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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)： | 6/40頁 |
| 文件大小： | 311K |
| 代理商： | AN-30 |
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AN-30
6
B
12/02
Voltage drops on diodes have subscripts with the prefix
for the
conduction drop and
for the reverse blocking voltage. The
only exception to this convention is for drain-to-source voltages,
which will be obvious from context.
Figure 4 also shows series resistances that the designer can
include to get better predictions of performance.
Detailed Design Procedure
This methodology guides the designer through a procedure that
determines parameters for prototype hardware. After bench
evaluation, the designer refines the parameters to meet all
requirements.
The design can start with knowledge of only the most basic
system requirements. For example, the forward voltage drops
on diodes and the resistances of transformer windings are
seldom known very accurately at the beginning of a new design.
Results of the design with default values will guide the designer
to select particular components with known parameters.
Figure 5 gives an expanded flowchart that includes the detailed
steps which follow.
Step 1. Establish system requirements.
Determine the parameters in Table 1. These should be available
from a system specification of the power supply’s application.
The software will compute and display the maximum and
minimum DC bus voltages to the converter from the AC inputs.
The need to know maximum and minimum voltages is obvious.
The optional nominal input voltage V
helps determine the
turns ratios of the transformer. The goal is to set the unregulated
output voltages at their nominal values when the input is at its
nominal value. The designer may choose any value between
V
ACMAX
and V
ACMIN
to be the nominal value.
The peak DC bus voltage (non-doubled) is
(1)
while the DC bus voltage at the valley of the ripple at the
minimum steady state AC input is
(2)
where P
is the total output power, t
is the conduction time of
the bridge rectifier,
η
is the efficiency exclusive of losses in
the AC input circuit, and C
IN
is the capacitance at the input to the
converter. Use 3 ms for t
and use the total system efficiency
η
for
η
if no better estimates are available. A good initial
value for C
IN
is 1
μ
F per watt multiplied by P
O
.
The designer should carefully choose the value of t
when
using passive PFC input (a large inductor in the AC line), since
this approach significantly increases the diode conduction time.
Also, the voltage waveform will deviate from a sinusoid,
causing some error in the prediction of Equations (1) and (2).
Remember to use the input voltage to linear regulators, not the
regulated output voltage, to compute the total output power.
The dissipation in the linear regulator is part of the load on the
converter.
The nominal DC bus voltage is defined to be
(3)
This is simply the midpoint between the peak and valley of the
ripple voltage on the input capacitor (non-doubled).
Step 2. Set ripple current in the output inductors.
Choose the ripple current factor K
. Figure 6 shows how it is
related to the average output current. K
is a useful parameter
for design because it directly influences the size of the output
inductor. It also affects the peak primary current and the RMS
current in the output capacitors.
PI-2820-121301
+
–
+
+
–
–
V
MAIN
V
DMAINC
V
DMAINF
L
MAIN
N
P
R
P
R
LMAIN
R
SMAIN
N
MAIN
Forward
Diode
Catch
Diode
Figure 4. Output Circuit with Parameter Definitions.
V
V
MAX
ACMAX
=
2
V
V
P
f
t
C
MIN
ACMIN
O
L
C
DC
IN
=
η
2
2
1
2
2
V
V
V
P
f
t
C
NOM
ACNOM
ACNOM
O
L
C
DC
IN
=
+
η
2
2
1
2
2
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