PRODUCT SPECIFICATION
ML4800
Functional Description
The ML4800 consists of an average current controlled,
continuous boost Power Factor Corrector (PFC) front end
and a synchronized Pulse Width Modulator (PWM) back
end. The PWM can be used in either current or voltage
mode. In voltage mode, feedforward from the PFC output
buss can be used to improve the PWM’s line regulation.
In either mode, the PWM stage uses conventional trailing-
edge duty cycle modulation, while the PFC uses leading-
edge modulation. This patented leading/trailing edge modu-
lation technique results in a higher usable PFC error ampli-
fier bandwidth, and can significantly reduce the size of the
PFC DC buss capacitor.
The synchronization of the PWM with the PFC simplifies the
PWM compensation due to the controlled ripple on the PFC
output capacitor (the PWM input capacitor). The PWM sec-
tion of the ML4800 runs at the same frequency as the PFC.
In addition to power factor correction, a number of protec-
tion features have been built into the ML4800. These include
soft-start, PFC overvoltage protection, peak current limiting,
brownout protection, duty cycle limiting, and under-voltage
lockout.
line voltage. One of these conditions is that the output volt-
age of the boost converter must be set higher than the peak
value of the line voltage. A commonly used value is
385VDC, to allow for a high line of 270VAC
rms
. The other
condition is that the current drawn from the line at any given
instant must be proportional to the line voltage. Establishing
a suitable voltage control loop for the converter, which in
turn drives a current error amplifier and switching output
driver satisfies the first of these requirements. 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 that 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), 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 propor-
tional to 1/V
IN
2, which linearizes the transfer function of the
system as the AC input voltage varies.
Since the boost converter topology in the ML4800 PFC is of
the current-averaging type, no slope compensation is
required.
Power Factor Correction
Power factor correction makes a nonlinear load look like a
resistive load to the AC line. For a resistor, the current drawn
from the line is in phase with and proportional to the line
voltage, so the power factor is unity (one). A common class
of nonlinear load is the input of most power supplies, which
use a bridge rectifier and capacitive input filter fed from the
line. The peak-charging effect, which occurs on the input fil-
ter capacitor in these supplies, causes brief high-amplitude
pulses of current to flow from the power line, rather than a
sinusoidal current inphase with the line voltage. Such sup-
plies present a power factor to the line of less than one (i.e.
they cause significant current harmonics of the power line
frequency to appear at their input). If the input current drawn
by such a supply (or any other nonlinear load) can be made
to follow the input voltage in instantaneous amplitude, it will
appear resistive to the AC line and a unity power factor will
be achieved.
To hold the input current draw of a device drawing power
from the AC line in phase with and proportional to the input
voltage, a way must be found to prevent that device from
loading the line except in proportion to the instantaneous line
voltage. The PFC section of the ML4800 uses a boost-mode
DC-DC converter to accomplish this. The input to the con-
verter is the full wave rectified AC line voltage. No bulk fil-
tering is applied following the bridge rectifier, so the input
voltage to the boost converter ranges (at twice line fre-
quency) from zero volts to the peak value of the AC input
and back to zero. By forcing the boost converter to meet two
simultaneous conditions, it is possible to ensure that the cur-
rent drawn from the power line is proportional to the input
PFC Section
Gain Modulator
Figure 1 shows a block diagram of the PFC section of the
ML4800. 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 volt-
age, 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 I
AC
. 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.
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
RMS
. The gain modulator s output is
inversely proportional to V
RMS2
(except at unusually
low values of V
RMS
where special gain contouring takes
over, to limit power dissipation of the circuit
components under heavy brownout conditions). The
relationship between V
RMS
and gain is called K, and is
illustrated in the Typical Performance Characteristics.
The output of the voltage error amplifier, VEAO. The
gain modulator responds linearly to variations in this
voltage.
7
2.
3.
REV. 1.0.5 9/25/01
FAIRCHILD [ FAIRCHILD SEMICONDUCTOR ]
