Design circuit

AN2623

10/25

An empirical formula that gives an indication regarding the needed area Product for

magnetic core that has to be selected for the application is shown in Equation 17:

Equation 17

where

∆B is maximum flux density swing in Tesla for normal operation and its typical value is

within 0.2-0.3T in the case of the forward converter. This value has to be chosen in order to

avoid saturation and to limit core losses; we chose 0.2T. The selected core is ETD39

transformer's max temperature rise

Equation 18

and the result of the relative max allowed core loss:

Equation 19

density swing is:

Equation 20

Equation 21

The turn ratio n between primary and secondary side is defined as:

Equation 22

Equation 23

AP

min

11.1

P

i

⋅

0.141

B

∆

f

sw

⋅⋅

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

⎝⎠

=

=

P

LOSTtrasfTOTAL

T

∆

R

thCORE

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

2.5 W

=

=

P

fe

2P

LOSTtrasfTOTAL

⋅

k

1

2

+

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

1.21 W

=

=

B

∆

2

P

LOSTtrasfTOTAL

V

e

k

0

f

k

2

sw

⋅⋅

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

⎝⎠

⎜⎟

⎛⎞

1

k

1

-----

0.146 T

=

⋅

=

N

1min

V

dcmin

D

max

⋅

A

e

f

sw

B

∆

⋅⋅

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

42

=

=

n

N

1

N

2

-------

V

dcmin

V

2min

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

D

max

V

dcmin

⋅

V

out

V

dropR

+

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

1.15

=

=

=

=

L

m

A

L

N

2

1

10

9

–

L

m

3.8 mH

≅

⇒

⋅⋅

=