ited by core saturation or losses), determining the core size, defining the primary and secondary wind-

ings (turns number and wire gauge) as well as calculating the air-gap necessary to achieve the desired

inductance. Moreover, additional considerations concerning the assembly are needed for meeting safety

requirements, maximising magnetic coupling and minimising parasitic high frequency effects, not to

mention the constraints imposed by the specific application, if any.

Some parameters are needed to start the design of the transformer. The (maximum) primary inductance

≤

1

2

⋅

F2

1

⋅

2

,

or by simply looking up the diagram of fig. 10, where the primary inductance required for 1W input power

value taken from fig. 10 (in mH), will be divided by the maximum input power to get the actual primary in-

ductance required by the specific application.

The primary-to-secondary turns ratio will be given by:

n

=

.

With the peak and RMS current values calculated in the "Energetic relationships" section, the design can

be carried out just like for any conventional flyback transformer, thus no particular procedure will be con-

sidered.

Anyway, as a design aid to core selection, two expressions for determining the minimum required core

Area-Product (winding window area times effective magnetic cross section) will be provided:

=







460

F2







1.316

(17);

=







480

F2







1.585

2

]

0.66

(18);

.

2 10

4

3 10

4

4 10

4

5 10

4

6 10

4

5

25

45

65

85

L(

)

,

0.5 fmin

L(

)

,

1 fmin

L(

)

,

1.5 fmin

L(

)

,

2 fmin

L(

)

,

2.5 fmin

fmin

2 10

4

3 10

4

4 10

4

5 10

4

6 10

4

0

100

200

300

400

L(

)

,

0.5 fmin

L(

)

,

1 fmin

L(

)

,

1.5 fmin

L(

)

,

2 fmin

L(

)

,

2.5 fmin

fmin

Lmax

[mH·W]

Lmax

[mH·W]

fswmin [Hz]

fswmin [Hz]

110Vac or

Wide-range Mains

220Vac Mains

1

1.5

2

2.5

1

1.5

2

2.5

Figure 10. Maximum specific primary inductance required

AN1059 APPLICATION NOTE

9/20