High Efficiency Single Phase Switched Capacitor AC to DC Step Down Converter

Transcription

1 Available online at ScienceDirect Procedia - Social and Behavioral Sciences 195 ( 215 ) World Conference on Technology, Innovation and Entrepreneurship High Efficiency Single Phase Switched Capacitor AC to DC Step Down Converter Golam Sarowar a *, Md. Ashraful Hoque a a Department of Electrical and Electronic Engineering, Islamic University of Technology, Board Bazar, Gazipur-174, Bangladesh Abstract A new topology of single-phase AC-DC converter using Buck-Boost conversion with high efficiency at extremely low duty cycle is proposed. Proposed double stage converter consists of single phase rectifier followed by a switched capacitor buck-boost DC- DC converter. The input current THD is kept low and the input power factor is kept high with two-loop feedback control. The proposed scheme can be used for new generation LED lighting. 215 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license 215 The Authors. Published by Elsevier Ltd. ( Peer-review Peer-review under under responsibility responsibility of Istanbul of Istanbul Univeristy. University. Keywords: AC-DC power converters; input power factor; total harmonic distortion; conversion efficiency. 1. Introduction Application of electronic circuits in various electrical equipment are common in these modern days. Power supply unit plays critical role in any electronic circuit. Generally AC power available from utility is converted to DC using various configurations of converter circuits. Switch mode power supply is considered the efficient way of obtaining DC power. Conventional rectifiers using diode and filters suffer from the difficulties of non-sinusoidal input current and low input power factor (Erickson, Maksimovic, & ebrary Inc., 21; Mohan, 212; Wu, 26). In order to reduce total harmonic distortion (THD) and to improve power factor in the input side various methods have been proposed (Alam, Eberle, & Dohmeier, 214; Li, Dusmez, Akin, & Rajashekara, 214; Lung-Sheng & En-Chih, * Corresponding author. Tel.: ; fax: address: asim@iut-dhaka.edu The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license ( Peer-review under responsibility of Istanbul Univeristy. doi:1.116/j.sbspro

2 2528 Golam Sarowar and Md. Ashraful Hoque / Procedia - Social and Behavioral Sciences 195 ( 215 ) ; Mahesh & Panda, 212; Musavi, Eberle, & Dunford, 21; Ohnuma & Itoh, 212; Rezaei, Golbon, & Moschopoulos, 214). Filters comprising inductor and capacitor in the input side are discouraged as the circuit becomes large and bulky. In addition it only reduces the THD but the improvement in the power factor is insignificant. To overcome above problems, a number of power factor correction (PFC) AC-DC converters have been proposed and developed (Alam, et al., 214; Li, et al., 214; Lung-Sheng & En-Chih, 211; Mahesh & Panda, 212; Musavi, et al., 21; Ohnuma & Itoh, 212; Rezaei, et al., 214). Generally, techniques involving two powerprocessing stages are used to solve the problems. The input PFC stage improves the power factor as well as maintains a constant DC link voltage. The most common PFC stage employ a Boost converter (Lung-Sheng & En- Chih, 211; Mahesh & Panda, 212; Mahesh, Panda, & Keshavan, 212). Buck, Buck-Boost, uk, SEPIC and ZETA converters are also employed for the same purpose with different input/output voltage gain relationships. In output stage, high frequency DC-DC converter (Das, Pahlevaninezhad, Drobnik, Moschopoulos, & Jain, 213; Kai, Xinbo, Xiaojing, & Zhihong, 212) acts as high frequency output current/voltage chopper which is reflected at the input as high frequency chopped AC current. This input current is then filtered by a small filter to obtain near sinusoidal input current with high power factor (Chang-Yeol et al., 213; SangCheol, Gwan-Bon, & Gun-Woo, 213). This is possible only when the output is full wave pulsed DC. To obtain quality power, the output is desired to be DC with low ripple which requires large output filter capacitor. This output filter with large capacitance draws pulsed current, and the pulsed ac current is reflected to the input side. Thus additional control is required to maintain sinusoidal shape of the input current. In some works the unidirectional switch used in the DC-DC converters stage of boost-regulated AC-DC conversion are operated in critical mode (Fengfeng & Lee, 2; Jongbok, Jongwon, Jang, & Bohyung, 211) that is, the power switch should be turned ON at the instant of zero current in the boost diode. Thus variable switching frequency operation of the DC-DC converter is required due to load or the input voltage changes. Another approach for boost-regulated rectifier involves controlling a constant level of average current at the boost diode. In order to keep the average current constant through the boost diode, the duty cycle must be modulated over the line cycle. Bridge-less configurations (Alam, et al., 214; Jauch & Biela, 212) and two-diode, two-switch rectifiers are also reported in literatures for AC-DC conversion having the above features of boost-regulated rectifier (Singh et al., 23). The reported bridgeless single phase AC-DC converters use more than one unidirectional switches or one bidirectional switch composed of two unidirectional switches antiparallel with two diodes. The efficiency of the converters vary with the change in duty cycle. The efficiency tends to reduce with extremely low duty cycle (Mohan, Undeland, & Robbins, 23). Therefore scope is there to design an AC-DC converter to step the voltage down at very low level with high efficiency. Recently much interests are focused on the applications of low voltage DC appliances. As for example lighting technology using LEDs is becoming very popular because of its efficiency, long life and low cost (Yang, Wu, Zhang, & Qian, 211; Ye, Greenfeld, & Liang, 28). Since LEDs are operated in the range of very low DC voltage, therefore conversion of energy from AC to DC for this case is extremely critical in terms of input power factor and THD. So far transformer or other voltage step-down mechanism are employed where question of efficiency is becoming a great concern. On the other hand, some proposals are reported with transformerless control techniques where power factor and THD are not improved much. In this paper, double stage single phase AC-DC buck-boost converter is designed using switched capacitor circuitry (Axelrod, Berkovich, & Ioinovici, 28) and a suitable feedback control technique is used to achieve high efficiency, high input power factor and low THD for input current at extremely low duty cycles. 2. Proposed Circuit Configuration and Operation The circuit in Fig. 1 illustrates the proposed single phase AC-DC converter using Buck-Boost topology in two stage. The first stage is basic single phase rectifier followed by high frequency switched capacitor DC-DC second stage. The proposed circuit comprises two inductors (Lin and ), four capacitors ( to, Cin), eight diodes (D1- D8) and a switch. Here work as Buck-boost inductors. The inductor Lin and capacitor Cin are used as input filter. and are the output filter capacitor and load of the converter respectively.

