THE ac ac converters have been widely used in various

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1 IEEE TRANSACTIONS ON INDUSTRIA EECTRONICS, VO. 59, NO. 1, JANUARY Cascade Three-evel AC/AC Direct Converter ei i, Member, IEEE, and Dongcai Tang Abstract This paper proposes a novel family of cascade threelevel (T) ac ac direct converters based on ac switch cells, which transfer unsteady high ac voltage with distortion into regulated sinusoidal voltage with low total harmonic distortion (THD). The topological family includes buck T boost, buck boost T, and buck T boost T modes. In order to achieve a reliable T ac ac conversion, a double transient voltage feedback control strategy of the output voltage and the voltage across the flying capacitor is introduced in this paper. A 500-VA 220-V ±10% 50-Hz ac/220-v 50-Hz ac prototype is presented with the experimental results to prove that the converters have four improved advantages simultaneously, including lower voltage across power switches, bidirectional power flow, low THD of output voltage, and higher input power factor. Index Terms AC switch cell, ac ac direct converter, double transient voltage feedback control, three-level (T). I. INTRODUCTION THE ac ac converters have been widely used in various industrial domains in recent years. However, recent research on the ac ac converter technology mainly focuses on two-level ac ac converters and ac dc ac-type multilevel ac ac converters [1] [4]. The former includes ac ac converters with electrical isolation and the ones without any electrical isolation such as ac choppers, thyristor phase-controlled cycloconverters, or matrix converters. The latter includes ac ac converters with no electrical isolation as well as the ones with low or middle frequency electrical isolation. Nowadays, the ac ac converters are required not only for the low-voltage but also for the high-voltage input applications. In these fields, a multilevel technique is effective to reduce the voltage across power switches with improved output voltage. A multilevel technique was firstly proposed in inverters [5] [13] and then developed in dc dc converters and rectifiers [14] [17]. So far, a multilevel technique used in ac ac converters has been mainly limited to ac dc ac-type ac ac converters, which have many shortcomings such as more power stages, unidirectional power flow, low input power factor, and weak adaptability to various loads [18], [19]. Therefore, a cascade three-level (T) Manuscript received August 14, 2010; revised December 7, 2010 and February 17, 2011; accepted March 29, Date of publication April 19, 2011; date of current version October 4, This work was supported in part by the Natural Science Foundation of China under Awards and , by the Natural Science Foundation of Jiangsu Province, China, under Award BK , by the Outstanding Scholar Project, and by the Nanjing University of Science and Technology Research Funding under Award 2010ZYTS043.. i is with the College of Automation Engineering, Nanjing University of Science and Technology, Nanjing , China ( lileinjust@ mail.njust.edu.cn). D. Tang is with Emerson Electronics Company, Shenzhen , China ( w200200@163.com). Digital Object Identifier /TIE Fig. 1. AC switch cells. (a) Two-level ac switch cell. (b) T ac switch cell. ac ac direct converter was proposed in order to improve the multilevel ac ac converters [20]. This paper proposes a novel family of cascade T ac ac direct converters based on ac switch cells. In order to achieve a reliable