February 2011

Doc ID 16820 Rev 3

1/36

AN3112

Application note

Solution for designing a fixed off-time controlled PFC pre-regulator

with the L6564

Introduction

In this document we propose a third approach to the operation of PFC pre-regulators. In

addition to the transition mode (TM) and the fixed-frequency continuous conduction mode

(FF-CCM) operation of PFC pre-regulators, an alternative approach is offered that couples

the simplicity and affordability of TM operation with the high-current capability of FF-CCM

operation. This solution is a peak current-mode control with fixed-off-time (FOT). Design

equations are given and a practical design for a 400 W board is illustrated and evaluated.

Two methods of controlling power factor corrector (PFC) pre-regulators, based on boost

topology, are currently in use: the fixed-frequency (FF) PWM and the transition mode (TM)

PWM (fixed on-time, variable frequency). The first method employs average current-mode

control, a relatively complex technique requiring sophisticated controller ICs (e.g. the

L4981A/B from STMicroelectronics) and a considerable component count. The second uses

the more simple peak current-mode control, which is implemented with cheaper controller

ICs (e.g. the L6561, L6562, L6562A and L6564 from STMicroelectronics), and much fewer

external parts making it far more cost efficient. In the first method the boost inductor works

in a continuous conduction mode (CCM), while TM makes the inductor work on the

boundary between continuous and discontinuous mode. For a given power throughput, TM

operation involves higher peak currents compared to FF-CCM (

Figure 1 and Figure 2).

This demonstration, consistent with the above mentioned cost considerations, suggests the

use of TM in a lower power range, while FF-CCM is recommended for higher power levels.

This criterion, though always true, is sometimes difficult to apply, especially for a mid-range

power level of around 150-300 W. Assessing which approach gives the better

cost/performance trade-off needs to be done on a case-by-case basis, considering the cost

and the stress of both power semiconductors and magnetics, but also of the EMI filter. At the

same power level, the switching frequency component to be filtered out in a TM system is

twice the line current, whereas it is typically 1/3 or 1/4 in a CCM system.

Figure 1.

Line, inductor, switch and diode

currents in FF-CCM PFC

Figure 2.

Line, inductor, switch and diode

currents in TM PFC

www.st.com