3 Golam Sarowar and Md. Ashraful Hoque / Procedia - Social and Behavioral Sciences 195 ( 215 ) Lin Cin D1 D3 D2 D4 D6 D5 D7 D8 Fig. 1. Proposed circuit for single phase AC-DC Buck-Boost converter in two stage. In double stage converter the input AC chopping at high frequency provides switched AC current. A small input filter makes it near sinusoidal. As a result, the input current THD reduces. To increase the input power factor proper feedback is required. The operating principle of the proposed converters is described below Buck-boost double stage The Buck-Boost topology in double stage has four operating states as shown in Fig. 2. State A and B represent the positive half cycle operation with switch ON and OFF positions, whereas, state C and D represent the negative half cycle with switch ON and OFF positions respectively. (a) (b) (c) (d) Fig. 2. Four states of operation of proposed double stage single phase AC-DC Buck-Boost converter, (a) State A, circuit when the switch is ON during positive half cycle of the AC supply, (b) State B, circuit when the switch is OFF during positive half cycle of the AC supply, (c) State C, circuit when the switch is ON during negative half cycle of the AC supply, (d) State D, circuit when the switch is OFF during negative half cycle of the AC supply. 3. Open Loop Simulations Simulation of the proposed circuits of Fig. 1 is performed with PSIM professional version and OrCAD Capture CIS version 9.2. And the control circuit is designed using MATLAB.

4 253 Golam Sarowar and Md. Ashraful Hoque / Procedia - Social and Behavioral Sciences 195 ( 215 ) Circuit Parameters For the open loop simulation of double stage buck-boost configuration, an input ac source of 22V amplitude with frequency of 5 Hz is employed. An MOSFET is used for switching purpose. In double stage topology the inductors and Lin have the values of 4uH and 4mH respectively, the capacitor and Cin have the value of 5 μf and to have the values of 1μF each. A resistor of 1 is used as load. The proposed circuit topologies have been compared with the conventional double stage single phase AC-DC buck-boost configuration converter Analyses and Results Typical input voltage & current and the output voltage waveforms of the proposed AC-DC double stage Buckboost converter circuits are given in Fig. 3 for a voltage gain of Input Voltage (V)-- And Input Current (A)*1 2-2 Output Voltage (V) Time (sec) Time (sec) (a) (b) Fig. 3. Input voltage and input current*1 (a) and output voltage waveform; (b) of proposed single stage buck-boost converter at voltage gain of Quantitative Comparison The input voltage & current of the Fig. 3 (a) clearly shows that, the THD (%) of the input current of the proposed converter is considerably low, but the input power factor is very low. The input current is almost 9 out of phase with the input voltage. The average output voltage of the proposed converter is shown in Fig. 3(b). Average output voltage is approximately V. For performance comparison among the proposed and conventional scheme, results are evaluated in terms of efficiency (%) of conversion, THD (%) of input current and input power factor. The outcomes of the investigation are discussed below with diagrams presented in Fig. 4 to 6. The performance curve shown in Fig. 4 indicates that, the conversion efficiency is reasonable high for the proposed scheme at extremely low duty cycles (.5-.15). THD (%) of input current of the proposed converter is reasonably well compare to the conventional one at extremely low duty cycles which is the point of interest (Fig. 5). The proposed converter exhibit low input power factor throughout all the duty cycles as well the conventional one (Fig. 6). Feedback control scheme needs to adopt to improve the input power factor and to reduce the size of the input filter which will be discussed in the next section.