T ac ac conversion, a strategy of the double transient voltage feedback control is presented also. The converters proposed in this paper have single-stage power conversion (low-frequency alternate-current FAC-FAC), bidirectional power flow, and higher input power factor compared with the ac dc ac-type T ac ac converters. Moreover, the converters have lower voltage across power switches compared with the two-level ac ac converters. The converters are targeted to be used on a new type of regulated sinusoidal ac power supply, electronic transformer, and ac regulator in which highvoltage input (output) and/or bidirectional power flow are needed. II. CONVERTER TOPOOGY As shown in Fig. 1, two-level (u i, 0) and T (u i, u i /2, 0)ac switch cells are presented in this paper. A T ac switch cell is produced by two-level ac switch cells in series /$ IEEE

2 28 IEEE TRANSACTIONS ON INDUSTRIA EECTRONICS, VO. 59, NO. 1, JANUARY 2012 A. Operation Mode A Power switches S 1 S 6 chop with high frequency, and power supply delivers power to the ac load. Topological states during one T s in mode A are shown in Fig. 3. State 1 [t 0 t 1 ] [refer to Fig. 3(a)]: S 2, S 4, and S 5 are on. C b and are both charged by power supply, and voltage u T = u i /2. transfers power to the ac load. The voltage across is u = u i u Cb, so inductor current i increases linearly. The change of i is given by Δi 1 = t 1 t 0 u i u Cb dt = u i u Cb (t 1 t 0 )= u i 2 (t 1 t 0 ). (1) State 2 [t 1 t 2 ] [refer to Fig. 3(b)]: S 1, S 4, and S 5 are on. S 2 is off. is still charged by power supply, and u T = u i. delivers power to the ac load. Voltage u = u i,soi still increases linearly. The change of i is obtained as Δi 2 = t 2 t 1 u i dt = u i (t 2 t 1 ). (2) State 3 [t 2 t 3 ] [refer to Fig. 3(c)]: S 1, S 3, and S 6 are on. S 4 and S 5 are off. C b and transfer power to and the ac load, and u T = u i /2. Voltage u = u Cb u o,soi starts to decrease linearly. The change of i can be given by the following: Fig. 2. Cascade T ac ac direct converter. (a) Buck T boost mode. (b) Buck boost T mode. (c) Buck T boost T mode. Based on ac switch cells, a novel family of cascade T ac ac direct converters shown in Fig. 2 is proposed. The topological family includes buck T boost, buck boost T, and buck T boost T modes. According to different output voltages, pulsewidth modulation (PWM) controlled T ac ac converters chopper in different operation modes with three voltage levels. Therefore, the converters can directly transfer unsteady high ac voltage with distortion into regulated sinusoidal voltage with low total harmonic distortion (THD). III. OPERATING PRINCIPES To simplify the steady-state analysis, the following assumptions are made: 1) Switching and conduction losses of the components are neglected; 2) input and output voltages are considered constant during one switching period T s ; 3) parasitic parameters of inductor for energy storage, flying capacitor C b, and filter capacitor are neglected; and 4) flying capacitor C b is large enough to be considered as a constant dc voltage source with value u i /2. According to the polarities of input voltage u i and current of inductor for energy storage i,the buck T boost mode cascade T ac ac direct converter can work in four kinds of operation modes: A (u i > 0,i > 0), B (u i > 0,i < 0),C(u i < 0,i < 0), and D (u i < 0,i > 0). Δi 3 = t 3 t 2 u Cb u o dt = u i/2 u o (t 3 t 2 ). (3) State 4 [t 3 t 4 ] [refer to Fig. 3(d)]: S 2, S 3, and S 6 are on. S 1 is off. supplies power to and