FAN7930 Fairchild Semiconductor, FAN7930 Datasheet - Page 12

• Manufacturer Part Number: FAN7930
• Description: Critical Conduction Mode PFC Controller
• Manufacturer: Fairchild Semiconductor

© 2010 Fairchild Semiconductor Corporation
FAN7930 • 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) and positive voltage is induced (see
equation 2) when the power switch turns off.
where, V
T
respectively, V
V
Because auxiliary winding voltage can swing from
negative voltage 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.
To check the boost inductor current zero instance,
auxiliary winding voltage is used. When boost inductor
current becomes zero, there is a resonance between
boost inductor and all capacitors at MOSFET drain pin,
V
V
AUX
OUT_PFC
AUX
AUX
are boost inductor turns and auxiliary winding turns
=
=
Figure 26.
−
T
is output voltage from the PFC converter.
T
AUX
AUX
T
IND
T
AUX
IND
Figure 25.
is the auxiliary winding voltage, T
⋅
AC
(
⋅
V
V
PFCOUT
AC
is input voltage for PFC converter and
Auxiliary Voltage Depends
on MOSFET Switching
−
Circuit Near ZCD
V
AC
)
IND
and
(1)
(2)
12
including C
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 as
detecting zero current of boost inductor and valley
position of MOSFET voltage stress. 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,
When no ZCD signal is available, the PFC controller
cannot turn on MOSFET, so the controller checks every
switching off time and forces MOSFET turn on when the
off time is longer than 150μs. It is called restart timer.
Restart timer triggers MOSFET turn on at startup and
may be used at the input voltage zero cross period.
Figure 27.
150
Figure 28.
internal
μ
OSS
s
of the MOSFET; an external capacitor at
logic
Auxiliary Voltage Threshold
Restart Timer at Startup
turns
on
the
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MOSFET.