DFS04M datasheet(11/42 Pages) STMICROELECTRONICS

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AN1262 APPLICATION NOTE

Due to the moderate switching frequency, Bmax 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 Bmax equal to 0.28-0.30 T may be selected to

maximize core utilization, or Bmax = 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 Ippkx 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 (Ilim = 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

1) Choose core size. Transformer's core must be able to handle the power throughput PinT 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:

Core

[cm3]

[cm2]

[cm4]

[cm]

Rth

[°C/W]

THOMSON (B2)

EF1505A

0.51

0.15

0.022

29.7

-0.68

2.63

0.92

EF2007A

1.46

0.31

0.26

0.081

61.1

-0.7

3.65

1.32

EF2509A

3.3

0.58

0.4

0.232

103

-0.73

4.64

1.64

E2006A

1.5

0.32

0.35

0.112

62.2

-0.7

3.9

1.18

E2507A

3.2

0.55

0.6

0.33

-0.73

5.2

1.54

PHILIPS (3C85)

E16/8/5

0.75

0.201

0.216

0.043

42.2

-0.7

3.3

0.94

E20/10/6

1.49

0.32

0.35

0.112

62.2

-0.69

3.9

1.18

E25/13/7

2.99

0.52

0.56

0.291

-0.73

4.9

1.56

EPCOS (ex S+M) (N67)

E16/8/5

0.76

0.2

0.22

0.044

42.2

-0.7

3.4

E20/10/6

1.49

0.32

0.34

0.109

62.2

-0.69

4.12

1.25

E25/13/7

3.02

0.52

0.61

0.317

-0.73

1.56

TDK (PC30)

EI16-Z

0.67

0.198

0.267

0.053

-0.57

3.31

0.86

EI22-Z

1.63

0.42

0.2

0.084

85.4

-0.71

3.86

0.845

EI25-Z

1.93

0.41

0.425

0.174

119

-0.57

4.94

0.98

MMG - NEOSID (F44)

EF16

0.754

0.225

0.216

0.049

42.2

-0.7

3.3

EF20

1.5

0.314

0.348

0.109

62.2

-0.69

3.9

1.2

EF25

3.02

0.515

0.564

0.29

-0.73

4.8

1.6

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