User`s guide

SMPS AC/DC Reference Design User’s Guide
DS70320B-page 34 © 2008 Microchip Technology Inc.
To compute the operating flux density and decide on the number of turns, it is assumed
that the maximum allowable transformer temperature rise is 80ºC. The power converter
will have forced air cooling from a lid mounted fan so the actual thermal resistance will
be about a third less at around 10ºC/W. This means that the total power dissipation can
be as much as 8W, with the losses split equally in the copper windings and the ferrite
cores. From the core loss curves shown in Figure 2-7, the operating AC peak-to-peak
flux density can be as high as 150 mT. The minimum number of turns at the given
maximum operating duty is given by Equation 2-14.
EQUATION 2-14:
Therefore, N
min
is 58, which means that the secondary winding must have either four
or five turns to give the required turns ratio computed above. Based on this, a good
solution is 64 turns on the primary and five turns on the secondary with a turns ratio, n,
of 12.8.
FIGURE 2-7: N87 POWER LOSS CURVES
With the selected turns ratio from Equation 2-12, the actual operating duty, D, is 0.82.
Therefore, the operating peak-to-peak flux density is 130 mT at 250 kHz. The core loss
in a N87 core can be computed by Equation 2-15.
EQUATION 2-15:
max
min
max
2
in
e
VD T
N
AB
=
41.592.74
1.36 10
core sw e
P
fBV
−
=×
where f
sw
is in kHz and B is in mT.