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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(shè)計多路輸出電源的的TOPSwitch |
| 文件頁數(shù)： | 5/24頁 |
| 文件大?。?/td> | 569K |
| 代理商： | AN-22 |
C
5/98
AN-22
5
example uses the spreadsheet for continuous conduction mode.
The techniques described in the following sections to extend
the standard single output transformer design to multiple
outputs are the same for either spreadsheet.
Spreadsheet Transformer Design
Figure 2 shows the spreadsheet for a transformer that meets the
output power and input voltage specification of Table 1. A full
explanation of the use of the spreadsheet is provided in AN-17,
but a brief overview will suffice for this explanation.
The first section of the spreadsheet is used to input the
application variables. Note that only the 5 V output is needed
to determine the number of turns of the primary, while the total
output power for all outputs is specified in this section to select
the transformer core, primary inductance and wire gauge.
Initial design requirements may not be firm enough to
determine which
TOPSwitch
will be used in the final product.
The designer usually has to choose between two likely
candidates (see AN-21). In all designs, whether single or
multiple output, the transformer design should accommodate
the largest
TOPSwitch
that might be used with it. A designer
may find it necessary to use the larger
TOPSwitch
(with a lower
on-resistance) to permit the use of a smaller heatsink, for
example.
Thus, although the circuit of Figure 1 specifies the TOP223Y,
the spreadsheet uses the upper current limit value for the
TOP224Y/TOP224P. The higher value is used here to ensure
flexibility to allow the use of the TOP224 should the application
require it. The change may be necessary if mechanical
restrictions in the available space of the power supply
’
s
enclosure force the use of a smaller heatsink.
The upper current limit is subsequently used in the spreadsheet
to determine the peak flux density B
, which should be limited
to prevent excessive core saturation under overload and start
up conditions.
The ferrite core used here is the industry standard ETD29. This
is used as an example only. Other standard cores such as the
EE or EER families can be substituted as desired.
The design is based on a margin wound construction, where
3 mm margins are provided at each side of the bobbin to give
a total of 6 mm primary to secondary creepage distance. This
is the standard creepage distance allowed for mains input
power supplies meeting IEC950 (or equivalent) isolation.
Local safety agency requirements for creepage and clearance
should be obtained before committing a design to manufacture.
Other transformer construction techniques, such as slotted
bobbin, concentric bobbin or the use of triple insulated wire,
are equally applicable. The bobbin style does not influence the
calculation of the primary inductance, but specific bobbin
width must be input to determine the physical space available
for the primary winding. Although triple insulated wire
techniques are not normally favored in applications requiring
a high number of secondary turns, transformer suppliers should
be consulted for advice on the optimum construction technique
in a particular application.
The spreadsheet defines two layers for the primary winding to
minimize construction costs. If other cores with reduced
bobbin widths are used, additional layers may be necessary to
satisfy recommendations for current capacity (CMA). It
should be noted that an even number of layers will ease
construction because the start and finish of the primary winding
will be at the same side of the bobbin.
The remaining sections of the spreadsheet provide the
transformer design that results from the input variables described
above. The key parameters that must be checked before a
design can be deemed acceptable are detailed in AN-17 and
summarized in Appendix A.
Since the spreadsheet is written for single output supplies, the
‘
Transformer Secondary Design Parameters
’
show values
assuming the total output power is provided by the 5 V output.
It is therefore necessary to extend these calculations to account
for the partitioning of output power defined in the power
supply specification of Table 1. The following section
provides the equations necessary to assign appropriate numbers
of turns and wire gauges to each output.
Calculation of Secondary Turns
From the spreadsheet, the 5 V output winding is defined as
having 4 turns. The voltage on the cathode of D2 in Figure 1
is 5 V. Therefore, 4 turns produce the output voltage plus the
forward drop of the output diode D2.
The volts per turn V
PT
is defined as:
V
V
V
N
PT
O
D
S
=
+
(
)
(1)
where:
V
PT
=
volts per turn
V
O
= output voltage (5 V)
V
= output diode forward voltage drop (typically 0.7 V
for ultra fast PN power diodes and 0.4 V for
Schottky diodes)
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