the ac load, and u T =0. Voltage u = u o,soi still decreases linearly. The change of i is Δi 4 = t 4 t 3 u o dt = u o (t 4 t 3 ). (4) In the steady state, the change of i during one T s must be zero, i.e., Δi 1 +Δi 2 +Δi 3 +Δi 4 =0. From (1) (4), (5) is obtained as u i (t 1 t 0 )/2+u i (t 2 t 1 )+(u i /2 u o )(t 3 t 2 ) u o (t 4 t 3 )=0. (5) Then, the ratio of the output root mean square (rms) voltage to the input rms voltage of the converter in continuous conduction mode is given by U o = (t 2 t 0 )+(t 3 t 1 ) U i 2[(t 4 t 0 ) (t 2 t 0 )] = D + D 2(1 D) where D =(t 2 t 0 )/(t 4 t 0 ) is the duty cycle of S 4, S 5 (S 4,S 5) and D =(t 3 t 1 )/(t 4 t 0 ) is the duty cycle of S 1 (S 1). (6)

3 I AND TANG: CASCADE THREE-EVE AC/AC DIRECT CONVERTER 29 B. Operation Mode B Power switches S 1 S 6 chop with high frequency, and the load delivers power to the power supply. Voltage u T varies with u i /2, u i, u i /2, and 0. Topological states during one T s in mode B are shown in Fig. 4. Operation modes C and D are similar to A and B, respectively, and we do not provide detailed analysis in this paper. IV. DESIGN CONSIDERATIONS Design specifications of the buck T boost mode cascade T ac ac direct converter are defined as follows: input voltage U i = V (50 Hz) ac, output voltage U o = 220 V (50 Hz) ac, rated capacity S = 500 VA, switching frequency f s = 100 khz, Δu o 2%u o, Δu Cb 5%u Cb, and Δi 20%i. To ensure the operation of the converter, circuit parameters, including D,, C b,, and S 1 S 6,are determined. A. Designing Duty Cycle D In order to simplify the design of D,, and C b, the current i of inductor and current i Co of filter capacitor are considered as constant during one switching period T s.key waveforms of the converter during one T s are shown in Fig. 5, where u Cb, i Cb, and u Co are the voltage across flying capacitor C b, the current of C b, and the voltage across filter capacitor, respectively. During t 0 t 2, u Co decreases linearly, and then, the decrement Δu Co is Δu Co =Δu o = t 2 t 0 i o dt = i o (t 2 t 0 )= i o T s D. During t 2 t 4, u Co increases linearly, and the increment Δu Co+ can be derived as Δu Co+ = t 4 t 2 i Co+ (7) dt = i Co+ T s (1 D). (8) However, Δu Co =Δu Co+ in one T s, and then, i Co+ = Di o /(1 D), and i = i Co+ + i o = i o /(1 D). During t 0 t 1, u Cb increases linearly, and the increment of u Cb is given by Δu Cb+ = t 1 t 0 i i o dt = C b C b (1 D) (t 1 t 0 ). (9) During t 2 t 3, u Cb decreases linearly, and the decrement of u Cb can be obtained Fig. 3. Topological states during one switching period T s in mode A. (a) State 1 [t 0 t 1 ]. (b) State 2 [t 1 t 2 ]. (c) State 3 [t 2 t 3 ]. (d) State 4 [t 3 t 4 ]. Δu Cb = t 3 t 2 i i o dt = C b C b (1 D) (t 3 t 2 ). (10)

4 30 IEEE TRANSACTIONS ON INDUSTRIA EECTRONICS, VO. 59, NO. 1, JANUARY 2012 Fig. 5. Key waveforms of buck T boost mode cascade T ac ac direct converter. Fig. 6. Principal waveforms of transient output voltage feedback control strategy. However, Δu Cb+ =Δu Cb in one T s, and then, t 1 t 0 = t 3 t 2, and D = D. From(6),U o /U i can be given by the following: U o = D U i 1 D. (11) Therefore, the maximum and the minimum duty cycles are determined by Fig. 4. Topological states during one T s in mode B. (a) State 1. (b) State 2. (c) State 3. (d) State 4. D max =1/(1+ U i,min /U o )= 1/(1+ 198/220)= (12) D min =1/(1+ U i,max /U o )= 1/(1+ 242/220)= (13)

