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參數(shù)資料
| 型號： | AN-21 |
| 英文描述： | TOPSwitch-II Flyback Quick Selection Curves |
| 中文描述： | 的TOPSwitch -Ⅱ激快速選擇曲線 |
| 文件頁數(shù)： | 1/8頁 |
| 文件大小： | 400K |
| 代理商： | AN-21 |
Introduction
This application note is for engineers starting a flyback power
supply design with
TOPSwitch-II
. It offers a quick method to
select the proper
TOPSwitch-II
device from parameters that
are usually not available until much later in the design process.
The
TOPSwitch-II
Flyback Quick Selection Curves provide
the essential design guidance.
Efficiency and
TOPSwitch-II
power dissipation are two
important performance parameters to the flyback power supply
designer. Both can be easily measured or accurately estimated
after the power supply is designed. But what if the designer
must make project and resource decisions before actually
committing to and starting development This application
note helps the designer quickly select the optimum
TOPSwitch-II
device from simple curves of estimated efficiency
and
TOPSwitch-II
power dissipation.
Typical Power Supply Losses
Power supplies have an input power which, because of internal
dissipation, can be significantly higher than the output power.
Efficiency, defined as the ratio of output power to input power,
indicates how much power is dissipated in the power supply.
In the typical
TOPSwitch-II
flyback power supply shown in
Figure 1, most of the power dissipation occurs in output
rectifier D2, Zener diode VR1 (or equivalent clamp circuit)
and the
TOPSwitch-II
device. Other components, such as
output filter inductor L1, input common mode inductor L2, and
bridge rectifier BR1, contribute lesser power dissipation terms.
Overview
of Quick Selection Curves
The
TOPSwitch-II
Flyback Quick Selection Curves consider
these dissipation terms (and others as well) to provide a good
estimate of expected efficiency for both Universal input and
230 VAC mains applications. Figure 2 (for +12 V outputs) and
Figure 3 (for +5 V outputs) show a set of curves for efficiency
and
TOPSwitch-II
power dissipation versus output power for
the entire family of TOP221-TOP227 devices. These curves
assume operation from a low line AC input voltage of 85 VAC,
which is a suitable value for all Universal input applications.
TOPSwitch-II
Flyback
Quick Selection Curves
Application Note AN-21
For higher nominal mains voltages, including 208, 220, 230,
and 240 VAC, a low line AC input voltage of 195 VAC is used
to generate similar curves found in Figure 4 (for +12 V outputs)
and Figure 5 (for +5 V outputs). For all curves, the maximum
AC mains voltage is assumed to be 265 VAC.
For each
TOPSwitch-II
device, a family of efficiency curves
(solid lines) is plotted on the Y-axis as a function of output
power on the X-axis.
TOPSwitch-II
power dissipation is
plotted separately on the same graph as a family of constant
power dissipation contours (dashed lines).
1) Determine which graph (Fig. 2, 3, 4 or 5) is
closest to your application.
Example: Use Figure 2 for Universal input,
12 V output.
2) Find your power requirement on the X- axis.
3) Move vertically from your power
requirement until you intersect with a
TOPSwitch-II
curve (solid line).
4) Read the associated efficiency on the
Y- axis.
5) Determine if this is the appropriate
efficiency for your application. If not,
continue to the next
TOPSwitch-II
curve.
6) Read
TOPSwitch-II
power dissipation from
the dashed contours to determine heatsink
requirements.
7) Start the design. Use the Transformer
Design Spreadsheet from AN-17.
Note: See Selection Curve Assumptions for limits of use.
QUICK START
April
1998
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