Datasheet FSD176MRT (ON Semiconductor) - 10

3. Feedback Control: This device employs currentmode 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 RSENSE 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 2.5 V, 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. 1. Startup: At startup, an internal high-voltage current
source supplies the internal bias and charges the
external capacitor (CVcc) connected to the VCC pin, as
illustrated in Figure 17. When VCC reaches 12 V, the
FSD176MRT begins switching and the internal highvoltage current source is disabled. The FSD176MRT
continues normal switching operation and the power is
supplied from the auxiliary transformer winding unless
VCC goes below the stop voltage of 7.5 V. 3.1 Pulse-by-Pulse Current Limit: Because currentmode control is employed, the peak current through
the SenseFET is limited by the inverting input of the
PWM comparator (VFB*), as shown in Figure 18.
Assuming that the 90 μA current source flows only
through the internal resistor (3R + R = 27 kΩ), the
cathode voltage of diode D2 is about 2.5 V. Since D1
is blocked when the feedback voltage (VFB) exceeds
2.5 V, 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.
Figure 17. Startup Block 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 RSENSE
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 tLEB (300 ns) after the SenseFET is
turned on. 2. Soft-Start: The internal soft-start circuit increases the
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 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. FSD176MRT — Green-Mode Fairchild Power Switch (FPS™) Functional Description Figure 18. Pulse Width Modulation Circuit
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FSD176MRT • Rev. 1.0.1 www.fairchildsemi.com
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