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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)： | 8/24頁 |
| 文件大小： | 569K |
| 代理商： | AN-22 |
AN-22
C
5/98
8
493 and 197 CMA respectively) allows acceptable power
dissipation in the majority of applications, depending on the
conditions of maximum ambient temperature and efficiency
requirements. In the United States, it is common to use the
reciprocal of current density expressed as circular mils per
ampere (CMA). One mil is 0.001 inch, and the area in circular
mils is the square of the wire diameter in mils. One circular mil
is 7.854
×
10
-7
in
2
or 5.067
×
10
-4
mm
2
.
Based on 9 A/mm
2
(219 CMA), using the RMS current
calculated above, the minimum bare copper diameter for each
output is:
5 V output
—
0.66 mm (22 AWG)
12 V output
—
0.51 mm (24 AWG)
30 V output
—
0.07 mm (41 AWG)
The above calculations define the minimum wire diameter
specifications. However, practical considerations of
transformer manufacture determine the actual wire gauges
used. For example, two or three parallel windings on the higher
current outputs can reduce the required wire diameter while
optimizing coverage of the bobbin. These issues are discussed
in detail next.
Transformer Construction
Primary winding techniques are well documented in AN-18
where I
SRMS
and I
O
are from the spreadsheet.
To find the RMS current in a winding, we simply multiply its
average current by K
RA
.
I
I
K
RMSX
X
RA
=
×
(4)
Hence, the RMS current in the 5 V winding is
I
RMS
5
2. A
1 524
.
=3 05
=
×
A
and the RMS current on the 12 V winding is
I
RMS
12
1 2
.
1 524
.
1 83
.
=
×
=
A
A
Similar calculations for the 30 V output yield
I
RMS
30
30. mA
=
The wire diameter can be chosen on the basis of the total
dissipation in the output winding. One can find the resistance
of the winding from the resistance per unit length of a particular
wire gauge and the length of the wire associated with each
output winding. However, a calculation based on the current
density can be used to make a first estimate of the required wire
gauge on each output.
A current density between 4 and 10 A/mm
2
(corresponding to
Table 3. Comparison of Secondary Winding Techniques in Margin Wound Transformers.
DISADVANTAGES
1. Poor regulation of lightly loaded
outputs due to peak charging.
2. Generally higher manufacturing
costs.
3. More pins on bobbin.
1. Winding with lowest or highest
voltage output must be placed
closest to the primary winding
– no flexibility to reduce leakage
inductance of outputs with higher
currents.
WINDING TECHNIQUE
Separate Output Windings
Stacked Output Windings
ADVANTAGES
1. Flexibility in winding
placement; Output with
highest current can be
positioned closest to primary
to minimize energy lost from
leakage inductance.
1. Improved cross regulation.
2. Generally lowest cost
manufacturing technique.
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