FAN6791 Datasheet, PDF (13/19 Pages) Fairchild Semiconductor – Highly Integrated, Dual-PWM Combination Controller

Functional Description
The highly integrated FAN6791 / FAN6793 dual-PWM
combination controller provides several features to
enhance the performance of converters.
Proprietary interleave switching synchronizes the
flyback and forward PWM stages. This reduces
switching noise.
The proprietary frequency jittering function for the
flyback and forward PWM stages helps reduce
switching EMI emissions.
For the flyback and forward PWM, the synchronized
slope compensation ensures the stability of the current
loop under continuous-mode operation. In addition,
FAN6791/3 provides complete protection functions,
such as brownout protection and RI open/short.
Startup Current
For startup, the HV pin is connected to the line input or
bulk capacitor through external resistor RHV,
recommended as 100KΩ. Typical startup current drawn
from pin HV is 2mA and it charges the hold-up capacitor
through the resistor RHV. When the VDD capacitor level
reaches VDD-ON, the startup current switches off. At this
moment, the VDD capacitor only supplies the FAN6791/3
to maintain the VDD before the auxiliary winding of the
main transformer provides the operating current.
Oscillator Operation
A resistor connected from the RI pin to the GND pin
generates a constant current source for the FAN6791/3
controller. This current is used to determine the center
PWM frequency. Increasing the resistance reduces
PWM frequency. Using a 24KΩ resistor results in a
corresponding 65kHz PWM frequency. The switching
frequency is programmed by the resistor RI connected
between RI pin and GND. The relationship is:
f_PWM
=
1560 _(kHz)
R_I (kΩ)
(1)
The range of the PWM oscillation frequency is designed
as 33KHz ~ 130KHz. FAN6791/3 integrates frequency
hopping function internally. The frequency variation
ranges from around 61KHz to 69KHz for a center
frequency 65KHz. The frequency hopping function helps
reduce EMI emission of a power supply with minimum
line filters.
For power saving, flyback PWM stage has a green
mode function. Frequency linearly decreases when VFB
is within VG and VN. Once VFB is lower than VG,
switching frequency disables, and it enters burst mode.
Figure 23. Oscillation Frequency in Green Mode
Line Voltage Detection (V_RMS)
Figure 24 shows a resistive divider with low-pass
filtering for line-voltage detection on VRMS pin. The
VRMS voltage is used for the PFC multiplier and
brownout protection. For brownout protection, when the
VRMS voltage drops below 0.8V, OPFC turns off.
Figure 24. Line-Voltage Detection on VRMS Pin
Remote On/Off
Figure 25 shows the remote on / off function. When the
supervisor FPO pin pulls down and enables the system
by connecting an opto-coupler, VREF applies to the
ON/OFF pin to enable forward PWM stage.
Figure 25. Remote On/Off
© 2008 Fairchild Semiconductor Corporation
FAN6791 / FAN6793 • Rev. 1.0.2
13
www.fairchildsemi.com