AN4068 datasheet(25/63 Pages) STMICROELECTRONICS

25 / 63 page

AN4068

ST7580 reference design description

Doc ID 022923 Rev 2

25/63

L1 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.

is the

series of L1 and the leakage inductance of the coupling transformer T1, adding about 1 µH

to L1.

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

L1 and C4 values, however, leads to limited attenuation due to either parasitic impedance or

resonance selectivity. If loaded with 5

Ω line impedance, the coupling circuit shows a Q

factor equal to 2 and a -3 dB bandwidth of 40 kHz (typical values).

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

characteristics are listed in Table 4. 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 L1 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 [2].

In Figure 11 the measured response of the whole transmission coupling, loaded with the

LISN impedance as set by the EN50065-1 document, is reported. The image highlights a

further filtering effect added by the passive L-C series resonant combined with the LISN

reactive load.

Table 4.

Line coupling transformer specifications

Parameter

Value

Turn ratio

1:1

Shunt inductance

≥ 1 mH

Leakage Inductance

≤1.5 µH

DC total resistance

≤0.5 Ω

DC saturation current

≥ 15 mA

Inter-winding capacitance

≤30 pF

Withstanding voltage

≥ 4 kV

1 C

⋅

=85 kHz

′

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