AN628 Datasheet, PDF (1/35 Pages) STMicroelectronics – DESIGNING A HIGH POWER FACTOR SWITCHING

AN628
APPLICATION NOTE
DESIGNING A HIGH POWER FACTOR SWITCHING
PREREGULATOR WITH THE L4981 CONTINUOUS MODE
by Ugo Moriconi
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
April 2004
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