A simple single-switch single-stage AC/DC converter with harmonic current correction

Transcription

1 A simple single-switch single-stage AC/DC converter with harmonic current correction Chun-Lin Yeh, Ting-Wei Hou a), and Chien-Ming Chao Department of Engineering Science, National Cheng Kung University No. 1, Sec. 1, Ta-Hsueh Rd., East District, Tainan City 701, Taiwan (R.O.C.) a) Abstract: A simple single-switch single-stage AC/DC converter based on the flyback topology with the high power factor correction, high efficiency and small size is proposed and implemented. It complies with IEC for low frequency harmonics under a power range (20-50 W) at 110 VAC input. The proposed converter introduces the auxiliary winding of the transformer to replace the bulk inductor of the conventional converter topologies. The voltage of the bulk capacitor is (1) clamped under the rectified line voltage and (2) not influenced by the output load. The efficiency reaches 89 90% and the power factor is between 0.94 and Keywords: AC/DC converter, power factor correction, IEC Classification: Science and engineering for electronics References 1] J. Sun and H. Grotstollen, Averaged modeling of switching power converters: reformulation and theoretical basis, Proc. IEEE PESC, Toledo, Spain, pp , June ] M. M. Jovanovic, D. M. C. Tsang, and F. C. Lee, Reduction of voltage stress in integrated high-quality rectifier regulator by variable frequency control, Proc. IEEE APEC, pp , ] N. Vazquez, J. Lopez, J. Arau, C. Hernandez, and E. Rodriguez, A different approach to implement an active input current shaper, IEEE Trans. Ind. Electron., vol. 52, no. 1, pp , Feb ] T. W. Hou, C. L. Yeh, and J. T. Lai, Analysis and simulation for a novel single-switch single-stage AC/DC converter, Proc. IEEE TENCON 2007, pp. 1 4, Oct. 30 -Nov. 2, ] L. K. Chang and H. F. Liu, A novel forward AC/DC converter with input current shaping and fast output voltage regulation via reset winding, IEEE Trans. Ind. Electron., vol. 52, no. 1, pp , Feb ] O. Garcia, J. A. Cobos, P. Alou, R. Prieto, and J. Uceda, A simple singleswitch single-stage AC/DC converter with fast output voltage regulation, IEEE Trans. Power Electron., vol. 17, no. 2, pp , March

2 1 Introduction In recent years, a variety of AC/DC converters with power factor correction (PFC) have been presented. The major advantages of such converters are the high power factor correction (HPFC), high efficiency, small size and low cost. There are two basic kinds of PFC converters: single- and two-stage. Two-stage converters have the PFC stage and DC/DC stage to comply with low frequency harmonic regulation and to obtain the desired regulated output voltage. They have the advantages of HPFC and low voltage stress. More recently, single-stage converters have been developed to reduce the size and cost and increase the efficiency of two-stage converters 2, 3, 4, 5, 6], by sharing a common switch and controller for both PFC and DC/DC stages. Single-stage converters are thus becoming more widely utilized in low power applications due to their simple power stages and control circuits. However, the main drawback of single-stage converters is a high voltage stress across the bulk capacitor (DC-bus) at a light load and high line voltage 2]. It is thus necessary to find a way to reduce the voltage stress. In this paper, we propose a single-stage converter that clamps the voltage stress across the bulk capacitor under the rectified line voltage, and there is no relation between the voltage of the bulk capacitor and the load. We present a detailed analysis of the steady-state and the experimental results in this paper. This proposed converter is depicted in Fig. 1. It is derived from the flyback topology with a single switch and a single control loop. The key idea is the use of a multi-winding transformer, so that the bulk inductor is replaced by the auxiliary winding of the transformer to reduce the volume and weight of the magnetic material used, and thus reduces costs. For the single-stage topology, we just use one transformer made of magnetic material, but 6] uses two transformers and 3] uses one bulk inductor and one transformer. We compare the efficiency of our system with that of those contained in these earlier studies in Section 4. Though a similar approach to that adopted in this work appears is in 5], this earlier paper presents a forward topology, in contrast to the flyback topology of our system. This proposed single-stage converter operates at 110 VAC input with HPFC, high efficiency and fast output voltage regulation. The measured power factor (PF) of the prototype is within the range of 0.94 to 0.95 under a power range (20-50 W). The efficiency is 89 90%. The line harmonic cur- Fig. 1. Proposed single switch single stage converter 1758

