FAN7528 Fairchild Semiconductor, FAN7528 Datasheet - Page 13

• Manufacturer Part Number: FAN7528
• Description: Dual Output Critical Conduction Mode PFC Controller
• Manufacturer: Fairchild Semiconductor

FAN7528 Rev. 1.0.1
Applications Information
1. Error Amplifier Block
The error amplifier block has several functions such as dual out-
put function, over voltage protection function and disable func-
tion.
1.1 Dual Output Function
Unlike conventional CRM PFC controllers, the FAN7528 has the
dual output control function according to the AC line voltage
without sensing the rectified AC line voltage. Because the out-
put voltage of the boost converter is proportional to the peak
voltage of the input AC line voltage before the boost converter
starts switching, the INV pin voltage represents the peak AC line
voltage. When the AC line is connected to the boost converter,
Vcc voltage starts to increase from zero voltage. If the Vcc volt-
age reaches 8.5V, the dual output reference generator com-
pares the INV pin voltage with 1.3V reference and if the INV pin
voltage is lower than 1.3V the dual output reference generator
sets the reference voltage of the error amplifier to be 1.5V. If the
INV pin voltage is higher than 1.3V, the reference voltage is set
to be 2.5V. That means if the output voltage of the boost con-
verter is set to be 400V at high line, the output voltage is
240V(400V*1.5/2.5) at low line. If the output voltage is set to be
390V at high line, the output voltage is 234V at low line.
Because this block does not need the input voltage sensing net-
work, the power loss and cost related with the sensing network
can be saved. The reference voltage of the error amplifier is not
reset until the Vcc voltage goes below 4.5V.
1.2 Over Voltage Protection Function
Figure 35. Error amplifier block
Disable
Error
OVP
Amp
2
COMP
Gm
Dual Output
Reference
Generator
2.66V
0.45V 0.35V
1.5V/2.5V
2.55V
1
INV
Vout
13
The control speed of the PFC converter is very slow, therefore
the over voltage protection of the output voltage is very impor-
tant. The FAN7528 provides a precise OVP function that shuts
down the drive circuit when the INV pin voltage exceeds 2.66V,.
and there is 0.11V hysteresis.
1.3 Disable Function
If the INV pin voltage is lower than 0.45V, most of the internal
block is disabled and the operating current is reduced to be
65uA, and there is 0.1V hysteresis in the comparator.
1.4 Error Amplifier
The error amplifier is a transconductance type amplifier. The
output current of the amplifier is proportional to the voltage dif-
ference between the inverting input and the non inverting input
of the amplifier. Some resistors and capacitors should be con-
nected to the error amplifier output pin, the COMP pin, for the
output voltage loop compensation.
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. If the volt-
age of the ZCD pin goes higher than 1.5V then the ZCD com-
parator waits until the voltage goes below 1.4V. If the voltage
goes below 1.4V, the zero current detector turns on the MOS-
FET. The ZCD pin is protected internally by two clamps, 6.7V
high clamp and 0.6V low clamp. The 160us timer generates a
MOSFET turn-on signal if the drive output has been low for
more than 160us from the falling edge of the drive output.
3. Saw Tooth Generator Block
The output of the error amplifier and the output of the saw tooth
generator are compared to determine the MOSFET turn-off
instance. The slope of the saw tooth is determined by an exter-
nal resistor connected to the MOT pin. The voltage of the MOT
pin is 1V and the slope is proportional to the current flowing out
of the MOT pin. The internal ramp signal has 1V offset, there-
fore the drive output is shut down if the voltage of the COMP pin
is lower than 1V. The MOSFET on-time is maximum when the
COMP pin voltage is 5V. According to the slope of the internal
ramp, the maximum on-time can be programmed. The neces-
sary maximum on-time depends on the boost inductor, lowest
Vin
Figure 36. Zero current detector block
R
ZCD
ZCD
5
6.7V
1.4V
1.5V
Zero Current
Detector
160us
Timer
R
S
www.fairchildsemi.com
Q
Turn-on
Signal