Things get more complicated when an electronic ballast can supply two lamps and is required to work

even if one lamp is not used or is exhausted, so that it is expected to work at half load as well.

The L6561, thanks to its highly linear, wide dynamics multiplier, extends the use of TM PFC boost pre-

regulators to applications that experience a wide range of operating conditions, both in terms of input

voltage variations and load change. High power (60 to70 W) AC-DC adapters for portable equipment

and computer monitor SMPS’ are the most noticeable examples.

This, however, calls for a more accurate design of the control loop than the one illustrated in Ref.[1]. The

control goal will no longer be to achieve only a low crossover frequency but also an adequate phase

margin. Besides ensuring stability over a large variety of operating conditions, this is necessary to pre-

vent dangerous oscillations of the output voltage as a result of load changes.

PFC Boost Preregulator Control Loop

To the aim of finding a compensation network able to achieve the above mentioned control goal, it is

necessary to get an insight into the control loop of such systems. This can be synthesised as shown in

the block diagram of fig. 2.

Virms

ERROR AMPLIFIER

G1(s)

MULTIPLIER

G2

PWM MODULATOR

POWER STAGE

G4(s)

FEEDBACK

H

G3

+

-

+

-

ZCD

Vref

Vo

Figure 2. Control loop of a PFC Preregulator: Block Diagram

DRIVER

-

+

PWM

COMPARATOR

Rs

E/A

VREF

(2.5V)

L6561

Vo

L

D

Co

7

1

2

4

Q

3

R

Q

5

S

MULTIPLIER

ZCD

STARTER

E/A

COMPENSATION

NETWORK

INV

COMP

MULT

CS

ZCD

GD

G4(s)

G3

H

G1(s)

G2

Vi

R7

R8

R9

R10

Figure 3. Control loop of a PFC Preregulator: electrical circuit and main quantities

AN1089 APPLICATION NOTE

2/12