FAN6791 Fairchild Semiconductor, FAN6791 Datasheet - Page 14

• 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
Functional Description
The
combination controller provides several features to
enhance the performance of converters.
Proprietary
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
recommended as 100kΩ. Typical startup current drawn
from pin HV is 2mA and it charges the hold-up capacitor
through the resistor R
reaches V
moment, the V
to maintain the V
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 R
between RI pin and GND. The relationship is:
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 V
is within V
switching frequency disables, and it enters burst mode.
f
PWM
=
R
capacitor
highly
1560
I
(k
DD-ON
Ω
G
)
(
interleave
kHz
DD
and V
, the startup current switches off. At this
integrated
capacitor only supplies the FAN6791/3
)
DD
through
before the auxiliary winding of the
N
HV
. Once V
. When the V
switching
external
FAN6791/3
FB
is lower than V
synchronizes
DD
resistor
capacitor level
I
dual-PWM
connected
R
(1)
the
HV
FB
G
,
,
14
Line Voltage Detection (V
Figure 24 shows a resistive divider with low-pass
filtering for line-voltage detection on VRMS pin. The
V
keeps the constant power limit and provides brownout
protection. For brownout protection, when the V
voltage drops below 0.8V, OPWM turns off.
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, V
ON/OFF pin to enable forward PWM stage.
Figure 23. Oscillation Frequency in Green Mode
Figure 24. Line-Voltage Detection on VRMS Pin
RMS
voltage is used for high/low line compensation that
Figure 25. Remote On/Off
RMS
REF
)
applies to the
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RMS