3 rent complies with IEC Class D. Although a preliminary version of this study, which only has simulation results, appears in 4], in this paper we have a complete presentation, with both the steady state analysis and the implementation of the prototype to demonstrate its feasibility and examine its performance. 2 Proposed circuit and operation principle The proposed single-switch single-stage AC/DC converter is shown in Fig. 1. Based on a flyback converter, it is composed of transformer T, transistor Q, bridge rectifier, diodes (D 1, D 2 and D 3 ), bulk capacitor C b and output capacitor C o, operating in the input line voltage 110 V, while the output regulated voltage is 24 VDC under a load range (20-50 W). The magnetic current of transformer T determines whether the circuit operation mode is the discontinuous current mode (DCM) or continuous current mode (CCM). The turn ratio n 1 /n 2 of transformer T determines the voltage stress across the bulk capacitor, and the turn ratio n 1 /n 3 of transformer T determines the output voltage. This proposed converter operates in DCM. The control stage is similar to a DC/DC converter, since the output voltage is the only controlled magnitude. The circuit operation of one switching period T s is divided into four stages. The waveforms of V N1,V N2,i Cb,i Co and the switch (S) overone switching period are illustrated in Fig. 2 (a). The equivalent circuits of the operation stages over one switching period T s are in Fig. 2 from (b) to (e). The integrity functions V N1,i N1,V N2,i N2,V N3, i N3,V Co,i Co derived from stages 1 to 4 are in 4]. 1) Stage 1: t 0 < = t < = t 1 When S turns on, stage 1 begins. Both the transformer s primary and auxiliary windings are charged with the line rectified voltage (V r ) and bulk capacitor voltage (V Cb ), respectively, so i N1 and i N2 are increased. The output diode (D 1 ) is turned off because the polarity of V N3 is inversed; the energy stored on the output capacitor is delivered to the load. When S turns off, this stage ends. 2) Stage 2: t 1 < = t < = t 2 When S turns off, stage 2 starts. All the transformer s windings change their polarity. The energy stored on the transformer s primary winding is delivered 1759

4 Fig. 2. The waveforms over one switching period T s are in (a), and the equivalent circuits for stages 1 to 4 are in (b) to (e), respectively to the output capacitor and load, so i N3 continues to increase. At the same time, the bulk capacitor is charged with some of i N1.Wheni N1 reaches zero, stage 2 ends. 3) Stage 3: t 2 < = t < = t 3 During stage 3, the stored energy on the transformer s output winding is delivered to the output capacitor and load. At the same time, V Cb is clamped. When the remaining energy of the transformer is completely transferred to the load, this stage ends. 4) Stage 4: t 3 < = t < = t 4 During stage 4, the energy of the transformer keeps at zero at all time, showing that the transformer works in DCM. When S is turned on, this stage ends. 3 Steady-State Analysis There are four energy storage elements L N1, L N2, C b and C o in the proposed converter. Bulk capacitor voltage V Cb and output capacitor voltage V Co are taken as state variables. The magnetic inductor currents of the transformer vanish at the initiation and the end of every switching period T s. Hence, L N1 and L N2 are not considered as state variables. Over one switching period T s, we perform the steady-state analysis with four stages of d 1 T s, d 2 T s, d 3 T s and d 4 T s, as shown in Fig. 2 (a). The signals V N1, V N2, i Cb and i Co are considered. The average state-variable equations and constraint equations of this proposed converter are C b d v Cb (t) C o d v Co (t) = ī Cb (t) = ī Co (t) (1) (2) 1760

