A Buck-Boost AC-AC Converter Topology Eliminating Commutation Problem with Multiple Mode of Operations

Transcription

1 RESEARCH ARTICLE A Buck-Boost AC-AC Converter Topology Eliminating Commutation Problem with Multiple Mode of Operations Mr. Harikrishnan U 1, Dr. Bos Mathew Jos 2, Mr.Thomas P Rajan 3 1,2,3 ( Department of EEE, MA College of Engineering, M.G University, India) OPEN ACCESS Abstract: AC - AC power conversions were traditionally done by using thyristor power controllers, phase angle control or by integral cycle control, but had low PF and other disadvantages. Variable voltage, variable frequency high power conversions are nowadays use DC link and Matrix converters, with higher efficiency and better regulation. But in situations where only voltage regulation is required and the circuit need to be simple and less complicated, directed PWM AC-AC converters are more preferred, due to reduced size and components. This project presents the design and simulation of a new type of AC-AC converter which can operate as traditional non-inverting buck and boost converters, and inverting buck-boost converter as well. This converter uses six unidirectional current flowing and bidirectional voltage blocking switches, implemented by six reverse blocking IGBTs or series MOSFET-diode pairs, two input and output filter capacitors, and one inductor. It has no shoot-through problem of voltage source (or capacitor) even when all switches are turned-on and therefore; PWM dead times are not needed resulting in high quality waveforms, and solves the commutation problem without using bulky and lossy RC snubbers or dedicated soft-commutation strategies. It has smaller switching losses because; only two switches out of six are switched at high frequency during each half cycle of input voltage, and it can use power MOSFETs as body diode never conducts, making it immune from MOSFET failure risk.. Keywords- PWM AC-AC Converters, Inverting Buck Boost Converter, Non Inverting Buck Boost Converter, THD I. INTRODUCTION Traditionally in industry, the ac-ac power conversions are performed by using ac thyristor power controllers, which use the phase angle or integral cycle control on input ac voltage, to get the desired output ac voltage. However, the obvious disadvantages of ac thyristor controllers such as low power factor, large total harmonic distortion in source current and lower efficiency, have limited their use [3]. For ac-ac power conversions with variable frequency and voltage, the use of indirect ac-ac converters with dc-link,and matrix converters have been advanced because they can obtain better power factor and efficiency, and smaller filter requirements. However, for applications in which only voltage regulation is needed, the direct PWM ac-ac converters All of these direct PWM AC - AC converters in are obtained from their dc-dc counterparts, where all the unidirectional switches are replaced with bidirectional devices.however, each topology has its own limitations; the buck type ac-ac converter, can only step-down the input voltage while boost type can only step-up the input voltage. The buck-boost and Cuk topology can both step-up and step-down the input voltage, however, the phase angle is reversed. Moreover, both topologies have disadvantage of higher voltage stress across switches, and there are discontinuous input and output currents in case of the buck-boost converter. The Cuk topology can overcome the currents discontinuity but at the cost of additional passive components; increasing the size and cost of converter and decreasing the efficiency.all of the direct PWM ac-ac converters have a common commutation problem, which occurs because compared to the ideal situation in which the complementary. ISSN: Page 88

