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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)： | 33/40頁 |
| 文件大?。?/td> | 311K |
| 代理商： | AN-30 |
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AN-30
33
B
12/02
Circuit for Reduction of Maximum Duty Ratio
Figure C1 shows the external circuit for limiting the maximum duty cycle of the
TOPSwitch-GX
.
Figure C1: Components that make up external duty-ratio
limiting circuit.
Step (r). Selection of duty-cycle limiting circuit components:
R
A
, R
B
, R
C
, R
D
, V
Z
, C
VS
Selection of
R
, R
These resistors provide L pin current that is proportional to the input voltage. The calculation formula for these resistors is as in
Equation (35) of AN-30.
R
= R
B
=
2.25
M
.
Selecting closest standard values:
Select
R
A
=
2.26
M
, 0.5
W
Select
R
B
=
2.26
M
, 0.5
W
(RA, F137)
(RB, F138)
Selection of Zener:
V
The Zener diode increases the dynamic range of operation by allowing the low line duty-ratio to be a higher value, while still
avoiding saturation.
The Zener diode should be used only in designs that operate over a very wide range of duty-cycle. Whenever needed, a 4 V-9 V
Zener diode should be used. In EP-12 this value is set at 6.8 V, as this is the maximum value of the Zener that avoids core
saturation at low line, while allowing extended operating range. Choose any standard low power (500 mW) Zener diode.
Choose 6.8 V Zener diode BZX79-C6V8, VZ = 6.8 V
(VZ F134)
Selection of Duty-Ratio limits:
D
,
D
Note that the spreadsheet takes default inputs corresponding to
D
and
D
. These values are based on the low-end parts. It
can be seen by looking at the curves for core saturation (
D
)
and the operating duty cycle (
D
), that the attempt here is to place
the maximum limiting duty-ratio (
D
MAX
) curve approximately mid-way these two limit curves.
Figure C2 shows an example curve taken from the design spreadsheet where the
D
RESET
limit is violated. Figure C3 shows an
example where the regulation limit,
D
ACTUAL
is violated.
Selection of Overvoltage onset level VOV
This determines the voltage at which frequency reduction commences. In EP-12 the default value was used.
VOV
= 380
(VOV, F135)
R
A
C
VS
R
B
Bias
Winding
TOPSwitch-GX
L-Pin
TOPSwitch-GX
SOURCE -Pin
From DC Bus
R
D
R
C
V
Z
6.8 V
PI-2888-061302
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