5 L N1 dī N1 (t) = v N1 (t) =0 L N2 dī Cb (t) = v N2 (t) =0 (3) (4) The output equation is v o (t) = v Co (t) (5) The following equations can be derived from the waveforms in Fig. 2 (a) d v C Cb (t) b = ī Cb = (d 1 ) 2 v C b L N (d 2 ) 2 v ] C b L N1 +0 d v C Co (t) o =ī Co = 1 v ] Co R (d 1+d 2 +d 3 +d 4 ) (d 2+d 3 ) 2 T 2 v Co S L N3 ] v N1 = 1 v r d 1 n 1 n 3 v Co (d 2 + d 3 ) =0 ] v N2 = 1 v Cb d 1 + n 2 n 3 v Co (d 2 + d 3 ) =0 (6) (7) (8) (9) The operation points of the steady state are v r = V r, d 1 = D 1, v Cb = V Cb and v Co = V o. The differentiation items in both (6) and (7) are set to zero. From (6) to (9), V Cb and V o can thus be obtained V Cb = n 2 V r (10) n 1 RT s V o = D 1 V r (11) 2L N1 From Eq. (10), we know that the voltage across on the bulk capacitor is dependent on the turn ratio n 2 /n 1 and V r. There is no relation between V Cb and load R. Hence, there is no high stress voltage across on the bulk capacitor when the load is light. Eq. (11) shows that the output voltage is directly proportional to the square root of load R. Hence, controlling the load variation regulates the output voltage. 4 Experimental Results of the Proposed Circuit The prototype of the proposed converter in Fig. 3 (a) is designed and implemented. The transformer core PQ-3220 is employed. The specifications and component values of this prototype are as follows: (1) the input rating:110 VAC/60 Hz, (2) output voltage: 24 VDC, (3) switching frequency f s : 50 khz, (4) magnetizing inductor L N1 : 250 µh, (5) turns ratio n 1 :n 2 :n 3 : 1761

6 Fig. 3. The results of the experiment include the (a) photograph of the proposed circuit prototype, (b) input voltage and current waveforms, (c) V Cb and V r waveforms, (d) V Cb versus output power, (e) PF versus output power, (f) efficiency versus output power, (g) harmonic contents of the input current and (h) efficiency comparison with 3] and 6] 24:24:8, (6) bulk capacitor C b :1 µf, (7) output capacitor C o : 3000 µf and (8) output power ranges: W. The experimental results are shown in Fig. 3 (b)-(h). In Fig. 3 (b), the waveforms of input voltage and current are shown at full load. In Fig. 3 (c), the waveforms of V Cb and V r show that V Cb is clamped under the input rectified voltage (V r ). In Fig. 3 (d), the curve of V Cb versus the output power shows that there is no relation between V Cb and the load. In Fig. 3 (e), the curve of PF versus the output power shows the range of PF is In Fig. 3 (f), the curve of efficiency versus the output power shows the range of efficiency is 89%-90%. In Fig. 3 (g), the harmonic contents of the input current are shown, which ensures the standard IEC class D is fulfilled. In Fig. 3 (h), we show the efficiency of the prototype, on average, is 12.2% better than in 3] and 2.37% than in 6]. 1762

7 5 Conclusions In this paper, a simple single-switch single-stage AD/DC converter with harmonic current correction is designed and implemented. During a power range of between 20 and 50 W, the power factor is between 0.94 and 0.95 and the efficiency is from 89% to 90%. The input line current meets the standard IEC class D requirements. The voltage of the bulk capacitor is not influenced by the output load and is clamped under the input rectified voltage. The proposed converter introduces the auxiliary winding of the transformer to replace the bulk inductor of the conventional converter topologies, and this also reduces the volume and weight of magnetic material required, and thus saves costs. The goals of HPFC, high efficiency, low voltage stress across the bulk capacitor, small size and low cost are achieved in this proposed converter. The proposed converter can be applied in low power applications such as battery chargers or LED lighting power drivers. In future applications, it can achieve greater efficiency by moving from hard-switching to soft-switching. 1763