AN1262 APPLICATION NOTE
(that is a larger input capacitor) is of help. Some iterations, involving a recheck of the points mentioned in "Pre-
liminary Calculations - step 2", may be necessary.
If no solution can be found, either some specification should be relaxed or the power handled by the converter
should be derated.
9
FLYBACK TRANSFORMER DESIGN
To complete the set of data needed to design the flyback transformer, the primary inductance value (L ) and the
p
primary-to-secondary turns ratio (n) are still to be defined.
The primary inductance will be chosen so that the converter is operated on the boundary between DCM and
CCM at V = V
:
inmin
in
[(Vinmin – VDS(on)x)) DX]2
Lp= -------------------------------------------------------------------------
(4)
2 fsw PinT
while the primary-to-secondary turns ratio is defined so as to get the desired reflected voltage V :
R
V R
n = ------------------------
(5)
V
out + VF
With the complete set of specification, the transformer design can start with the selection of the magnetic core
material and geometry.
Table 9. Ferrite Materials selection
Saturation flux density
3
Grade
Manufacturer
Specific Power Losses @100 °C [W/cm ]
[T]
B2
0.36
THOMSON
P Fe = 1.15 10–5 ∆B2.26
PFe = 1.54 10–7 ∆B2.62
PFe = 8.53 10–7 ∆B2.54
PFe = 1.59 10–6 ∆B2.58
PFe = 2.39 10–6 ∆B2.23
f
f
f
f
f
1.11
sw
3C85
N67
0.33
0.38
0.39
0.4
PHILIPS
EPCOS (ex S+M)
TDK
1.54
sw
1.36
sw
PC30
F44
1.32
sw
MMG
1.26
sw
As to the magnetic material, a standard soft ferrite for power applications (gapped core-set with bobbin) is the
usual choice: the switching frequency is not so high thus special grades for high frequency operation are not
required. Table 9 shows some suitable materials.
The geometry will be usually a popular E or E-derived type. Other configurations, such as RM or PQ cores, are not
recommended because they are inherently high leakage geometries, since they result in narrower and thicker wind-
ings. Consider that minimizing leakage inductance is one of the major tasks in the design of a flyback transformer.
Among the various shapes and styles offered by manufacturers the most suitable one will be selected with technical
and economic considerations. Table 10 shows some possible choices with the relevant data useful for the design.
The next quantity to be defined is the peak flux density B
which the transformer will be operated at. Being
max
∆
this a DCM design, B
will also equal the maximum flux density swing B
.
max
max
10/42
STMICROELECTRONICS [ ST ]
