AN3213 datasheet(30/64 Pages) STMICROELECTRONICS

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ST7570 demonstration board description

AN3213

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Doc ID 17429 Rev 1

Figure 20.

Montecarlo simulation of the reception passive filter response

7.1.3

Power line coupling

The coupling to the power line requires some passive components in addition to the active

filtering stage. In particular, it includes the DC decoupling capacitor C2, the line transformer

T1, the power inductor L2, and the Y2 safety capacitor C4.

L2 has been accurately chosen to have high saturation current (> 2 A) and very low

equivalent series resistance (< 0.1

Ω), to limit distortion and insertion losses even with

heavy line load.

Center frequency for the series resonance can be calculated at first approximation as:

Equation 9

provided that the capacitance of C2 is much greater than the C4 capacitance. L2 is the

series of L2 and the leakage inductance of the coupling transformer T1, adding about 1 µH

to L2. The Q factor of this coupling circuit is driven by the mains line impedance: the choice

of the L2 and C4 values leads to 3 dB maximum attenuation when line impedance modulus

goes down to 5

Ω.

Particular attention has been paid to choosing the line transformer. The required

characteristics are listed in Table 5.

In order to have a good signal transfer and minimize the insertion losses, it is recommended

to choose a transformer with a primary (shunt) inductance of 1 mH or greater, a leakage

inductance much smaller than L2, and a total DC resistance lower than 0.5

Ω.

The 4 kV insulation voltage requirement, the last specified parameter, is described and

codified by the EN50065-4-2 CENELEC document (References 4).

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