AN3112 Datasheet, PDF (1/36 Pages) STMicroelectronics – Solution for designing a fixed off-time controlled PFC pre-regulator

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).
Figure 1. Line, inductor, switch and diode
currents in FF-CCM PFC
Figure 2. Line, inductor, switch and diode
currents in TM PFC
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.
February 2011
Doc ID 16820 Rev 3
1/36
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