AN1262 Datasheet, PDF (11/42 Pages) STMicroelectronics – APPLICATION NOTE

AN1262 APPLICATION NOTE
Due to the moderate switching frequency, B_max will be limited by core saturation and not by core losses. This
means that transformer's power losses will be located mostly in the windings.
As shown in table 9, ferrites saturate above 0.3 T thus a value of B_max equal to 0.28-0.30 T may be selected to
maximize core utilization, or B_max = 0.25 T can be chosen for a more conservative design.
This maximum peak flux density will occur when the peak primary current is maximum. However, it is not suffi-
cient to consider the peak current Ip_pkx resulting from table 6. To guarantee that the transformer does not sat-
urate even under short circuit conditions, the maximum peak primary current to be considered is the maximum
value of the OCP threshold (I_lim = 0.7A, from the datasheet).
Now a step-by-step procedure for the design of the transformer will be given.
Table 10. Core list and significant design data
Ve
Ae
Aw
AP
Core
[cm³]
[cm²]
[cm²]
[cm⁴]
K1
THOMSON (B2)
EF1505A 0.51
0.15
0.15
0.022
29.7
EF2007A 1.46
0.31
0.26
0.081
61.1
EF2509A 3.3
0.58
0.4
0.232
103
E2006A
1.5
0.32
0.35
0.112
62.2
E2507A
3.2
0.55
0.6
0.33
90
PHILIPS (3C85)
E16/8/5 0.75
0.201
0.216
0.043
42.2
E20/10/6 1.49
0.32
0.35
0.112
62.2
E25/13/7 2.99
0.52
0.56
0.291
90
EPCOS (ex S+M) (N67)
E16/8/5 0.76
0.2
0.22
0.044
42.2
E20/10/6 1.49
0.32
0.34
0.109
62.2
E25/13/7 3.02
0.52
0.61
0.317
90
TDK (PC30)
EI16-Z
0.67
0.198
0.267
0.053
66
EI22-Z
1.63
0.42
0.2
0.084
85.4
EI25-Z
1.93
0.41
0.425
0.174
119
MMG - NEOSID (F44)
EF16
0.754
0.225
0.216
0.049
42.2
EF20
1.5
0.314
0.348
0.109
62.2
EF25
3.02
0.515
0.564
0.29
90
K2
-0.68
-0.7
-0.73
-0.7
-0.73
-0.7
-0.69
-0.73
-0.7
-0.69
-0.73
-0.57
-0.71
-0.57
-0.7
-0.69
-0.73
Lt
[cm]
WB
[cm]
Rth
[°C/W]
2.63
0.92
75
3.65
1.32
45
4.64
1.64
30
3.9
1.18
46
5.2
1.54
40
3.3
0.94
65
3.9
1.18
46
4.9
1.56
40
3.4
1
65
4.12
1.25
46
5
1.56
40
3.31
0.86
44
3.86
0.845
33
4.94
0.98
31
3.3
1
65
3.9
1.2
46
4.8
1.6
40
1) Choose core size. Transformer's core must be able to handle the power throughput P_inT without saturating
and with acceptable power losses, with the minimum size. Determining its optimum size is a trial-and-error
process and a proper starting point may reduce considerably the number of iterations needed.
A most common way of describing core size is the so-called Area Product (AP), which is the product of the
effective cross-sectional area of the core times the window area available to accommodate the windings. It
is possible to define the minimum AP required by a specific application.
The following equation can be useful to estimate the minimum AP (in cm⁴) required:
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