2 Figure 1: Schematic of type Non Inverting buck-boost The switches do not have any overlap or dead-time; however, practically there exists a small overlap or dead-time owing to different time delays of gating signals and limited speed of switching devices. During the overlap time between complementary switches, either a short-circuit of voltage source (or capacitor) occurs, or two capacitors with different voltages become in parallel to each other; both of which results in current spikes which may damage the switching devices. During the dead-time, there is no current path for the flow of inductor current [6], or two inductors become in series ; resulting in voltage spikes which may also damage the switching devices. To solve this commutation problem, the PWM dead-times are intentionally added in switching signals to avoid overlap time, and then bulky and lossy RC snubbers are used to protect switching. Figure 2 : Inverting buck-boost type converter II. BUCK BOOST AC-AC CONVERTER Figure 3: AC-AC Converter Topology A. AC AC Converter This section Figure 3.1 shows the circuit topology of the AC-AC converter consisting of six unidirectional current flowing bidirectional voltage blocking switches S1 - S6, one inductor L, and two input and output filter capacitors C in and Co. The six unidirectional current switches can be realized by series combination of power MOSFETs with externa lfast recovery diodes, as shown in Figure 3.1. In this figure, body diodes of MOSFETs are not shown as they never conduct, and thus, their poor reverse recovery problem is eliminated. For high power applications, it can either use six reverse blocking IGBTs(RB-IGBTs), or six IGBTs (without body diode) with external fast recovery diodes in series. This converter can operate as traditional non-inverting buck and boost converters with voltage gain of D and 1/(1-D), respectively, and also as inverting buckboost converter with voltage gain of ( D / (1 - D ). By using only six switches, it can combine the functionality of eight switches non-invertinboost converter shown in Fig. 1.1 and four switches buck- inverting buck-boost converter shown in Fig 1.2. Therefore,it can be used as non-inverting buck-boost converter to replace the traditional inverting buckboost converter in various ac-aapplications For its as DVR, the non-inverting buck- conversion boost mode can be used to compensate voltage sags(which occurs more often), and inverting buck- occurs less boost mode for voltage swells (which often). The current and voltage stresses of the components in the converter are calculated and In this section, the design parameters of inductor and switches will be determined based on maximum values of their stresses for its operation B. Non Inverting Buck Mode ISSN: Page 89

3 V L = -V O Figure 4: Non Inverting Buck mode Figure 6. Operation during negative half of input ac voltage (a)during (D) (b) During (1-D) [1]. Operating principle The PWM switching sequence of the converter during non-inverting buck mode operation and key waveforms are shown in Figure 4.The modes of operations include. a. Postive Cycle Figure 5 shows operation circuit Positive half of input ac voltage (vin > 0 ), switches S1 S3 S6 are always turn on and S4 S5, are always turn off, while switch S2 is switched at high frequency. Fig.3.3 shows the equivalent circuits of the proposed converter for vin > 0. For Vin < 0, switches S2,S4,S5 are always turn on while switches S3 S6 are always turn off, and S1 becomes high frequency switch. The operation for vin 0 is same as explained for vin < 0, with only difference is that now the switch S1 performs same ass2 (for vin > 0 ), and vice-versa. The equivalent circuits during this negative half-cycle are shown in Fig. 6 (a) and (b). By applying volt-sec balance condition on inductor L from above equations, the gain in this buck mode is given by V O /V i = D It can be concluded that the voltage gain of the proposed ac-ac converter in this operation mode is same as that of non-inverting buck ac-ac converter. C. Non Inverting Boost Mode The switching sequence of the converter during noninverting boost mode operation and key waveforms are shown in below figure 7. Figure 5. Operation during positive half of input ac voltage (a)during (D) (b) During (1-D) The circuit shown in Fig.5 (a) is during DT interval in which switch S2 is turned on and the input energy is stored in inductor L. Even though the switch S1 is also turned on, however, its external series diode becomes reverse biased because of inverse input voltage v in across it. Therefore, no current flows through switch S1 during this interval, as shown in Fig.6 (a). Applying KVL, we get VL = V in - V o. b. Negative Cycle During (1 - D )T interval as shown in Fig.6(b), switch S2 is turned off whiles1 conducts in this interval as its series diode becomes forward biased due to freewheeling action of inductor L current. Energy stored in inductor is released to load in this interval. Applying KVL yields, Figure 7: Non Inverting Boost mode a. Positive Half Cycle For v in > 0, switches S2,S3,S6 are always turn on and S1 S4 are always turn off, while switch S5 is switched at high frequency. Fig.8 shows the equivalent circuits of the proposed converter for vin 0. The circuit shown in Fig. 8(a) is during DT interval in which switch S5 is turned on and the input energy is stored in inductor L. The switchs6 is also turned on, however, its external series diode becomes reverse biased because of ISSN: Page 90

