ML4800
FUNCTIONAL DESCRIPTION
(Continued)
shut down. The PWM section will continue to operate.
The OVP comparator has 250mV of hysteresis, and the
PFC will not restart until the voltage at V
FB
drops below
2.50V. The V
FB
should be set at a level where the active
and passive external power components and the ML4800
are within their safe operating voltages, but not so low as
to interfere with the boost voltage regulation loop.
Error Amplifier Compensation
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 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
returned to V
REF
to produce a soft-start characteristic on
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.
There are two major concerns when compensating the
voltage loop error amplifier; stability and transient
response. Optimizing interaction between transient
response and stability requires that the error amplifier’s
open-loop crossover frequency should be 1/2 that of the
line frequency, or 23Hz for a 47Hz line (lowest
anticipated international power frequency). The gain vs.
input voltage of the ML4800’s voltage error amplifier has a
specially shaped non-linearity such that under steady-state
operating conditions the transconductance of the error
amplifier is at a local minimum. Rapid perturbations in
line or load conditions will cause the input to the voltage
error amplifier (V
FB
) to deviate from its 2.5V (nominal)
value. If this happens, the transconductance of the voltage
error amplifier will increase significantly, as shown in the
Typical Performance Characteristics. This raises the gain-
bandwidth product of the voltage loop, resulting in a
much more rapid voltage loop response to such
perturbations than would occur with a conventional linear
gain characteristic.
The current amplifier compensation is similar to that of the
voltage error amplifier with the exception of the choice of
crossover frequency. The crossover frequency of the
current amplifier should be at least 10 times that of the
voltage amplifier, to prevent interaction with the voltage
loop. It should also be limited to less than 1/6th that of the
switching frequency, e.g. 16.7kHz for a 100kHz switching
frequency.
There is a modest degree of gain contouring applied to the
transfer characteristic of the current error amplifier, to
increase its speed of response to current-loop
perturbations. However, the boost inductor will usually be
the dominant factor in overall current loop response.
Therefore, this contouring is significantly less marked than
that of the voltage error amplifier. This is illustrated in the
Typical Performance Characteristics.
For more information on compensating the current and
voltage control loops, see Application Notes 33 and 34.
Application Note 16 also contains valuable information for
the design of this class of PFC.
VREF
VBIAS
PFC
OUTPUT
VEAO
VFB
15
2.5V
IAC
2
VRMS
4
ISENSE
3
VEA
–
+
RBIAS
16
IEAO
1
VCC
IEA
+
–
+
–
0.22µF
CERAMIC
ML4800
GND
15V
ZENER
GAIN
MODULATOR
Figure 2. Compensation Network Connections for the
Voltage and Current Error Amplifiers
REV. 1.0.2 3/7/2001
Figure 3. External Component Connections to V
CC
9
FAIRCHILD [ FAIRCHILD SEMICONDUCTOR ]