5 I AND TANG: CASCADE THREE-EVE AC/AC DIRECT CONVERTER 31 Fig. 7. Control structure block diagram of the presented control strategy. B. Designing Filter Capacitor From (7), (12), and Δu o 2%u o, must satisfy I o T s D max 2%U o = S T s D max 0.02U 2 o = =2.72 (μf). (14) The maximum voltage across is 2U o = = 311 (V), so is chosen as 4.7 μf/630 V. C. Designing Flying Capacitor C b To simplify the control, let (t 1 t 0 )/T s =0.25. According to (9), (11), u Cb = u i /2, and Δu Cb 5%u Cb, C b must be satisfied with the following expression: C b i o (t 1 t 0 ) (1 D) 5% u i /2 = 40S (t 1 t 0 ) D max (1 D max ) 2 U 2 o = ( ) =2.42 (μf). (15) The maximum voltage across C b is 2U i,max /2= 2 242/2 = 171 (V), soc b is selected as 4.7 μf/630 V. D. Designing Inductor for Energy Storage During t 0 t 2, current i of inductor increases. From (11), u Cb = u i /2, D =(t 2 t 0 )/T s, and (t 1 t 0 )/T s = 0.25, the maximum change of i is Δi = u i u Cb (t 1 t 0 )+ u i (t 2 t 1 ) = (2D 0.25) (1 D) T s u o. (16) 2D Fig. 8. Prototype of cascade T ac ac direct converter. According to (16), Δi 20%i, and i = i o /(1 D), must satisfy (2D min 0.25) (1 D min ) 2 T s U 2 o 0.4D min S = ( ) ( ) =1.186 (mh). (17) Select =1.2 mh. E. Determining Power Switches S 1 S 6 The voltage across S 1 S 4 is obtained as 2Ui,max /2= 2 242/2 = 171(V). (18) Moreover, the voltage across S 5 S 6 is 2Uo = = 311(V). (19)

6 32 IEEE TRANSACTIONS ON INDUSTRIA EECTRONICS, VO. 59, NO. 1, JANUARY 2012 Fig. 9. Experimental waveforms with resistive load (R =96Ω). (a) Input voltage u i and output voltage u o. (b) Voltage across C b u Cb and u i. (c) Voltage across S 1 u ds and u i. (d) Voltage u T. (e) Voltage u T. (f) Current i of inductor for energy storage. The maximum rms current of S 1 S 6 is given by I,max = I o,max 500 = =5.33 (A). (20) 1 D max 198 ( ) Then, MOSFET IRFP460 (500 V/20 A) is chosen for S 1 S 6. V. M ECHANISM FOR CONTROING VOTAGE ACROSS FYING CAPACITOR A strategy of the transient output voltage feedback control, shown in Fig. 6, is introduced for the converters. According to the polarity of u i and i, the converter will work in four modes: A, B, C, and D. If voltage u Cb across flying capacitor C b is out of control, T waveforms of u T cannot be achieved. Therefore, a new double transient voltage feedback control strategy of u o and u Cb is presented, whose control structure block diagram is shown in Fig. 7. EA represents the error amplifier. Sample signal u cf of u Cb is compared with sample signal u if of u i, and then, erroramplified signal u EA c can be obtained. Meanwhile, sample signal u of of u o is compared with reference voltage u ref, and then, another error-amplified signal u EA o can be got. Voltage u EA1 can be gained by adding u EA c to u EA o. By comparing u EA1 and u EA1 with carrier waves u RAMP, PWM signals u 3 u 6 can be obtained. Similarly, u EA2 can be gained by adding u EA c to u EA o. By comparing u EA2 and u EA2 with u RAMP, PWM signals u 1 u 2 can be got. In the positive (negative) half cycle of u o, once u Cb < u i /2, u EA c is positive; then, u EA1 ( u EA1 ) increases, and u EA2 ( u EA2 ) decreases. As the results, the pulses of S 4 and S 5 (S 4 and S 5) turn wider, and the pulse of S 1 (S 1) turns narrower. The charge time and the discharge time of C b turn longer and shorter, respectively, so u Cb can be controlled to be u i /2. On the other hand, if sample signal u of of u o is less than reference voltage u ref, u EA o is positive, and then, u EA1 and u EA2 ( u EA1 and u EA2 ) rise. Therefore, the pulses of S 1, S 4, and S 5 (S 1, S 4, and S 5) turn wider, so u o can be increased to be the expected value.

