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參數(shù)資料
| 型號(hào)： | AN-31 |
| 英文描述： | DPA-Switch Forward Converter Design Guide |
| 中文描述： | 政治部開關(guān)正激變換器的設(shè)計(jì)指南 |
| 文件頁數(shù)： | 5/20頁 |
| 文件大?。?/td> | 245K |
| 代理商： | AN-31 |
AN-31
5
B
4/03
voltage regulation is not quite as good as with the output
coupled inductor bias. However, the solution does provide a
reasonably constant bias voltage over a variety of input
voltage and output load conditions. This solution works best
if the independent inductor is maintained in the continuous
conduction mode. The solution can be implemented with a
low current, low cost (off-the-shelf) inductor, but the
inductance value will be high enough to ensure continuous
conduction mode over the majority of operating conditions.
Transformer Design
The power transformer is critical to the success of the converter
design. Requirements for efficiency, component height and
footprint will determine the details of construction. System
engineers and circuit designers may choose to specify the
electrical parameters and mechanical limits, and delegate the
construction details to a supplier of custom transformers. Use
the
PI Expert
design tool to determine the proper parameters.
This section gives guidance for specification of the transformer.
Turns Ratio
The most important parameter for the power transformer is the
primary-to-secondary turns ratio. It must be low enough to
provide the regulated output voltage at the minimum input
voltage. Determine the minimum input voltage from the system
specification and the tolerance of the line under-voltage lockout
circuit.
Whereas the minimum input voltage may be specified at 36 V,
worst case tolerances of the under-voltage circuit are likely to
allow the
DPA-Switch
to operate at an input as low as 29 V.
From this voltage, subtract the estimated drain-to-source voltage
of
DPA-Switch
at the maximum load. Reduce it further by an
estimate of the voltage drop from the high frequency AC
resistance of the transformer windings at full load.
Multiply the result by the maximum guaranteed duty ratio and
divide by the sum of the output voltage and the drop on the
output rectifier at full load. The duty ratio can be greater than
50% because
DPA-Switch
uses a voltage mode control. The
quotient is the upper limit for the turns ratio.
Core and Copper
The actual number of turns for the transformer will depend on
the dimensions of the particular core. The core material should
be low loss at the
DPA-Switch
operating frequencies. Technical
data on properties of Ferrite Cores are available from several
suppliers. See references [1], [2] and [3]. Skin effect and
proximity effect will set a practical limit for wire size. Foil
windings become attractive when the output current is higher
than about 6 amperes.
Thermal considerations often dominate selection of the core.
The selection of the core is a complex trade-off between
winding area, core cross-section and ratio of core surface area
to core volume. These parameters determine the power loss as
well as the thermal resistance of the transformer. A small core
may meet the requirements in every respect except temperature
rise, forcing the use of a larger core. The only practical way to
check temperature rise is with bench evaluation of a prototype.
Temperature must be measured at the hottest spot in the
transformer, which is usually next to the center of the core under
the windings. Wire temperatures above 110
°
C need special
considerations and UL Class F materials.
Other Practical Considerations
Minimize the number of turns within the limits of other
constraints. Resistive losses depend on the length of the wire.
Maximize the amount of copper (wire) that can be fitted within
the winding window. Leakage inductance must be kept low to
reduce losses associated with clamp components. This is best
Input Voltage
Range (V)
Bias Type
DC Input
Derived Bias
Efficiency
Cost Complexity
Comment
Recommended
for 18 to 36 V only
Transformer Bias
(unregulated)
Output Coupled
Inductor Bias
Transformer
Bias (regulated)
18 to 36
36 to 72
36 to 72
36 to 72
Recommended for
low-cost design
Only recommended if
supply already requires
coupled output inductor
Recommended for high
efficiency designs
Table 2. Bias Voltage Solution Comparison.
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