INTRODUCTION

PFC preregulators based on the boost topology working in Transition Mode (TM, see fig. 1) have been

widespread in electronic lamp ballast systems. This kind of equipment almost always works under a sin-

gle mains supply (110 or 220 VAC, with some tolerance) and the use of a PFC preregulator is mainly

aimed at optimising the downstream half-bridge lamp driver and improving their inherent extremely poor

PF.

The PFC preregulator sees the downstream stage as a constant load, so it is requested to work under a

limited range of operating conditions. From the control loop standpoint, this means that the frequency

compensation of the error amplifier can be very simple, typically just a feedback capacitor. Its capaci-

tance will be high enough to ensure the crossover frequency of the open loop gain is low, so as to

achieve a high PF (see Ref. [1]).

March 2000

®

AN1089

APPLICATION NOTE

CONTROL LOOP MODELING OF L6561-BASED TM PFC

by Claudio Adragna

This paper provides a model and a tool for evaluating and improving the control loop characteristics

of L6561-based PFC preregulators in boost topology and operated in Transition Mode (TM).

Such a subject is now becoming topical since TM PFC preregulators are more and more used in sys-

tems other than electronic lamp ballast where the input voltage range is limited and the load current

is almost constant.

The ability to operate under large variations of both input voltage and load current, as well as the use

of TM PFC systems as preregulators for switching converters, requires a more accurate design of the

control loop. The goal will be not only to ensure a narrow bandwidth in order to achieve a high Power

Factor, but also to have enough phase margin so as to make sure the system is stable over a large

range of operating conditions.

8

3

R9

R10

Vac

5

6

L6561

7

21

R7

Co

Rs

R8

+

-

4

Vcc

Vo

Compensation

Network

Figure 1. Typical L6561-based TM PFC preregulator

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