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Procedia - Social and Behavioral Sciences 195 (2015) 2527 - 2536

World Conference on Technology, Innovation and Entrepreneurship

High Efficiency Single Phase Switched Capacitor AC to DC Step

Down Converter

Golam Sarowara*, Md. Ashraful Hoquea

aDepartment of Electrical and Electronic Engineering, Islamic University of Technology, Board Bazar, Gazipur-1704, 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.

©2015The Authors.Publishedby Elsevier Ltd. This is an open access article under the CC BY-NC-ND license

(http://creativecommons.Org/licenses/by-nc-nd/4.0/).

Peer-reviewunderresponsibilityoflstanbulUniveristy.

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., 2001; Mohan, 2012; Wu, 2006). 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, 2014; Li, Dusmez, Akin, & Rajashekara, 2014; Lung-Sheng & En-Chih,

* Corresponding author. Tel.: +880-02-9291254; fax: +880-02-9291260. E-mail address: asim@iut-dhaka.edu

1877-0428 © 2015 The Authors. Published by Elsevier Ltd. This is an open access article under the CC BY-NC-ND license

(http://creativecommons.org/licenses/by-nc-nd/4.0/).

Peer-review under responsibility of Istanbul Univeristy.

doi: 10.1016/j.sbspro.2015.06.437

2011; Mahesh & Panda, 2012; Musavi, Eberle, & Dunford, 2010; Ohnuma & Itoh, 2012; Rezaei, Golbon, & Moschopoulos, 2014). 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., 2014; Li, et al., 2014; Lung-Sheng & En-Chih, 2011; Mahesh & Panda, 2012; Musavi, et al., 2010; Ohnuma & Itoh, 2012; Rezaei, et al., 2014). Generally, techniques involving two power-processing 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, 2011; Mahesh & Panda, 2012; Mahesh, Panda, & Keshavan, 2012). Buck, Buck-Boost, Cuk, 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, 2013; Kai, Xinbo, Xiaojing, & Zhihong, 2012) 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., 2013; SangCheol, Gwan-Bon, & Gun-Woo, 2013). 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, 2000; Jongbok, Jongwon, Jang, & Bohyung, 2011) 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., 2014; Jauch & Biela, 2012) 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., 2003). 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, 2003). 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, 2011; Ye, Greenfeld, & Liang, 2008). 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, 2008) 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 L1), four capacitors (C1 to C3, Cin), eight diodes (D1-D8) and a switch M1. Here L1 work as Buck-boost inductors. The inductor Lin and capacitor Cin are used as input filter. C1 and R1 are the output filter capacitor and load of the converter respectively.

Vin K;

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.

0i4= R1|Q)

T R1;i

(a) -)->-

01^ R1:

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 9.0.3.400 and OrCAD Capture CIS version 9.2. And the control circuit is designed using MATLAB.

3.1. Circuit Parameters

For the open loop simulation of double stage buck-boost configuration, an input ac source of 220V amplitude with frequency of 50 Hz is employed. An MOSFET is used for switching purpose. In double stage topology the inductors L1 and Lin have the values of 400uH and 40mH respectively, the capacitor C1 and Cin have the value of 50 ^F and C2 to C3 have the values of 1^F each. A resistor of 100'Q is used as load. The proposed circuit topologies have been compared with the conventional double stage single phase AC-DC buck-boost configuration converter.

3.2. Analyses and Results

Typical input voltage & current and the output voltage waveforms of the proposed AC-DC double stage Buck-boost converter circuits are given in Fig. 3 for a voltage gain of 0.25.

? -50-

10 9.8 9.85 9.9 9.95 10

Time (sec)

Fig. 3. Input voltage and input current*10 (a) and output voltage waveform; (b) of proposed single stage buck-boost converter at voltage gain of 0.25.

3.3. 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 900 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 133.72V. 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 (0.05-0.15). 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.

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.

4. 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, 2012). 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.

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),

(l—D)(2—D) , J

Where,

GpSi (s ) = Power stage transfer function of the converter for current control loop, GpSv (s ) = Power stage transfer function of the converter for voltage control loop, Vin = Peak input voltage, Vo = Average output voltage, iL (s ) = Inductor current perturbation, d (s ) = Duty cycle perturbation, V0 ( s ) = Output voltage perturbation, D = Duty cycle.

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 -1800. Hence the current controller in average current mode control introduces a pole-zero pair to provide a phase margin of approximately 600 at the loop crossover frequency. The Bode plot of the inner current control loop is shown in Fig. 8. The phase boost of 600 is provided at crossover frequency of 10 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. 10.

Fig. 8. Compensated Bode plot for the current control loop of the two stage AC-DC buck-boost converter.

Fig. 9. Compensated Bode plot for the voltage control loop of the two stage AC-DC buck-boost converter.

Fig. 10. PFC controlled single phase two-stage AC-DC buck-boost converter.

5. Analyses and Results with Feedback

The feedback control circuit is designed to achieve an average output voltage of 100 V. The simulation result of the input current and voltage are shown in Fig. 11 (a) where input current is multiplied with 10 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.

Time (sec)

& -400)

1 -6°>

Ï -8° ° -100

-120 9.8

Time (sec) (b)

Fig. 11. Input voltage and input current*10 (a) and the output voltage; (b) with the feedback control optimized for average output voltage of 100V.

Fig. 12. Comparison of the input power factor of proposed AC-DC buck-boost converter in two-stage with and without the feedback control.

Golam Sarowar and Md. Ashraful Hoque /Procedia - Social and Behavioral Sciences 195 (2015) 2527 - 2536 2535 Table 1. Comparison of input power factor with and without feedback control._

Voltage gain Two-stage Buck-boost PF Two-stage Buck-boost PF

Without Controller With Controller

0.05 0.020 0.979

0.11 0.062 0.985

0.18 0.127 0.997

0.25 0.208 0.994

0.43 0.427 0.992

0.67 0.691 0.970

1.00 0.919 0.435

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 20mH and 1^F 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 0.997 at the voltage gain of 0.18 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.

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