CM6800AGIP datasheet(10/18 Pages) CHAMP

10 / 18 page

CM6800A

LOW START-UP CURRENT PFC/PWM CONTROLLER COMBO

2006/10/11 Rev. 1.3

Champion Microelectronic Corporation

Page 10

OSCILLATOR

ISENSE

RAMP1

GAIN

MODULATOR

VRMS

IAC

VFB

PFC OUT

7.5V

REFERENCE

IEAO

VCC

VEAO

VREF

0.5V

0.3V

-1V

VCC

2.5V

17.9V

VCC

GND

2.75V

MNPFC

GMi

VCC OVP

MPPFC

GMv

3.5K

PFC ILIMIT

3.5K

PFC OVP

CLK

LOW POWER

DETECT

PFC CMP

POWER

FACTOR

CORRECTOR

TRI-FAULT

Figure 1. PFC Section Block Diagram

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

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.

PFC Voltage Loop

There are two major concerns when compensating the

voltage loop error amplifier, VEAO; 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 CM6800A’s voltage error amplifier, VEAO 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 perturbation 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

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 characteristics.

The Voltage Loop Gain (S)

EAO

OUTDC

EAO

OUT

EAO

OUT

≈

ZCV: Compensation Net Work for the Voltage Loop

GMv: Transconductance of VEAO

PIN: Average PFC Input Power

VOUTDC: PFC Boost Output Voltage; typical designed value is

380V.

CDC: PFC Boost Output Capacitor

PFC Current Loop

The current amplifier, IEAO compensation is similar to that of

the voltage error amplifier, VEAO with 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.

The Current Loop Gain (S)

OUTDC

SENSE

EAO

OFF

ISENSE

≈

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