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參數資料
| 型號： | 935269484115 |
| 廠商： | NXP SEMICONDUCTORS |
| 元件分類： | 穩壓器 |
| 英文描述： | SWITCHING CONTROLLER, 115 kHz SWITCHING FREQ-MAX, PDSO5 |
| 封裝： | 1.50 MM, PLASTIC, MO-178, SOT-25, SOT-23, SOP-5 |
| 文件頁數： | 4/21頁 |
| 文件大小： | 298K |
| 代理商： | 935269484115 |
Philips Semiconductors
Product data
SA57251-XX
CMOS switching regulator (PWM controlled)
2001 Aug 01
12
Flyback converter
The SA57251-XX can also be used to create a flyback converter,
also known as an isolated boost converter. The advantage of a
flyback converter is that the input voltage can go higher or lower
than the output voltage without affecting the operation of the
converter. The only restrictions are the breakdown voltage of the
NPN transistor and the feedback (VFB) pins.
One transformer can accommodate a variety of output voltages in
different applications, because the circuit will change the on and
off–times to provide the desired output voltage.
The output voltage of the flyback can be changed by using a
SA57251-XX with the desired output voltage, with no other changes
to the circuit.
Selecting the components
It is best to operate the transformer in the continuous-mode where
the highest expected peak primary current is below the maximum
current rating of the NPN transistor.
Begin with a peak current equal to or less than the maximum current
rating of the NPN transistor. A reasonable value of the primary
inductance can be found in Equation (9).
L
pri t 5VIN(min)
Ton
I
peak
Eqn. (9)
Where:
Ipeak is the current rating of the NPN transistor.
Ton is the maximum expected on-time of the switch (≈10 s).
VIN(min) is the lowest expected input voltage (V).
Then select an off-the-shelf transformer such as the Coiltronics
CTX100–1P, a 1:1 turns ratio transformer that has a primary
inductance of 100
H. It does not reach saturation until the primary
current reaches 440 mA, which is above the expected peak current
of the flyback converter. The 1:1 turns ratio should work for output
voltages from 0.8 to 2 times the highest input voltage, and produce
the output voltage set by the SA57251-XX. The only other restriction
is that the input voltage plus the output voltage must be less than
the breakdown voltage of the NPN transistor.
Use Equation (8) to determine the minimum value for the input
capacitor. A 0.1 V drop is desired across this capacitor.
C
IN u
(0.3
A)(10
ms)
0.1
V
+ 30mF
A 47
F at 6 V tantalum capacitor would be suitable.
For the design example, the output voltage will be +3.3 V with a
maximum output current of 50 mA. The input voltage can vary
between +1.8 V and 4.0 V. The design can be seen in Figure 23,
and the expected waveforms can be seen in Figure 24.
SL01498
SA57251-XX
VIN
CIN
ON/OFF
GND
FB
COUT
VOUT
COILTRONICS
CTX100-1P
DRIVE
Figure 23. Flyback converter circuit.
SL01468
DIODE
CURRENT
(A)
SWITCH
CURRENT
(A)
SWITCH
VOLTAGE
(V)
SECONDARY
VOLTAGE
(V)
VIN + VOUT
(1:1 TRANSFORMER)
+VOUT
GROUND (0 V)
–VIN
ISW(peak)
Idiode(peak)
VIN
Ipeak = Idiode
(1:1 TRANSFORMER)
VIN
Figure 24. Flyback converter waveforms.
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