AN3065 datasheet(27/33 Pages) STMICROELECTRONICS

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AN3065

EMI filtering and conducted EMI pre-compliance measurements

Doc ID 16279 Rev 2

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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 remind that typically a PFC produces a significant differential mode noise

with respect to other topologies and therefore in case an additional margin with respect to

the limits is required, we suggest trying to increase the across-the-line (X) capacitors or the

capacitor after the rectifier bridge C5. This will be more effective and cheaper than

increasing the size of the common-mode filter coil that would filter the differential mode

noise by the leakage inductance between the two windings only.

In order 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 in the same conditions. If the peak measurement is used as

in the figures below, some countermeasures will have to be used, like 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 35.

EVL6563S-100W TM PFC CE peak

measurement at 100 Vac, 50 Hz, full

load, phase

Figure 36.

EVL6563S-100W TM PFC CE peak

measurement at 100 Vac, 50 Hz, full

load, neutral

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