NCS12 Series

Isolated 12W 4:1 Input Single & Dual Output DC/DC Converters

KDC_NCS12C.A02 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

1μF Polyester or ceramic capacitor

C3

47μF Electrolytic capacitor

C4 & C5 10nF 250 VAC Y Rated (15nf SXX03 & SXX05)

L1

1μH Common-mode choke (Murata part number 51105C)

The following filter circuit shows the input filter typically required to meet CISPR22 Quasi-PeakCurve B.