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參數(shù)資料
| 型號： | AN-30 |
| 英文描述： | TOPSwitch-GX Forward Design Methodology |
| 中文描述： | 的TOPSwitch - GX系列正向設(shè)計方法 |
| 文件頁數(shù)： | 3/40頁 |
| 文件大?。?/td> | 311K |
| 代理商： | AN-30 |
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AN-30
3
B
12/02
gives the basic expressions illustrating the methodology. The
PI Expert
software uses more complex versions of these
expressions containing additional parameters to account for
non-ideal effects. Thus, results from the software may not
exactly match the computations from expressions in this
document.
This document assumes a non-doubled input configuration.
PI Expert
includes modified expressions for both doubler and
non-doubler input configurations. To simplify the expressions,
all outputs are assumed to operate in continuous conduction
mode, consistent with the worst case design at maximum load.
At lower load conditions it is possible for individual outputs to
operate in discontinuous conduction mode.
The methodology begins with an explanation of the general
converter topology. It then presents the design flow, showing
the major tasks in a high level flowchart. After a review of the
nomenclature and definitions of variables, it discusses the
details of the design procedure. Rationale, assumptions and
expressions are given to help the designer enter parameters and
interpret results. A complete list of variables used in the
expressions follows in Appendix A. Appendix B offers a
procedure for hardware verification. A worked example is
presented in Appendix C.
General Converter Topology
Figure 1 shows a typical single-ended forward converter using
TOPSwitch-GX
. Detail is focused on the primary side of the
transformer because the circuits on the secondary are
conventional and covered in other literature.
Resistors R
and R
B
set the under-voltage lockout threshold.
Resistor network R
, R
, R
, and R
with capacitor C
adjusts
the maximum duty ratio as a function of the input voltage. This
methodology gives the procedure to determine proper values
for the resistors and the capacitor.
Another key element in the use of
TOPSwitch-GX
is the
primary clamp (C
, D1, VR1, VR2 and VR3 in Figure 10)
which resets the transformer flux and limits the maximum drain
voltage. This methodology assumes use of this Zener-capacitor
clamp circuit. Guidance for selection of components for this
particular clamp is included in this application note.
The topic of clamp circuits is deferred to a separate application
note. Designers may choose to use their own clamp circuits
with the restriction that
resonant clamps, (for example, LCD
clamps–inductor/capacitor/diode) and reset windings are not
recommended
. The internal current sense of
TOPSwitch-GX
does not allow the high reset current of a resonant clamp to be
excluded from the sensed drain current.
This methodology uses an ordinary optically isolated feedback
circuit that is common in voltage mode systems with a two-pole
response. The frequency compensation will in general require
two zeros and two poles to obtain the phase margin desired for
most applications. While the design of the feedback circuit is
a separate topic beyond the scope of this application note, the
general topology of the circuit is discussed.
Output Options
Salient features of the output circuits are illustrated in
Figure 2. Multiple secondary windings of the transformer may
be configured in many different ways to give several options for
regulated and unregulated output voltages.
All applications will have only one main output. This is the
voltage that is regulated directly by
TOPSwitch-GX
through the
optically isolated feedback circuit. In general, any number of
auxiliary outputs may be derived from other secondary windings
and regulated indirectly by means of a coupled inductor that
they share with the main output.
The secondary windings for the auxiliary outputs may be
configured in two different ways. The conventional
configuration connects one side of the auxiliary winding to the
main output return. This connection is used when the auxiliary
output is the opposite polarity of the main output. An alternative
configuration, sometimes known as the DC stacked connection,
has one side of the auxiliary winding referenced to the main
output instead of the output return. It has the advantage of better
regulation of the auxiliary output voltage than the non-stacked
arrangement, but is limited to outputs that are greater in
magnitude and of the same polarity as the main output voltage.
Any number of unregulated output voltages may be derived
from circuits that do not share an inductor with any other
outputs. They are related to the main output only through
separate secondary windings on the transformer. Their inductors
are independent of the others. These outputs typically are
referenced to the output return, but alternatively they may be
referenced to any potential that the isolation of the transformer
will tolerate.
Multiple tightly regulated voltages may be obtained with
either linear or switching post regulators. These external
regulators may be added to any output, including the main
output. They are simply additional loads on those output
voltages.
A particularly useful type of switching post regulator is the
magnetic amplifier, which uses a saturating magnetic element
as an independently controlled switching device. While a
magnetic amplifier can in theory be operated from any output,
this methodology restricts the connection to the main output
only.
Since it is not possible to treat every combination of output
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