Input Filter Design for Power Factor Correction Converters Operating in Discontinuous Conduction Mode zy - zy Diode =Filter - - Vlad Grigore, Jorma Kyyr;d Institute of Intelligent Power Electronics Helsinki University of Technology P.O.Box zyxwvuts 3000 FIN-02015 HUT, Finland - Power Factor - Correction zyxwvu 1 DCDC 1 - 1 z Abstract: Power Factor Correction (PFC) converters operating in Discontinuous Conduction Mode (DCM) are very attractive for use in lowcost and low-power applications, due to simpler control when compared with Continuous Conduction Mode (CCM) operation. Design of the EM1 input filter for DCM operation has different constraints in certain aspects, when compared with the CCM case. This paper focuses on particularities of the input filter design in the DCM case. INTRODUCTION In recent years requirements for power quality in power supply systems have been rising. Various international and national standards limiting the harmonic content of input current of equipment connected to the public network have come or will come into force [I]. For these reasons, producers of power supply systems have had to reduce the harmonic content of the input current of their rectifier systems by introducing a supplementary Power Factor Cmection (PFC) stage. Typical configuration of a power supply with PFC and output voltage regulation is shown in Figure zyxwvutsrq 1. The PFC stage consists of a DC-DC converter whose average input current tracks a sinusoidal waveform. The Boost converter, presented in Figure 2a) is widely used for this purpose. Continuous Conduction Mode (CCM) is used for power levels exceeding few hundred watts (e.g. 300W), and the corresponding input current is shown in Figure 2b). A current loop in the control circuit is needed to make the average input current track a sinusoidal reference. For lower power, Discontinuous Conduction Mode (DCM) is attractive. Input current in DCM is shown in Jyri Rajamaki Safety Technology Authority P.O. Box. 123 (Liinnrotinkatu 37) FIN-00 18 1 Helsinki, Finland Figure 2c). In this operation mode the converter has inherent PFC properties, meaning that, at constant duty-cycle zyx D =TON /Ts , the average input current automatically tracks to some extent the sinusoidal shape of the input voltage [2]. This is realized without the need of sensing and controlling the input current, thus simplifying control circuit. Moreover, such feature zyxw can be used to integrate the PFC stage with the output voltage regulation stage into a single- switch converter. Several integrated topologies are reported [3], [41, and [SI, in which the Boost inductor is operated in DCM. This approach is deemed to be a low-cost one, since one switch and its control circuit are saved. The PFC stage has pulsating input current, which gives EM1 levels above the limits specified in [6] virtually in all practical cases. An input filter is needed to restrict EMI. At same power level, it is expected that this problem is more severe in DCM when compared with CCM, because of the larger high frequency ripple. This paper aims to provide design criteria for the input filter of a PFC stage based on Boost converter in DCM, and to compare these criteria with the CCM case. Generation of differential-mode conducted EM1 is analyzed, based on the waveform of the input current. Firstly, we describe the models used in the analysis and characterize the differential-mode EM1 without an input filter. Secondly, we analyze the requirements for the input filter: attenuation level to meet regulatory specifications, low input displacement angle between filter input voltage and current, minimum interaction with the PFC stage and system stability. Finally, conclusions are presented. Figure 1. Typical configuration of a power supply with PFC and output voltage regulation. iL h+-:$-fqR iLGp_-- jLK;A, a) b) 4 Figure 2. Boost converter for PFC a) Schematic; b) Input current in CCM; c) Input current in DCM. 0-7S03-5057-W99~$l0.00 0 1999 IEEE 145