ML4805
FUNCTIONAL DESCRIPTION
The ML4805 consists of a combined average-current-
controlled, continuous boost Power Factor Corrector (PFC)
front end and a synchronized Pulse Width Modulator
(PWM) back end. It is distinguished from earlier combo
controllers by its dramatically reduced start-up and
operating currents. The PWM section 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 modulation technique
results in a higher useable PFC error amplifier bandwidth,
and can significantly reduce the size of the PFC DC buss
capacitor.
No filtering is applied following the bridge rectifier, so the
input voltage to the boost converter ranges, at twice line
frequency, 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 current which the converter draws from the power
line matches the instantaneous line voltage. One of these
conditions is that the output voltage 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 . The other condition is that the
rms
current which the converter is allowed to draw from the
line at any given instant must be proportional to the line
voltage. The first of these requirements is satisfied by
establishing a suitable voltage control loop for the
converter, which sets an average operating current level
for a current error amplifier and switching output driver.
The second requirement is met by using the rectified AC
line voltage to modulate the input of the current 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), from
introducing distortion back through the voltage error
amplifier, the bandwidth of the voltage loop is
The synchronization of the PWM with the PFC simplifies
the PWM compensation due to the reduced ripple on the
PFC output capacitor (the PWM input capacitor). The
PWM section of the ML4805 runs at twice the frequency
of the PFC, which allows the use of smaller PWM output
magnetics and filter capacitors while holding down the
losses in the PFC stage power components.
In addition to power factor correction, a number of
protection features have been built into the ML4805.
These include soft-start, PFC over-voltage protection, peak
current limiting, brown-out protection, duty cycle limit,
and under-voltage lockout.
deliberately kept low. A final refinement is to adjust the
2
overall gain of the PFC such to be proportional to 1/V
,
IN
which linearizes the transfer function of the system as the
AC input voltage varies.
POWER FACTOR CORRECTION
Since the boost converter topology in the ML4805 PFC is
of the current-averaging type, no slope compensation is
required.
Power factor correction makes a non-linear 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 non-linear load is the input of most
power supplies, which use a bridge rectifier and
PFC SECTION
Gain Modulator
capacitive input filter fed from the line. The peak-
charging effect which occurs on the input filter capacitor
in such a supply causes brief high-amplitude pulses of
current to flow from the power line, rather than a
sinusoidal current in phase with the line voltage. Such a
supply presents a power factor to the line of less than one
(another way to state this is that it causes significant
current harmonics to appear at its input). If the input
current drawn by such a supply (or any other non-linear
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.
Figure 1 shows a block diagram of the PFC section of the
ML4805. 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 an external resistor and is then fed into the
gain modulator at I . Sampling current in this way
AC
To maintain the input current of a device drawing power
from the AC line in phase with, and proportional to, the
input voltage, a way must be found to cause that device
to load the line in proportion to the instantaneous line
voltage. The PFC section of the ML4805 uses a boost-
mode DC-DC converter to accomplish this. The input to
the converter is the full wave rectified AC line voltage.
minimizes ground noise, as is required in high power
switching power conversion environments. The gain
modulator responds linearly to this current.
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 V . The gain modulator’s output is
RMS
6
MICRO-LINEAR [ MICRO LINEAR CORPORATION ]
