FAN4800 Low Start-Up Current PFC/PWM Controller Combos
© 2005 Fairchild Semiconductor Corporation www.fairchildsemi.com
FAN4800 Rev. 1.0.5 10
3. The output of the voltage error amplifier, V
EAO
. The
gain modulator responds linearly to variations in this
voltage.
The output of the gain modulator is a current signal, in
the form of a full wave rectified sinusoid at twice the line
frequency. This current is applied to the virtual ground
(negative) input of the current error amplifier. In this way,
the gain modulator forms the reference for the current
error loop and ultimately controls the instantaneous cur-
rent draw of the PFC from the power line. The general
form of the output of the gain modulator is:
More precisely, the output current of the gain modulator
is given by:
where K is in units of V
-1
.
Note that the output current of the gain modulator is lim-
ited around 228.57µA and the maximum output voltage
of the gain modulator is limited to 228.57µA x 3.5K =
0.8V.
This 0.8V also determines the maximum input power.
However, I
GAINMOD
cannot be measured directly from
I
SENSE
. I
SENSE
= I
GAINMOD
– I
OFFSET
and I
OFFSET
can
only be measured when V
EAO
is less than 0.5V and
I
GAINMOD
is 0A. Typical I
OFFSET
is around 60µA.
1.2 Selecting R
AC
for I
AC
pin
I
AC
pin is the input of the gain modulator. I
AC
is also a
current mirror input and requires current input. Selecting
a proper resistor R
AC
provides a good sine wave current
derived from the line voltage and helps program the
maximum input power and minimum input line voltage.
R
AC
= V
IN
peak x 7.9K. For example, if the minimum line
voltage is 80V
AC
, the R
AC
= 80 x 1.414 x 7.9K = 894kΩ.
1.3 Current Error Amplifier, IEAO
The current error amplifier’s output controls the PFC duty
cycle to keep the average current through the boost
inductor a linear function of the line voltage. At the invert-
ing input to the current error amplifier, the output current
of the gain modulator is summed with a current, which
results from a negative voltage being impressed upon
the I
SENSE
pin.
The negative voltage on I
SENSE
represents the sum of all
currents flowing in the PFC circuit and is typically derived
from a current sense resistor in series with the negative
terminal of the input bridge rectifier.
The inverting input of the current error amplifier is a vir-
tual ground. Given this fact, and the arrangement of the
duty cycle modulator polarities internal to the PFC, an
increase in positive current from the gain modulator
causes the output stage to increase its duty cycle until
the voltage on I
SENSE
is adequately negative to cancel
this increased current. Similarly, if the gain modulator’s
output decreases, the output duty cycle decreases to
achieve a less negative voltage on the I
SENSE
pin.
1.4 Cycle-By-Cycle Current Limiter and Selecting R
S
As well as being a part of the current feedback loop, the
I
SENSE
pin is a direct input to the cycle-by-cycle current
limiter for the PFC section. If the input voltage at this pin
is ever less than -1V, the output of the PFC is disabled
until the protection flip-flop is reset by the clock pulse at
the start of the next PFC power cycle.
R
S
is the sensing resistor of the PFC boost converter.
During the steady state, line input current x R
S
equals
I
GAINMOD
x 3.5K.
Since the maximum output voltage of the gain modulator
is I
GAINMOD
maximum x 3.5k = 0.8V during the steady
state, R
S
x line input current is limited to below 0.8V as
well. Therefore, to choose R
S
, use the following equation:
For example, if the minimum input voltage is 80V
AC
and
the maximum input RMS power is 200Watt,
R
S
= (0.8V x 80V x 1.414) / (2 x 200) = 0.226Ω.
1.5 PFC OVP
In the FAN4800, the PFC OVP comparator serves to pro-
tect the power circuit from being subjected to excessive
voltages if the load changes suddenly. A resistor divider
from the high-voltage DC output of the PFC is fed to V
FB
.
When the voltage on V
FB
exceeds 2.78V, the PFC output
driver is shut down. The PWM section continues to oper-
ate. The OVP comparator has 280mV of hysteresis and
the PFC does not restart until the voltage at V
FB
drops
below 2.50V. V
CC
OVP can also serve as a redundant
PFC OVP protection. V
CC
OVP threshold is 17.9V with
1.5V hysteresis.
2
1
AC EAO
GAINMOD
RMS
IV
IV
V
×
=×
(3)
( 0.625)GAINMOD
EAO AC
IKV I=× − ×
(4)
(5)
0.8
2
INPEAK
S
VV
R
LineInput Power
×
=
×
