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