5 Golam Sarowar and Md. Ashraful Hoque / Procedia - Social and Behavioral Sciences 195 ( 215 ) Fig. 4. Comparison of conversion efficiency (%) between conventional and proposed scheme. Fig. 5. Comparison of input current THD (%) between conventional and proposed scheme. Fig. 6. Comparison of input power factor between conventional and proposed scheme.

6 2532 Golam Sarowar and Md. Ashraful Hoque / Procedia - Social and Behavioral Sciences 195 ( 215 ) Two Loop Feedback Control to Improve Input Power Factor The input power factor of the proposed converter scheme is very low without PFC control. Proper feedback control can improve the input power factor of the converter. Reported PFC feedback control consists of two loops (Mohan, 212). The inner current loop and the outer voltage loop. Average current mode control is applied to the inner current control loop. The small signal model of the double stage proposed buck-boost converter of Fig. 1 is given in Fig. 7. L C r R vo Fig. 7. Small signal model of the proposed double stage AC-DC buck-boost converter. The power stage transfer function for the inner current control loop and the outer voltage control loop are derived and shown in equation (1) and (2), G PSi s i 1 ˆ L s Vo 3 2D d s D D sl (1) Where, GPSi GPSv G PSv s v s D D R i s D D src (2) o L s Power stage transfer function of the converter for current control loop, s Power stage transfer function of the converter for voltage control loop, Vˆin Peak input voltage, Vo Average output voltage, L i s Inductor current perturbation, ds Duty cycle perturbation, o v s Output voltage perturbation, D Duty cycle.

7 Golam Sarowar and Md. Ashraful Hoque / Procedia - Social and Behavioral Sciences 195 ( 215 ) The power-stage transfer function for the current control loop in equation (1) is an approximation, valid at high frequencies and not a pure integrator. Therefore to have a high DC loop gain and a zero DC steady state error, the current controller transfer function must have a pole at the origin. In the current control loop the phase due to the pole at the origin of the controller and that of the power stage transfer function of equation (1) add up to -18. Hence the current controller in average current mode control introduces a pole-zero pair to provide a phase margin of approximately 6 at the loop crossover frequency. The Bode plot of the inner current control loop is shown in Fig. 8. The phase boost of 6 is provided at crossover frequency of 1 khz. The objective of the outer voltage control loop is to generate the peak of the reference current for the current control loop. In the voltage loop the bandwidth is limited to approximately 15 Hz. The power-stage transfer function for the voltage control loop at these low perturbation frequency is shown in equation (2). Because of such low bandwidth it is perfectly reasonable to assume that the current loop to be ideal at low frequency around 15 Hz. To achieve zero steady state error, the voltage controller should have a pole at the origin. A transfer function is used for the voltage controller, where a pole is placed at the voltage-loop crossover frequency (which is below 15 Hz) is often used for simplicity. The Bode diagram of the voltage control loop is shown in Fig. 9. The PFC controller for the double stage AC-DC buck-boost converter is shown in Fig Inner Current Control Loop Magnitude (db) Phase (deg) Frequency (rad/s) Fig. 8. Compensated Bode plot for the current control loop of the two stage AC-DC buck-boost converter. Phase (deg) Magnitude (db) Outer Voltage Control Loop Frequency (rad/s) Fig. 9. Compensated Bode plot for the voltage control loop of the two stage AC-DC buck-boost converter.

8 2534 Golam Sarowar and Md. Ashraful Hoque / Procedia - Social and Behavioral Sciences 195 ( 215 ) D5 D6 D7 D8 Voltage Control Loop V o + - V o Measured Voltage Controller sin t I L i L = I L sin t i L Measured + - Current Control Loop Current Controller v r v c 5. Analyses and Results with Feedback Fig. 1. PFC controlled single phase two-stage AC-DC buck-boost converter. The feedback control circuit is designed to achieve an average output voltage of 1V. The simulation result of the input current and voltage are shown in Fig. 11 (a) where input current is multiplied with 1 to show in the same scale with input voltage. The average output voltage is shown in Fig. 11 (b). The comparison of the input power factor with and without the feedback control is shown in Fig. 12 and the data is tabulated in Table 1. Input Voltage (V) -- and Input Current (A)* Time (sec) (a) Output Voltage (V) Time (sec) (b) Fig. 11. Input voltage and input current*1 (a) and the output voltage; (b) with the feedback control optimized for average output voltage of 1V. Fig. 12. Comparison of the input power factor of proposed AC-DC buck-boost converter in two-stage with and without the feedback control.

