7/14

AN1621 APPLICATION NOTE

where Ton is in sec,

∆B in Tesla and Ae in mm2.

The turns ratio is defined as:

where:

Dmax = 0.48 (maximum duty cycle):

Vf =1V (voltage drop of the output diode);

Vp = 0.5V (voltage drop of the output inductor).

The primary and secondary turns number are Np = 32, Ns = 10.

As for the wire selection, the skin effect is not negligible at this frequency.

To reduce this effect, the wire diameter should not exceed two times the penetration depth:

and therefore a wire diameter of 0.36mm (AWG27) is suitable. However, to have a sufficient copper area, a cer-

tain number of these wires will be twisted together.

To define the numbers of wires this equation can be used:

where:

R = resistance of the wire per cm

lw = winding length (cm)

As to the primary side:

The RMS secondary current is 3.4 times the primary one. Considering the copper losses equally dissipated be-

tween primary and secondary windings, this yields 3 wires in parallel (Npwire) for the primary winding and 10

wires (Nswire) for secondary winding.

isolation requirements and fill factor must be taken into account. The total copper area is:

that fits the window.

The windings are interleaved:

one layer of 16 turns (first half of the primary), one layer of 10 turns (the secondary) and another layer of 16

turns (second half of the primary).

With this arrangement the transformer used in the application has a leakage inductance of 15uH (about 0.5%

of the primary inductance, which is 2.7 mH).

The magnetising current is:

N

P

V

inmindc

T

on max

()

⋅

B

∆

A

e

⋅

--------------------------------------------------- 10

4

⋅

≥

200 2.4

⋅

0.130 125

⋅

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

29.5

==

n0.9

V

in min

⋅

V

o

V

f

V

p

++

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

⋅

0.9

200 0.48

⋅

2410.5

++

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

⋅

3.38

==

=

δ

7.5

f

--------

0.17mm

==

N

wire

RN

turns

I

w

I

rms

2

P

loss

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

⋅⋅

⋅

=

I

rmsp

P

o

η V

inmindc

D

max

⋅⋅

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

312

0.9 200

0.48

⋅⋅

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

2.23A

==

=

I

m

V

in min

⋅

L

p

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

200 2.4 10

6

–

⋅⋅

2.7 10

3

–

⋅

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

180mA

==

=