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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)： | 11/24頁 |
| 文件大?。?/td> | 569K |
| 代理商： | AN-22 |
C
5/98
AN-22
11
Also, the multiple parallel strands of thinner wire can be placed
flat for good coverage of the bobbin as shown in Figure 5. This
will insure that the winding is well coupled to the primary and
to the other secondaries that are wound afterwards.
In this example we chose to split the 5 V winding into six
conductors to fit the pin arrangement of the bobbin. One pin
can accommodate three wires. Since each wire carries one
sixth the current, or 0.84 A RMS, we may use a wire diameter
of 0.4 mm (27 AWG), which is much easier to handle during
manufacture.
The 12 V winding must handle a total of 1.86 A RMS (I
RMS12
+ I
RMS30
). To maintain a maximum current density below
9 A/mm
2
(219 CMA) we can use the same 0.4 mm (27 AWG)
wire with the number of parallel strands reduced to 2. Again
this should be wound evenly across the bobbin with turns
distributed to provide the optimum coupling with the 5 V and
primary winding. Appendix C shows how to put both windings
on the same layer for best coupling.
Finally, the 30 V winding is added across the entire bobbin
width. This winding carries the current for only the 30 V load;
therefore, we can use a single strand of the 0.4 mm diameter
(27 AWG) wire. If desired, a thinner wire gauge may be
specified to reduce the volume occupied by the winding. The
same wire may be used in all windings to reduce cost. Appendix
C illustrates these methods with complete construction details
of the transformer used in this Application Note.
The techniques detailed above should be used in the transformer
construction to optimize cross regulation. However, additional
external circuit techniques to further enhance cross regulation
are discussed in the section on circuit performance.
Output Rectifier Specification
As in single output converters, the proper choice of output
rectifiers in multiple output converters is essential to achieve
desired performance and reliability. It is important to use only
Schottky and ultra fast PN junction rectifiers. The effects of the
reverse recovery characteristics on the primary circuit are
amplified in multiple output applications because the output
rectifiers are effectively in parallel. Refer to AN-19 for a
discussion of how the selection of output rectifiers influences
efficiency.
The specification on each output rectifier diode is determined
on the basis of the required voltage and current rating. The
peak inverse voltage (PIV) on each diode is given by:
PIV
V
V
N
N
X
X
MAX
X
P
=
+
×
(5)
where V
is the voltage of the particular output, N
X
is the
number of output turns on the particular output and N
is the
transformer primary turns. V
is the maximum primary DC
rail voltage, which for 230 VAC input applications is typically
375 VDC (peak value of 265 VAC).
For the transformer in this example,
N
P
=
77 turns
V
MAX
=
375 V
Hence, for the 5 V output,
Figure 6 (a). Cross Regulation with Feedback from 5 V Only.
Response to a 5 V Load.
95
0.50
0.75
1.00
(a) 5 V Load (A)
1.25
1.50
O
P
105
100
5 V
12 V
30 V
1.75
2.00
95
0.50
0.75
1.00
(b) 5 V Load (A)
1.25
1.50
O
P
105
100
1.75
2.00
5 V
12 V
30 V
Figure 6 (b). Cross Regulation with Feedback from 5 V and 12 V.
Response to a 5 V Load.
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