Closing the Control Loop

The control loop of a high-PF flyback converter based on the L6561, can be synthesised as in the block

diagram of fig. 13.

Unlike conventional converters, in such regulators the control loop will have quite a narrow bandwidth so

sure a high PF. On the other hand, it is not possible to achieve a very high PF (>0.99), thus it makes no

sense to have a very narrow bandwidth (<20 Hz) like in boost PFC preregulators. This would degrade

the transient response to line and load changes without any benefit. A compromise will then be found

between these two contrasting terms.

To the aim of deriving the transfer functions of the blocks in fig. 13, the narrow bandwidth of the control

loop allows to assume that the control action takes place on the peak amplitude of the various quantities.

The error amplifier (E/A) of the L6561 is compensated as illustrated in fig. 14. The transfer function

G1(s) will be then:

G1

(s) =

= −

⋅

1

1

.

ERROR AMPLIFIER

MULTIPLIER

POWER STAGE

FEEDBACK

+

-

G1(s)

G2(s)

G4(s)

H(s)

Vref

Vout

Vcomp

PWM MODULATOR

G3(s)

+

-

Vin

ZCD

Vcx

Ve

Figure 13. Block Diagram of the Control Loop of an L6561 - based high-PF flyback.

COMP

INV

2.5V

E/A

+

_

L6561

C2

R7

R6

TO MULTIPLIER

R8

Figure 14. Compensation of the Error Amplifier.

AN1059 APPLICATION NOTE

13/20