AN3142 Datasheet, PDF (1/44 Pages) STMicroelectronics – Solution for designing a 400 W fixed-off-time controlled

AN3142
Application note
Solution for designing a 400 W fixed-off-time controlled
PFC preregulator with the L6563S and L6563H
Introduction
In addition to the transition mode (TM) and fixed-frequency continuous conduction mode
(FF-CCM) operation of PFC preregulators, a third approach is proposed 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) preregulators 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 simpler peak current-mode control, which is implemented with cheaper controller ICs
(e.g. the L6561, L6562, L6562A and L6563S from STMicroelectronics), and far fewer
external parts, therefore it is far less expensive. In the first method the boost inductor works
in continuous conduction mode (CCM), while TM makes the inductor work on the boundary
between continuous and discontinuous mode, by definition. For a given power throughput,
TM operation involves higher peak currents as 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 midrange
power level, around 150-300 W. The assessment of 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 not only power semiconductors and magnetic 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 17005 Rev 3
1/44
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