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ML4800_0109 Fiches technique(PDF) 9 Page - Fairchild Semiconductor

No de pièce ML4800_0109
Description  Power Factor Correction and PWM Controller Combo
Download  14 Pages
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Fabricant  FAIRCHILD [Fairchild Semiconductor]
Site Internet  http://www.fairchildsemi.com
Logo FAIRCHILD - Fairchild Semiconductor

ML4800_0109 Fiches technique(HTML) 9 Page - Fairchild Semiconductor

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PRODUCT SPECIFICATION
ML4800
REV. 1.0.5 9/25/01
9
Error Amplier 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 VREF 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 cross-
over 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 (VFB) 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 Charac-
teristics. 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.
Figure 2. Compensation Network Connections for the
Voltage and Current Error Amplifiers
Figure 3. External Component Connections to VCC
15
VEAO
IEAO
VFB
IAC
VRMS
ISENSE
2.5V
+
16
2
4
3
VEA
+
IEA
+
VREF
1
PFC
OUTPUT
GAIN
MODULATOR
ML4800
VCC
GND
VBIAS
0.22
µF
CERAMIC
15V
ZENER
RBIAS


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