FAN6791 Fairchild Semiconductor, FAN6791 Datasheet - Page 15

• Manufacturer Part Number: FAN6791
• Description: The highly integrated FAN6791/3 dual PWM combination controller provides several features to enhance the performance of converters
• Manufacturer: Fairchild Semiconductor

© 2008 Fairchild Semiconductor Corporation
FAN6791 / FAN6793 • Rev. 1.0.3
Interleave Switching
The
synchronize the stand-by PWM / forward PWM stages.
This reduces switching noise and spreads the EMI
emissions. Figure 26 shows that an off-time t
inserted in between the turn-off of the stand-by gate
drives and the turn-on of the forward PWM.
Slope Compensation
The stand-by PWM and forward PWM stage are
designed for flyback and forward power converters.
Peak-current-mode control is used to optimize system
performance. Slope compensation is added to stabilize
the
synchronized, positively sloped ramp at each switching
cycle. The positively sloped ramp is represented by the
voltage signal V
Gate Drivers
FAN6791/3 output stages are fast totem-pole gate
drivers. The output driver is clamped by an internal 18V
Zener diode to protect the power MOSFET.
current
FAN6791/3
Figure 27. Slope Compensation
Figure 26. Interleaved Switching
s-comp
loop.
uses
in Figure 27.
The
interleaved
FAN6791/3
switching
inserts
OFF
to
is
a
15
Constant Power Control
To limit the output power of the converter constantly, a
power-limit function is included. Sensing the converter
input voltage through the VRMS pin, the power limit
function generates a relative peak-current-limit threshold
voltage for constant power control, as shown in Figure 28.
Protections
The FAN6791/3 provides full protection functions to
prevent the power supply and the load from being
damaged. The protection features include:
V
forward PWM stages will be disabled whenever the V
voltage exceeds the over-voltage threshold.
AC Under-Voltage Protection. The VRMS pin is used to
detect the AC input voltage. When voltage is lower than
the brownout threshold, voltage disables both forward
and stand-by PWM.
RI Pin Open / Short Protection. The RI pin is used to set
the switching frequency and internal current reference.
The stand-by PWM and forward PWM stages are
disabled whenever the RI pin is short or open.
Open-Loop Protection. The stand-by PWM and forward
PWM stages of FAN6791/3 is disabled whenever the
FBFYB / FBPWM pin is open.
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Over-Voltage Protection. The stand-by PWM and
Figure 28. Constant Power Control
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