Conducted Emissions and SusceptibilityConducted emissions are simpler to investigate than radiated emissionsbecause only the product’s power cord needs to be measured. Measurements of conducted emissions are made using a line impedancestabilization network (LISN) that is inserted between the product andthe power distribution network.

product LISN

phase

neutral

safety

phase

neutral

safety

a.c. powerdistribution

spectrumanalyzer

Figure 1

The purpose of the LISN is twofold:1) present the same impedance out of the product’s power cord;2) reduce disturbances due to the a.c. power distribution.

The LISN used for FCC conducted emission tests is:

Product

greenwire

LISN

neutral

phase50H

50H

1F1F1F

PI

NI

50 1k1k

50Dummyloadreceiver

+

-

PV

+

-

NV

Figure 2

The values of the inductors and the capacitors are chosen so that in the frequency range of interest the inductors have a large impedance and the capacitors are almost short circuits.

PI

NI

In this way, the phase current and the neutral current are related to thevoltages by:

)2(ˆ50ˆ

)1(ˆ50ˆ

NN

PP

IV

IV

Note that for the CISPR 22 conducted emission test, the frequency rangeis different (150 KHz-30 MHz) and both capacitors and inductors musthave other values to obtain the desired behavior.

Common Mode and Differential Mode CurrentsThe simplified representation of a LISN as two 50 resistor is the following:

50 PV

50 NV

phase

neutral

Green wire

DCP III ˆˆˆ

DI

CI

CI

and, similarly to what we did for the radiated emissions, we introduce common and differential mode currents as:

(4) 21

(3) 21

NPC

NPD

III

III

Figure 3

In the case of conducted emissions, common mode currents can be of the order of differential mode currents. If this is the case, then and are different since they are given by:

PV

NV

(6) 50

(5) 50

DCN

DCP

IIV

IIV

Usually one component dominates the other so that the magnitudes of and are the same.PV NV

Conducted emissions are usually reduced by introducing a power filter. A power filter contains elements each one of them reducing eithercommon or differential mode currents

An usual way to block common mode currents is the green wire inductor:

GWL

GWL

Productphase

neutral

Green wire

CI

CI

CI2

Figure 4

Other ways of reducing emissions are considered in the following.

We want to remind that almost all electronics products contain some form of power filter where the power cord exists the product. Sometimes the filter is simply a large transformer or a linear power supply that provide inherent filtering; for all other cases an “intentional”power filter is required.

A typical topology of a power supply filter is shown in the following:

Figure 5

: green wire inductor - blocks common mode currents;GWL

DRDL CC , : divert differential mode currents - these are line-to-line capacitors or X caps;

CRCL CC , : divert common mode currents - these are line-to-ground capacitors or Y caps;

L, L: two coupled inductors -- block common mode currents

The purpose of the filter is to reduce so that:CD II ˆ,ˆ

(8) )ˆˆ(50

(7) )ˆˆ(50''

''

DCN

DCP

IIV

IIV

are below the limits.

Let us consider the operation of the power supply filter when onlycommon mode currents are present:

Figure 6

Common mode currents are represented with current sources. Becauseof the symmetry of the circuit, we obtain the equivalent model:

L+M

GWL2Figure 7

Note: the common mode choke appears as an inductance L+M, and the green wire inductor appears doubled because the current through it is doubled

The operation of the power supply filter under differential mode currentsis understood by considering the following:

Figure 8

Again, in this case, differential mode currents are represented with current sources; due to the symmetry of the circuit we obtain the equivalent representation:

L-MFigure 9

Note: line to line capacitors appear twice as large to differential mode currents; line to ground capacitors also affect differential mode currents. In the ideal case, L-M=0 and differential model currents are completely blocked.

Separation of common mode and differential mode currentsThe actual behavior of a power supply filter is not the ideal one and in apractical situation the conducted emissions may behave as in the following:

The total current is given by:

DCT III (9)

And when one of the components ( ) is much larger than the other.The total current is dominated by the larger component.

CD II or

Figure 10

Therefore, in order to reduce the total emissions, one should identify (ina given frequency range) the dominant current component and changethe elements of the power supply filter to reduce its emissions.

The common mode and differential mode components are identified viaa diagnostic tool, such as the one shown in the following:

Figure 11

Additional considerations

Even with the use of power supply filters, only a certain degree of emission reduction can be accomplished. In general, the best way tosuppress conducted emissions is to reduce them at their source.

However, this is not always possible, as in the case of the noise due tosharp rise and fall times of clock waveforms

For example, the switched-mode power supplies that are found in many products are generally the leading cause of conducted emissions.

The operation of these power supplies relies on short rise/ fall timesin order to increase the energy conversion efficiency.

Another simple way to reduce conducted emissions is based on theappropriate location of the power supply and the power filter.

Let us consider the two following configurations:

Case (a) represents a poor choice for the location of the filter and thepower supply because the fields inside the product cabinet may couplewith the wires that lead to the power cord.

Case (b) shows the proper location of both the filter and the power supply. In this way, the emissions transmitted to the power cord are reduced to minimum level provided by the filter.

In general, the filter should be placed against the cabinet to minimize the coupling with the internal fields. For the same reason, the power supply should go as close as possible to the filter.

FilterPowerSupply

Product cabinet

Internal fields

FilterPowerSupply

Product cabinet

Internal fields

Figure 12(a) (b)