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TOPSwitch -GX Flyback
Quick Selection Curves
Application Note AN-29
Introduction
QUICK START
Thisapplicationnoteisintendedforengineersstartingaflyback
power supply design with TOPSwitch-GX. It offers a quick
1) Determine which graph (Fig. 1, 2, 3 or 4) is closest to
yourapplication. Example:UseFigure1forUniversal
Input, 12 V output.
method to select the proper TOPSwitch-GX device from
parameters that are usually not available until much later in the
design process.
2) Find your power requirement on the X-axis.
Curves estimating the efficiency of the power supply and the
correspondingTOPSwitch-GXdevicedissipationareprovided.
They form a powerful tool for estimating cost and project
requirementsbeforeevencommittingtoorstartingdevelopment.
This application note is similar to both AN-21 for
TOPSwitch-II and AN-26 for TOPSwitch-FX.
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.Ifnot,continuetothenextTOPSwitch-GX
curve.
Overview of Quick Selection Curves
The TOPSwitch-GX Quick Selection Curves (Figures 1-4)
showtheexpectedpowersupplyefficiencyandTOPSwitch-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.
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.
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
extendstothemaximumpowercapabilityofthedevice, 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.
8) Start the design. Use the TOPSwich-GX Transformer
Design Spreadsheet or PI Expert.
Note:See‘SelectionCurveAssumptions’ forlimitsofuse.
have lower current limits than their Y packaged counterparts to
match device dissipation to package capability.
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.
Higheroutputcurrentsarepossiblebutsuchadesignistypically
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.
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
wouldincreasetoapproximately85%. Similarlyanopenframe
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
dissipationmustbelimitedto0.85W. Thisisduetothethermal
constraints of the P package. The P and G parts intentionally
February 2003
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