AN2317 datasheet(22/27 Pages) STMICROELECTRONICS

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External Circuit Design

AN2317 - Application

22/27

1.5.3

EMC Filter

EMC has become an important power supply parameter. In order to deal with common and

differential mode noise, a two-part AC filter is added (see

Figure 11 on page 18).

●

Differential filter (Filter 1)

Inductors L1/L2, and C1 represent a differential filter for DM (differential mode) noise

trying to enter the power supply. DM noise is produced by current flowing along either

the Line or Neutral conductor, and returning by the respective other. This produces a

noise voltage between the Line and Neutral conductors.

The filter will be designed for at least 10 times the line frequency, thereby resulting in a

frequency of 600Hz. The indication is then, that the cutoff frequency (fC) must not be

below 600Hz.

Capacitor C1 is X Class capacitor, used to reduce differential noise. To ensure that C1

does not fail because of the surge or short circuit current, it must be able to withstand

twice the mains voltage value. Keeping this requirement in mind, fC is calculated as

follows:

Equation 35

Note:

Generally, a specific fC value is chosen, then the inductors are tuned to that value.

π L

() C

⋅

-----------------------------------------------7.59Hz

≈

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