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參數(shù)資料
| 型號： | AN-22 |
| 英文描述： | OBSOLETE when inventory is depleted. 10% tolerance no l |
| 中文描述： | 設計多路輸出電源的的TOPSwitch |
| 文件頁數(shù)： | 10/24頁 |
| 文件大?。?/td> | 569K |
| 代理商： | AN-22 |
AN-22
C
5/98
10
arranged such that the 5 V and 12 V outputs have the best
coupling to the primary winding.
An arrangement that has the 30 V winding closest to the
primary may show the same primary leakage inductance as the
preferred structure when measured with the standard technique
of shorting all outputs together. In the application, however,
efficiency will be reduced since the leakage inductance
associated with the 5 V and 12 V outputs will be higher.
The use of separate output windings provides complete
flexibility in the winding arrangement. In this case the optimum
configuration for separate layers might be to wind the 5 V
output first followed by the 12 V winding and finally the 30 V
output. That is, the winding with the greatest output current
would go next to the primary. An even better arrangement
would have the two highest current windings share a single
layer using the nesting technique illustrated in Appendix C.
Separate windings, however, tend to increase the cost of the
transformer since every output winding is a separate operation.
The alternative stacking technique described below improves
the regulation, particularly on lightly loaded outputs.
Stacked Output Windings
Figure 3(b) shows a stacked output winding configuration,
which is generally favored by transformer manufacturers. The
windings of the 5 V output provide the return and part of the
windings for the 12 V output. Similarly, the 30 V output uses
the turns of the 5 and 12 V outputs and additional turns to make
up the full winding. The wire for each output must be sized to
accommodate its output current plus the sum of the currents for
the other outputs stacked on top of it.
The stacked configuration improves cross regulation while
reducing construction costs. Consider this example where the
5 V output is fully loaded but the 12 V and 30 V outputs have
minimum load applied. With separate output windings, the
capacitors on the 12 V and 30 V outputs would tend to peak
charge under the influence of leakage inductance. However,
with a stacked winding, the fact that the 5 V output forms part
of the 12 V and 30 V windings reduces the impedance of these
windings and reduces the effect of peak charging.
The only disadvantage of this winding technique is that there
is little flexibility in the placement of the windings relative to
the primary. Either the 30 V or 5 V winding must form the start
of the output windings closest to the primary. In this case, since
the 5 V has the highest loading, it is defined as the start of the
secondary winding.
Since the stacking technique generally offers the best cross
regulation, the winding construction of Figure 3(b) was chosen
for the example circuit in this application note, as illustrated in
Figure 4. The only difference between T1 in Figures 1 and 4
is the use of the stacked winding technique on the transformer
in Figure 4.
Construction to Improve Cross Regulation
The cross regulation is a measure of how well the output
voltages regulate under the influence of varying load conditions
on other outputs. The quality of cross regulation depends on
the coupling between the various output windings. The better
coupled these windings are, the better the cross regulation.
As such, it is recommended that each individual winding is
wound to cover the complete bobbin width. Therefore, the
easiest way to wind the transformer is to use several parallel
wires of the same gauge to insure the bobbin is well covered.
In this case, the total copper area used by the 5 V winding must
handle the total RMS current of all outputs.
The total output RMS current is:
I
I
I
I
RMSTOT
RMS
RMS
RMS
=
+
+
=
5
12
30
5 03
.
A
This summation is possible only when the currents have the
same shape, which is a valid simplifying assumption for the
design.
Based on a current density of 9 A/mm
2
(219 CMA), the copper
diameter of a single wire would need to be 1.03 mm (20 AWG).
However, if the wire is split into several parallel sections, each
carrying an equal share of the current, we may use a smaller
diameter wire which is much easier to handle during
manufacture.
Figure 5. Cross Section of Bobbin Showing Five Interleaved Turns
of Four Parallel Conductors on a Single Layer.
Turn
1
}
FINISH
Turn
5
}
Turn
2
}
Turn
4
}
Turn
3
}
START
PI-2128-120297
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