AN4205 datasheet(10/23 Pages) STMICROELECTRONICS

10 / 23 page

Input stage of the AC/DC converter

AN4205

10/23

Doc ID 023929 Rev 1

In order to calculate the wire dimension, we have to impose the maximum losses on the

copper. In this case we choose a dissipation level similar to the one of the core (Pfe) plus a

safety margin, for practical purposes we have chosen 0.5 W. Now we assume that the

primary and secondary winding losses are half of the selected total power:

PCUprimary = 0.25 W, PCUsecondary = 0.25 W

From Joule’s law, we have:

Equation 21

Equation 22

Knowing the copper resistivity at 100 °C (

ρ100=2.303*10-6 Ωcm) and the average winding

length (Lt=3.4 cm, from the datasheet of the core), we have:

Equation 23

Equation 24

Therefore, the diameter of the wire is:

Equation 25

Equation 26

In accordance with the above specifications, the final transformer has been provided by

MAGNETICA. The following table summarizes the most important parameters of this

transformer.

CUprimary

PIPrms

≅

→

ondary

sec

SIPrms

0.5

≅

→

PCU

100NPLt

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

3.46

–

•

SCU

100NSLt

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

4.5

–

•

6.6

–

•

23.4

–

•

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