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IEEE TRANSACTIONS ON POWER ELECTRONICS, VOL. 23, NO. 1, JANUARY 2008 229 Unity-Power-Factor Operation of Three-Phase AC–DC Soft Switched Converter Based On Boost Active Clamp Topology in Modular Approach H. M. Suryawanshi, Member, IEEE, M. R. Ramteke, K. L. Thakre, and V. B. Borghate Abstract—In this paper, a three-phase ac–dc converter using three single-phase pulse width modulated active clamped, zero-voltage-switched boost converter in modular approach is presented. The active clamp technique is used for zero-voltageswitching of the main and auxiliary switches. The operating modes, analysis, and design considerations for the proposed converter are explained. To evaluate the performance of the proposed converter, finally simulation and experimental results for a 500-V, 1.5-kW prototype converter are presented. The proposed converter operates at almost unity power factor with reduced output filter size. The output voltage is regulated without affecting zero-voltage-switching, even under unbalanced three-phase input voltages. Index Terms—Active clamp, boost converter, three-phase ac–dc converter, unity power factor. I. INTRODUCTION U SAGE of power electronic (PE) converters are ever increasing in the processing of electrical energy in industrial applications such as adjustable speed drives (ASD), SMPSs, UPSs, etc. [1]. Therefore, the converters with high power factor are increasingly required in industries. In high-power range, mainly a three-phase system is employed. Most of the PE systems which get connected to ac utility mains use diode rectifiers at the input. The nonlinear nature of diode rectifiers cause significant line current harmonic generation; thus, they degrade power quality, increase losses, failure of some crucial medical equipment, and so on. Therefore, stringent international harmonic standards are imposed, and hence power factor correction (PFC) circuits are incorporated in PE systems. Earlier, to reduce rectifier-generated harmonics, expensive and bulky filter inductors and capacitors were installed, but they effectively eliminate only certain harmonics [2]. The active power line conditioners (APLCs) used for harmonic reduction are generally hard switched; hence, the components are subjected to high-voltage stresses which increases further with increase in the switching frequency. Also, hard switching results in low efficiency, large EMI, etc., as discussed in [3]. Manuscript received January 25, 2007; revised May 23, 2007. Recommended for publication by Associate Editor P. M. Barbosa. The authors are with the Department of Electrical Engineering, Visvesvaraya National Institute of Technology, Nagpur-440011, India (e-mail: [email protected]; [email protected]; klthakre@ vnitnagpur.ac.in; [email protected]). Digital Object Identifier 10.1109/TPEL.2007.911842 The soft-switched resonant converters are also reported in [4]–[6]. However, some of their characteristics such as large conduction losses, high component stresses, load limitation, and high cost restrict the practical use of these converters as discussed in [7]. Such converters are usually operated in variablefrequency mode, and thus components are required to be designed at the lowest operating frequency. Also, resonant tank circuits are required to be designed at a much higher kVA/kW rating. The active clamp technique is one of the most attractive zerovoltage-switching (ZVS) topologies [8], [9]. Medium and high power ac–dc converters usually make use of continuous conduction mode (CCM) boost topology as it gives near to unity power factor (UPF) at the AC input [10]. Industries are focusing on extension of existing and wellproven single-phase technology for the development of highpower converters. A modular approach provides a convenient paralleling of modules, thus facilitating power expandability. Modules being identical, reserve inventory requirement, manufacturing cost, and time are also reduced. It would also reduce a problem like arduous heat dissipation and expensive components of high rating which may occur in single high power design [11], [12]. Modular approach is also presented in [13] and [14]. The proposed converter has higher efficiency and lower THD in input current than [13]. In [14], there exist interactions between different modules which causes boost inductor current of the same module to be different during the off interval of the main switches. In the