ml4801 Fairchild Semiconductor, ml4801 Datasheet - Page 7

• Manufacturer Part Number: ml4801
• Description: Variable Feedforward Pfc/pwm Controller Combo
• Manufacturer: Fairchild Semiconductor

More exactly, the output current of the gain modulator is
given by:
FUNCTIONAL DESCRIPTION
3) The output of the voltage error amplifier, VEAO. The
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
current draw of the PFC from the power line. The general
form for the output of the gain modulator is:
More exactly, the output current of the gain modulator is
given by:
where K is in units of V
Note that the output current of the gain modulator is
limited to ≅ 500µA.
REV. 1.1 3/9/2001
inversely proportional to V
low values of V
takes over to limit power dissipation of the circuit
components under heavy brownout conditions). The
relationship between V
gain modulator responds linearly to variations in this
voltage.
I
I
15
GAINMOD
GAINMOD
2
4
3
8
7
I AC
V FB
V RMS
I SENSE
RAMP 1
R T C T
2.5V
= ×
=
+
-
K
VEA
I
AC
VEAO
MODULATOR
(
RMS
V
VEAO
´
RMS
GAIN
VEAO
16
where special gain contouring
-1
2
−
.
RMS
0 625
.
1.6kΩ
´
RMS
1
and gain is designated as K.
V
V
1.6kΩ
)
2 (except at unusually
×
+
–
IEA
I
AC
IEAO
8V
Figure 1. PFC Section Block Diagram
1
OSCILLATOR
DUTY CYCLE
(Continued)
LIMIT
POWER FACTOR CORRECTOR
CONTROLLER
÷2
PFC
(1)
(1)
Current Error Amplifier
The current error amplifier’s output controls the PFC duty
cycle to keep the current through the boost inductor a
linear function of the line voltage. At the inverting 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
pin (current into I
voltage on I
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. In higher power applications,
two current transformers are sometimes used, one to
monitor the I
the I
input of the current error amplifier is a virtual 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 will cause the
output stage to increase its duty cycle until the voltage on
I
current. Similarly, if the gain modulator’s output
decreases, the output duty cycle will decrease to achieve
a less negative voltage on the I
Cycle-By-Cycle Current Limiter
The I
feedback loop, is a direct input to the cycle-by-cycle
current limiter for the PFC section. Should the input
voltage at this pin ever be more negative than -1V, the
output of the PFC will be disabled until the protection
flip-flop is reset by the clock pulse at the start of the next
PFC power cycle.
SENSE
F
2.75V
SENSE
of the boost diode. As stated above, the inverting
-1V
is adequately negative to cancel this increased
PFC I LIMIT
pin, as well as being a part of the current
SENSE
OVP
+
+
-
-
D
of the boost MOSFET(s) and one to monitor
SENSE
represents the sum of all currents
≅ V
SENSE
SENSE
/1.6kΩ). The negative
OUTPUT
DRIVER
REFERENCE
PFC
pin.
7.5V
13
V CC
ML4801
PFC OUT
V REF
14
12
SENSE
7