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參數(shù)資料
| 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)： | 6/12頁 |
| 文件大?。?/td> | 91K |
| 代理商： | AN-29 |
AN-29
6
D
2/03
The choice of V
can affect the efficiency greatly. For
example increasing the V
(turns ratio) may allow a Schottky
diode on the output, for higher efficiency, by reducing the
diode inverse voltage. However it will increase secondary
reflected leakage and therefore clamp dissipation.
Lowering the V
reduces secondary reflected leakage, reducing
clamp dissipation but at the expense of higher primary RMS
currents, increasing the
TOPSwitch-GX
conduction losses.
For low voltage outputs, the secondary currents and their
associated losses can become significant. Close attention
must be paid to the ‘ESR’ (Equivalent Series Resistance) of
the output capacitor in particular. The values in Tables 2 and
4 for the 5 V Quick Selection Curves (Figures 2 and 4) use
capacitors with very low-ESR.
Energy stored in the leakage inductance is dumped into
primary clamp (RCD clamp or Zener clamp) when the
TOPSwitch-GX
turns off. Therefore the efficiency will fall
significantly if the leakage inductance is too high. Refer to
Example 3 of AN-26 to see how the effective in-circuit
leakage should be measured and how secondary trace
inductance reflects into the primary. For low voltage outputs
at high power, it is critical to minimize leakage inductance.
Factors which can improve performance:
For more experienced designers, there are ways to improve the
performance indicated by the Quick Selection curves. Some of
these are now mentioned briefly:
The recommended capacitance per Watt is based on the
optimum cost to performance ratio. Better performance can
certainly be obtained in terms of efficiency,
TOPSwitch-GX
dissipation and life expectancy of the input bulk capacitor,
by using a higher capacitance per Watt than recommended.
If the intended application is for 100/115 VAC only, the
clamp voltage and V
may be raised by a calculated amount
provided no voltage doubler is being used at the input of the
power supply. This will enhance the overall efficiency and
lower the device dissipation.
The recommended primary inductances in Tables 1 to 4 are
based on the minimum permissible K
at the maximum
power capability of the device. In other words, the primary
inductance along any given solid curve corresponding to a
particular device has been kept a constant.
However in an adapter application for example, the output
power is limited by thermal considerations to a value much
less than the maximum power capability of the
TOPSwitch-GX
device. This presents an opportunity to
improve efficiency and lower device dissipation by increasing
the primary inductance while ensuring that the K
at the
actual power requirement stays within recommended design
limits.
Since the Quick Selection Curves are based on a
TOPSwitch-GX
junction temperature of 100
°
C at low line,
full load, better performance is possible if the
TOPSwitch-GX
runs cooler. Good heatsinking will help in
achieving higher efficiency.
Increasing the V
can be helpful in some cases. A high V
decreases the reverse voltage stress on the output diodes.
This may allow the use of 45 V Schottky output diodes for
high voltage outputs, resulting in a significant improvement
in the efficiency.
This step should be taken only after considering the overall
impact. It should be mentioned that increasing the V
causes an increase in the duty cycle and a corresponding
reduction in the RMS currents and conduction losses in the
TOPSwitch-GX
device provided the overall efficiency is not
adversely affected due to increased clamp loss.
Conclusions
The
TOPSwitch-GX
devices may be considered to be an
extension of the
TOPSwitch-FX
family. The P-package options
have reduced current limits to match the device current limit to
the thermal dissipation capability of the package. This allows
for a smaller transformer in adapter designs. However for the
same conditions both the P/G and Y packaged devices will
dissipate the same power. Therefore the Quick Selection curves
are valid for either package (up to the point where current
limiting takes place).
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| AN-32 | TOPSwitch-GX Flyback Design Methodology |
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