ML4800CP【PDF 9/14 页】IC PFC CTRLR AVERAGE CURR 16DIP

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PRODUCT SPECIFICATION

ML4800

REV. 1.0.5 9/25/01

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

ampliers. Figure 2 shows the types of compensation

networks most commonly used for the voltage and current

error ampliers, along with their respective return points.

The current loop compensation is returned to V

REF

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 amplier; stability and transient response.

Optimizing interaction between transient response and

stability requires that the error ampliers 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 ML4800s

voltage error amplier has a specially shaped non-linearity

such that under steady-state operating conditions the

transconductance of the error amplier is at a local

minimum. Rapid perturbations in line or load conditions

will cause the input to the voltage error amplier (V

) to

deviate from its 2.5V (nominal) value. If this happens, the

transconductance of the voltage error amplier will increase

signicantly, 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 amplier compensation is similar to that of the

voltage error amplier with the exception of the choice of

crossover frequency. The crossover frequency of the current

amplier should be at least 10 times that of the voltage

amplier, 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 amplier, 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 signicantly less marked than that of the

voltage error amplier. 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 V

VEAO

IEAO

RMS

SENSE

2.5V

VEA

IEA

REF

PFC

OUTPUT

GAIN

MODULATOR

ML4800

GND

BIAS

0.22礔

CERAMIC

15V

ZENER

BIAS

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