ML4804
FUNCTIONAL DESCRIPTION
The ML4804 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
modulation technique results in a higher useable PFC
error amplifier 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 section of the ML4804 runs at the same frequency
as the PFC.
In addition to power factor correction, a number of
protection features have been built into the ML4804.
These include soft-start, PFC over-voltage protection, peak
current limiting, brownout protection, duty cycle limiting,
and under-voltage lockout.
POWER FACTOR CORRECTION
PFC SECTION
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
capacitive input filter fed from the line. The peak-
charging effect, which occurs on the input filter capacitor
in these supplies, 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
supplies 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 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.
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
ML4804 uses a boost-mode DC-DC converter to
accomplish this. The input to the converter is the full
wave rectified AC line voltage. No bulk 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 drawn from the power line is proportional to the
input line voltage. One of these conditions is that the
Gain Modulator
Figure 1 shows a block diagram of the PFC section of the
ML4804. 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 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.
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
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.
3) The output of the voltage error amplifier, VEAO. The
gain modulator responds linearly to variations in this
voltage.
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
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 proportional
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 ML4804 PFC is
of the current-averaging type, no slope compensation is
required.
7
MICRO-LINEAR [ MICRO LINEAR CORPORATION ]
