參數(shù)資料
型號(hào)： AN-31
英文描述： DPA-Switch Forward Converter Design Guide
中文描述： 政治部開(kāi)關(guān)正激變換器的設(shè)計(jì)指南
文件頁(yè)數(shù)： 15/20頁(yè)
文件大?。?/td> 245K
代理商： AN-31
AN-31
15
B
4/03
Operation at small duty ratios requires a larger capacitor to keep
the bias voltage above its minimum required value of 8 V. In a
trade-off with size, cost and efficiency, the best solution to a
requirement to operate with no load is to include a small preload
in parallel with the output capacitors. The amount of the load is
determined empirically to supplement the natural loading from
the other small-signal circuits that get their power from the
output.
Synchronous Rectification
The use of synchronous rectification can yield a substantial
increase in efficiency over passive Schottky rectifiers on the
output. For a 5 V output, an efficiency of 85% with Schottky
rectifiers would typically go to 90% or higher with synchronous
rectifiers. Synchronous rectification gives the benefit of greater
efficiency at lower output voltages as shown in Table 3.
DPA-Switch
has features that can simplify the design of
synchronous rectifier circuits that are in common use. Circuits
for synchronous rectification with
DPA-Switch
fall into three
categories of increasing complexity.
a) Winding Driven DC Coupled
b) Winding Driven AC Coupled
c) Actively Driven
The first two are shown in Figures 11 and 12. MOSFETs Q1 and
Q2 conduct at appropriate times to reduce the voltage drops
associated with the output rectifiers of a forward converter. Q2
performs the function of the forward rectifier. Q1 operates as
the catch rectifier with a parallel Schottky diode. The voltage
drop of each synchronous rectifier is dominated by the on-
resistance of the MOSFETs multiplied by the RMS load
current, rather than by the average current times the minimum
voltage of a Schottky barrier.
Winding Driven DC Coupled Synchronous Rectifier
The simplest way to drive synchronous rectifiers with
DPA-Switch
is shown in Figure 12 (a). The gate-to-source
voltage that turns on the MOSFETs is essentially the voltage at
the secondary winding of the transformer. The channel of the
MOSFET will conduct as long as the gate-to-source voltage
exceeds the threshold voltage.
The forward rectifier MOSFET Q2 turns on when the
DPA-Switch
turns on to apply the DC input voltage across the
primary winding. The direction of current in Q2 is from source
to drain. When the
DPA-Switch
turns off, the reset voltage on
the transformer forces a negative gate-to-source voltage on Q2
and a positive gate-to-source voltage on Q1. Schottky diode D3
conducts until the gate-to-source voltage on Q1 rises sufficiently
to exceed the threshold voltage.
Suitable MOSFETs for this application have threshold voltages
typically between 4 V and 5 V. The permissible maximum gate-
to-source voltage is usually 15 V to 20 V. These restrictions
limit the range of input voltage for converter operation.
The integrated line overvoltage feature of
DPA-Switch
simplifies
the design of winding driven synchronous rectifiers. In most
cases it eliminates the need for Zener diodes to protect the gates
of the MOSFETs from excessive voltage. Excess voltage will
not appear on the secondary of the transformer because the
DPA-Switch
will not operate when the input voltage is too high.
DC coupling of the gates in this configuration permits a mode
of operation that may not be desirable in some applications.
During shutdown, the voltage across the output inductor will go
to zero after its current decays to zero. The remaining output
voltage will then appear across Q1 and D3.
If the output voltage is high enough (above the gate threshold
of Q2) it will turn on Q2, allowing reverse current to flow
through L2 and the transformer secondary. The voltage on the
secondary winding will saturate the transformer, abruptly turning
off Q2 and generating a voltage spike on the gate of Q1. This
Q1
Q2
D3
R15
L2
PI-2989-081502
Q1
Q2
D3
D4
R15
R16
C17
L2
PI-3475-031303
(a)
(b)
Figure 12. Synchronous Rectification (a) Winding Driven DC
Coupled. (b) Winding Driven AC Coupled.
Efficiency Gain Over
Diode Rectification
+3%
+6%
+8%
5 V
3.3 V
2.5 V
Table 3. Efficiency Gain vs. Output Voltage for Synchronous
Rectification.
Output Voltage
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