AN2794 datasheet(14/39 Pages) STMICROELECTRONICS

14 / 39 page

Design considerations

AN2794

14/39

Doc ID 14827 Rev 2

The Kg constant is related to the core geometrical parameters by the following equation:

Equation 32

where Wa is the core window area, Ac is the core cross sectional area and MLT is the mean

length per turn.

For example, choosing an E55/28/21 core with N27 ferrite, having

●

Wa= 2.8 cm

●

Ac= 3.5 cm

●

MLT= 11.3 cm

the resulting Kg factor is:

●

Kg= 0.91 cm

which is then suitable for this application.

Once the core has been chosen, it is possible to calculate the number of primary turns as

follows:

Equation 33

The primary inductance value is:

Equation 34

and the number of secondary turns is:

Equation 35

At this point wires must be selected in order to implement primary and secondary windings.

At 100 kHz the current penetration depth is:

Equation 36

Then, the wire diameter can be selected as follows:

Equation 37

MLT

g =

turns

max

min

5800

•

turns

•

0209

0418

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