ml4827cs-2 Fairchild Semiconductor, ml4827cs-2 Datasheet - Page 8

• Manufacturer Part Number: ml4827cs-2
• Description: Fault-protected Pfc And Pwm Controller Combo
• Manufacturer: Fairchild Semiconductor

ML4827
2.
3.
The output of the gain modulator is a current signal, in the
form of a full wave rectified sinusoid at twice the line fre-
quency. This current is applied to the virtual-ground (nega-
tive) 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 average 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
rent into I
I
circuit, and is typically derived from a current sense resistor
in series with the negative terminal of the input bridge recti-
fier. In higher power applications, two current transformers
are sometimes used, one to monitor the I
8
SENSE
A voltage proportional to the long-term RMS AC line
voltage, derived from the rectified line voltage after
scaling and filtering. This signal is presented to the gain
modulator at V
inversely proportional to V
low values of V
over, to limit power dissipation of the circuit compo-
nents under heavy brownout conditions). The relation-
ship between V
illustrated in the Typical Performance Characteristics.
The output of the voltage error amplifier, VEAO. The
gain modulator responds linearly to variations in this
voltage.
represents the sum of all currents flowing in the PFC
SENSE
I
GAINMOD
I
GAINMOD
≅ V
RMS
SENSE
RMS
RMS
=
. The gain modulator’s output is
-1
K
where special gain contouring takes
and gain is termed K, and is
.
/3.5kΩ). The negative voltage on
=
×
I
--------------------------------
(
AC
VEAO 1.5V
V
RMS
×
RMS
VEAO
2
–
(except at unusually
2
D
×
of the boost MOS-
1V
)
SENSE
×
I
AC
pin (cur-
(1)
FET(s) and one to monitor the I
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
increased 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
back loop, is a direct input to the cycle-by-cycle current lim-
iter 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
exceeds 2.7V, the PFC output driver is shut down. 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
level where the active and passive external power compo-
nents and the ML4827 are within their safe operating volt-
ages, but not so low as to interfere with the boost voltage
regulation loop.
OUTPUT
PFC
Figure 2. Compensation Network Connections for the
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-
SENSE
FB
drops below 2.58V. The V
is adequately negative to cancel this
–
+
VEA
VEAO
MODULATOR
GAIN
16
SENSE
F
FB
of the boost diode. As
. When the voltage on V
PRODUCT SPECIFICATION
pin.
+
–
IEA
FB
REV. 1.0.1 6/27/01
IEAO
should be set at a
V REF
1
+
–
FB