AN4149 Datasheet, PDF (1/43 Pages) STMicroelectronics – Designing a CCM PFC pre-regulator based on the L4984D

AN4149
Application note
Designing a CCM PFC pre-regulator based on the L4984D
By Hiroshi Andrea Fusillo
Introduction
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).
With the first method the boost inductor works in a continuous conduction mode (CCM) and
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 one uses the more simple peak current-mode control and makes the inductor
work on the boundary between continuous and discontinuous mode, which is implemented
with cheaper controller ICs (e.g. the L6562A, L6563x and L6564x from STMicroelectronics),
and much fewer external parts, making it far more cost efficient. For a given power
throughput, TM operation involves higher peak currents compared to FF-CCM (see the
figures below).
Figure 1. Line and inductor currents in Figure 2. Line and inductor currents in TM
CCM PFC
PFC
AM13300v1
AM13301v1
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.
In the power range 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
stress of both power semiconductors and magnetic components, 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. In this
document the CCM using a fixed-off-time (FOT) control mode, fully integrated in the
controller, is proposed, coupling simplicity and low port count similar to a TM control. The
design procedure is explained too.
June 2013
DocID023523 Rev 2
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