FAN7930C Fairchild Semiconductor, FAN7930C Datasheet - Page 12

• 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
4. Zero-Current Detection: Zero-current detection
(ZCD) generates the turn-on signal of the MOSFET
when the boost inductor current reaches zero using an
auxiliary winding coupled with the inductor. When the
power switch turns on, negative voltage is induced at the
auxiliary winding due to the opposite winding direction
(see Equation 1). Positive voltage is induced (see
Equation 2) when the power switch turns off.
Figure 25.
Figure 26.
Two Cases of RDY Triggered HIGH
Two Cases of RDY Triggered LOW
12
Because auxiliary winding voltage can swing from
negative to positive voltage, the internal block in ZCD
pin has both positive and negative voltage clamping
circuits. When the auxiliary voltage is negative, internal
circuit clamps the negative voltage at the ZCD pin
around 0.65V by sourcing current to the serial resistor
between the ZCD pin and the auxiliary winding. When
the auxiliary voltage is higher than 6.5V, current is
sinked through a resistor from the auxiliary winding to
the ZCD pin.
The auxiliary winding voltage is used to check the boost
inductor current zero instance. When boost inductor
current becomes zero, there is a resonance between
boost inductor and all capacitors at the MOSFET drain
pin: including C
capacitor at the D-S pin to reduce the voltage rising and
falling slope of the MOSFET; a parasitic capacitor at
inductor; and so on to improve performance. Resonated
voltage is reflected to the auxiliary winding and can be
used for detecting zero current of boost inductor and
valley position of MOSFET voltage stress.
V
V
AUX
AUX
where:
V
T
T
V
V
Figure 28.
AUX
IND
IND
AC
OUT_PFC



T
is input voltage for PFC converter; and
T
is boost inductor turns;
auxiliary winding turns;
is the auxiliary winding voltage;
AUX
T
IND
T
AUX
IND
Figure 27.
is output voltage from the PFC converter.



V
V
PFCOUT
AC
OSS
Auxiliary Voltage Depends on
of the MOSFET; an external
MOSFET Switching

Circuit Near ZCD
V
AC

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(1)
(2)