DIRECT BUCK-TYPE AC/AC CONVERTER BASED ON SWITCHED-CAPACITOR

Transcription

1 DREC BCK-YPE ACAC CVERER BASED SWCHED-CAPACR elles Brunelli Lazzarin, Marcos Paulo Moccelini, Barbi Federal niversity of Santa Catarina - FSC, Power Electronics nstitute - EP P box 59, ZP code , Florianpolis, SC, BRAZL telles@inep.ufsc.br, moccelini@inep.ufsc.br, ibarbi@inep.ufsc.br Abstract Switched-capacitor (SC) converters mainly non-isolated DC-DC converters have been a very important research topic for many years. Recently, switched capactitors have also been applied in direct AC- AC conversion, and studies have presented two different bidirectional SC ac-ac converters, which intended to replace the conventional auto-transformer in commercial and residential applications. he analysis of one of the presented converters shows that, by sacrificing the bidirectionality, the circuit can be shortened, resulting in a reduction of its overall cost and size. he proposed change modifies the behaor of the converter in each operational stage, thus a new analysis is needed. herefore, this paper studies, describes, analyzes, designs and tests the proposed new topology. he paper also compares the new converter with the preous topology and presents its advantages, disadvantages and applications. n order to demonstrate the performance of that converter, the design example and experimental results of a prototype of kw, 22 V inputoutput ltages, and switching frequency of khz, are reported herein. he maximum and nominal efficiencies shown by the prototype were 97.6% and 96.%, respectively. Keywords buck-type, direct ac-ac converter, switchedcapacitor.. RDC t is common to find commercial and residential deces that use a different ltage from the one available in the electrical grid, and, thus, generally a low powerlow ltage autotransformer must be employed as a solution. However, as the auto-transformer is an electromagnetic transformer, it suffers from low efficiency, audible noise and the heavy weight of the magnetic core. Another important matter to take into consideration is the global shortage of copper, commonly used to construct the windings of the transformer [], [2], [3]. he Switched-Capacitor (SC) has been an important research topic for many years, mainly in relation with nonisolated DC-DC static power conversion [4], [5]. he main applications of the SC converters are: power supplies for mobile electronics systems [6], [7]; electric vehicles [8]; battery equalizer circuits [9] and ltage-balancing circuits for multilevel inverters [], []. As SC power converters are composed only of capacitors and electronic switches, it is possible to achieve smaller size and weight comparing to common switched-mode power supplies, which use magnetic deces. Also, the SC converter behaor can be described by simple equivalent circuits. he switched capacitor principle was recently extended to AC-AC static conversion for the first time []. n the article, a brief analysis and experimental results for a stepdownstep-up converter with rated power of 6 W, 22 V, 6 Hz, switching frequency of 5 khz and a measured peak efficiency of 95.6% were presented. he converter supplies a differential output ltage and it employs two SC s and eight unidirectional switches. Another publication [2] proposed a new bi-directional topology with rated power of kw, 22 V or 22 V, 6 Hz and switching frequency of khz and a measured peak efficiency of 97.8% were presented. he converter employs two fixed capacitors and one SC, it supplies an output ltage which has a common reference with the input ltage, and it uses four bi-directional switches. he purpose of this paper is to present the theoretical analysis and practical results for a reduced version of the topology presented in [2]. By sacrificing the bi-directionality of the converter, one of the fixed capacitors can be removed from the circuit, reducing its overall cost and size. he absence of one of the capacitors changes the behaor of the circuit in each operational stage, thus a new analysis is needed.. HE PRPSED SWCHED-CAPACR AC-AC CVERERS he original topology introduced by [2] consists of four bidirectional switches (S,,, ) and three capacitors: the SC C and the fixed capacitors C 2 and (Figure ). C Fig.. riginal bidirectional converter topology proposed in [2]. he new proposed SC AC-AC converter topology is shown in Figure 2 (a). t is obtained by the elimination of one of the fixed capacitors from the original circuit, C 2. he elimination of C 2 takes away the bidirectionality characteristic of the converter and therefore only the stepdown operation becomes possible. he elimination of the capacitor instead of C 2 is possible for a step-up operation S C $3. 23 EEE 23