proposed converter, no such interaction takes place between modules. The active clamp technique is well known for ZVS operation in various converters, but its application in three-phase, high input power factor ac–dc converters in modular approach is not yet reported in the literature. In this paper, a three-phase ac–dc converter with input powerfactor almost unity and soft-switching topology in modular approach is presented. An identical single-phase boost type active clamped ac–dc converter is connected in each line of a three-phase AC source. Outputs of all the three converter modules are connected in parallel to raise the power level. Each single-phase module is operated from 115-V, 50-Hz input to give regulated output of 500 V with a switching frequency of 50 KHz. The power rating of each module is 500 W. The circuit configuration, operation, analysis, and design considerations of the proposed converter are presented. Finally, simulation and experimental results obtained with the prototype are presented to 0885-8993/$25.00 © 2007 IEEE 230 IEEE TRANSACTIONS ON POWER ELECTRONICS, VOL. 23, NO. 1, JANUARY 2008 Fig. 2. Single-phase module of the proposed converter. Fig. 1. Three-phase modular converter system. demonstrate the performance and design considerations. Similar to a three-phase active rectifier, the output voltage has a dominant ripple of six times the line frequency. Thus, a smaller output filter capacitor is required. It also works under unbalanced input ac voltage maintaining almost unity power factor at the input. As a separate PFC-PWM controller is provided for each module, the independent control is facilitated even under unbalanced input voltages. The PFC-PWM controllers are not very costly; therefore, increase in the cost is marginal. Reliability in term of availability of output power to some extent in case of failure of a module or single phasing is also achieved. The proposed converter maintains an input power-factor almost unity, regulated output voltage, and ZVS for all the switches even under variable load conditions. It has only two switches in each module, and has lesser components and lesser complexity compared to [3], [5], and [15]. The proposed converter has applications such as drives, power conditioning stages of UPSs, etc. IC (UC3854) and fed to Driver IC (UC3706). Drive IC provides complementary gate drive pulses with sufficient dead band. A PCB-mounted miniature current LEM is used for sensing the boost inductor current. When boost inductor current exceeds the set limit, drive pulses are disabled, hence the converter is protected. Proposed converter uses average current mode control. In average current mode control, boost inductor current is continuously monitored and controlled to follow the reference signal proportional to ac line voltage. Thus, input current is sinusoidal. To regulate output voltage, a multiplier circuit controls the amplitude of the sinusoidal current reference signal in accordance with the voltage error signal generated using the output voltage and rectified input ac voltage. When the load decreases, the output voltage increases. To maintain constant load voltage, the control circuit senses the load voltage and the pulse width is automatically reduced in the switching cycle and the output voltage is regulated and maintained almost constant. The control circuit varies the duty ratio in switching cycles over the input supply voltage cycle, as the instantaneous input supply voltage is varying over the cycle. B. Operating Principle II. CIRCUIT DESCRIPTION AND OPERATING PRINCIPLE A. Circuit Description Fig. 1 shows simplified block-diagram of a three-phase ac–dc converter in a modular system. Fig. 2 shows circuit diagram of the proposed single-phase module. The proposed converter and folconsists of a small line filter comprising of lowed by single-phase line rectifier and a very small . Unlike the conventional high-frequency bypass capacitor and the boost converter, in addition to the boost inductor high-frequency (HF) rectifier output diode ; the resonant inductor in series and resonant capacitor in parallel are . The auxiliary switch with connected to the main switch is connected between series connected clamping capacitor and the cathode of the . The small capacthe drain of the itor is used as a high-frequency bypass filter at the output of each module. Both the switches are driven in a complemenis used at the tary manner. The single output