AN1889 datasheet(13/33 Pages) STMICROELECTRONICS

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AN1889

Application description

13/33

Equation 20

For practical consideration, to better optimize the transformer window utilization and in the

main time in accordance with the above calculation, it has been chosen:

●

dp = 0.025 [cm]

●

ds =0.05 [cm] (2 in parallel)

The choice at secondary side of 2 wires of 0.05 mm in parallel vs. one of 0.074 mm has

been made to minimize the skin effect.

During the transformer construction particular care must be put in minimizing the parasitic

impedances, in particular the leakage inductance and the winding capacitance. Practically

the leakage inductance should not exceed 3% of the primary inductance. The use of

interleaved winding is a common method to reduce leakage inductance. Practically half of

the primary turns is firstly wound, then the secondary turns and finally the last half of the

primary that must result series connected with the first half-wind of the primary. To avoid an

excessive increase of the winding capacitance the second half of the primary must be

connected with the switch.

The final transformer has been realized from TDK in accordance with the above

specification, its relative part number is SRW2924ED-E02V015. The table below

summarizes the most important parameters of the above-mentioned transformer:

Table 2.

Transformer parameters

Symbol

Description

Values

Dimension

Effective cross section area

85.78

mm2

Effective magnetic path

58.65

Effective core volume

5030

mm3

ACW

Cross sectional winding area

115.45

mm2

Air gap length

0.8

Single turn inductance

130

nH/n2

SCU

100

•

------------------------------------

4.30

–

0.074 cm

[]

⇒

•

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