Datasheet

NCS12 Series
Isolated 12W 4:1 Input Single & Dual Output DC/DC Converters
KDC_NCS12C.B02 Page 11 of 14
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EMC FILTERING AND SPECTRA
FILTERING
The module includes a basic level of filtering, sufficient for many applications. Where lower noise levels are desired, filters can easily be added to achieve any required
noise performance.
A DC/DC converter generates noise in two principle forms: that which is radiated from its body and that conducted on its external connections. There are three separate
modes of conducted noise: input differential, output differential and input-output.
This last appears as common mode at the input and the output, and cannot therefore be removed by filtering at the input or output alone. The first level of filtering is to
connect capacitors between input and output returns, to reduce this form of noise. It typically contains high harmonics of the switching frequency, which tend to appear
as spikes on surrounding circuits. The voltage rating of this capacitor must match the required isolation voltage. (Due to the great variety in isolation voltage and required
noise performance, this capacitor has not been included within the converter.)
Input ripple is a voltage developed across the internal Input decoupling capacitor. It is therefore measured with a defined supply source impedance. Although simple
series inductance will provide filtering, on its own it can degrade the stability. A shunt capacitor is therefore recommended across the converter input terminals, so that it
is fed from a low impedance.
If no filtering is required, the inductance of long supply wiring could also cause a problem, requiring an input decoupling capacitor for stability. An electrolytic will perform
well in these situations. The input-output filtering is performed by the common-mode choke on the primary. This could be placed on the output, but would then degrade
the regulation and produce less benefit for a given size, cost, and power loss.
Radiated noise is present in magnetic and electrostatic forms. The latter is suppressed by the metal case, which is connected to the output return, typically a zero-volt
point. Thanks to the small size of these units, neither form of noise will be radiated “efficiently”, so will not normally cause a problem. Any question of this kind usually
better repays attention to conducted signals.
EMC FILTER AND VALUES TO OBTAIN SPECTRA AS SHOWN
C1
L1
C2 C3
DC
DC
C4
C5
C1, C2 1F Polyester or ceramic capacitor
C3 47F Electrolytic capacitor
C4 & C5 10nF 250 VAC Y Rated (15nf SXX03 & SXX05)
L1 1mH Common-mode choke (Murata part number 51105C)
The following filter circuit shows the input filter typically required to meet CISPR22 Quasi-PeakCurve B.