filter capacitor output of the proposed three-phase converter whose size get reduced drastically owing to the fact that, like the three-phase active rectifier, the dominant ripple frequency is six times the input source frequency. By sensing boost inductor current, output dc and input ac voltages, gating pulses are generated accordingly using PFC-PWM To simplify the analysis and operating modes of the circuit, the following assumptions are made. • The semiconductor devices, inductors, and capacitors are ideal. • The output filter capacitor is large enough to maintain constant output voltage . is constant over one • The rectified output voltage switching cycle as switching frequency is very high compared to ac input frequency. is much larger than ; and • The boost inductor is much larger than capacitor . clamping capacitor • The energy stored in is larger than energy stored in . The circuit behavior during one switching cycle can be explained in seven modes with the help of key waveforms in Fig. 3 and operating states of the converter in Fig. 4 under steady state. The sequential circuit states are described below. : Fig. 4(a) shows the operating state of the Mode 1 is already in circuit during this interval. The auxiliary switch starts conan off-state. The body diode of the main switch ducting, and the output diode continues conduction. During continues to decrease this mode, the boost inductor current also starts decreasing. The negand the output diode current continues to decrease. This ative resonant inductor current mode ends when it decreases to zero, and the body diode of the SURYAWANSHI et al.: UNITY-POWER-FACTOR OPERATION OF THREE-PHASE AC–DC SOFT SWITCHED CONVERTER 231 : As shown in Fig. 4(d), the current, which Mode 4 was flowing through the main switch, is diverted to resonant . The voltage across the main switch starts capacitor increasing from zero, and when it reaches , this mode ends. During this mode (4) (4a) Fig. 3. Key waveforms of the proposed converter. main switch ceases to conduct. To turn-on the main switch with ZVS, a gate pulse must be applied during this interval. During this mode (1) (1a) (1b) (1c) Mode 2 : Fig. 4(b) shows the operating state of the turns on circuit during this interval. At , the main switch and starts carrying the positive resonant inductor current . is achieved. The boost inductor current Thus, ZVS of switch continues to decrease. This mode ends when output diode becomes zero. During this mode current where and . As soon as the main switch voltage reaches approximately starts conducting. The body equal to , the output diode diode of the auxiliary switch starts conducting only after . Since the interval the main switch voltage reaches between the start of conduction of the diode and the start of conduction of the body diode of auxiliary switch is very small compared to switching cycle, this is not treated as a separate continues until the body mode. Actually, current through starts conducting. diode of the auxiliary switch : Fig. 3(e) shows the operating state of Mode 5 the circuit during this interval. The boost inductor current starts decreasing. The output diode continues conducting, starts increasing. If clamping capacitor and its current is large, then clamping capacitor voltage is almost constant, and inductor current decreases linearly, otherwise varies resonantly. This mode ends when the resonant inductor current becomes zero. Thus, the body diode of the auxiliary switch ceases to conduct. During this interval, a gate pulse must be applied to auxiliary switch in order to achieve ZVS. During this mode (5) (5a) (2) (5b) (2a) (5c) (5d) (2b) Mode 3 : During this mode, the main switch continues to conduct as shown in Fig. 4(c). A load is supplied by the output filter capacitor . Input power is stored in boost inductor and resonant inductor . Therefore, the boost inductor curstarts increasing. The resonant inductor current conrent is tinues to increase. This mode ends when the main switch turned off. During this mode where and Mode 6 : As shown in Fig. 4(f), the auxiliary switch starts conducting. Thus, ZVS of switch is achieved. The boost inductor current continues to decrease. This mode ends when the auxiliary switch is turned off. During this mode (3) (6a) (3a) where is the duty cycle of the main switch, and duty cycle for the interval . is the (6) (6b) (6c) (6d) 232 IEEE TRANSACTIONS ON POWER ELECTRONICS, VOL. 23, NO. 1, JANUARY 2008 Fig. 4. Operating