ML4802
FUNCTIONAL DESCRIPTION
(Continued)
On an instantaneous basis, an increase in VOUT above its
programmed value will cause the error voltage presented
to VDC to decrease. This will shut off PWMOUT to keep
the loop in regulation. If the output voltage goes below its
intended level, VDC will increase. When the feedback
voltage VDC rises above VTH2, PWMOUT is re-enabled
causing the output voltage to increase. This series of
actions will repeat, maintaining the average VOUT at its
design value. Since the PWM error amplifier gain is quite
high in the average configuration, this action introduces
no appreciable ripple on the PWM’s DC output(s). One
item to note here is that, to keep the pulse skipping
action as clean as possible (that is, to prevent pulse
grouping), a relatively fast error amplifier with an
electrically quiet feedback path to VDC is desirable.
When the PWM enters its pulse-skipping mode, the PFC is
shut off completely. The PWM then runs off of the energy
stored in the PFC buss capacitor. During this period, the
voltage on the buss capacitor will decay. When VBUSS
falls below a user-set threshold VPFC1 (typically 382V),
the PFC is turned on again, charging its output capacitor
back to a higher voltage VPFC2 (typically 425V).
Simultaneously, the threshold to which VDC is compared
is switched back to VTH1. As soon as the output voltage
of the PFC exceeds VPFC2, the PFC shuts off and VDC is
again compared to VTH2. This cycle repeats as long as
the power consumption from the PWM remains below the
Green Mode threshold.
Exiting Green Mode
The ML4802 enters Green Mode at any time that VDC <
VTH1. In order to reliably exit Green Mode, VTH1 must
be used as the exit criterion as well (using VTH2 as a
comparison voltage to exit Green Mode would eliminate
the part’s ability to skip pulses throughout the Green
Mode power range). Therefore, once the voltage on VDC
has set the part into Green Mode operation, the ML4802
can only exit Green Mode when the PFC is recharging the
buss capacitor. As noted above, VDC is compared against
VTH1 during the PFC recharge time. Another way of
viewing this is as follows: every time the PFC turns on,
the ML4802 exits Green Mode, and will either return to
Green Mode or remain in continuous-mode operation
depending upon whether the voltage on VDC exceeds
VTH1. Note that this means that there will be brief
periods of continuous PWM operation even while the
output power drawn from the PWM is within the Green
Mode range. This is a normal and harmless consequence
of the ML4802’s Green Mode logic.
GREEN MODE THRESHOLD
To a first approximation, the Green Mode Threshold as a
percentage of the PWM’s maximum rated power output is
given by:
PGMT = (VGMT/VCURRENT LIMIT(PWM)) x POUT(MAX)
PGMT @ (0.25V/1.5V) = 0.167 x POUT(MAX)
For example, a flyback supply designed for 100W
maximum output will nominally enter and exit Green
Mode operation at 17W. Similarly, a 200W forward
converter would have a Green Mode threshold of about
34W. In actual designs, the Green Mode threshold will
often be at a slightly lower power level than is given by
this simplified equation. This is principally due to the fact
that VFB is an average-responding voltage, while POUT is
inferred from the instantaneous peak current through
RSENSE(PWM). On a short-term basis, the output current
demand as sensed by VFB is essentially a DC level. This is
not true of V(RAMP1), however: V(RAMP2) is given by
(RSENSE(PWM) x IPRIMARY(PWM)), which for most
designs is a combination of DC (pedestal) and peak
(ramp) currents. It is the ramp current portion of
IPRIMARY(PWM) which causes real-world designs to
typically enter Green Mode at several percentage points
lower output power than would otherwise occur.
POWER FACTOR CORRECTION
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
ML4802 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
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. For the
ML4802, a good value to use is 425V DC out, to allow for
a high line of 270V AC while in Green Mode. 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
August 2000
Datasheet
7
MICRO-LINEAR [ MICRO LINEAR CORPORATION ]
