User`s guide

Hardware Design
© 2008 Microchip Technology Inc. DS70320B-page 35
Therefore, the predicted core loss is actually 2.9W. The next stage is now to optimize
the winding designs to minimize the losses, especially the high-frequency AC losses
due to skin-effect and the proximity effect in multilayer windings (see Reference 5 and
Reference 6 in Appendix C. “References”). The available winding width, b
w
, must be
reduced to accommodate a 3 mm creepage border on each side of the bobbin, leaving
around 13 mm available for the windings. The total height of the two windings must be
less than 4.5 mm which takes into account the layers of 0.05 mm inter-winding tape.
The secondary transformer winding rms current for the current-doubler synchronous
rectifier, ignoring inductor ripple current, is given by the relationship shown in
Equation 2-16.
EQUATION 2-16:
The secondary rms current is therefore 16.5A, but will be slightly higher in practice due
to the magnetizing ramp component of current in the output inductors. For high current
windings, copper foil is better suited to utilize the available winding area, and minimize
AC copper losses. The secondary winding is a 5 turn strip of copper, and the ideal foil
height, h
id
, in mm is given by Equation 2-17.
EQUATION 2-17:
So h
id
at 250 kHz is 0.088 mm. The resistance factor, F
R
, is given by Equation 2-18.
EQUATION 2-18:
Therefore, for a practical foil thickness, h, of 0.1 mm, F
R
= 1.56. The total resistance
including AC effects is given by Equation 2-19.
EQUATION 2-19:
The realistic foil width for the ETD29 is 13.0 mm. This means that the secondary
resistance is 1.4 mΩ, which leads to a secondary winding copper loss of about 0.5 W.
The current density is actually 14A/mm
2
and, although very high, the power loss is
acceptable.
sec
2
o
I
I
=
%
3
9.74 10
id
ssw
h
Nf
×
=
4
1
1
3
R
id
h
F
h
⎛⎞
=+
⎜⎟
⎝⎠
when
1.4
id
h
h
<
6
45 10
Rm
w
Fl
r
bh
=
×
where l
m
is the mean turn length
and b
w
is the foil width.