2 S S S S C (a) Buck-type topology (b) Boost-type topology Fig. 2. nidirectional topologies. C C 2 C (a) Fig. 4. v o C (b) Buck converter topological states. v o (boost-type), as shown in Figure 2 (b). Both cases must be feeding on a source with ltage characteristics. his paper studies in detail the step-down version (buck-type) presented in Figure 2 (a).. HERECAL AALYSS A detailed low-frequency (input ltage frequency) and high-frequency (switching frequency) theoretical analysis of the step-down converter is presented in this section. B. High-frequency (HF) analysis he operational stages of the converter are discussed in this section. he closed-switch resistance non-ideality (R on ), as well as a load resistance R o >> R on are taken into consideration. he analysis is detailed only for the positive half-cycle of the input ltage because the operational stages are very similar in the negative half-cycle; however, the ltages and the directions of the currents in the elements are the opposite from those of the positive half-cycle. A. opological stages he proposed converter presents two topological stages during its operation, which are controlled by the switch gate signals shown in Figure 3. he switches S and operate together, as well as and. he duty-cycle of all the switches is.5, and S gate signals are complimentary of the gate signals. he switching logic is very simple and can be achieved with a single C3525 C. C 2R on C 2R on S S R o Δt A Δt B (a) First stage ( t ) R o 2R on R o 2R on R o C C Δt 2A Δt 2B (b) Second stage ( t 2) Fig. 3. Gate signals of the bi-directional switches. Fig. 5. Buck converter operational stages in the positive half-cycle. n the first stage, S and are turned on, and and are turned off. he converter operates with C and seriesconnected, as shown in Figure 4 (a). n this stage, the circuit becomes a simple ltage dider and output ltage is 2. n the second stage, and are turned on, and S and are turned off. he converter works with C and parallel-connected, as shown in Figure 4 (b). n this stage, the input ltage source is disconnected from the circuit and the capacitor C works to maintain a ltage equilibrium with. herefore, the SC C applies and maintains a ltage in equal to 2, i.e., the converter supplies an output ltage equal to 2. First stage ( t ) starts when the switches S and are closed. During t A, the capacitors are series-connected and receive energy from the ltage source. When the current in reaches zero, it starts discharging ( t B ). C charges during the whole stage. he switches S and are then opened and the stage is concluded. Figure 5 (a) illustrates this topological stage. he currents in S,,,, C and and the ltages, v o and across C during this stage ( t ) are presented in Figure 6 (a) for the positive half-cycle and in Figure 6 (b) for the negative half-cycle. Second stage ( t 2 ) starts when the switches and are closed. he capacitors are parallel-connected 23

3 and the ltage source is disconnected from the circuit. C charges C2 during t2a. When the current in C2 reaches zero, both capacitors start discharging ( t2b ). he opening of S2 and S4 marks the end of the stage. Figure 5 (b) illustrates this topological stage. he currents and the ltages during this stage ( t2 ) are also presented in Figure 6 (a) for positive half-cycle and Figure 6 (b) for negative half-cycle. Gv = vc3 = = =.5 2 _max2 + input ltage. he theoretical LF waveforms of the proposed converter are shown in Figures 7 (a), 7 (b) and 7 (c). max _max 2 is3 is3 ωt (b) Capacitor ltages ωt vs =vs2 =vs3 =vs4 _max2 (a) nput and output ltages vc =vc3 is () ωt is (c) Switch ltages Fig. 7. Main low frequency waveforms. is2 is4 D. Equivalent model According to [2], an equivalent circuit for the proposed converter, shown in Figure 8, can be obtained. he calculations are based on (2) presented in [2], where fs is the switching frequency, Req is the equivalent resistance and D, the duty-cycle. is2 is4 ic ic3 ic ic3 Req = fs C e (2fs Ron C) vc ( D) D DS Δt2B (-D)S (a) ΔtA (2).5 vc ΔtA ΔtB Δt2A e (2fs Ron C) + e (2fs Ron C) + e (2fs Ron C).5 ΔtB Δt2A DS Req Δt2B (-D)S _eq Ceq Ro_eq (b) Fig. 6. High-frequency waveforms: (a) positive half-cycle, (b) negative half-cycle. Fig. 8. Equivalent circuit of the proposed converter. C. Low-frequency (LF) analysis Considering C and C3 equal capacitors, the ltage across each one of them tends to be 2 in both series and parallel stages. he maximum ltage across the switches is also 2. As is the ltage across C3 and vc3 is ideally 2, the ltage gain (Gv ) is calculated by (). hus, the converter presents the behaor of a ltage dider for any type of input ltage (AC or DC). herefore, if a sinusoidal ltage is connected to the input of the converter, it outputs a sinusoidal ltage equal to half the 232 As shown in [2], the lowest Req is obtained with D =.5 and, consequently, the ltage drop and energy losses are minimized for that duty-cycle value. Eq. (3) shows the calculation of Req for a 5% duty-cycle. Req(D=.5) = + e (2fs Ron C) fs C e (2fs Ron C) (3) able summarizes the calculated value of the circuit elements.

