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參數(shù)資料
| 型號(hào)： | AN-31 |
| 英文描述： | DPA-Switch Forward Converter Design Guide |
| 中文描述： | 政治部開關(guān)正激變換器的設(shè)計(jì)指南 |
| 文件頁數(shù)： | 14/20頁 |
| 文件大小： | 245K |
| 代理商： | AN-31 |
AN-31
14
B
4/03
on the control loop is minor, appearing at P6, well beyond the
0 dB crossover frequency.
The zero at Z2 provides partial cancellation of the pair of poles
P3, P4 that originate from the output inductor and output
capacitors of the forward converter. The network of C16, R6
and R12 gives additional cancellation with a zero at Z3. The
ESR of the output capacitors gives a final zero at Z4. The
internal high frequency filter of the
DPA-Switch
provides the
pole at P5.
The magnitude of the gain at frequencies greater than Z1 is
related directly to the current transfer ratio (CTR) of the
optocoupler. Therefore, the CTR must be controlled to maintain
a stable and well-behaved system. Designers should choose an
optocoupler that has a CTR in the range of 100% to 200% at the
maximum CONTROL pin current of 12 mA. The phototransistor
of the optocoupler must also have a breakdown voltage greater
than the maximum bias voltage.
Figure 10 shows that this example has a desirable phase margin
of 60 degrees and a comfortable gain margin of 20 dB. Sufficient
margin is required in the design of the feedback loop to allow
for tolerances in the CTR of the optocoupler, changes in ESR of
the output capacitor, and the change in gain with operating
voltage. The ESR can change significantly with temperature.
This should be a primary consideration in the selection of output
capacitors. The design must also allow for tolerance variations
in all other components.
Operation at No Load
Those who design or specify DC-DC converters should pay
particular attention to requirements for minimum load. The
control characteristics are different for operation in the
continuous conduction mode (moderate to heavy loads) and
discontinuous conduction mode (light loads). The boundary
between the two modes occurs at the load where K
Ι
= 2
(without synchronous rectification).
The two modes have different control characteristics. The
converter in discontinuous conduction mode will usually have
a slower response to transients and higher ripple voltage at the
output than in continuous conduction mode. In extreme cases,
a converter that is well-behaved in continuous conduction mode
may actually become unstable at light load or with no load
unless correctly designed. Many commercial DC-DC converter
modules specify a large minimum load to prevent operation in
discontinuous conduction mode.
A converter that operates deeply in discontinuous conduction
mode requires a very small duty ratio. Operation at very light
loads is not a problem for
DPA-Switch
because it automatically
reduces the effective switching frequency by skipping cycles to
give duty ratios less than about 5%.
U1
DPA425R
D1
BAV
19WS
D2
C5
220 nF
VR1
SMBJ
150
C6
68
μ
F
10 V
C7
1 nF
1.5 kV
R14
10
T1
R3
18.2 k
1%
R1
619 k
1%
R4
1.0
C10
100
μ
F
10 V
C11
100
μ
F
10 V
C12
1
F
10 V
36-75 VDC
L1
1
H
2.5 A
L2
C1, C2 & C3
1
F
100 V
D
L
S
X
F
C
CONTROL
U2
PC357N1T
PI-3472-040903
DPA-Switch
5 V, 6 A
RTN
Q2
Si4888
DY
R15
10
R17
10
R16
10 k
D4
BAV19WS
Q1
Si4888
DY
C4
4.7
μ
F
20 V
U2
D3
BAV19WS
U3
LM431AIM3
R9
220
R11
10.0 k
1%
R10
10.0 k
1%
R12
5.1
R7
10 k
C14
1
μ
F
C13
10
μ
F
10 V
C16
100 nF
R6
150
V
+
V
IN
C17
3300 pF
Figure 11. Example of DPA-Switch in a Single-Ended DC-DC Forward Converter with Synchronous Rectification.
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