modes of the proposed converter. Mode 7 : Fig. 4(g) shows the operating state of the concircuit during this interval. The boost inductor current continues to decrease due tinues to decrease. The voltage . When becomes zero, the to negative inductor current starts conducting and this body diode of the main switch and the resonant capacmode ends. The resonant inductor form a resonance circuit. To achieve ZVS of the main itor switch , must reach zero at the end of this mode. This SURYAWANSHI et al.: UNITY-POWER-FACTOR OPERATION OF THREE-PHASE AC–DC SOFT SWITCHED CONVERTER requirement compels that stored energy in the resonant inductor must be greater than resonant capacitor : 233 TABLE I COMPONENTS VALUES (7) (7a) (7b) and where . III. DESIGN CONSIDERATION Neglecting transition intervals between two switches and based on voltage-second balance of the boost inductor , from (1)–(3) and (5) and (6); we get (8) Therefore, output voltage is given by completely Thus (16) (9) If and , we get (10) If peak-to-peak ripple current of the boost inductor Neglecting interval and , the required value of By choosing a suitable value of can be found out. plays an important role. The main The clamp capacitor . If switch voltage is clamped to the voltage level is small, will be relatively large. Thus, voltage stresses on the switches will increase, and hence design of is crucial. is approximately The duration required to reach to can be one half of the off period of the main switch, thus expressed as (11) Therefore, from (10) and (11), we get ; thus, the time required for discharging is given by (17) where (12) is the switching frequency. will be maximum at full load with minimum rectified line . Also, peak-to-peak ripple voltage of clamp output voltage capacitor can be expressed as (13) will be maximum for the value calculated by (13) at value of boost inductor . should be greater than ; therefore, minimum (18) is (14) As per discussion in Mode 7, energy stored in the resonant inmust be greater than the energy stored in the resonant ductor to achieve ZVS for the main switch . Thus capacitor (15) discharges At the end of Mode 7, the resonant capacitor to zero in the quarter cycle of the resonant frequency Peak-to-peak ripple voltage of clamp capacitor is generally kept of the maximum capacitor dc voltage below 20% calculated using (17) above. If , using (17) can be obtained as in and (18), the value of clamp capacitor [10] (19) The value of the output filter capacitor is dependent on allowable peak-to-peak value of output voltage ripple and maximum load current and input line frequency . has to limit the six times the line frequency component The 234 IEEE TRANSACTIONS ON POWER ELECTRONICS, VOL. 23, NO. 1, JANUARY 2008 Fig. 5. Simulation results at full load. (a) Clamp capacitor voltage (V ), (b) boost inductor current (i ), and (c) resonant inductor current (i ). Fig. 6. Simulated input currents (i at full load. ;i ;i ) and input R-phase voltage (v ) Fig. 8. Experimental input R-phase voltage (v ) and input currents (i ; i ; i ) at (a) full load (b) 50% of full load (c) 25% of full load, and (d) full load with unbalanced input voltage (v = 115 V, v = 126:5 V, and v = 103:5 V),(scales: 50 V/div., 2 A/div., 5 ms/div.). of the rectifier output voltage. For the given can be obtained [5], [15] as Fig. 7. Experimental main switch voltage (v ) and main switch current (i ), and auxiliary switch voltage (v ) and auxiliary switch current (i ) at full load (scales: 200 V/div., 5 A/div., 1 s/div. (zoom view)). and , (20) SURYAWANSHI et al.: UNITY-POWER-FACTOR OPERATION OF THREE-PHASE AC–DC SOFT SWITCHED CONVERTER 235 Fig. 10. Experimental main diode current (i ) and resonant inductor current (i ) at full load (scales: 5 A/div., 5 s/div.). Fig. 9. Experimental performance characteristics of the proposed converter (a) efficiency, (b) power factor, and (c) % THD. IV. SIMULATION AND EXPERIMENTAL RESULTS The converter shown in Figs. 1 and 2 is designed as discussed in Section III and simulated. The converter behavior is studied at different load conditions as well under unbalanced input voltages. The components’ values (shown in Table I) and other specifications of the single-phase module of the proposed converter are as follows. Specifications: : 115 V , 50 Hz. Input voltage : 500 V. Output voltage : 500 W. Output power Total output power W. : 50 kHz. Switching frequency For inductors, Litz wires NELC175/40SPSN which has 175 