4 ABLE Circuit elements Parameter Proposed converter riginal converter [2] C eq C eql = 2C C eql = 3C R eq R eql = R eq R eql = R eq V i eq V i 2 V i 2 V. EXPERMEAL RESLS n order to verify the proposed converter operation, a prototype was built in laboratory. he experimental results and a comparison with the original bidirectional converter is presented within this section. able shows the converter specifications, which are similar to the specifications of the original converter, since that allows a better comparison. he waveforms were recorded with rated power output and a.4 duty-cycle (. dead-time). ABLE Converter specifications Description Value nput Voltage 22 V utput Voltage V utput Power W nput Voltage Frequency 6 Hz Switching Frequency khz Capacitors 2 µf MSFE R on.6 mω Fig.. nput and output ltages ( Hz). Figure 7. he ltage stresses on internal components is one half of the input ltage. Figure 2 shows v S detailed at switching frequency, which reveals the waveform does not present overltage. Figures 9 and show the input and output ltages. As expected, the converter works with a ltage gain of.5, i.e., v o is one half of. he frequency of the input ltage is 6 Hz in Figure 9 and, in Figure, is Hz. his shows the converter operates normally for different input ltage frequencies. Fig.. Voltages across S, C and (v o). Fig. 9. nput and output ltages (6 Hz). he ltages across the switch S and capacitors C and can be seen in Figure. hose experimental waveforms are very similar to the theoretical waveforms presented in Figure 3 shows the ltage and current outputs supplied by the converter at rated power ( kw) with a resistive load. he measured output ltage was 6.6 V, which correponds to a regulation near 96.9%. he input ltage and the input current at the rated power can be seen in Figure 4. he input current i i presents a high frequency component and it leads the ltage by approximately, which is expected due to the topological stages of the converter and the circuit being capacitive. f the ltage source can not supply the high frequency component of the input current, a filter must be added to the converter. he frequency of the current ripple is high ( khz), and, thus, a small LC filter is sufficient. hat configuration was also tested, with C = µf and L = 5µH. he current i s supplied by the source, i.e., before the LC filter, can be also seen in Figure 4. he current i s is approximately sinusoidal and without distortion. 233

5 ABLE Results comparison Description riginal [2] Proposed Peak efficiency 97.8% 97.6% Efficiency ( kw) 96.2% 96.% Power factor ( kw) 96.9% 98.5% Voltage regulation 96.9% 96.9% original bidirectional converter. A negligible loss of efficiency (due to the disconnection of the ltage source in the second operational stage), similar regulation and higher power factor are observed in the new converter results, as able shows. Fig. 2. Voltage across S detailed at khz. Fig. 5. Efficiency Fig. 3. utput ltage and current. he experimental efficiency curve plotted in Figure 5 shows that, for a wide range of loads, the experimental efficiency is higher than 96%. he measured efficiency at rated power was 96.% and the efficiency peak were 97.6% and occurred at approximately 4 W. Figure 5 also shows the efficiency curve of the original converter studied in [2], making edent the proposed circuit reduction does not decrease significantly the efficiency of converter. Fig. 4. nput ltage (), current supplied by the source (i s) and converter input current (i i). Figures 5, 6 and 7 show the efficiency, regulation and power factor, respectively, of the proposed converter and the Fig. 6. Regulation he output regulation curve can be seen in Figure 6. A 234

