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ML4805

Variable Feedforward PFC/PWM Controller Combo

Fairchild Semiconductor 

FUNCTIONAL DESCRIPTION (Continued)

VFB

17

2.5V

IAC

2

VRMS

4

ISENSE

3

RAMP 1

8

RTCT

7

18

VEAO

1

IEAO

VEA

-

+

1.6kΩ IEA

+

–

GAIN

MODULATOR

8V

1.6kΩ

POWER FACTOR CORRECTOR

2.75V

PFC

CONTROLLER

-1V

OSCILLATOR

÷2

DUTY CYCLE

LIMIT

OVP

+

-

PFC ILIMIT

+

-

Figure 1. PFC Section Block Diagram

ML4805

15

VCC

7.5V

REFERENCE

VREF

16

PFC

OUTPUT

DRIVER

PFC OUT

14

inversely proportional to VRMS2 (except at unusually

low values of VRMS where special gain contouring

takes over to limit power dissipation of the circuit

components under heavy brownout conditions). The

relationship between VRMS and gain is designated as K.

3) The output of the voltage error amplifier, VEAO. The

gain modulator responds linearly to variations in this

voltage.

The output of the gain modulator is a current signal, in the

form of a full wave rectified sinusoid at twice the line

frequency. This current is applied to the virtual-ground

(negative) input of the current error amplifier. In this way

the gain modulator forms the reference for the current

error loop, and ultimately controls the instantaneous

current draw of the PFC from the power line. The general

form for the output of the gain modulator is:

IGAINMOD

=

IAC ´ VEAO

VRMS 2

´ 1V

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giveenn bbyy::

IGAINMOD = K × (VEAO − 0.625V) × IAC

(1)

where K is in units of V-1.

Note that the output current of the gain modulator is

limited to ≅ 500µA.

Current Error Amplifier

The current error amplifier’s output controls the PFC duty

cycle to keep the current through the boost inductor a

linear function of the line voltage. At the inverting input

to the current error amplifier, the output current of the

gain modulator is summed with a current which results

from a negative voltage being impressed upon the ISENSE

pin (current into ISENSE ≅ VSENSE/1.6kΩ). The negative

voltage on ISENSE represents the sum of all currents

flowing in the PFC circuit, and is typically derived from a

current sense resistor in series with the negative terminal

of the input bridge rectifier. In higher power applications,

two current transformers are sometimes used, one to

monitor the ID of the boost MOSFET(s) and one to monitor

the IF of the boost diode. As stated above, the inverting

input of the current error amplifier is a virtual ground.

Given this fact, and the arrangement of the duty cycle

modulator polarities internal to the PFC, an increase in

positive current from the gain modulator will cause the

output stage to increase its duty cycle until the voltage on

ISENSE is adequately negative to cancel this increased

current. Similarly, if the gain modulator’s output

decreases, the output duty cycle will decrease to achieve

a less negative voltage on the ISENSE pin.

Cycle-By-Cycle Current Limiter

The ISENSE pin, as well as being a part of the current

feedback loop, is a direct input to the cycle-by-cycle

current limiter for the PFC section. Should the input

voltage at this pin ever be more negative than -1V, the

output of the PFC will be disabled until the protection

flip-flop is reset by the clock pulse at the start of the next

PFC power cycle.

REV. 1.1 3/9/2001

7

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