参数资料
| 型号： | FSGM300N |
| 厂商： | Fairchild Semiconductor |
| 文件页数： | 9/16页 |
| 文件大小： | 0K |
| 描述： | IC PWM/SENSEFET SMPS 8-DIP |
| 标准包装： | 50 |
| 系列： | FPS™ |
| 输出隔离： | 隔离 |
| 频率范围： | 61kHz ~ 73kHz |
| 输入电压： | 13 V ~ 22.5 V |
| 输出电压： | 650V |
| 功率（瓦特）： | 40W |
| 工作温度： | -40°C ~ 125°C |
| 封装/外壳： | 8-DIP（0.300"，7.62mm） |
| 供应商设备封装： | 8-DIP |
| 包装： | 管件 |
Functional Description
1. Startup: At startup, an internal high-voltage current
source supplies the internal bias and charges the
external capacitor (C Vcc ) connected to the V CC pin, as
illustrated in Figure 17. When V CC reaches 12V, the
FSGM300N begins switching and the internal high-
voltage current source is disabled. The FSGM300N
continues normal switching operation and the power is
supplied from the auxiliary transformer winding unless
V CC goes below the stop voltage of 7.7V.
Figure 17. Startup Block
2. Soft-Start : The FSGM300N has an internal soft-start
circuit that increases PWM comparator inverting input
voltage, together with the SenseFET current, slowly
after it starts. The typical soft-start time is 15ms. The
pulse width to the power switching device is
progressively increased to establish the correct working
3. Feedback Control : This device employs current-
mode control, as shown in Figure 18. An opto-coupler
(such as the FOD817) and shunt regulator (such as the
KA431) are typically used to implement the feedback
network. Comparing the feedback voltage with the
voltage across the R SENSE resistor makes it possible to
control the switching duty cycle. When the reference pin
voltage of the shunt regulator exceeds the internal
reference voltage of 2.5V, the opto-coupler LED current
increases, pulling down the feedback voltage and
reducing drain current. This typically occurs when the
input voltage is increased or the output load is decreased.
3.1 Pulse-by-Pulse Current Limit : Because current-
mode control is employed, the peak current through
the SenseFET is limited by the inverting input of PWM
comparator (V FB *), as shown in Figure 18. Assuming
that the 150 μ A current source flows only through the
internal resistor (3R + R =16k ? ), the cathode voltage
of diode D2 is about 2.4V. Since D1 is blocked when
the feedback voltage (V FB ) exceeds 2.4V, the
maximum voltage of the cathode of D2 is clamped at
this voltage. Therefore, the peak value of the current
through the SenseFET is limited.
3.2 Leading-Edge Blanking (LEB) : At the instant the
internal SenseFET is turned on, a high-current spike
usually occurs through the SenseFET, caused by
primary-side capacitance and secondary-side rectifier
reverse recovery. Excessive voltage across the R SENSE
resistor leads to incorrect feedback operation in the
current mode PWM control. To counter this effect, the
FSGM300N employs a leading-edge blanking (LEB)
circuit. This circuit inhibits the PWM comparator for
t LEB (400ns) after the SenseFET is turned on.
conditions for transformers, inductors, and capacitors.
The voltage on the output capacitors is progressively
increased to smoothly establish the required output
voltage. This helps prevent transformer saturation and
reduces stress on the secondary diode during startup.
Figure 18. Pulse Width Modulation Circuit
? 2009 Fairchild Semiconductor Corporation
FSGM300N ? Rev. 1.0.3
9
www.fairchildsemi.com
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