ABSTRACT The electrolytic capacitors used in rectification are usually bulkier and more expensive than the other components composing a power converter. Therefore, the compactness and cost of topologies are often sacrificed. This paper presents a new simple approach to reduce the size of the input capacitor of a switch mode power supply. The method-proposed provides an indirect charging path for the capacitor, thus avoiding direct connection to the supply line. During this disconnected period, where the line voltage is appreciably high, power is directly supplied to the output. Within this very period the input capacitor is charged periodically but indirectly through the ripple voltage across the inductor of the output LC filter. The capacitor automatically couples to the line only when the line voltage has dropped to the required level whereby the voltage rating of the capacitor has been reduced. Moreover the input line current has improved its shape and therefore the power factor has been improved compared to a peak detecting type rectifier. Due to the low voltage associated, the size of the filter capacitor becomes much smaller, whereby an economical design is made possible. In this paper the performance has been experimentally verified and compared with a commercial computer power supply to confirm the validity of the circuit. Index Terms-Power Factor, Switch Mode Power Supply, Filter Capacitor Size 1.0 INTRODUCTION With the advance of switching and controller topologies new methods have been introduced to reduce the value of the capacitive filter of the rectifier stage. However, the size reduction is badly affected by the voltage and the ripple voltage requirements of the filter capacitor. Since the capacitor directly appears across the line, capacitor voltage rating essentially must be above 325V for a 230 V supply. To bear this large ripple voltage while keeping the life expectancy beyond a minimal prescribed value, expensive capacitor has to be used. As illustrate in Fig. 1, the conventional implementations unnecessarily strain the capacitor for small energy and output voltage requirements. 1 Transactionsof the IEE Sri Lanka September 2002 Figure 1 - Capacitor discharge for a rectifier stage in a conventional system. Identifying the fact that the required DC output is lower, the proposed method reduces the upper voltage level of charging the capacitor. The capacitor is connected to the output line only during the discharging period. Thus the ripple voltage substantially decreases as shown in the Fig. 2 (a) and (b). (a) Suppressed Input Capacitor Voltage (b) Experimental Voltage Waveforms across C S - bottom trace; Input DC bus voltage-upper trace. Fig. 2. Voltage waveforms of C S , dc bus, and line 2 D. B. Ekanayake, S. G. Abeyratne A Compact Rectifier Stage with an Improved Power Factor D.B. Ekanayake, S. G. Abeyratne Department of Electrical and Electronics Engineering, University of Peradeniya, Sri Lanka. UPPER CHARGING LEVEL RIPPLE WITHSTANDING LEVEL RIPPLE WITHSTANDING LEVEL CAPACITOR ENERGY SUPPLY PERIOD voltage ripple