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參數(shù)資料
| 型號： | AN-19 |
| 英文描述： | TOPSwitch Flyback Power Supply Efficiency |
| 中文描述： | TOPSwitch的反激式電源供應(yīng)器效率 |
| 文件頁數(shù)： | 7/20頁 |
| 文件大?。?/td> | 177K |
| 代理商： | AN-19 |
A
6/96
AN-19
7
1.3
Maximum Duty Cycle (DMAX)
I
1.2
1.1
0.9
0.8
0.7
0.6
0.5
0.4
0.45
0.5
0.55
0.6
1
P
Irms
Ipeak
ST204A
OPERATING POINT
Figure 11. Output Diode Peak Inverse Voltage at Maximum Input
Voltage vs. Maximum Duty Cycle
for ST204A.
Maximum Duty Cycle
For a flyback power supply, the choice of maximum duty cycle
at minimum input voltage (D
) determines in part the
apportioning of losses between the primary and secondary
components of the power supply. The maximum duty cycle
discussed here does not refer to the internal duty cycle limit of
TOPSwitch
, but to an operating point set externally to
TOPSwitch
. The power supply duty cycle for a given input and
output voltage is set by the ratio of secondary to primary turns
of the flyback transformer. A lower ratio of secondary to
primary transformer turns (N
S
/N
P
) results in a higher D
MAX
,
governed by the formula:
D
V
V
V
N
N
V
V
MAX
O
D
MIN
S
P
O
D
=
+
×
+
+
(1)
D
is the duty cycle for minimum input voltage, V
is the
output voltage, V
is the forward voltage drop of the output
rectifier, N
is the number of transformer secondary turns, and
N
is the number of primary turns. The maximum
TOPSwitch
duty cycle limit provides a limit point for the duty cycle. The
choice of secondary to primary turns ratio is not completely
arbitrary. In addition to setting the operating duty cycle of the
supply, the turns ratio also determines the magnitude of the
reflected secondary voltage across the primary winding of the
transformer during the
TOPSwitch
off time, according to the
equation:
V
N
N
V
V
OR
P
S
O
D
=
×
+
(
)
(2)
V
is the reflected value of the secondary voltage across the
primary of the transformer. During the
TOPSwitch
off time, the
drain voltage is equal to the sum of the primary DC bus voltage
and the reflected secondary voltage at the transformer primary,
Figure 10. Output Filter Capacitor RMS Ripple current vs.
Maximum Duty Cycle for ST204A.
130
P
120
110
90
80
70
60
0.4
0.45
Maximum Duty Cycle (DMAX)
0.5
0.55
0.6
100
P
ST204A
OPERATING POINT
Figure 8. Primary Peak, RMS Current vs. Maximum Duty Cycle for
ST204A.
R
2.6
2.2
2
1.8
1.6
0.4
0.45
0.5
0.55
0.6
2.4
P
ST204A
OPERATING POINT
Maximum Duty Cycle (DMAX)
I
8
7
5
4
3
2
0.4
0.45
0.5
0.55
0.6
6
ST204A
OPERATING POINT
P
Irms
Ipeak
Maximum Duty Cycle (DMAX)
Figure 9. Secondary Peak, RMS Current vs. Maximum Duty Cycle
for ST204A.
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|---|---|
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| AN-30 | TOPSwitch-GX Forward Design Methodology |
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