AN4149 datasheet(1/43 Pages) STMICROELECTRONICS

Numer części	AN4149

Szczegółowy opis	Designing a CCM PFC pre-regulator based on the L4984D

PDF	43 Pages

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Producent	STMICROELECTRONICS [STMicroelectronics]

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June 2013

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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).

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.

Figure 1. Line and inductor currents in

CCM PFC

Figure 2. Line and inductor currents in TM

PFC

AM13300v1

AM13301v1

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