參數(shù)資料
型號(hào)： LM3101
廠商： National Semiconductor Corporation
英文描述： Secondary-Side PWM Controller
中文描述： 二次側(cè)PWM控制器
文件頁(yè)數(shù)： 15/20頁(yè)
文件大?。?/td> 319K
代理商： LM3101
Typical Applications
(Continued)
TL/H/11436–16
Output Voltage, 1V/div.
Horizontal Time Base: 500
m
s/div.
FIGURE 10. Output Voltage Start-up
At the end of the start-up sequence, the circuit is in steady-
state or normal PWM operation.
Design Procedure
For the Offline Voltage Mode Flyback Regulator (Figure 7),
the specifications for the power transformer, MOSFET
switch, the switch snubber, and the output rectifier can be
calculated based on the system specifications:
System specifications:
V
O
e
5 VDC
V
I
Range
e
90 VAC–132 VAC
I
O
Range
e
0.5A–10A
Efficiency (
h
)
&
80%
F
O
e
500 kHz.
TRANSFORMER SPECIFICATIONS
Manipulating the transfer function of a flyback regulator re-
sults in a calculation for the turns ratio of the power trans-
former, involving the minimum input voltage, the output volt-
age, and the maximum duty cycle (D):
V
O
a
V
F
e
(V
IN(MIN)
–V
SW(ON)
)
#
(N
S
/N
P
)
#
(D
(MAX)
/(1–D
(MAX)
))
v
N
S
/N
P
e
[
(V
O
a
V
F
)/(V
IN(MIN)
–V
SW(ON)
)
]
#
((1–D
(MAX)
)/D
(MAX)
)
Assume that the diode forward voltage (V
F
) is about 0.7V
and the drain-to-source voltage when the switch is on
(V
SW(ON)
) is approximately 0.9V. Selecting a 28% maximum
duty cycle results in a turns ratio of:
N
S
/N
P
e
(5.7V/126.1V)
#
(1–0.28)/0.28
e
0.12
(N
P
/N
S
e
8.5/1).
Assuming an efficiency (
h
) of 80%, the average input cur-
rent (at the maximum load current and for the entire period)
is:
I
IN
e
(V
O
) (I
O
)/ (V
IN(MIN)
#
h
)
e
(50W)/(127V
#
0.80)
0.49A.
The average current when the switch is on is the average
current over the entire period divided by the duty cycle:
I
IN(TON)
e
I
IN
/D
e
(0.49A)/(0.28)
e
1.77A.
Selecting the primary inductance ripple current (
D
I
P
) to be a
certain percentage of I
IN(TON)
, and combining that with the
duty cycle, input voltage, and operating frequency, gives the
primary inductance by the equation:
L
P
e
(V
IN(MIN)
–V
SW(ON)
)
#
D
(MAX)
/ (
D
I
P
#
F
O
)
Assuming the percentage to be 46% in the example, then:
L
P
e
126.1V
#
0.28/(0.81A
#
500 kHz)
j
87
m
H.
MOSFET PARAMETERS
The peak current through the primary inductance and the
Power MOSFET is the average current when the switch is
on plus one-half the primary inductance ripple current:
I
PRI(PK)
e
I
IN(TON)
a
(
D
I
P
/2)
e
1.77A
a
(0.81A/2)
e
2.18A
Assuming ideal conditions, the maximum voltage at the
drain of the Power MOSFET when the switch is off is:
V
SW(OFF)
e
(V
O
a
V
F
) (N
P
/N
S
)
a
V
IN(MAX)
e
(5.7V) (8.5)
a
185V
e
233V
x
250V.
However, leakage inductance exists in the transformer,
causing a voltage spike immediately after the switch turns
off. This voltage spike will add to the rest of the drain volt-
ages, making V
SW(OFF)
even greater. With a leakage induc-
tance that is 2% of the transformer primary inductance and
selecting a switch which has a fall time of 2% the total off-
time, the added voltage will be:
V
LL
e
2%
#
L
P
#
I
PRI(PK)
#
F
O
/
[
2%
#
(1–D
(MAX)
)
]
.
The maximum duty cycle of 28% is used for worst case
purposes. Thus, the leakage inductance voltage spike is:
V
LL
e
0.02
#
87
m
H
#
2.18A
#
500 kHz/
[
0.02
#
(1–0.28)
]
e
130V
x
150V.
This means the actual peak drain voltage is approximately
400V. When choosing the Power MOSFET, add some mar-
gin to this number. A 500V MOSFET was used in this appli-
cation.
SNUBBER DESIGN
A ‘‘snubber’’ circuit, consisting of a 1N4937 fast recovery
diode and a parallel RC network, is inserted around the
transformer primary to clamp the voltage spike. This is to
reduce the switch voltage stress when it is off. The ‘‘snub-
ber’’ components are calculated in the following manner:
C
SN
t
0.02
#
L
P
#
I
P(PK)2
/(V
MAX2
b
V
SN2
)
e
0.02
#
87
m
H
#
(2.18A)
2
/
[
(255V)
2
b
(250V)
2
]
&
3.3 nF
and
R
SN
s
[
(V
MAX
a
V
SN
b
V
IN
)/2
]
2
#
[
100/(F
O
#
L
P
#
I
P(PK)2
)
]
e
[
(255V
a
250V
b
185V)/2
]
2
#
[
100/(500 kHz
#
87
m
H
#
(2.18A)
2
)
]
&
12 k
X
.
In the Offline Flyback Regulator application, a 0.01
m
F ca-
pacitor and a 10 k
X
resistor are used as the snubber com-
ponents. V
MAX
is the selected maximum voltage at the drain
of the MOSFET. Usually the RC values are selected so that
V
MAX
is 5V to 10V higher than V
SN
. The power dissipation
of the resistor is:
P
e
[
(V
MAX
a
V
SN
b
V
IN
)/2
]
2
/R
e
[
(255V
a
250V
b
185V)/2
]
2
/10 k
X
e
2.56W.
To add some margin, a 4W resistor is chosen.
The fast recovery diode must have a reverse voltage rating
greater than V
MAX
. The 1N4937 has a 600V rating.
OUTPUT DIODE PARAMETERS
The peak secondary current can be calculated using the
peak primary current and the turns ratio (this equation is for
single output flyback regulators):
I
SEC(PK)
e
I
PRI(PK)
#
(N
/N
S
)
e
2.18A
#
8.5
e
18.43A
x
20A.
15
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