU] is equal to one for an applied voltage equal to
the rated voltage, but reduces to 0.06 if the capacitor is derated by
50%, and drops to 0.001 if the capacitor voltage is de-rated by 30% of
applied voltage [the applied voltage is 30% of the rated voltage]. The
correction factor [FT] is equal to one for an operating
temperature of 85oC, and drops to 0.2 for a temperature of
55oC. An example of the Basic Failure rate
[FB] might be 1%/1000 hours. The correction factor
[FR] is equal to one for a resistance of 0.1 ohms, and
drops to 0.2 for a series resistance of 1 ohm.
A manufacturers list of Capacitors
Back to the main Component Derating page.
Engineering Key words: Component Derating, Derate, Capacitor, Sizing, Rated Temp, Temperature, Guide, Guideline, Example, Class.
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