strands of 40 AWG is used. This reduces the effective resistance , of inductor as well as its skin effects. For boost inductor , N87 ferrite core (Siemens make) and for resonant , , N87 ferrite core (Siemens make) is inductor used. , Fig. 5 shows the simulated clamping capacitor voltage , and resonant inductor current boost inductor current at full load, respectively. Fig. 5(b) of boost inductor current shows that the converter operates in continuous conduction is mode (CCM). The maximum resonant inductor current . The voltage the same as that of maximum boost inductor is maximum at full load, across the clamping capacitor which is 23.5 V in this case. Fig. 6 shows the simulated threeat phase input currents with input R-phase input voltage full load. It is observed that ac input line currents follow their sinusoidal input voltages; thus, input power-factor is almost unity. Fig. 11. Output voltage under step change in load. (a) From full load to half load. (b) From half load to full load. (scales: 100 V/div., 5 ms/div). The experimental switch voltages and currents at full load are shown in Fig. 7, and it is evident that both the switches operate with ZVS. Fig. 8 shows the experimental three-phase input current with R-phase voltage under variable load conditions from full load to 25% of full load with balanced three-phase input voltages. From Fig. 8(a)–(c), it is evident that the proposed converter operates at almost unity power factor over a wide range of load variation, and input current decreases with decrease in load. Fig. 8(d) shows the experimental input currents under uninput voltages ( V, balanced Rated voltage V, and V) at full load. It is seen that the proposed converter maintains unity power factor even under unbalanced input voltages. Under unbalanced input voltages, neutral current will have triple harmonics. The performance characteristics of the proposed converter under variable load condition are given in Fig. 9. It is observed that the efficiency varies from 94.9% to 92.5%, power factor varies from 0.999 to 0.997, and total harmonic distortion (THD) varies from 1.1% to 2.7% from full load to 25% of full load. In a hard-switched converter, efficiency is nearly about 90% [16]. The proposed converter has a higher efficiency compared to [13] and [16]. Fig. 10 shows and main diode current .A resonant inductor current SiC diode is used as an output diode . Therefore, it has no reverse recovery characteristics [17]. However, if a fast recovery 236 IEEE TRANSACTIONS ON POWER ELECTRONICS, VOL. 23, NO. 1, JANUARY 2008 Si diode is used, its reverse recovery current is controlled by resconnected in series with main switch . onant inductor For the given output voltage, reverse recovery current will decrease with increase in . Fig. 11 shows the variation in output voltage under step-change in load condition. It is observed that the converter maintains almost constant load voltage. V. CONCLUSION High power quality three-phase ac–dc converters are being widely used in the industries. In this paper, a three-phase ac–dc converter using three single-phase modules adopting active clamped boost topology has been presented. The operating modes, analysis of the circuit, and design considerations are explained. The simulation and experimental results on laboratory prototype (500 V, 1.5 kW) are presented. The experimental results are in good agreement with simulation results. The proposed converter has smaller output filter capacitor and lesser component count as compared to other topologies. It operates at almost unity power factor, low THD, and high efficiency. In addition, it maintains unity power factor and regulated output voltage with ZVS over the wide range of the load even-with unbalanced input voltages. [13] M. L. Heldwein, A. Ferrari de Souza, and I. Barbi, “A simple control strategy applied to three-phase rectifier units for telecommunication applications using single-phase rectifier modules,” in Proc. IEEE PESC ’99, pp. 795–800. [14] G. Spiazzi and F. C. Lee, “Implementation of single-phase boost power-factor correction circuits in three-phase applications,” IEEE Trans. Ind. Electron., vol. 44, no. 3, pp. 365–371, Jun. 1997. [15] M. A. Chaudhari and H. M. Suryawanshi, “High power factor operation of three-phase ac-to-dc resonant converter,” IEE Proc. Elect. Power Appl., vol. 153, pp. 873–882, Nov./Dec. 2006. [16] H. Bodur and A. F. Bakan, “A new ZVT-PWM dc-dc converter,” IEEE Trans. Power Electron., vol. 