ML4826
FUNCTIONAL DESCRIPTION
The ML4826 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 ML4826-1 runs at the same frequency
as the PFC. The PWM section of the ML4826-2 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 ML4826. These
include soft-start, PFC over-voltage protection, peak
current limiting, brown-out protection, duty cycle limit,
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 a 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 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.
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
ML4826 uses a boost-mode DC-DC converter to
accomplish this. The input to the converter is the full wave
rectified AC line voltage. 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
Gain Modulator
Figure 1 shows a block diagram of the PFC section of the
ML4826. 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 brown-out conditions). The
relationship between V
RMS
and gain is designated as K,
and is illustrated in the Typical Performance
Characteristics.
conditions, it is possible to ensure that the current which
the converter draws from the power line agrees with 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
rms
. The other condition is that the 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 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)
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 ML4826 PFC is
of the current-averaging type, no slope compensation is
required.
7
MICRO-LINEAR [ MICRO LINEAR CORPORATION ]
