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參數(shù)資料
| 型號： | NCP1351 |
| 廠商： | ON SEMICONDUCTOR |
| 英文描述： | Variable Off Time PWM Controller |
| 中文描述： | 變關(guān)閉時(shí)間PWM控制器 |
| 文件頁數(shù)： | 9/19頁 |
| 文件大小： | 222K |
| 代理商： | NCP1351 |
NCP1351
http://onsemi.com
9
Figure 6. The Voltage on the Current Sense Pin
Figure 7. The Voltage Across the Sense
Resistor
Current Sense Pin
Current Sense Resistor
Below are a few recommendations concerning the wiring
and the PCB layout:
A small 22 pF capacitor can be placed between the CS
pin and the controller ground. Place it as close as
possible to the controller.
Do not place the offset resistor in the vicinity of the
sense element, but put it close to the controller as well.
Regulation by frequency
The power a flyback converter can deliver relates to the
energy stored in the primary inductance
L
p
and obeys
the following formulae:
1
Pout_DCM
(eq. 5)
Pout_CCM
12LP(Ipeak2
Ivalley2)FSW
(eq. 6)
Where:
η
(eta) is the converter efficiency
I
peak
is the peak inductor current reached at the on time
termination
I
valley
represents the current at the end of the off time. It
equals zero in DCM.
F
SW
is the operating frequency.
Thus, to control the delivered power, we can either play on
the peak current setpoint (classical peak current mode
control) or adjust the switching frequency by keeping the
peak current constant. We have chosen the second scheme
in this NCP1351 for simplicity and ease of implementation.
Thus, once the peak current has been selected, the feedback
loop automatically reacts to satisfy Equations 5 and 6. The
external capacitor that you connect between pin 2 and
ground (again, place it close to the controller pins) sets the
maximum frequency you authorize the converter to operate
up to. Normalized values for this timing capacitor are
270 pF (65 kHz) and 180 pF (100 kHz). Of course, different
combinations can be tried to design at higher or lower
frequencies. Please note that changing the capacitor value
does not affect the operating frequency at nominal line and
load conditions. Again, the operating frequency is selected
by the feedback loop to cope with Equations 5 and 6
definitions.
The feedback current controls the frequency by changing
the timing capacitor end of charge voltage, as illustrated by
Figure 8.
Figure 8. The Current Injected into the Feedback Loop Adjusts the Switching Frequency
I
Ct
= 10 A
Controlled by the
FB Current
V
Ct
Maximum Frequency
Minimum Frequency
P
out
Decreases
P
out
Increases
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| NCP1351BDR2G | 功能描述:電流型 PWM 控制器 OSPI PWM CONTROLLER RoHS:否 制造商:Texas Instruments 開關(guān)頻率:27 KHz 上升時(shí)間: 下降時(shí)間: 工作電源電壓:6 V to 15 V 工作電源電流:1.5 mA 輸出端數(shù)量:1 最大工作溫度:+ 105 C 安裝風(fēng)格:SMD/SMT 封裝 / 箱體:TSSOP-14 |
| NCP1351BPG | 功能描述:電流型 PWM 控制器 VRIABLE OFF TM CNTRL RoHS:否 制造商:Texas Instruments 開關(guān)頻率:27 KHz 上升時(shí)間: 下降時(shí)間: 工作電源電壓:6 V to 15 V 工作電源電流:1.5 mA 輸出端數(shù)量:1 最大工作溫度:+ 105 C 安裝風(fēng)格:SMD/SMT 封裝 / 箱體:TSSOP-14 |
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