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参数资料
| 型号： | NCP1280DR2 |
| 厂商： | ON Semiconductor |
| 文件页数： | 13/17页 |
| 文件大小： | 0K |
| 描述： | IC CTRLR PWM PROG OVP HV 16SOIC |
| 标准包装： | 2,500 |
| 输出隔离： | 隔离 |
| 频率范围： | 140kHz ~ 320kHz |
| 输入电压： | 7 V ~ 25 V |
| 工作温度： | -40°C ~ 150°C |
| 封装/外壳： | 16-SOIC（0.154"，3.90mm 宽） |
| 供应商设备封装： | 16-SOIC N |
| 包装： | 带卷 (TR) |
NCP1280
SOFT ? START
V AUX(on)
V AUX
V AUX(off)
0V
2V
0V
0V
UV/OV Voltage
Soft ? Start Voltage
OUT2
0V
OUT1
0V
Figure 31. Soft ? Start Timing Diagram (Using Auxiliary Winding)
T + 1 +
+ 5.0 m s
IFF +
IFF +
+ 55.2 m A
+
RFF + in * 12.0 k W +
* 12.0 k W [ 1.82 M W
Feedforward Ramp Generator
The NCP1280 incorporates line feedforward (FF) to
compensate for changes in line voltage. A FF Ramp
proportional to V in is generated and compared to V EA . If the
line voltage changes, the FF Ramp slope changes
accordingly. The duty cycle will be adjusted immediately
instead of waiting for the line voltage change to propagate
around the system and be reflected back on V EA .
A resistor between V in and the FF pin (R FF ) sets the
feedforward current (I FF ). The FF Ramp is generated by
charging an internal 10 pF capacitor (C FF ) with a constant
current proportional to I FF . The FF Ramp is finished
(capacitor is discharged) once the Oscillator Ramp reaches
2.0 V. Please refer to Figure 2 for a functional drawing of the
Feedforward Ramp generator.
I FF is usually a few hundred microamps, depending on the
operating frequency and the required duty cycle. If the
operating frequency and maximum duty cycle are known,
I FF is calculated using the equation below:
CFF VDC(inv) 125 k W
6.7 k W ton(max)
where V DC(inv) is the voltage on the inverting input of the
Max DC Comparator and t on(max) is the maximum ON time.
Figure 18 shows the relationship between I FF and DC MAX .
For example, if a system is designed to operate at 200 kHz,
with a 60% maximum duty cycle at 100 V, the DC MAX pin
can be grounded and I FF is calculated as follows:
1
f 200 kHz
ton(max) + DCMAX T + 0.6 5.0 m s + 3.0 m s
CFF VDC(inv) 125 k W
6.7 k W ton(max)
10 pF 0.888 V 125 k W
6.7 k W 3.0 m s
For a minimum line voltage of 100 V, the required
feedforward resistor is calculated using the equation below:
V 100 V
IFF 55.2 m A
From the above calculations it can be observed that I FF is
controlled predominantly by the value of R FF , as the
resistance seen into the FF pin is only 12 k W . If a tight
maximum duty cycle control overtemperature is required,
R FF should have a low thermal coefficient.
http://onsemi.com
13
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