6 ltage drop of less than 3.5% was noted for rated power compared to the no-load output ltage. Again, the proposed and original converters showed similar results. he input power factor curves plotted in Figure 7 show that the power factor is close to one at rated power, but it drops considerably as active power decreases. hat is a consequence of the capacitive circuit and the parameters chosen. he comparison between the original and proposed converters demonstrates that the latter performs better in terms of input power factor, measured 98.5% at rated power. hose characteristics were expected due to fact the total capacitance of the circuit was reduced. Fig. 7. Power Factor V. CCLSS A new switched-capacitor AC-AC converter was proposed with the following characteristics: no inductors or other magnetic elements present, only capacitors and switches employed; operation in open-loop; common reference between input and output ltages; and step-down operation with a ltage gain of.5. he proposed converter topology was based on another SC AC-AC converter, but with the number of capacitors reduced. Consequently, the cost and size of converter were also reduced, although, as a disadvantage, the converter became unidirectional. he paper presented the theoretical analysis of the converter and its operation was verified through experimental results. he measured maximum and rated power efficiencies were 97.6% and 96.%, respectively. he ltage gain measured in rated power was.485. A comparison between the new and the original converter showed that both presented similar performance. herefore, the proposed converter presents the advantage of a reduced number of capacitors and the disadvantage of being unidirectional. hose characteristics indicate the new converter is an appropriate alternative solution for the auto-transformer in unidirectional and low cost applications. REFERECES []. B. Lazzarin, R. L. Andersen, G. B. Martins, and. Barbi, A 6-w switched-capacitor ac ac converter for 22 v v and v22 v applications, EEE ransactions on Power Electronics, l. 27, no. 2, pp , 22. [2] R. Andersen,. Lazzarin, and. Barbi, A kw step-upstep-down switched-capacitor ac-ac converter, l. 28, pp , 23. [3] R. B. Gordon, M. Bertram, and. Graedel, Metal stocks and sustainability, Proceedings of the ational Academy of Sciences of the nited States of America, l. 3, no. 5, pp , 26. [4] A. oinoci, Switched-capacitor power electronics circuits, Circuits and Systems Magazine, EEE, l., no. 3, pp , 2. [5] M. S. Makowski and D. Maksimoc, Performance limits of switched-capacitor dc-dc converters, in Power Electronics Specialists Conference, 995. PESC 95 Record., 26th Annual EEE, l. 2. EEE, 995, pp [6] F. Z. Peng, F. Zhang, and Z. Qian, A magnetic-less dc-dc converter for dual-ltage automotive systems, ndustry Applications, EEE ransactions on, l. 39, no. 2, pp. 5 58, 23. [7] F. Zhang, L. Du, F. Z. Peng, and Z. Qian, A new design method for high-power high-efficiency switchedcapacitor dc dc converters, Power Electronics, EEE ransactions on, l. 23, no. 2, pp , 28. [8] Z. Amjadi and S. S. Williamson, A novel control technique for a switched-capacitor-converter-based hybrid electric vehicle energy storage system, ndustrial Electronics, EEE ransactions on, l. 57, no. 3, pp , 2. [9] C. Pascual and P.. Krein, Switched capacitor system for automatic series battery equalization, in Applied Power Electronics Conference and Exposition, 997. APEC 97 Conference Proceedings 997., welfth Annual, l. 2. EEE, 997, pp [] B. Axelrod, Y. Berkoch, and A. oinoci, A boostswitched capacitor-inverter with a multilevel waveform, in Circuits and Systems, 24. SCAS 4. Proceedings of the 24 nternational Symposium on, l. 5. EEE, 24, pp. V 884. [] Y. Hinago and H. Koizumi, A switched-capacitor inverter using seriesparallel conversion with inductive load, ndustrial Electronics, EEE ransactions on, l. 59, no. 2, pp , 22. [2] J. W. Kimball and P.. Krein, Analysis and design of switched capacitor converters, in Applied Power Electronics Conference and Exposition, 25. APEC 25. wentieth Annual EEE, l. 3. EEE, 25, pp