7 I AND TANG: CASCADE THREE-EVE AC/AC DIRECT CONVERTER 33 Fig. 10. Experimental waveforms with R (R =72Ω, = 200 mh) and RC load (R =72Ω, C =50μF). (a) Output voltage u o and output current i o. (b) Output voltage u o and output current i o. VI. PROTOTYPE The designed and developed prototype is as follows: Buck T boost mode circuit topology, double transient voltage feedback control strategy, rated capacity S = 500 VA, input voltage U i = V (50 Hz) ac, output voltage U o = 220 V (50 Hz) ac, duty cycle D = , switching frequency f s = 100 khz, inductance for energy storage = 1.2 mh, flying capacitance C b =4.7 μf/630 V, filter capacitance =4.7 μf/630 V, MOSFET IRFP460 (500 V/20 A) for S 1 S 6, and load power factor cos ϕ = The prototype shown in Fig. 8 has the following good performances: rated capacity S = 500 VA, input voltage U i = V (50 Hz) ac, precision of output voltage 1.5 V, load power factor cos ϕ = , output voltage THD < 3.5%, conversion efficiency at rated power for different types of loads η %, line power factor at rated different nature load cos ϕ , operational time of 120 min at 110% rated load, weight < 2.5 kg, and bulk < 175 mm 170 mm 130 mm. Experimental waveforms of the converter are shown in Figs. 9 and 10. The experimental results have verified that the converter has the following advantages such as low THD of u o, symmetrical voltage u Cb and u Cb be controlled as u i /2, lower voltage across the power switches in the buck T stage (u i /2), T (u i,u i /2, 0) in voltage u T, strong adaptability to various loads, etc. Fig. 11 illustrates how the curves of the line power factor, THD of u o and u i, and conversion efficiency vary with the load. According to the results, the converter achieves high conversion Fig. 11. ine power factor, THD, and conversion efficiency versus the load. (a) ine power factor versus output power. (b) THD of input and output voltages versus output power at U i = 220 V. (c) Conversion efficiency versus output power. efficiency, higher line power factor, and low THD of output voltage. VII. CONCUSION In this paper, a novel family of cascade T ac ac direct converters has been proposed based on ac switch cells. The converters directly transfer unsteady high ac voltage with distortion into regulated sinusoidal voltage with low THD. The

8 34 IEEE TRANSACTIONS ON INDUSTRIA EECTRONICS, VO. 59, NO. 1, JANUARY 2012 topological family includes buck T boost, buck boost T, and buck T boost T modes. By introducing the double transient voltage feedback control strategy, the T ac ac conversion and lower voltage across power switches can be reliably achieved. This paper also describes the design and the development of a 500-VA 220-V ±10% 50-Hz ac/220-v 50-Hz ac prototype. Experimental results show that the converters reduce the voltage across the power switches in the T stage to u i /2, which is only a half of the traditional two-level ac ac converters. The input power factor is higher than at rated capacity, which is better than the ac dc ac-type T ac ac converters. Furthermore, low THD of output voltage and the function of bidirectional power flow are also demonstrated in this paper. REFERENCES [1] R. K. Gupta, K. K. Mohapatra, A. Somani, and N. 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IEEE IPEMC, 2009, pp ei i (M 09) received the B.S. degree from the Department of Electrical Engineering, Shandong University of Science and Technology, Qingdao, China, in 1997, and the Ph.D. degree from the Department of Electrical Engineering, Nanjing University of Aeronautics and Astronautics, Nanjing, China, in He is currently an Associate Professor with the College of Automation Engineering, Nanjing University of Science and Technology, Nanjing. He has published more than 50 technical papers. His research interests include multilevel technique, high-frequency power conversion, and control technique. Dr. i was the recipient of one first class reward production of science and technology of Jiangsu Province and is the holder of three China patents. Dongcai Tang received the B.S. degree from the Department of Electrical Engineering, Xuzhou Teachers College, Xuzhou, China, in 2007, and the M.S. degree from the College of Power Engineering, Nanjing University of Science and Technology, Nanjing, China, in He is currently an Engineer with the Emerson Electronics Company, Shenzhen, China. He has published several technical papers. His research interests include multilevel technique and ac/ac converters.