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| 型號： | AN-29 |
| 英文描述： | TOPSwitch-GX Flyback Quick Selection Curves |
| 中文描述： | 的TOPSwitch - GX系列激快速選擇曲線 |
| 文件頁數(shù)： | 1/12頁 |
| 文件大小： | 91K |
| 代理商： | AN-29 |
February
2003
TOPSwitch
Quick Selection Curves
Application Note AN-29
-GX
Flyback
QUICK START
Introduction
This application note is intended for engineers starting a flyback
power supply design with
TOPSwitch-GX
. It offers a quick
method to select the proper
TOPSwitch-GX
device from
parameters that are usually not available until much later in the
design process.
Curves estimating the efficiency of the power supply and the
corresponding
TOPSwitch-GX
device dissipation are provided.
They form a powerful tool for estimating cost and project
requirements before even committing to or starting development.
This application note is similar to both AN-21 for
TOPSwitch-II
and AN-26 for
TOPSwitch-FX
.
Overview of Quick Selection Curves
The
TOPSwitch-GX
Quick Selection Curves (Figures 1-4)
show the expected power supply efficiency and
TOPSwitch-GX
dissipation for typical flyback applications. Power supplies
with either a 5 V or a 12 V output, operating with either
‘Universal Input’ (85 VAC-265 VAC) or ‘Single 230 VAC
Input’ (195 VAC-265 VAC) are described.
The solid lines in the Quick Selection Curves give a typical
efficiency figure for a given load, depending upon the
TOPSwitch-GX
device used. Each solid line efficiency curve
extends to the maximum power capability of the device, limited
by device current limit. The superimposed dashed lines are
contours of constant
TOPSwitch-GX
device dissipation, the
intersections of these dashed lines with the solid lines provide
the corresponding dissipation at different loads. Interpolation
or extrapolation can find the dissipation at intermediate points.
The shaded region indicates the output power where a flyback
design at the given output voltage is no longer practical. This
limit has been shown at an output current of 10 A and above.
Higher output currents are possible but such a design is typically
not cost effective due to the size of the output diode and
capacitors. Higher output power can be obtained if the output
voltage is higher.
The curves can be used for both P (DIP-8), G (SMD-8) and Y
(TO-220) packaged devices, however for the P and G parts the
dissipation must be limited to 0.85 W. This is due to the thermal
constraints of the P package. The P and G parts intentionally
have lower current limits than their Y packaged counterparts to
match device dissipation to package capability.
When using the curves for different output voltages the reader
should be aware that altering the output voltage will give
dramatic changes in efficiency.
For voltages between 5 V and 12 V the data from both curves
can be used to extrapolate an intermediate point. Lower
voltages will give lower efficiencies and limit maximum power
capability. Higher voltages will give higher efficiencies and
greater power. For example from the curves a 12 V, 70 W
universal design using the TOP249 has an estimated efficiency
of 79.5%. If the output voltage were increased to 19 V this
would increase to approximately 85%. Similarly an open frame
1) Determine which graph (Fig. 1, 2, 3 or 4) is closest to
your application. Example: Use Figure 1 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-GX
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-GX
curve.
6) Read the
TOPSwitch-GX
power dissipation from the
dashed contours to determine heatsink requirements.
7) If the device dissipation is 0.85 W then the lower cost
P/G packages can be considered.
8) Start the design. Use the
TOPSwich-GX
Transformer
Design Spreadsheet or PI Expert.
Note: See ‘Selection Curve Assumptions’ for limits of use.
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