ML4802
FUNCTIONAL DESCRIPTION (Continued)
PFC SECTION
control loop for the converter, which in turn drives a
current error amplifier and switching output driver. The
second requirement is met by using the rectified AC line
voltage to modulate the output of the voltage control
loop. Such modulation causes the current error amplifier
to command a power stage current which varies directly
with the input voltage. In order to prevent ripple which
will necessarily appear at the output of the boost circuit
(typically about 10VAC on a 385V DC level, or about
40VAC during Green Mode operation) from introducing
distortion back through the voltage error amplifier, the
bandwidth of the voltage loop is deliberately kept low. A
final refinement is to adjust the overall gain of the PFC
such to be proportional to 1/VIN2, which linearizes the
transfer function of the system as the AC input voltage
varies.
Gain Modulator
Figure 3 shows a block diagram of the PFC section of the
ML4802. The gain modulator is the heart of the PFC, as it
is this circuit block which controls the response of the
current loop to line voltage waveform and frequency, rms
line voltage, and PFC output voltage. There are three
inputs to the gain modulator. These are:
1) A current representing the instantaneous input voltage
(amplitude and waveshape) to the PFC. The rectified
AC input sine wave is converted to a proportional
current via a resistor and is then fed into the gain
modulator at IAC. Sampling current in this way
minimizes ground noise, as is required in high power
switching power conversion environments. The gain
modulator responds linearly to this current.
Since the boost converter topology in the ML4802 PFC is
of the current-averaging type, no slope compensation is
required.
2) 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 VRMS. The gain modulator’s output
2
is inversely proportional to VRMS (except at unusually
low values of VRMS where special gain contouring
takes over to limit power dissipation of the circuit
components under heavy brownout conditions). The
relationship between VRMS and gain is designated as
K, and is illustrated in the Typical Performance
Characteristics.
3) The output of the voltage error amplifier, VEAO. The
gain modulator responds linearly to variations in this
voltage.
VCC
13
VEAO
16
IEAO
1
POWER FACTOR CORRECTOR
+
-
VEA
7.5V
REFERENCE
2.75V
VREF
OVP
VFB
1.6kΩ
14
12
15
-
IEA
+
–
2.5V
+
PFC
CONTROLLER
IAC
VRMS
2
4
3
8
-1V
+
-
8V
GAIN
MODULATOR
PFC
PFC ILIMIT
1.6kΩ
OUTPUT
DRIVER
PFC OUT
ISENSE
RAMP 1
÷2
FROM
GREEN MODE
CONTROLLER
OSCILLATOR
RT/CT
7
DUTY CYCLE
LIMIT
Figure 3. PFC Section Block Diagram
Datasheet August 2000
8
MICRO-LINEAR [ MICRO LINEAR CORPORATION ]
