AN1262 APPLICATION NOTE
Due to the moderate switching frequency, B
will be limited by core saturation and not by core losses. This
max
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 equal to 0.28-0.30 T may be selected to
max
maximize core utilization, or B
= 0.25 T can be chosen for a more conservative design.
max
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 resulting from table 6. To guarantee that the transformer does not sat-
pkx
urate even under short circuit conditions, the maximum peak primary current to be considered is the maximum
value of the OCP threshold (I = 0.7A, from the datasheet).
lim
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
Lt
[cm]
WB
[cm]
Rth
[°C/W]
Core
K1
K2
3
2
2
4
[cm ]
THOMSON (B2)
[cm ]
[cm ]
[cm ]
EF1505A
EF2007A
EF2509A
E2006A
0.51
0.15
0.31
0.58
0.32
0.55
0.15
0.26
0.4
0.022
0.081
0.232
0.112
0.33
29.7
61.1
103
62.2
90
-0.68
-0.7
2.63
3.65
4.64
3.9
0.92
1.32
1.64
1.18
1.54
75
45
30
46
40
1.46
3.3
1.5
3.2
-0.73
-0.7
0.35
0.6
E2507A
-0.73
5.2
PHILIPS (3C85)
E16/8/5
E20/10/6
E25/13/7
0.75
0.201
0.32
0.52
0.216
0.35
0.043
0.112
0.291
42.2
62.2
90
-0.7
-0.69
-0.73
3.3
3.9
4.9
0.94
1.18
1.56
65
46
40
1.49
2.99
0.56
EPCOS (ex S+M) (N67)
E16/8/5
E20/10/6
E25/13/7
TDK (PC30)
EI16-Z
0.76
1.49
3.02
0.2
0.32
0.52
0.22
0.34
0.61
0.044
0.109
0.317
42.2
62.2
90
-0.7
-0.69
-0.73
3.4
4.12
5
1
65
46
40
1.25
1.56
0.67
1.63
1.93
0.198
0.42
0.41
0.267
0.2
0.053
0.084
0.174
66
-0.57
-0.71
-0.57
3.31
3.86
4.94
0.86
0.845
0.98
44
33
31
EI22-Z
85.4
119
EI25-Z
0.425
MMG - NEOSID (F44)
EF16
EF20
EF25
0.754
1.5
0.225
0.314
0.515
0.216
0.348
0.564
0.049
0.109
0.29
42.2
62.2
90
-0.7
-0.69
-0.73
3.3
3.9
4.8
1
65
46
40
1.2
1.6
3.02
Choose core size.
1)
Transformer's core must be able to handle the power throughput P without saturating
inT
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.
4
The following equation can be useful to estimate the minimum AP (in cm ) required:
11/42
STMICROELECTRONICS [ ST ]
