User`s guide

Hardware Design

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

, 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

, in mm is given by Equation 2-17.

EQUATION 2-17:

So h

at 250 kHz is 0.088 mm. The resistance factor, F

, is given by Equation 2-18.

EQUATION 2-18:

Therefore, for a practical foil thickness, h, of 0.1 mm, F

= 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

and, although very high, the power loss is

acceptable.

sec

9.74 10

ssw

⎛⎞

⎜⎟

⎝⎠

when

1.4

45 10

where l

is the mean turn length

and b

is the foil width.