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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)： | 20/24頁 |
| 文件大?。?/td> | 569K |
| 代理商： | AN-22 |
AN-22
C
5/98
20
V
PT
=
1 33
.
V per turn
From (2), the turns required on the 5 V output are:
N
S
=
4 06
.
turns
This result demonstrates that this choice of turns and output
diodes yields an almost perfect integer turns ratio between the
3.3 V and 5 V outputs. It is a very popular solution for this
reason.
The coupling between the output windings is still a crucial
factor to insure that the turns ratio calculated above does
indeed result in the required output cross regulation. Since so
few turns are involved in these outputs, it is usual for multiple
parallel wire strands to be used on each output winding, and for
the 3.3 V and 5 V outputs to be constructed as separate
windings. Stacked windings are not appropriate in this case.
As discussed in the body of the application note, windings
should be constructed in 2 layers and interleaved across the
bobbin width to optimize coupling with the primary winding.
2. Foil windings
An alternative technique is to use foil instead of multiple
strands of copper wire. Using this technique, the turns ratio of
3 turns on the 3.3 V output and 4 turns on the 5 V output is
retained. The foil is cut to the required length with appropriate
termination points included prior to winding. The foil is then
wound as a single operation. Termination to the transformer
pins is performed afterwards. Figure 2 illustrates this
technique.
The foil is prepared to fit the bobbin width of the chosen
transformer exactly, and is backed with insulation material
which is wrapped around the foil to provide creepage
distances appropriate to the required isolation requirements of
the application.
Although this technique may add some cost to the transformer
construction, the fact that the foil is prepared to the exact
bobbin width provides excellent coupling with the primary
winding. In addition, the 3.3 V and 5 V windings have very
good mutual coupling that improves cross regulation. This
mutual coupling makes the stacked winding construction the
preferred technique when using foil windings.
As shown in Figure 3, subsequent output windings can be
stacked on the foil windings, though the total RMS current
requirements must accounted for in the choice of the foil.
Independent of whether copper wire or foil winding
techniques are used, the output feedback configuration must be
determined according to the load and regulation requirements.
Figure 4 shows the use of a TL431 where the feedback is
derived from both the 3.3 V and 5 V outputs. The proportion
of feedback from each output can be adjusted as required, and
is discussed in detail in the body of the Application Note.
Figure 2. Preparation of Foil Windings for 5 V and 3.3 V Outputs.
PI-2134 -121897
Foil
Prepared
to Exact
Bobbin
Width
Return
Termination
Point
3.3 V Output
Termination
Point
5 V Output
Termination
Point
Insulated
Backing
Wrapped to
Provide
Creepage
Distance
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