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參數(shù)資料
| 型號： | AN-31 |
| 英文描述： | DPA-Switch Forward Converter Design Guide |
| 中文描述： | 政治部開關正激變換器的設計指南 |
| 文件頁數(shù)： | 7/20頁 |
| 文件大?。?/td> | 245K |
| 代理商： | AN-31 |
AN-31
7
B
4/03
if system constraints prevent good thermal design. Multiplication
of the R
by the square of the RMS current in the primary
gives a reasonable estimate of the power dissipation in the
DPA-Switch
. The
DPA-Switch
dissipates approximately 25%
of the total system loss in designs without synchronous rectifiers.
If power dissipation is a problem with the smallest device,
select the next larger device and program the current limit with
the X pin to 10% above the peak primary current. This is done
to limit overload power capability. Refer to the
DPA-Switch
data sheet to determine the value of the resistor on the X pin that
corresponds to the desired current limit.
Figure 3 illustrates how the efficiency is related to the selection
of the
DPA-Switch
. Devices with lower R
dissipate less
power where resistive voltage drop dominates the loss. Thus,
the efficiency is higher for larger devices at low input voltage.
At higher input voltages the RMS current in the
DPA-Switch
decreases and the loss from capacitance on the drain increases,
so the lower R
DS(ON)
has virtually no effect on efficiency.
Clamp Circuit
All applications must protect the
DPA-Switch
from excessive
drain voltage. Figure 1 shows a simple and effective solution.
A Zener diode from the drain to source provides a hard clamp.
The 30 W prototype example (Table 1), uses a 150 V Zener to
guarantee substantial margin from the breakdown voltage of
220 V. A small capacitor across the primary of the transformer
may be necessary in conjunction with the Zener clamp (see
Figure 4).
The designer should put a placeholder for this capacitor on the
initial prototype. In some designs there is sufficient stray
capacitance on the primary of the transformer to remove the
need for this clamp capacitor. Bench tests will determine
whether the capacitor is required to maintain safe drain-to-
source voltages. In normal steady-state operation, the capacitor
C
across the primary of the transformer absorbs energy from
leakage inductance to keep the drain-to-source voltage below
the Zener voltage. There is an optimum value for C
that
typically ranges between 10 pF and 100 pF for converters in the
range of 10 W to 40 W.
The value of C
depends on the leakage inductance and the
peak current. The proper value of capacitance will allow most
of the energy in the leakage inductance to be recovered during
the next switching cycle. Too little capacitance will cause the
Zener diode to conduct. Dissipation in the Zener will reduce
efficiency. Too much capacitance will also reduce efficiency
because it will increase turn-on losses in the
DPA-Switch
and
may also interfere with the reset of the transformer.
The Zener diode does not conduct during normal steady-state
operation, but it is required to limit the drain voltage during
start-up, transient loading and overload conditions.
At higher powers, the clamp capacitor value (C
), becomes a
limiting factor on the efficiency of the power supply. Different
techniques can be used for these higher power applications
PI-3474-032603
D
S
CONTROL
DPA-Switch
DC
Input
Voltage
D2
L1
D1
C1
V
1
V
DS
+
Figure 5. LC (Inductor Capacitor) Reset and Clamp.
Figure 4. Components of the Transformer Clamp and Reset Circuit.
PI-2875-120902
D
S
C
CONTROL
VR1
C
S
R
S
– DC INPUT
+ DC INPUT
DPA-Switch
U1
C
CP
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