AN4077

EMI filtering and conducted EMI pre-compliance measurements

Doc ID 022997 Rev 1

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EMI filtering and conducted EMI pre-compliance

measurements

The following figures show the peak measurement of the conducted noise at full load and

nominal mains voltages for both mains lines. The limits shown in the diagrams are EN55022

class-B which is the most popular regulation for domestic equipment using a two-wire mains

connection.

It is also useful to remember that, typically, a PFC produces a significant differential mode

noise with respect to other topologies and therefore, if an additional margin with respect to

the limits is required, it is suggested to try to increase the across-the-line (X) capacitors or

the capacitor after the rectifier bridge C5. This is more effective and cheaper than increasing

the size of the common mode filter coil which would only filter the differential mode noise by

the leakage inductance between the two windings.

To recognize if the circuit is affected by common mode or differential mode noise it is

sufficient to compare the spectrum of phase and neutral line measurements: if the two

measurements are very similar, the noise is almost totally common mode. If there is a

significant difference between the two measurement spectrums, their difference represents

the amount of differential mode noise. Of course, to obtain a reliable comparison the two

measurements must be done in the same conditions. If the peak measurement is used (like

in the following figures), some countermeasures must be used, such as synchronizing the

sweep of the spectrum analyzer with the input voltage. This is necessary with TM PFC

having a switching frequency that is modulated along the sine wave.

Because the differential mode produces the common mode noise by the magnetic field

induced by the current, decreasing the differential mode consequently limits the second one.

Figure 37.

EVL6564H-100W conducted

emission peak measurement at 100

Figure 38.

EVL6564H-100W conducted

emission peak measurement at 100