The Capacitor
The effect of the application of DC voltage is shown in
Figure 4. The voltage coefficient is more pronounced for
higher K dielectrics. These figures are shown for room tem-
perature conditions. The combination characteristic known
as voltage temperature limits which shows the effects of
rated voltage over the operating temperature range is
shown in Figure 5 for the military BX characteristic.
"Q" Factor
“Q” vs. Frequency
2000
1600
1200
AVX
C0G (NP0)
T.C.
Cap. Change vs. D.C. Volts
AVX X7R T.C.
Capacitance Change Percent
2.5
0
-2.5
-5
-7.5
-10
25%
50%
75%
Percent Rated Volts
100%
800
400
AVX X7R T.C.
0
1
KHz
10
KHz
100
KHz
1
MHz
10
MHz
100
MHz
1
GHz
Frequency
Figure 7
Effects of Frequency –
Frequency affects capacitance
and dissipation factor as shown in Figures 6 and 7.
Variation of impedance with frequency is an important con-
sideration for decoupling capacitor applications. Lead
length, lead configuration and body size all affect the
impedance level over more than ceramic formulation varia-
tions. (Figure 8)
Effects of Time –
Class 2 ceramic capacitors change
capacitance and dissipation factor with time as well as
temperature, voltage and frequency. This change with time
is known as aging. Aging is caused by a gradual re-align-
ment of the crystalline structure of the ceramic and
produces an exponential loss in capacitance and decrease
in dissipation factor versus time. A typical curve of aging
rate for semistable ceramics is shown in Figure 9 and a
table is given showing the aging rates of various dielectrics.
If a ceramic capacitor that has been sitting on the shelf for
a period of time, is heated above its curie point, (125°C for
4 hours or 150°C for
1
⁄
2
hour will suffice) the part will
de-age and return to its initial capacitance and dissipation
factor readings. Because the capacitance changes rapidly,
immediately after de-aging, the basic capacitance
measurements are normally referred to a time period some-
time after the de-aging process. Various manufacturers use
different time bases but the most popular one is one day or
twenty-four hours after “last heat.” Change in the aging
curve can be caused by the application of voltage and
other stresses. The possible changes in capacitance due to
de-aging by heating the unit explain why capacitance
changes are allowed after test, such as temperature
cycling, moisture resistance, etc., in MIL specs. The
application of high voltages such as dielectric withstanding
voltages also tends to de-age capacitors and is why
re-reading of capacitance after 12 or 24 hours is allowed in
military specifications after dielectric strength tests have
been performed.
Figure 4
Typical Cap. Change vs. Temperature
AVX X7R T.C.
Capacitance Change Percent
+20
+10
0VDC
0
-10
-20
-30
-55 -35
-15
+5
+25 +45 +65 +85 +105 +125
RVDC
Temperature Degrees Centigrade
Figure 5
Cap. Change vs. Frequency
Capacitance Change Percent
AVX C0G (NP0) T.C.
0
AVX X7R T.C.
-10
-20
-30
1
KHz
10
KHz
100
KHz
1
MHz
10
MHz
100
MHz
1
GHz
Frequency
Figure 6
6
AVX [ AVX CORPORATION ]
