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FAN7930B Datasheet(PDF) 14 Page - ON Semiconductor

Part # FAN7930B
Description  Critical Conduction Mode PFC Controller
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Manufacturer  ONSEMI [ON Semiconductor]
Direct Link  http://www.onsemi.com
Logo ONSEMI - ON Semiconductor

FAN7930B Datasheet(HTML) 14 Page - ON Semiconductor

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14
The roles of PFC controller are regulating output voltage
and input current shaping to increase pow er factor. Duty
control based on the output voltage should be fast
enough to compensate output voltage dip or overshoot.
For the pow er factor, how ever, the control loop must not
react to the fluctuating A C input voltage. These tw o
requirements conflict; therefore, w hen designing a
feedback loop, the feedbac k loop should be least ten
times
slow er
than AC line frequency. That slow
response is made by C1 at the compensator. R1 makes
gain boost around operation region and C2 attenuates
gain at higher frequency. Boost gain by R1 helps raise
the response time and improves phase margin.
Freq.
C1
R1
Proportional
Gain
C2
Integrator
High-Frequency
Noise Filter
Gain
Figure 33.
Com pensators Gain Curve
For the transconductance error amplifier side, gain
changes based on differential input. When the error is
large, gain is large to suppress the output dip or peak
quickly. When the error is small, low gain is used to
improve pow er factor performance.
ICOMP
Powering
Braking
mho
250
mho
115
Figure 34.
Gain Characteristic
7. Soft-Start: When VCC reaches VSTART, the internal
reference voltage is increased like a stair step for 5 ms.
As a result, VCOMP is also raised gradually and MOSFET
turn-on time increases smoothly. This reduces voltage
and current stress on the pow er sw itch during startup.
VREF
SS
gM
VINV=0.4V
ISOURCE
COMP
VCOMP
ISOURCE
COMP
RCOMP=VCOMP
t
(VREF
SS-V
INV)
gM=ISOURCE
COMP
VREF
END=2.5V
5ms
VCC
VSTART=12V
Figure 35.
Soft-Start Sequence
8. Startup w ithout Overshoot: Feedback control speed
of PFC is quite slow . Due to the slow response, ther e is
a gap betw een output voltage and feedback control.
That is w hy over-voltage protection ( OV P) is critical at
the PFC controller and voltage dip caused by fast load
changes from light to heavy is diminished by a bulk
capacitor. OV P can be triggered during startup phase.
Operation on and off by OV P at startup may cause
audible noise and can increase voltage stress at startup,
w hich is nor mally higher than in nor mal operation. This
operation is improved w hen soft-start time is very long.
How ever, too much startup time enlarges the output
voltage building time at light load. FAN7930B has
overshoot protection at startup. During startup, the
feedback loop is controlled by an internal proportional
gain controller and w hen the output voltage reaches the
rated value, it sw itches to an external compensator after
a transition time of 30 ms. This internal proportional gain
controller
eliminates
overshoot
at startup and
an
external
conventional
compensator
takes
over
successfully afterw ard.
Depend on Load
VOUT
VCOMP
Startup
Overshoot
Internal Controller
t
Conventional Controller
Overshoot-less Startup Control
Control Transition
Figure 36.
Startup w ithout Overshoot


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