ML4804
FUNCTIONAL DESCRIPTION (Continued)
Overvoltage Protection
that the voltage from the bootstrap winding must equal
15.8V during regular circuit operation, and will increase
The OVP comparator serves to protect the power circuit
from being subjected to excessive voltages if the load
should suddenly change. A resistor divider from the high
to 17.2V at the point of V OVP shutdown. Then the
output voltage from the PFC will have increased from a
CC
noninal V
419VDC. When V
of 385VDC to (17.2/15.8) x 385V =
BUSS
voltage DC output of the PFC is fed to V . When the
reaches 419V, the PFC will shut
FB
BUSS
voltage on V exceeds 2.75V, the PFC output driver is
shut down. The PWM section will continue to operate. The
off, thereby protecting the output (BUSS) capacitor and
the semiconductors in both the PFC and PWM stages.
FB
OVP comparator has 250mV of hysteresis, and the PFC
will not restart until the voltage at V drops below 2.50V.
To assure reasonable headroom in which to operate this
FB
The V should be set at a level where the active and
device, V OVP tracks with UVLO. The V OVP
threshold is always at least 2V above that of the UVLO.
FB
CC CC
passive external power components and the ML4804 are
within their safe operating voltages, but not so low as to
interfere with the boost voltage regulation loop.
To assure reliable operation of the ML4804, V must be
CC
operated from a bootstrap winding on the PFC’s inductor,
or from an external power supply whose output is
regulated to 15.0V (nominal). In the case of a regulated
V
OVP
CC
The V OVP feature of the ML4804 works along with the
TriFault Detect as a redundant PFC buss voltage limiter,
power supply powering the ML4804, the V OVP function
CC
TM
CC
will be rendered non-operational.
to prevent a damaged and broken connection or
component from causing an unsafe fault condition.
Error Amplifier Compensation
V
OVP assumes that V is generated from a bootstrap
The PWM loading of the PFC can be modeled as a
negative resistor; an increase in input voltage to the PWM
causes a decrease in the input current. This response
dictates the proper compensation of the PFC's two
transconductance error amplifiers. Figure 2 shows the
types of compensation networks most commonly used for
the voltage and current error amplifiers, along with their
respective return points. The current loop compensation is
CC
CC
winding on the PFC boost inductor, or by some other
means whereby V is proportional to V . If the
proportionality is exact, then a nominal V
CC
BUSS
of 385V at
BUSS
V
= 15.0V will cause the V OVP comparator to shut
CC
CC
the PFC down when V
The PFC will then remain in the shutdown state until V
declines to 13.0V, at which time the PFC will restart. If
= [(16.4/15.0) x 385V] = 444V.
BUSS
CC
the PFC V again encounters an over voltage condition,
the protection cycle will repeat. Note that the PWM stage
of the ML4804 remains operational even when the PFC
returned to V
to produce a soft-start characteristic on
CC
REF
the PFC: as the reference voltage comes up from zero
volts, it creates a differentiated voltage on IEAO which
prevents the PFC from immediately demanding a full duty
cycle on its boost converter.
goes into V OVP shutdown.
CC
For a real-world example, assume that the bootstrap
supply is derived from a conventional boost inductor
winding and rectified using Shottky diodes. Then it follows
There are two major concerns when compensating the
V
REF
V
BIAS
R
PFC
OUTPUT
16
1
IEAO
VEAO
VEA
BIAS
V
FB
IEA
15
–
V
CC
+
–
0.22µF
CERAMIC
+
–
2.5V
AC
15V
ZENER
+
ML4804
GND
I
2
4
3
GAIN
MODULATOR
V
RMS
I
SENSE
Figure 2. Compensation Network Connections for the
Voltage and Current Error Amplifiers
Figure 3. External Component Connections to V
CC
9
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