参数资料
型号: FSD156MRBN
厂商: Fairchild Semiconductor
文件页数: 9/16页
文件大小: 0K
描述: IC FPS POWER SWITCH 650V 8-MDIP
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 ~ 23 V
输出电压: 650V
功率(瓦特): 40W
工作温度: -40°C ~ 125°C
封装/外壳: *
供应商设备封装: *
包装: *
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
FSD156MRBN begins switching and the internal high-
voltage current source is disabled. Normal switching
operation continues and the power is supplied from the
auxiliary transformer winding unless V CC goes below the
stop voltage of 7.5V.
Figure 17. Startup Block
2. Soft-Start : The internal soft-start circuit increases
PWM comparator inverting input voltage, together with
the SenseFET current, slowly after startup. The typical
soft-start time is 15ms. The pulse width to the power
switching device is progressively increased to establish
the correct working conditions for the transformers,
inductors, and capacitors. The voltage on the output
capacitors is progressively increased to smoothly
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 90 μ A current source flows only through the
internal resistor (3R + R =25k ? ), the cathode voltage
of diode D2 is about 2.8V. Since D1 is blocked when
the feedback voltage (V FB ) exceeds 2.8V, 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
leading-edge blanking (LEB) circuit inhibits the PWM
comparator for t LEB (300ns) after the SenseFET is
turned on.
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
? 2011 Fairchild Semiconductor Corporation
FSD156MRBN ? Rev. 1.0.0
9
www.fairchildsemi.com
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