參數(shù)資料
型號(hào)： 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(jì)方法
文件頁(yè)數(shù)： 14/40頁(yè)
文件大小： 311K
代理商： AN-30
AN-30
14
B
12/02
The resistor network also sets the threshold for line under-
voltage lockout. Protection from over-voltage is generally not
a concern for this topology since it uses a Zener clamp to
provide a hard limit on the drain-to-source voltage.
The resistors are matched to the capacitor to form an integrator
with an appropriate time constant to give a cycle-by-cycle duty
ratio limit. The integration of the voltage on the bias winding
gives the external duty ratio limit a desirable relationship to
the
flux in the transformer. The circuit adjusts the duty ratio limit
to set an upper bound on the volt-second product, and to balance
the volt-second product during
TOPSwitch-GX
on and off
times. The dynamic nature of the circuit allows greater freedom
and precision in the design without interference from the line
over-voltage threshold limit.
Figure 1 shows the locations of resistors R
, R
, R
and R
with
capacitor C
. Several important quantities related to their
values are illustrated in Figure 8. The broken vertical lines in
Figure 8 mark the boundaries of the DC bus voltage for
minimum and maximum operating voltages, the line under-
voltage lockout threshold, and the lowest input voltage that will
guarantee regulation of the output.
The broken horizontal line shows the maximum guaranteed
duty cycle of
TOPSwitch-GX
. A value of 74% is recommended
for design.
The lowest curve is the duty ratio D that corresponds to steady-
state operation at a given input voltage. The straight line with
negative slope is the maximum duty ratio D
that will still
guarantee reset of the transformer for a given V
DSOP
. The
converter must always operate with D less than D
saturation of the transformer. The curved line between the D
and D
lines is the external duty ratio limit D
XMAX
that is set
by the resistors.
The designer must choose the components to set the curve of
D
at a desired position between the boundaries of D
RESET
and D for a given set of specified voltages.
PI-Expert
prompts the user to enter several parameters that are
important to the computation of the resistor values. Some
parameters are from the
TOPSwitch-GX
data sheet while others
are design choices. The software suggests default and typical
values. The designer can enter maximum and minimum values
to check worst case situations.
The components are calculated to satisfy the constraints of four
parameters: D
(external duty ratio limit at V
), D
(external duty ratio limit at V
), V
(input voltage where
the
TOPSwitch-GX
starts switching), and the maximum transient
input voltage V
OV
that is greater than V
MAX
.
PI-2823-121701
D
LL_ACTUAL
D
XMAX
D
LL_RESET
D
RESET
D
HL_RESET
D
HL_ACTUAL
V
IN
DC
MAX
D
74%
100%
0%
D
MAX_RESET
D
XDO
D
MAX_ACTUAL
D
XHL
V
DROPOUT
V
UVLO
V
MIN
V
MAX
D
Figure 8. Boundaries of Voltages and Duty Ratio Related to the Selection of R
A
, R
B
, R
C
and R
D
with C
VS
in Figure 1.
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