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)： 11/20頁(yè)
文件大?。?/td> 245K
代理商： AN-31
AN-31
11
B
4/03
maximum input voltage with maximum load) and at the extremes
of the specified ambient temperature, since important component
parameters (especially capacitor ESR) can change greatly with
temperature.
Stabilizing a high frequency forward DC-DC converter presents
some challenges due to the inherently high bandwidth of this
topology. Many DC-DC converter designs use cycle-by-cycle
current-mode control. The
DPA-Switch
uses classic voltage
mode control to allow operation at duty ratios greater than 50%
without the need for the stabilizing ramp (“slope compensation”)
required with current-mode control. The fundamental system
characteristics of the forward converter in continuous conduction
mode with voltage mode control call for a compensation circuit
with multiple poles and zeros to achieve the desired loop
response.
The crossover frequency for a control loop that uses
DPA-Switch
in a forward converter with an optocoupler should
be limited to 10 kHz or less at maximum input voltage and room
temperature. The
DPA-Switch
has one internal pole at
approximately 30 kHz to filter switching noise. Other poles at
higher frequencies contribute additional phase shift at 30 kHz.
The optocoupler has two poles at approximately 100 kHz. The
phase shift from these poles, combined with the phase shift
introduced by the LC filter at the output of the converter, is
difficult to compensate above 10 kHz.
Feedback Design
Stability is an important consideration for a switching power
supply. Three parameters that describe the characteristics of the
control loop are crossover frequency, phase margin and gain
margin. The crossover frequency is the frequency where the
magnitude of the loop gain passes through 0 dB. It is a measure
of the system’s bandwidth.
The phase margin is specified at the crossover frequency. It is
the difference between the phase of the loop gain and 180
degrees. A stringent specification will call for a phase margin of
at least 60 degrees under worst case conditions. In no case
should the phase margin be less than 45 degrees. This means the
phase would have to decrease by that amount for the system to
become unstable. Phase margin is also related to the dynamic
characteristics of the system. A low phase margin suggests an
oscillatory response to a load step or other disturbance.
It is also important that the loop gain decrease in magnitude
beyond the crossover frequency. This requirement is generally
specified as gain margin. Gain margin is the difference between
0 dB and the magnitude of the loop gain at the frequency where
the phase is 180 degrees. An acceptable gain margin is greater
than 10 dB. This means the magnitude would have to increase
by that amount for the system to become unstable. Loop gain
should be measured at worst case conditions (generally
U1
U2
D
S
C
OUTPUT
OUTPUT
RETURN
+
–
R6
L2
R12
R10
C10
R11
C14
R9
R4
C5
U3
TL431
C6
C11
C16
PI-2876-112102
CONTROL
DPA-Switch
Input
Return
Figure 9. Essential Components of the Feedback Circuit. The Schematic Does Not Show ESR of the Output Capacitors (Component
Designators are the Same as in the EP-21 Prototype).
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