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參數(shù)資料
| 型號： | MIC2171 |
| 廠商： | Micrel Semiconductor,Inc. |
| 元件分類： | 基準電壓源/電流源 |
| 英文描述： | Isolated Flyback Switching Regulator with 9V Output |
| 中文描述： | 隔離反激式開關穩(wěn)壓9V輸出 |
| 文件頁數(shù)： | 8/10頁 |
| 文件大小： | 80K |
| 代理商： | MIC2171 |
MIC2171
Micrel
4-10
1997
For our practical example:
L1
4.178
×
2
.662
1 10
5
×
3.0
≥
×
(
)
0
2
L1
≥
12.4
μ
H (use 15
μ
H)
Equation (3) solves for L1’s maximum current value.
(3)
I
L1(peak)
=
V
IN
T
ON
L1
Where:
T
ON
=
δ
/ f
SW
= 6.62
×
10
-6
sec
I
4.178
6.62 10
15 10
L1(peak)
-6
-6
=
×
I
L1(peak)
= 1.84A
Use a 15
μ
H inductor with a peak current rating of at least 2A.
Flyback Conversion
Flyback converter topology may be used in low power appli-
cations where voltage isolation is required or whenever the
input voltage can be less than or greater than the output
voltage. As with the step-up converter the inductor (trans-
former primary) current can be continuous or discontinuous.
Discontinuous operation is recommended.
Figure 2 shows a practical flyback converter design using the
MIC2171.
Switch Operation
During Q1’s on time (Q1 is the internal NPN transistor—see
block diagrams), energy is stored in T1’s primary inductance.
During Q1’s off time, stored energy is partially discharged into
C4 (output filter capacitor). Careful selection of a low ESR
capacitor for C4 may provide satisfactory output ripple volt-
age making additional filter stages unnecessary.
C1 (input capacitor) may be reduced or eliminated if the
MIC2171 is located near a low impedance voltage source.
Output Diode
The output diode allows T1 to store energy in its primary
inductance (D2 nonconducting) and release energy into C4
(D2 conducting). The low forward voltage drop of a Schottky
diode minimizes power loss in D2.
Frequency Compensation
A simple frequency compensation network consisting of R3
and C2 prevents output oscillations.
High impedance output stages (transconductance type) in
the MIC2171 often permit simplified loop-stability solutions to
be connected to circuit ground, although a more conventional
technique of connecting the components from the error
amplifier output to its inverting input is also possible.
Voltage Clipper
Care must be taken to minimize T1’s leakage inductance,
otherwise it may be necessary to incorporate the voltage
clipper consisting of D1, R4, and C3 to avoid second break-
down (failure) of the MIC2171’s internal power switch.
Discontinuous Mode Design
When designing a discontinuous flyback converter, first de-
termine whether the device can safely handle the peak
primary current demand placed on it by the output power.
Equation (8) finds the maximum duty cycle required for a
given input voltage and output power. If the duty cycle is
greater than 0.8, discontinuous operation cannot be used.
(8)
δ
2 P
I
V
– V
OUT
CL
IN(min)
SW
≥
)
For a practical example let: (see Figure 2)
P
OUT
= 5.0V
×
0.5A = 2.5W
V
IN
= 4.0V to 6.0V
I
CL
= 2.5A when
δ
< 50%
1.67 (2 –
δ
) when
δ
≥
50%
Then:
V
= V
– I
R
IN min
V
IN(min)
= 4 – 0.78V
V
IN(min)
= 3.22V
δ
≥
0.74 (74%), less than 0.8 so discontinous is
permitted.
A few iterations of equation (8) may be required if the duty
cycle is found to be greater than 50%.
CL
SW
)
×
(
)
Calculate the maximum transformer turns ratio
a,
or
N
PRI
/N
SEC
, that will guarantee safe operation of the MIC2171
power switch.
(9)
a
≤
V
CE
F
CE
– V
IN(max)
V
SEC
Where:
a
= transformer maximum turns ratio
V
CE
= power switch collector to emitter
maximum voltage
F
CE
= safety derating factor (0.8 for most
commercial and industrial applications)
V
IN(max)
= maximum input voltage
V
SEC
= transformer secondary voltage (V
OUT
+ V
F
)
For the practical example:
V
CE
= 65V max. for the MIC2171
F
CE
= 0.8
V
SEC
= 5.6V
Then:
a
≤
65
×
0.8 – 6.0
5.6
a
≤
8.2 (N
PRI
/N
SEC
)
Next, calculate the maximum primary inductance required to
store the needed output energy with a power switch duty
cycle of 55%.
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