AN4205 datasheet(8/23 Pages) STMICROELECTRONICS

8 / 23 page

Input stage of the AC/DC converter

AN4205

8/23

Doc ID 023929 Rev 1

Then the value of the IRMS current must be determined:

Equation 11

Equation 12

2.1

Flyback transformer design

The transformer design mainly consists of fixing the magnetic core, its geometry and the

exact number of primary turns. The standard soft ferrite with gapped core and E-type

geometry is a common choice for flyback topology because it is low cost and suitable for

low/medium power applications. For this project E16/8/5 N87 material has been chosen.

First the power losses of the ferrite with regard to the maximum magnetic flux

ΔBmax must

be determined. From the datasheet of the N87 material, the dynamic magnetization curve

shows that

ΔBmax = 380 mT (@ T=100 °C) but we impose a ΔBmax = 200 mT to have a

safety margin. So, knowing the switching frequency (fs=100 kHz) and the magnetic flux, we

can determine the relative core losses ( referring to the datasheet of N87, see the figure

"relative core losses versus frequency"):

Therefore the power losses of the ferrite are:

Equation 13

where Ve=0.75 cm

3 is the total volume of the core.

Knowing that the thermal resistance Rth of the selected core is 65 °C/W, we have:

Equation 14

This result provides the thermal condition regarding the hypothesis of

ΔBmax. Next we have

to add the power losses due to the winding.

Now the number of primary turns must be calculated.

Equation 15

where Amin = 19.4 mm² is the minimum area of ferrite.

Prms

-------

ONmax

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

85mA

≅

Srms

S peak

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

reset

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

713mA

≅

400mW cm

⁄

vVe

0.75

400

•

300mW

ΔT

--------

ΔT19.5°C

DCminTONmax

ΔB

maxAmin

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

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