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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系列正向設計方法 |
| 文件頁數(shù)： | 13/40頁 |
| 文件大?。?/td> | 311K |
| 代理商： | AN-30 |
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AN-30
13
B
12/02
output. The computation is based on K
, which considers the
total ampere turns of the coupled inductor, not just the current
in one winding. The inductance of the winding for the main
output, valid for only the DC stacked configuration, is
(34)
PI Expert
gives the designer the turns ratio, the total ampere
turns, and the peak energy stored in the inductor. The designer
has the option to change these parameters by adjustment of the
K
I
for each inductor.
These quantities assist the designer to obtain an appropriate
inductor of either his own design or one from a qualified
supplier. Bench evaluation of the prototype will determine if
fine adjustment of the turns is necessary in the final configuration.
Step 12. Calculate inductance for independent outputs.
Calculation of the inductance for independent outputs is
straightforward and similar to the computation of the parameters
for the coupled inductor. Design of the component is simplified
because there is no turns ratio associated with an inductor that
has only one winding.
PI Expert
computes the inductance and
the peak stored energy. This information is useful for selection
of magnetic cores from catalogs.
Step 13. Calculate output inductance for the magnetic
amplifier.
PI Expert
computes the output inductor for a magnetic
amplifier post regulator in the same way as for an independent
output. It does not address the magnetic switching element.
Step 14. Adjust output inductors if necessary.
The designer may modify the K
of any inductor to accommodate
special requirements. If the value or the estimated physical size
of the computed inductor is not satisfactory, adjust the individual
K
I
to achieve the desired result.
Step 15. Calculate component values for external reduction
of DC
.
The maximum duty ratio (DC
)
of
TOPSwitch-GX
must be
restricted to avoid saturation of the transformer during transient
loading. A network of four resistors and a capacitor (R
, R
, R
C
,
V
, R
and C
in Figure 1 and Figure 1 of Appendix B)
determines a variable upper limit on the duty ratio. Adjustment
of the maximum duty ratio with input voltage allows enough
deviation beyond the steady-state operating point to respond to
transients while maintaining enough time in every switching
cycle for the transformer to reset.
I
K I
RMS
2 3
=
N
N
N
N
LMAIN
LAUX
MAIN
AUX
=
Calculations of the peak inverse voltage on the output rectifiers
use V
, V
, and the output voltages with the turns on the
transformer windings.
Calculations of worst case average current in the catch diodes
are with the duty ratio that corresponds to the maximum input
voltage. A very good approximation to the average rectifier
current is then just the output current. Current in the forward
diodes is computed with D
. Note that with DC stacked
outputs, the rectifiers on the main output must conduct the sum
of the currents of the main and auxiliary outputs.
In general, the stress will be different for the forward diode and
the catch diode on the same output. Designers will have to
consider the one with the greater stress when choosing
components that contain both diodes in the same package.
Step 10. Calculate RMS ripple currents in output capacitors.
Currents in the output capacitors are computed at the maximum
loads. In continuous conduction mode, the RMS ripple current
is given by
(32)
where K
is for the particular output under consideration. This
expression is reliable for independent outputs and for a main
output with no coupled inductors. For converters with auxiliary
outputs, Equation (32) is only an estimate. Ripple currents in
the individual windings of coupled inductors depend on magnetic
coupling coefficients, parasitic voltage drops, and other
quantities in the circuit that are difficult to predict. Therefore,
designers must evaluate prototype hardware on the bench to
confirm that the assumptions of the design are valid for a
particular application.
Step 11. Calculate parameters for the coupled inductor.
The coupled inductor allows the auxiliary outputs to have better
regulation than independent outputs, with the penalty of increased
complexity of the inductor.
PI Expert
allows two options for the topology of the auxiliary
output. The auxiliary output may be referenced to the main
output voltage for the best regulation or to output return when
necessary. The reference must be at output return to obtain a
negative auxiliary output with a positive main output.
Turns ratios for the coupled inductor are the same as the ratios
for the transformer. The turns ratio of a coupled inductor for a
converter that has one auxiliary output is, in terms of the actual
number of turns,
(33)
Inductance is computed for the winding that is on the main
L
V
V
K
I
I
N
N
f
MAIN
MAIN
DMAINC
I
MAIN
AUX
LAUX
LMAIN
S
=
+
+
+
0
1
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