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參數(shù)資料
| 型號： | 83F5958 |
| 英文描述： | IC-1.25A SWITCHING REGULATOR |
| 中文描述： | 集成電路1.25A的開關(guān)穩(wěn)壓器 |
| 文件頁數(shù)： | 14/16頁 |
| 文件大?。?/td> | 129K |
| 代理商： | 83F5958 |
MIC2172/3172
Micrel
4-26
1997
Finally, recalculate the transformer turns ratio to insure that
it is less than the value earlier found in equation (9).
(12)
a
≤
L
PRI
L
SEC
Then:
a
≤
1.8
×
10
-5
2.54
×
10
-5
a
≤
0.84 Use 0.8 (same as 1:1.25).
This ratio is less than the ratio calculated in equation (9).
When specifying the transformer it is necessary to know the
primary peak current which must be withstood without satu-
rating the transformer core.
(13)
I
PEAK(pri)
=
V
IN(min)
T
ON
L
PRI
So:
I
PEAK(pri)
= 4.0
×
5.5
×
10
-6
18
μ
H
I
PEAK(pri)
= 1.22A
Now find the minimum reverse voltage requirement for the
output rectifier. This rectifier must have an average current
rating greater than the maximum output current of 0.25A.
(14)
V
BR
≥
V
IN(max)
+ V
OUT
a
F
BR
a
)
Where:
V
BR
= output rectifier maximum peak
reverse voltage rating
a
= transformer turns ratio (0.8)
F
BR
= reverse voltage safety derating factor (0.8)
Then:
V
BR
≥
6.0 + 5.0
×
0.8
0.8
×
0.8
V
BR
≥
15.625V
A 1N5817 will safely handle voltage and current require-
ments in this example.
)
Forward Converters
Micrel’s MIC2172/3172 can be used in several circuit con-
figurations to generate an output voltage which is less than
the input voltage (buck or step-down topology). Figure 13
shows the MIC3172 in a voltage step-down application.
Because of the internal architecture of these devices, more
external components are required to implement a step-down
regulator than with other devices offered by Micrel (refer to
the LM257x or LM457x family of buck switchers). However,
for step-down conversion requiring a transformer (forward),
the MIC2172/3172 is a good choice.
A 12V to 5V step-down converter using transformer isolation
(forward) is shown in figure 14. Unlike the isolated flyback
converter which stores energy in the primary inductance
during the controller’s on-time and releases it to the load
during the off-time, the forward converter transfers energy to
the output during the on-time, using the off-time to reset the
transformer core. In the application shown, the transformer
core is reset by the tertiary winding discharging T1’s peak
magnetizing current through D2.
For most forward converters the duty cycle is limited to 50%,
allowing the transformer flux to reset with only two times the
input voltage appearing across the power switch. Although
during normal operation this circuit’s duty cycle is well below
C2
2.2μF
C1*
100μF
D2
R4
10
R2
1.2k
C4
1μF
C5
330μF
L1
100μH
R3
3.7k
D1
1N4148
V
IN
D3
1N4148
* Locate near MIC2172/3172 when supply leads > 2"
R3/R2 sets output voltage
5V, 0.1A to 1A
(I
LOAD
> 100mA)
MIC3172
V
IN
V
SW
FB
R3
470
C3
1μF
EN
GND
P1 P2 S
COMP
Figure 13. Step-Down or Buck Converter
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