AN2451 datasheet(21/55 Pages) STMICROELECTRONICS

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AN2451

Board description

Doc ID 12791 Rev 3

21/55

provided that the dc-decoupling capacitor C22 is much greater than C23 (in this case, 100

times greater) and that parasitic components of the transformer have negligible effects on

the filtering action.

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

characteristics are listed in Table 5.

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

recommended to choose a transformer with a primary (shunt) inductance greater than 1mH

and a series resistance lower than 0.5

Ω.

Another important parameter is the leakage inductance. If it has a relevant value (10 to 50

uH), this can be used to design the coupling filter without inserting series inductance (L5,

L6). The drawback, however, is the poor accuracy of this parameter, which can lead to a

shift of the filter response and to bad coupling. Consequently, a low leakage inductance

value (<1 µH) has been chosen, fixing the series inductance through a discrete component

with greater accuracy.

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

coded in the EN50065-4-2 CENELEC document.

Figure 12 shows the measured response of the Tx active and passive filters, loaded with the

CISPR network. The figure highlights a further filtering effect added by the passive L-C

series resonant combined with the CISPR reactive load.

Table 5.

Line coupling transformer specifications

Parameter

Value

Turn ratio

1:1

Magnetizing inductance

>1 mH

Leakage inductance

<1 µH

DC resistance

<0.5

DC saturation current

>2 mA

Interwinding capacitance

< 50 pF

Withstanding voltage

4 kV

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