9 Golam Sarowar and Md. Ashraful Hoque / Procedia - Social and Behavioral Sciences 195 ( 215 ) Table 1. Comparison of input power factor with and without feedback control. Voltage gain Two-stage Buck-boost PF Without Controller Two-stage Buck-boost PF With Controller The input power factor improved significantly with the adopted feedback controller compared to the uncontrolled converter. Also the inductor and the capacitor of the input filter reduces to 2mH and 1μF respectively without affecting converter efficiency and THD (%). 6. Conclusion In this paper a new AC-DC Buck-boost topology has been proposed for low voltage application. The topology has been derived from the conventional double-stage AC-DC converter adopting switched capacitor in the middle. Because of using switched capacitor topology, low voltage is achieved at relatively high duty cycle than that of the duty cycle of conventional double stage. The proposed converter without feedback control shows significant improvement in the conversion efficiency at extremely low duty cycles. THD (%) of the input current is also kept within IEEE-519 standard but it results in very low input power factor. This problem has been addressed with the help of two loop feedback control topology. The average current mode control is used in inner current control loop and voltage mode control in outer loop. Significant improvement of input power factor is noticed at low duty cycles and achieved maximum input power factor of.997 at the voltage gain of.18 in the proposed work. The conversion efficiency and the THD (%) was not affected by the feedback control. In addition, the feedback control reduced the size of the input filter. High efficiency low voltage can be used for lighting load specifically for new generation LEDs. References Alam, M., Eberle, W., & Dohmeier, N. (214, Sept. 214). An inrush limited, surge tolerant hybrid resonant bridgeless PWM AC-DC PFC converter. Paper presented at the Energy Conversion Congress and Exposition (ECCE), 214 IEEE. Axelrod, B., Berkovich, Y., & Ioinovici, A. (28). Switched-Capacitor/Switched-Inductor Structures for Getting Transformerless Hybrid DCȓDC PWM Converters. Circuits and Systems I: Regular Papers, IEEE Transactions on, 55(2), Chang-Yeol, O., Dong-Hee, K., Dong-Gyun, W., Won-Yong, S., Yun-Sung, K., & Byoung-Kuk, L. (213). A High-Efficient Nonisolated Single Stage On-Board Battery Charger for Electric Vehicles. Power Electronics, IEEE Transactions on, 28(12), Das, P., Pahlevaninezhad, M., Drobnik, J., Moschopoulos, G., & Jain, P. K. (213). A Nonlinear Controller Based on a Discrete Energy Function for an AC/DC Boost PFC Converter. Power Electronics, IEEE Transactions on, 28(12), Erickson, R. W., Maksimovic, D., & ebrary Inc. (21). Fundamentals of power electronicspp. xxi, 883 p.). Fengfeng, T., & Lee, F. C. (2, 2). A critical-conduction-mode single-stage power-factor-correction electronic ballast. Paper presented at the Applied Power Electronics Conference and Exposition, 2. APEC 2. Fifteenth Annual IEEE. Jauch, F., & Biela, J. (212, 4-6 Sept. 212). Single-phase single-stage bidirectional isolated ZVS AC-DC converter with PFC. Paper presented at the Power Electronics and Motion Control Conference (EPE/PEMC), th International. Jongbok, B., Jongwon, S., Jang, P., & Bohyung, C. (211, May June 3 211). A critical conduction mode bridgeless flyback converter. Paper presented at the Power Electronics and ECCE Asia (ICPE & ECCE), 211 IEEE 8th International Conference on. Kai, Y., Xinbo, R., Xiaojing, M., & Zhihong, Y. (212). Reducing Storage Capacitor of a DCM Boost PFC Converter. Power Electronics, IEEE Transactions on, 27(1), Li, X., Dusmez, S., Akin, B., & Rajashekara, K. (214, Sept. 214). Capacitor voltage balancing control of a fully integrated three-level isolated AC-DC PFC converter for reliable operations. Paper presented at the Energy Conversion Congress and Exposition (ECCE), 214 IEEE. Lung-Sheng, Y., & En-Chih, C. (211, Nov. 211). Analysis of Modified Single-Phase PFC AC-DC Buck-Boost Converter. Paper presented at the Robot, Vision and Signal Processing (RVSP), 211 First International Conference on. Mahesh, M., & Panda, A. K. (212, 15-2 Sept. 212). A high performance single-phase AC-DC PFC boost converter with passive snubber

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