AN-6026 Datasheet, PDF (3/23 Pages) Fairchild Semiconductor – Design of Power Factor Correction Circuit

AN6026
2. Device Block Description
2.1 Error Amplifier Block
The error amplifier block consists of a transconductance
amplifier, output OVP comparator, and disable comparator.
For the output voltage control, a transconductance amplifier
is used instead of the conventional voltage amplifier. The
transconductance amplifier (voltage controlled current
source) aids the implementation of OVP and disable func-
tion. The output current of the amplifier changes according
to the voltage difference of the inverting input and the non-
inverting input of the amplifier. The output voltage of the
amplifier is compared with the internal ramp signal to gener-
ate the switch turn-off signal. The OVP comparator shuts
down the output drive block when the voltage of the INV pin
is higher than 2.675V and there is 0.175V hysteresis. The
disable comparator disables the operation of the FAN7529
when the voltage of the inverting input is lower than 0.45V
and there is 100mV hysteresis. An external, small-signal
MOSFET can be used to disable the IC, as shown in Figure
4. The IC operating current decreases to under 65µA to
reduce power consumption if the IC is disabled.
OVP
2.675V 2.5V
Disable
0.45V 0.35V
Error
Amp
Gm
V_ref (2.5V)
V_OUT
INV
1
2
COMP
Disable
Signal
APPLICATION NOTE
the junction capacitor of the MOSFET resonates with the
boost inductor and the auxiliary winding voltage decreases
resonantly. If it reaches 1.4V, the zero current detector turns
on the MOSFET. The ZCD pin is protected internally by two
clamps: the 6.5V HIGH clamp and the 0.65V LOW clamp,
as shown in Figure 5.
Turn-on
Signal
Timer
V_IN
ZCD
5
S
Q
R
6.5V 1.4V 1.5V
Zero Current
Detector
Figure 5. Zero Current Detector Block
Figure 6 shows typical ZCD-related waveforms. Because the
ZCD pin has some capacitance, there can be some delay
caused by R_zcd and the turn-on time can be delayed.
I_PEAK
t_zero
Inductor
Current
0A
t_on t_dis
I_NEG
t_off
n·(V_OUT-V_IN)
V_AUX
0V
-n·V_IN
Figure 4. Error Amplifier Block
2.2 Zero Current Detection Block
The zero current detector (ZCD) generates the turn-on signal
of the MOSFET when the boost inductor current reaches
zero using an auxiliary winding coupled with the inductor.
Because the polarity of the auxiliary winding is opposite the
inductor winding, the auxiliary winding voltage is negative
and proportional to the rectified AC line voltage when the
MOSFET is turned on. If the MOSFET is turned off, the
voltage becomes positive and proportional to the difference
between V_OUT and V_IN. If the inductor current reaches zero,
ZCD
Voltage
OUT
V_th
V_OUT
Delay Time
R_ZCD Delay
V_clamp
0V
V_DS
Minimum
Voltage Turn-on _0V
Figure 6. Zero Current Detector Waveform
© 2006 Fairchild Semiconductor Corporation
Rev. 1.0.4 • 4/25/09
3
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