AN628 APPLICATION NOTE

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THE L4981 PFC CONTROLLER IC

The L4981 integrated circuit is a continous mode average current controller with several specific functions for

active power factor correction. It can operate in high quality, medium/high power conversion range and provides

all the necessary features to achieve a very high power factor, up to 0.99. Thanks to the BCD technology used,

operative switching frequency higher than 200kHz can be used.

The L4981 can be used in systems with universal input mains voltage without any line switch.

This new PFC offers the alternative of synchronization working at fixed frequency (L4981A), or working in mod-

ulated frequency (L4981B) to optimize the size of the input filter. Both devices control the conversion in average

current mode PWM to maintain a sinusoidal line current without slope compensation.

MAIN FEATURES:

P.F.C. BOOST TOPOLOGY OPERATION

The operation of the P.F.C. boost converter (see fig. 2) can be summarized in the following description.

The A.C. line voltage is rectified by a diode bridge and the rectified voltage delivered to the boost converter. The

boost converter section, using a PWM switching technique, boosts the rectified input voltage to a D.C. controlled

(D), an output capacitor (CO) and, obviously, a control circuitry.

Referring to the time-variable mains voltage (sine waveform), the converter produces a boost inductor average

current like the rectified input voltage, changing continuosly the duty-cycle of the active switch (Q).

The boosted D.C. voltage is controlled to a programmed value, higher than the maximum input instantaneous

Referring to the main currents shown in fig.2 schematic, the simplified formulae are (assuming: power efficiency

= 1; output ripple voltage = 0; high frequency inductor ripple current = 0):

1) Peak inductor (L), switch (Q) and diode (D) currents

2) RMS inductor current

I

Lpk

I

Qpk

I

Dpk

2

P

O

V

lpk

-----------

⋅

==

=

I

Lrms

2

P

O

V

lpk

-----------

⋅

=