17, no. 1, pp. 40–47, Jan. 2002. [17] M. M. Jovanovic and Y. Jang, “State-of-the-art, single-phase, active power-factor correction techniques for high-power applications-an overview,” IEEE Trans. Ind. Electron., vol. 52, no. 3, pp. 701–708, Jun. 2005. H. M. Suryawanshi (M’06) received the B.E. degree from Shivaji University, Kolhapur, India in 1988, the M.E. degree from IISc., Bangalore, India in 1994, and the Ph.D. degree from Nagpur University, Nagpur, India, in 1998, all in electrical engineering. He is currently working as an Assistant Professor in the Department of Electrical Engineering, Visvesvaraya National Institute of Technology, Nagpur. His research interests include the field of power electronics, emphasizing developmental work in the area of resonant converters, power factor correctors, active power filters, FACTs devices, and electric drives. REFERENCES [1] B. Singh, B. N. Singh, A. Chandra, K. Al-Haddad, A. Pandey, and D. P. Kothari, “A review of three-phase improved power quality ac–dc converters,” IEEE Trans. Ind. Electron., vol. 51, no. 3, pp. 641–660, Jun. 2004. [2] B. M. Saied and H. I. Zynal, “Minimizing current distortion of a threephase bridge rectifier based on line injection technique,” IEEE Trans. Power Electron., vol. 21, no. 6, pp. 1754–1761, Nov. 2006. [3] M. R. Ramteke, H. M. Suryawanshi, and K. L. Thakre, “Single-phase resonant converter in three-phase system in modular approach,” EPE J., vol. 16, no. 4, pp. 5–13, Dec. 2006. [4] A. K. S. Bhat and R. L. Zheng, “Analysis and design of a three-phase LCC-type resonant converter,” IEEE Trans. Aerospace Electron. Syst., vol. 34, no. 1, pp. 508–518, Apr. 1998. [5] S. S. Tanavade, H. M. Suryawanshi, and K. L. Thakre, “Novel single-phase ac-to-dc convertor using three-phase modified series-parallel resonant converter,” IEE Proc. Elect. Power Appl., vol. 152, pp. 1027–1035, Jul. 2005. [6] R. L. Steigerwald, “A comparison of half bridge resonant converter topologies,” IEEE Trans. Power Electron., vol. 3, no. 2, pp. 174–182, Apr. 1988. [7] T.-F. Wu and S.-A. Liang, “A systematic approach to developing single-stage soft switching PWM converters,” IEEE Trans. Power Electron., vol. 16, no. 5, pp. 581–593, Sep.. 2001. [8] R. Watson, F. C. Lee, and G. C. Hua, “Utilization of an active clamp circuit to achieve soft switching in flyback converters,” IEEE Trans. Power Electron., vol. 11, no. 1, pp. 162–169, Jan. 1996. [9] J. A. Cobos, O. Garcia, J. Uceda, J. Sebastian, and E. Cruz, “Comparison of high efficiency low output voltage forward topologies,” in Proc. IEEE PESC, 1994, pp. 887–894. [10] M. M. Jovanovic, “A technique for reducing rectifier reverse recovery related losses in high power boost converter,” IEEE Trans. Power Electron., vol. 13, no. 5, pp. 932–941, Sep. 1998. [11] R. Ayyanar, R. Giri, and N. Mohan, “Active input voltage and load current sharing in input-series and output parallel connected modular dc-dc converter using dynamic input-voltage reference scheme,” IEEE Trans. Power Electron., vol. 19, no. 6, pp. 1462–1473, Nov. 2004. [12] C. S. Moo, H. L. Cheng, and P. H. Lin, “Parallel operation of modular power factor correction circuits,” IEEE Trans. Power Electron., vol. 17, no. 3, pp. 398–404, May 2002. M. R. Ramteke received the M.Tech. degree in electronics engineering from Nagpur University, Nagpur, India, in 1994. He is currently pursuing the Ph.D. degree at Nagpur University. He is currently working as an Assistant Professor in the Department of Electrical Engineering, Visvesvaraya National Institute of Technology. (VNIT), Nagpur. His research interests includes resonant converters and power quality. K. L. Thakre received the B.E., M.E., and Ph.D. degrees in electrical engineering from Nagpur University, Nagpur, India, in 1971, 1973, and 2001, respectively. He is currently working as a Professor in the Department of Electrical Engineering, Visvesvaraya National Institute of Technology, Nagpur, India. His research interests include the fields of power electronics; AI techniques for control, protection, and monitoring of power systems; and power system operation in deregulated environment. V. B. Borghate received the B.E. and M.Tech. and Ph.D. degrees in electrical engineering from Nagpur University, Nagpur, India, in 1982, 1984, and 2007, respectively. Currently, he is working as an Assistant Professor in the Department of Electrical Engineering, Visvesvaraya National Institute of Technology, Nagpur, India.