Behind The Mystery of Electromagnetic Compatibility Design 16
Figure 5: EMC measurement with an aluminum polymer capacitor as input capacitor C
IN
(Source: Würth Elektronik Group)
It can be seen that the limit values of CISPR 32 class B are exceeded.
Noise levels of up to 100dBμV are detectable. But where do these
interfering signals come from? The capacitor as a real component
has parasitic effects, particularly the ESR and the parasitic effects
of the layout (the lead inductance) generate a high-frequency
voltage drop that can be detected by measurement. This is shown
schematically in Figure 4. An aluminum polymer capacitor can be
used as a first approach to achieve acceptable levels of emissions
and stay below the limits. The aluminum polymer capacitor's
electrical properties in terms of capacity and rated voltage are the
same as those of the aluminum electrolytic capacitor.
The design is also equivalent at the capacitance of 47μF, and the
capacitor fits the original land pattern. The aluminum polymer
capacitor used was a WCAP-PSLP 875 105 344 006 (Link to
REDEXPERT) with a capacitance of 47μF, rated voltage of 16V, and
an ESR of 20.7mΩ and ESL of 3.9nH. Because of the very low ESR
and ESL, the following measurement of the interference spectrum is
achieved (Figure 5).
R
EQ,C
C
IN
L
EQ,C
U
IN
U
SUPP
L
SUPP,1
L
SUPP,2
R
SUPP,2
R
SUPP,1
f
RES
C
IN
i
IN
i
IN -
i
C,IN
-20
0
20
40
60
80
100
100 kHz 1 MHz 10 MHz
dBµV
Frequency
Average QPeak CISPR 32 Average CISPR 32 QPeak
30 MHz
150 kHz
Figure 4: Schematic representation of the cause of the disturbances
(Source: Würth Elektronik Group)
