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參數(shù)資料
| 型號： | AN-31 |
| 英文描述： | DPA-Switch Forward Converter Design Guide |
| 中文描述： | 政治部開關(guān)正激變換器的設(shè)計指南 |
| 文件頁數(shù)： | 4/20頁 |
| 文件大小： | 245K |
| 代理商： | AN-31 |
AN-31
4
B
4/03
should be 12 V. The output coupled inductor and the regulated
transformer bias techniques give the highest efficiency of the
four solutions because the voltage across the optocoupler is
controlled. This is countered by increased complexity.
Optocoupler dissipation can be significant and should be verified.
Maximum optocoupler phototransistor current is equal to the
maximum CONTROL pin current (I
) for the selected
DPA-Switch.
Maximum dissipation therefore occurs at the
highest bias voltage (highest input voltage for (a) and (b)) and
minimum load. Table 2 provides a comparison of complexity vs
performance for all the solutions.
a) The DC input derived bias is the simplest of the three
solutions. It uses a Zener diode between the positive DC
input and the collector of the phototransistor of the
optocoupler to reduce the maximum collector-to-emitter
voltage, and more importantly, to limit the dissipation in the
optocoupler. The penalty for simplicity is a reduction in
efficiency that can be significant at high input voltages. This
alternative is best for industrial applications where the input
voltage is low (18 V to 36 V). The input voltage in industrial
applications is usually low enough to eliminate the Zener
diode because the breakdown voltages for standard
optocouplers can be as high as 70 V. Designers must check
the maximum power dissipation in the optocoupler in either
case.
b) The transformer bias (unregulated) is created from a winding
on the power transformer. The forward bias winding should
be connected to the rectifier in a polarity such that it conducts
when the
DPA-Switch
is on. Since the bias voltage is
proportional to the input voltage, efficiency is reduced at
high input voltages, but the effect is less than with the direct
connection to the input. Again, the designer needs to check
the power dissipation in the optocoupler at the maximum
bias voltage. For this bias type worst case is minimum output
load and high input voltage. Flyback bias windings are not
recommended for
DPA-Switch
applications since they will
affect the transformer reset.
c) Output coupled inductor bias uses a winding on the output
inductor to develop the bias voltage. This technique provides
a well regulated bias voltage when the converter operates in
the continuous conduction mode. Regulation is accomplished
by phasing the winding such that the bias voltage is
proportional to the output voltage by transformer action
when the
DPA-Switch
turns off. The penalty for the higher
efficiency is the cost and complexity of a custom output
inductor. The bias voltage can be adjusted by modifying
turns ratio, bias capacitor size and minimum load on the
main output. The designer should verify a minimum bias
voltage of 8 V at minimum load and maximum input voltage.
d) The transformer bias (regulated) solution peforms the same
function as the output coupled inductor bias (c). The bias
in greater detail in a separate section. Ultra-fast PN junction
diodes are not suitable at
DPA-Switch
operating frequencies.
Efficiency
Designing a DC-DC converter with
DPA-Switch
involves several
engineering tradeoffs that weigh efficiency against cost and
complexity. The circuit configuration in Figure 1 achieves
efficiencies greater than 85% over the range of input voltage at
medium loads. In typical applications without synchronous
rectifcation, approximately 25% of the total power loss will be
in the
DPA-Switch
(see
DPA-Switch
data sheet)
,
40% in the
output rectifiers, and 30% in the magnetic devices. The
remainder is distributed among other devices and circuit traces.
Higher efficiencies of approximately 91% can be obtained
when Schottky rectifiers are replaced by synchronous rectifiers,
allowing lower voltage drops. The efficiency can be raised even
higher with the use of the next larger device in the
DPA-Switch
family that has lower R
. Further increases in device size
may not improve the efficiency due to increased device switching
losses. Losses in the magnetic devices can be reduced by using
larger cores and by switching at 300 kHz instead of 400 kHz. All
these alternatives have compromises in size, cost and complexity
that the designer must evaluate.
Temperature
DC-DC converters usually must operate over an extended range
of temperature that goes beyond the limits for ordinary consumer
electronics. Designers should be aware that the characteristics
of passive components are likely to change significantly with
temperature. Attention to these effects to choose suitable
components can prevent unexpected and undesirable behavior.
Designers must pay particular attention to the selection of the
output capacitors and the components in the feedback circuit to
guarantee specified performance throughout the temperature
range. The details are addressed later in the sections on Output
Capacitor Selection and Feedback Design.
Bias Voltage
There are four ways to generate the bias voltage required for
operation of
DPA-Switch
:
(a) DC input derived
(b) Transformer bias (unregulated)
(c) Output coupled inductor winding
(d) Transformer bias (regulated)
Figure 2 illustrates the four alternatives. Each one must provide
a minimum of 8 V at the collector of the optocoupler under
worst case operating conditions (minimum input voltage and
minimum load). The lowest bias voltage under typical conditions
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