ml4841 Fairchild Semiconductor, ml4841 Datasheet - Page 8

• Manufacturer Part Number: ml4841
• Description: Ml4841 Variable Feedforward Pfc/pwm Controller Combo
• Manufacturer: Fairchild Semiconductor

ML4841
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 ≅ 200µA.
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 modu-
lator is summed with a current which results from a negative
voltage being impressed upon the I
I
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. In
higher power applications, two current transformers are
sometimes used, one to monitor the I
MOSFET(s) and one to monitor the I
As stated above, the inverting 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
cancel this increased current. Similarly, if the gain modula-
tor’s output decreases, the output duty cycle will decrease, to
achieve a less negative voltage on the I
There is a modest degree of gain contouring applied to the
transfer characteristic of the current error amplifier, to
increase its speed of response to current-loop perturbations.
However, the boost inductor will usually be the dominant
factor in overall current loop response. Therefore, this
contouring is significantly less marked than that of the
voltage error amplifier. This is illustrated in the Typical
Performance Characteristics.
8
SENSE
I
I
GAINMOD
GAINMOD
≅ V
SENSE
≅
≅
K
I
--------------------------------
AC
×
V
(
/3.5kΩ). The negative voltage on I
×
RMS
VEAO 1.5V
VEAO
-1
2
.
SENSE
–
×
1V
)
is adequately negative to
×
SENSE
I
D
F
AC
SENSE
of the boost diode.
of the boost
pin (current into
pin.
SENSE
(1)
Cycle-By-Cycle Current Limiter
The I
back 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.
Overvoltage Protection
The OVP comparator serves to protect the power circuit
from being subjected to excessive voltages if the load should
suddenly change. A resistor divider from the high voltage
DC output of the PFC is fed to V
V
The PWM section will continue to operate. The OVP
comparator has 125mV of hysteresis, and the PFC will not
restart until the voltage at V
should be set at a level where the active and passive external
power components and the ML4841 are within their safe
operating voltages, but not so low as to interfere with the
boost voltage regulation loop.
Error Amplifier Compensation
The PWM loading of the PFC can be modeled as a negative
resistor; an increase in input voltage to the PWM causes a
decrease in the input current. This response dictates the
proper compensation of the two transconductance error
amplifiers. Figure 2 shows the types of compensation
networks most commonly used for the voltage and current
error amplifiers, along with their respective return points.
The current loop compensation is returned to V
produce a soft-start characteristic on the PFC: as the
reference voltage comes up from zero volts, it creates a
differentiated voltage on IEAO which prevents the PFC from
immediately demanding a full duty cycle on its boost
converter.
OUTPUT
FB
PFC
Figure 2. Compensation Network Connections for the
exceeds 2.7V, the PFC output driver is shut down.
SENSE
15
2
4
3
I AC
V FB
V RMS
I SENSE
2.5V
Voltage and Current Error Amplifiers
pin, as well as being a part of the current feed-
+
-
VEA
VEAO
MODULATOR
GAIN
16
FB
drops below 2.58V. The V
FB
PRODUCT SPECIFICATION
. When the voltage on
-
+
IEA
IEAO
REV. 1.0.3 6/13/01
V REF
1
REF
+
-
to
FB