參數(shù)資料
型號(hào)： AN-31
英文描述： DPA-Switch Forward Converter Design Guide
中文描述： 政治部開(kāi)關(guān)正激變換器的設(shè)計(jì)指南
文件頁(yè)數(shù)： 6/20頁(yè)
文件大?。?/td> 245K
代理商： AN-31
AN-31
6
B
4/03
accomplished with a split primary construction that has the
secondary between the layers of the primary winding. Also, all
transformers should have no air gap.
If the transformer has a winding for the bias voltage, be sure that
it has enough turns to maintain a minimum of 8 V bias at the
lowest input voltage. Perform bench verification to confirm that
the converter shuts off at low input voltages by virtue of the
under-voltage lockout circuit, and not because the bias voltage
is too low.
With the actual number of turns on the transformer, verify that
the duty ratio to regulate the output at the minimum input
voltage is less than the minimum DC
MAX
specified for
DPA-Switch
.
The AC flux density contributes to the core losses. For this
reason the AC flux density should be maintained in the range
between 1000 and 1500 gauss (0.1 to 0.15 tesla).
Output Inductor
For a single output application with no bias winding, the
inductor can be a standard off-the-shelf component. Inductors
with multiple windings are typically custom designs.
The inductor value is determined chiefly by the amount of
current ripple that the designer is willing to tolerate. Higher
ripple current will allow an inductor that is smaller both
electrically and physically. The consequence of higher ripple
current is the requirement for more output capacitance with
lower equivalent series resistance (ESR) to meet the specification
for output ripple. Higher current ripple in the inductor also
translates to higher peak current in the
DPA-Switch
for a given
output power. It also leads to generally greater loss and lower
efficiency because the RMS value of all the currents will be
higher.
A convenient design parameter for selection of the inductor is
K
, defined as the ratio of the peak-to-peak ripple current to the
average current in the inductor. Smaller K
corresponds to
lower ripple and a larger inductor. Recommended values for K
I
are between 15 and 20 percent. The choice of K
involves a
trade-off between the size of the inductor, the number and type
of output capacitors, efficiency, and cost. Higher values of K
I
are not recommended, as these higher ripple currents increase
both the stress and the ripple voltage on the output capacitor.
Whether the inductor is standard off-the-shelf or custom, the
design should minimize the number of turns to reduce the
resistive loss. The construction should also use a low loss core
material.
With user input, the
PI Expert
design tool computes the
inductance, the RMS current and the peak stored energy to aid
in the selection or specification of the inductor. Peak stored
energy is a useful parameter to select designs that use a closed
toroid core, where magnetic saturation is generally a concern.
Additional Winding for Bias Voltage
If the configuration in Figure 2 (c) is chosen for generation of
the bias voltage, choose the number of turns on the bias winding
to give 12 V at the optocoupler under nominal conditions.
Compute the required number of turns from the lowest regulated
output voltage and the highest forward voltage drops for the
output rectifier and the bias rectifier. Check the bias voltage at
minimum load, maximum line, and add a preload if necessary
to maintain the bias voltage at 8 V minimum. It may also be
necessary to increase the bias winding turns to meet the minimum
voltage requirement with a reasonably small pre-load.
DPA-Switch
Selection
The first criterion for the selection of the
DPA-Switch
is peak
current capability. From the turns ratio of the transformer and
the peak current in the output inductor, estimate the peak current
in the primary of the transformer. The magnetization current of
the transformer should be negligible for this estimate. For
lowest cost, select the smallest
DPA-Switch
that has a minimum
current limit that is at least 10% greater than the maximum
primary current. The allowance of 10% greater current gives
design margin with the ability to respond to transient loading.
The second criterion for the selection is power dissipation. The
smallest
DPA-Switch
that will handle the current may dissipate
too much power to meet the efficiency requirements. Even if
efficiency is not a concern, the smallest device may get too hot
Figure 3. Efficiency of the Low Cost EP-21 Prototype with
Different Devices in the DPA-Switch Family
(Synchronous Rectification Would Improve Efficiency).
90%
86%
88%
35
40
45
DC Input Voltage
50
55
60
65
70
75
Efficiency vs. Input Voltage, 30 W Supply
E
P
80%
82%
84%
DPA424
DPA425
DPA426
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