FSGM0465R Fairchild Semiconductor, FSGM0465R Datasheet - Page 9

• Manufacturer Part Number: FSGM0465R
• Description: The FSGM0465R is an integrated Pulse Width Modulation (PWM) controller and SenseFET specifically designed for offline Switch-Mode Power Supplies (SMPS) with minimal external components
• Manufacturer: Fairchild Semiconductor

© 2009 Fairchild Semiconductor Corporation
FSGM0465R • Rev. 1.0.2
Functional Description
1. Startup: At startup, an internal high-voltage current
source supplies the internal bias and charges the
external capacitor (C
illustrated in Figure 17. When V
FSGM0465R begins switching and the internal high-
voltage current source is disabled. The FSGM0465R
continues normal switching operation and the power is
supplied from the auxiliary transformer winding unless
V
2. Soft-Start: The FSGM0465R 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
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.
CC
goes below the stop voltage of 7.5V.
Figure 17. Startup Block
Vcc
) connected to the V
CC
Figure 18. Pulse Width Modulation Circuit
reaches 12V, the
CC
pin, as
9
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
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
that the 210μA current source flows only through the
internal resistor (3R + R =11.6kΩ), the cathode
voltage of diode D2 is about 2.4V. Since D1 is
blocked when the feedback voltage (V
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
in the current mode PWM control. To counter this
effect, the FSGM0465R employs a leading-edge
blanking (LEB) circuit. This circuit inhibits the PWM
comparator for t
turned on.
SENSE
resistor leads to incorrect feedback operation
FB
*), as shown in Figure 18. Assuming
LEB
SENSE
(300ns) after the SenseFET is
resistor makes it possible to
FB
www.fairchildsemi.com
) exceeds