FAN6791 Datasheet, PDF (14/19 Pages) Fairchild Semiconductor – Highly Integrated, Dual-PWM Combination Controller

Interleave Switching
The FAN6791/3 uses interleaved switching to
synchronize the stand-by PWM / forward PWM stages.
This reduces switching noise and spreads the EMI
emissions. Figure 26 shows that an off-time tOFF is
inserted in between the turn-off of the stand-by gate
drives and the turn-on of the forward PWM.
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.
Figure 26. Interleaved Switching
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 current loop. The FAN6791/3 inserts a
synchronized, positively sloped ramp at each switching
cycle. The positively sloped ramp is represented by the
voltage signal Vs-comp in Figure 27.
Figure 27. Slope Compensation
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.
Figure 28. Constant Power Control
Protections
The FAN6791/3 provides full protection functions to
prevent the power supply and the load from being
damaged. The protection features include:
VDD Over-Voltage Protection. The stand-by PWM and
forward PWM stages will be disabled whenever the
VDD 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.
© 2008 Fairchild Semiconductor Corporation
FAN6791 / FAN6793 • Rev. 1.0.2
14
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