FAN7930C Fairchild Semiconductor, FAN7930C Datasheet - Page 13

• Manufacturer Part Number: FAN7930C
• Description: The FAN7930C is an active power factor correction (PFC) controller for boost PFC applications that operate in critical conduction mode (CRM)
• Manufacturer: Fairchild Semiconductor

© 2010 Fairchild Semiconductor Corporation
FAN7930C • Rev. 1.0.1
For valley detection, a minor delay by the resistor and
capacitor is needed. A capacitor increases the noise
immunity at the ZCD pin. If ZCD voltage is higher than
1.5V, an internal ZCD comparator output becomes
HIGH and LOW when the ZCD goes below 1.4V. At the
falling edge of comparator output, internal logic turns on
the
When no ZCD signal is available, the PFC controller
cannot turn on the MOSFET, so the controller checks
every switching off time and forces MOSFET turn on
when the off time is longer than 150μs. This is called the
restart timer, which triggers MOSFET turn-on at startup
and may be used at the input voltage zero-cross period.
Because the MOSFET turn-on depends on the ZCD
input, switching frequency may increase to higher than
several megahertz due to the miss-triggering or noise
on the nearby ZCD pin. If the switching frequency is
higher than needed for critical conduction mode (CRM),
operation mode shifts to continuous conduction mode
Figure 29.
Figure 30.
150

s
Auxiliary Voltage Threshold
Restart Timer at Startup
MOSFET.
13
(CCM). In CCM, unlike CRM where the boost inductor
current is reset to zero at the next switch on; inductor
current builds up at every switching cycle and can be
raised to very high current that exceeds the current
rating of the power switch or diode. This can seriously
damage the power switch and result in burn down. To
avoid this, maximum switching frequency limitation is
embedded. If ZCD signal is applied again within 3.3μs
after the previous rising edge of gate signal, this signal
is ignored internally and FAN7930C waits for another
ZCD signal. This slightly degrades the power factor
performance at light load and high input voltage.
5. Control: The scaled output is compared with the
internal reference voltage and sinking or sourcing
current is generated from the COMP pin by the
transconductance amplifier. The error amplifier output is
compared with the internal sawtooth waveform to give
proper turn-on time based on the controller.
Unlike a conventional voltage-mode PWM controller,
FAN7930C turns on the MOSFET at the falling edge of
ZCD signal. On-instance is determined by the external
signal and the turn-on time lasts until the error amplifier
output (V
load is heavy, output voltage decreases, scaled output
decreases, COMP voltage increases to compensate low
output, turn-on time lengthens to give more inductor
turn-on time, and increased inductor current raises the
output voltage. This is how PFC negative feedback
controller regulates output.
The maximum of V
the maximum turn-on time, and switching stops when
V
COMP
Figure 31.
is lower than 1.0V.
COMP
Figure 32.
) and sawtooth waveform meet. When
Maximum Switching Frequency
COMP
is limited to 6.5V, which dictates
Limit Operation
Control Circuit
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