6. Soft-Start: When VCC reaches VSTART, the internal
reference voltage is increased like a stair step for 5ms.
As a result, VCOMP is also raised gradually and MOSFET
turn-on time increases smoothly. This reduces voltage
and current stress on the power switch during startup.
0.155 V / μs
Figure 33. Turn-On Time Determination
The roles of PFC controller are regulating output voltage
and input current shaping to increase power factor. Duty
control based on the output voltage should be fast
enough to compensate output voltage dip or overshoot.
For the power factor, however, the control loop must not
react to the fluctuating AC input voltage. These two
requirements conflict; therefore, when designing a
feedback loop, the feedback loop should be least 10
times slower than AC line frequency. That slow
response is made by C1 at 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.
Figure 36. Soft-Start Sequence
7. Startup without Overshoot: Feedback control speed
of PFC is quite slow. Due to the slow response, there is
a gap between output voltage and feedback control.
That is why over-voltage protection (OVP) is critical at
the PFC controller and voltage dip caused by fast load
changes from light to heavy is diminished by a bulk
capacitor. OVP is easily triggered at startup phase.
Operation on and off by OVP at startup may cause
audible noise and can increase voltage stress at startup,
which is normally higher than in normal operation. This
operation is better when soft-start time is very long.
However, too much startup time enlarges the output
voltage building time at light load. FAN7930C has
overshoot avoidance at startup. During startup, the
feedback loop is controlled by an internal proportional
gain controller and, when the output voltage reaches the
rated value, it switches to an external compensator after
a transition time of 30ms. This internal proportional gain
controller eliminates overshoot at startup and an
external conventional compensator takes over
successfully afterward.
Figure 34. Compensators Gain Curve
For the transconductance error amplifier side, gain
changes based on differential input. When the error is
large, gain is large to make the output dip or peak to
suppress quickly. When the error is small, low gain is
used to improve power factor performance.
250μmho
115μmho
Figure 35. Gain Characteristic
© 2010 Fairchild Semiconductor Corporation
FAN7930C • Rev. 1.0.0
www.fairchildsemi.com
14
FAIRCHILD [ FAIRCHILD SEMICONDUCTOR ]
