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參數(shù)資料
| 型號： | AN-16 |
| 英文描述： | TOPSwitch Flyback Design Methodology |
| 中文描述： | TOPSwitch的反激式設計方法 |
| 文件頁數(shù)： | 14/32頁 |
| 文件大?。?/td> | 247K |
| 代理商： | AN-16 |
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AN-16
A
6/96
14
clamp voltage should not be used, because part of the stored
energy in the core would be delivered to the Zener, dramatically
increasing Zener dissipation.
The nominal clamp Zener voltage V
is usually specified at
low current values and at room temperature. High voltage
Zeners have a strong positive temperature coefficient and are
quite resistive. Consequently, the clamp voltage at high current
and high temperature V
can be much higher. Experimental
data has shown that the V
CLM
can be as high as 40% above the
specified V
CLO
V
V
CLM
CLO
=
×
1 4
This needs to be taken into consideration when choosing a
clamp Zener. In addition, it is important to allow an additional
20V for the spike due to the forward recovery time of the
blocking diode in series with the clamp Zener. With all those
factors considered, the maximum voltage that the
TOPSwitch
drain may experience is:
V
V
V
V
DRAIN
MAX
OR
=
+
×
+
1 4 1 5
.
)
20
To minimize power supply cost, it is important to maximize the
V
consistent with the
TOPSwitch
breakdown voltage rating
after taking into account all of the above effects. As will be seen
A typical flyback circuit using
TOPSwitch
is shown in Figure
1. When the
TOPSwitch
is off and the secondary is conducting,
the voltage on the secondary is reflected to the primary side of
the transformer by the turns ratio. This reflected voltage V
adds to the input DC voltage at the
TOPSwitch
drain node.
Worst case voltage at the drain occurs at high line when the DC
input voltage is at its maximum value. The maximum DC input
voltage can be calculated as:
V
V
MAX
ACMAX
=
×
2
In addition to V
+V
the drain also sees a large voltage spike
at turn off that is caused by the energy stored in the leakage
inductance of the primary winding (see Figures 8 and 9). To
keep this voltage spike from exceeding the rated minimum
drain breakdown voltage BV
, a clamp circuit is needed
across the primary winding. A Zener clamp as shown in
Figure 1 is highly recommended over the usual RC clamp as it
is much more effective in clamping the leakage energy during
start up transients. The nominal value of Zener clamp voltage
V
needs to be 50% (determined empirically) greater than the
reflected voltage so that the Zener clamps only the leakage
energy and does not impede the switch-over of current from the
primary to the secondary. Experimental measurements show
that this voltage margin is needed for the secondary current to
be quickly established through the leakage inductance. Lower
t
C
P
O
= Output Power
f
L
= Line Frequency
(50 or 60Hz)
t
C
= Conduction Angle
Use 3ms if unknown
η
= Efficiency - Assume
0.8 if unknown
V
V
ACMIN
MIN
×
2
V
V
P
f
t
C
MIN
ACMIN
O
L
C
IN
=
×
×
×
×
×
(
)
(
(
)
)
2
2
1
2
2
η
PI-1854-050696
V+
Figure 7. Input Voltage Waveform.
相關PDF資料 |
PDF描述 |
|---|---|
| AN-17 | Flyback Transformer Design for TOPSwitch Power Supplies |
| AN-18 | TOPSwitch Flyback Transformer Construction Guide |
| AN-19 | TOPSwitch Flyback Power Supply Efficiency |
| AN-20 | Transient Suppression Techniques for TOPSwitch Power Supplies |
| AN-22 | OBSOLETE when inventory is depleted. 10% tolerance no l |
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