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
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.
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
Ω
DC saturation current
>2 mA
Interwinding capacitance
< 50 pF
Withstanding voltage
4 kV
