1/35

AN628

APPLICATION NOTE

April 2004

INTRODUCTION

Conventional AC-DC converters usually employ a full wave rectifier bridge with a simple capacitor filter to draw

power from the AC line. This "bulk" capacitor must be big enough to supply the total power during most of each

half-cycle, while instantaneous line voltage is below the DC rectified voltage. Consequentely, the line current

waveform is a narrow pulse, and the power factor is poor (0.5-0.6) due to the high harmonic distortion of the

current waveform.

If a high power factor switching preregulator is interposed between the input rectifier bridge and the bulk filter

capacitor, the power factor will be improved (up to 0.99). Increasing in addition, the RMS current capability from

the mains, reducing the bulk capacitor peak current and the harmonic disturbances.

Switching at a frequency much higher than the line's one, the preregulator draws a sinusoidal input current, in

phase with the input line voltage.

There are several way that this can be accomplished. When the output voltage is higher than the input voltage

(Vo > Vin), BOOST topology and continuos inductor current control mode are well suited to produce a good

quality input sine current waveform. The input di/dt is low because the inductor is located between the bridge

and the switch. This minimizes line noise and the line spikes will be absorbed by the inductor.

Figure 1. L4981 Block Diagram

by Ugo Moriconi

DESIGNING A HIGH POWER FACTOR SWITCHING

PREREGULATOR WITH THE L4981 CONTINUOUS MODE