EMI filtering and conducted EMI pre-compliance measurements

AN3119

28/32

Doc ID 16849 Rev 2

7

EMI filtering and conducted EMI pre-compliance

measurements

Figures 40, 41, 42, and 43 show the measurement in average mode 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 standard for domestic equipment

using a two-wire mains connection.

It is worth remembering that typically a PFC produces a significant differential mode noise

with respect to other topologies. In case an additional margin, with respect to the limits, is

needed, increasing the differential attenuation by increasing the across the line (X)

capacitors or theC4 capacitor after the rectifier bridge is suggested. Sometimes a differential

mode coil connected as a pi-filter placed after, between the bridge rectifier and the boost

stage, is more effective and cheaper than increasing the size of the common mode filter coil

which would filter the differential mode noise by the leakage inductance only 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 get a reliable comparison the two

measurements have to be done under the same conditions.

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

induced by the current, decreasing the differential mode consequently limits the latter.

As visible in the diagrams, in all test conditions there is at least a 6dB margin of the

measurements with respect to the limits. The measurements have been done in AVG

detection to evaluate the benefit of the jittering effect of the TM control.

Figure 40.

EVL6563S-250W CE AVG

measurement at 115 Vac-60 Hz - full

load - phase wire

Figure 41.

EVL6563S-250W CE AVG

measurement at 115 Vac-60 Hz - full

load - neutral wire