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Revision: 10-Jun-16
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Document Number: 40024
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GUIDE TO APPLICATION
1. AC Ripple Current: subjecting a capacitor to an AC
voltage causes an AC current to flow through it. The
amplitude of the current is dependent on the impedance
of the capacitor at the frequency of the applied signal:
where:
I = ripple current
V = applied AC voltage
Z = impedance of capacitor (frequency dependent)
This current causes heating in the capacitor because of
I
2
R losses (R is the equivalent series resistance at the
applied frequency). This heating or power dissipation, is
one of the limiting factors of the capacitor’s ripple current
rating.
These power dissipation ratings are based on a
calculated +50 °C internal temperature rise in still air. The
maximum allowable ripple currents given in the Standard
and Extended Ratings tables are based on these ratings
and the maximum equivalent series resistance at that
frequency.
The relationship is written as follows:
where:
P = maximum power
I = maximum ripple current
R = equivalent series resistance
Therefore:
where:
R is in
P is in W
I is in A
RMS
2. AC Ripple Voltage: in operation, the peak voltage across
the capacitor (DC working voltage plus peak ripple
voltage) must not exceed the rated working voltage of the
capacitor. The DC component of the applied voltage
should be sufficiently large to prevent polarity reversal in
excess of 3 V at +85 °C or 2 V at 125 °C.
There will be a point at the lower frequency and
capacitance values when the peak AC voltage will be the
limiting factor on the ripple current - not its heating
effects.
For example:
given a 25 μF, 8 V capacitor in the “C” case code and
assuming a ripple current application at a frequency of
120 Hz, the total maximum allowable peak to peak
voltage at +25 °C is:
In order to allow the full swing of 11 V
pp
and not exceed
rated forward or rated reverse, a DC bias of 2.5 V is
assumed to be applied.
From the “Standard Ratings Table”, the maximum ripple
current at 40 kHz is 0.820 A. Compensating for the lower
frequency from the “Ripple Current Multipliers” tables:
This current rating is calculated strictly on the basis of
maximum power dissipation. Now calculate what
impressed voltage this amount of current will cause
across this capacitor.
Assuming a sinusoidal voltage, calculate the rated peak
to peak current:
where:
ESR = 4 (from “Standard Ratings” table)
Therefore:
and
Therefore, the peak voltage of the capacitor is the limiting
factor for the ripple current and can be calculated as
follows:
or
CASE CODE
MAXIMUM PERMISSIBLE
POWER DISSIPATION
AT +25 °C (W) IN FREE AIR
C1.00
F1.55
T1.75
K1.95
I
RMS
(120 Hz) 0.820 A x 0.6 0.492 A
RMS
==
I
pp
I
RMS
x 2 2 0.492 x 2.828 1.39 A
pp
===
V
pp impressed
I
pp
x Z
C
120 Hz=
Z
C120Hz
ESR
2
(X
C
(120 Hz)
+
2
=
X
C
1
2fC
1
2 12025 x 10
-6
53.1 == =
Z
C
4
2
53.1
2
+ 53.3 ==
V
pp impressed
1.39 A
pp
x 53.3 =
Max. I
pp
V
Cpp
allowed
Z
C
-----------------------------------------
11.0 V
53.3
------------------
0.206 A
pp
===
0.206
22
---------------
0.073 A
RMS
at 120 Hz=
