ML4827 Fairchild, ML4827 Datasheet - Page 7

• Manufacturer Part Number: ML4827
• Description: Fault-Protected PFC and PWM Controller Combo
• Manufacturer: Fairchild

PRODUCT SPECIFICATION
Functional Description
The ML4827 consists of an average current controlled, con-
tinuous 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 volt-
age 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 sec-
tion of both the ML4827-1 and the ML4827-2 run at the
same frequency as the PFC.
A number of protection features have been built into the
ML4827 to insure the final power supply will be as reliable
as possible. These include TriFault Detect, soft-start, PFC
over-voltage protection, peak current limiting, brown-out
protection, duty cycle limit, and under-voltage lockout.
Tri-Fault Detect protection
Many power supplies manufactured for sale in the US must
meet Underwriter’s Laboratories (UL) standards. UL’s speci-
fication UL1950 requires that no unsafe condition may result
from the failure of any single circuit component. Typical sys-
tem designs include external active and passive circuitry to
meet this requirement. TriFault Detect is an on-chip feature
of the ML4827 that monitors the V
undervoltage, or floating conditions which indicate that a
component of the feedback path may have failed. In such an
event, the PFC supply output will be disabled. These inte-
grated redundant protections assure system compliance with
UL1950 requirements.
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 in phase 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.
REV. 1.0.1 6/27/01
FB
pin for overvoltage,
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 ML4827 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 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
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 volt-
age control loop for the converter, which in turn drives a cur-
rent 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 modu-
lation 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 volt-
age error amplifier, the bandwidth of the voltage loop is
deliberately kept low. A final refinement is to adjust the over-
all gain of the PFC such to be proportional to 1/V
linearizes the transfer function of the system as the AC input
voltage varies.
Since the boost converter topology in the ML4827 PFC is of
the current-averaging type, no slope compensation is
required.
PFC Section
Gain Modulator
Figure 1 shows a block diagram of the PFC section of the
ML4827. 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
noise, as is required in high power switching power con-
version environments. The gain modulator responds lin-
early to this current.
AC
. Sampling current in this way minimizes ground
rms
. The other condition is that the current which the
IN
2
, which
ML4827
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