4 inverse output voltage v o across it. Therefore, no configuration requirement.the PWM switching current flows through switch S6during this interval, as frequency is chosen to be 25 Khzfor PWM Switched shown in Fig. 8a). Applying KVL, we get Switches and 50hz for other 4 switches. V L = V in During (1-D)T interval as shown in Fig. 3.6(b), switch Table 2: Simulation Parameters S5 is turned off while S6conducts in this interval as its series diode becomes forward biased due to freewheeling action of inductor L current. Energy stored in inductor is released to load in this interval. Applying KVL yields, V L = V in - V O Table 3: Prototype Parameters Figure 8: Operation during positive half of input ac voltage (a)during (D) (b) During (1-D) b. Negative Half Cycle For Vin < 0, switches S1,S4,S6 are always turn on while switches S2 S3, are always turn off, and S6 becomes high frequency switch. The operation for v in < 0 is same as explained for v in > 0, with only difference is that now the operation of switch S6 is same as that of S5 (for v in > 0 ), and vice- this half- versa. The equivalent circuits during cycle are shown in Fig. 3.7(a) and (b). By applying volt-sec balance conditionn on inductor L from above Equations, the voltage gain in this boost mode is given by 1 1 It can be concluded that the voltage gain of the proposed ac-ac converter in this operation mode is same as that of non-inverting boost ac-ac converter. III. SIMULATION RESULTS The simulation was done in MATLAB/SIMULINK 2014a. The simulation model andresult for the two stage configuration is shown. A constant output voltage of 110 V ismaintained for input voltages in each configuration. Simulation is done for an inputvoltage of 70 V for boost operation and 152V for Buck and Buck-Boost operations,withswitching frequency of converter,fs = 50 Khz and PWM frequency chosen is 25 Khz. Generation of switching signals involves switching pulses for converter switches S1to S6. For each configuration, the switching pulses for switches changes according tothe Figure 8: Gate signal to converter switches (a) S 1 S 3 (b) S 2 S 4 (c) S5 (d) S 6 ISSN: Page 91

5 verification. Experimentally obtained value is V which is nearly equals to the calculated value. Figure 9: a) Voltage and Current stress across switch S 6 (b) Voltage and Current stress across switch S 4 Figure 12:Hardware Prototype Figure 10: Output voltage and current Voltage and current of Buck Mode & Input Figure 13: PWM Gating Signal Figure 11 : Output voltage and current & Input voltage and current of Boost mode Figure 14: Gating Pulse at 50Hz IV. EXPERIMENTAL RESULTS Prototype with reduced voltage had been developed. For Boost converter mode an input voltage of 12 V is given with a duty ratio of 42% and with a switching frequency of 25 KHz for S 5 & S 6 and other switches S 1 - S 4 at 50hz and for buck converter mode an input voltage of 12 V is given with a duty ratio of 72% and with a switching frequency of 25 KHz for S 1 & S 2 and other switches S 3 - S 6 at 50hz The switching pulses were developed using PIC 16F877A and driver IC used is TLP250. An output voltage of about 18 V and 9 V was obtained. Table 3 shows the prototype parameters used for experimental Figure 15: The output voltage of the converter (Boost mode) ISSN: Page 92

6 other converters, this topology has less high frequency active switches. Six switch converter circuit with reduced stress, less component count and high gain is proposed for multiple mode operations. For an input voltage of 12 V, output voltages for boost, buck modes and buck boost mode can be obtained. A closed loop control of converter session can be done to maintain constant voltage at the output of the converter.. A prototype converter section is tested and results are experimentally verified. Future scope involves increasing the modes included in converter with less complexity in control. VI. ACKNOWLEDGMENT Figure 16: The output voltage of the converter (Buck mode) V. CONCLUSION This paper is based on the development and analysis of the multiple mode operations in a AC-AC Converter topology. Compared with popularly used It is a great pleasure to acknowledge all those who have assisted and supported me for successfully completing the paper. I express my deep sense of gratitude to Dr. Bos Mathew Jos,, for the valuable guidance as well as timely advice which helped me a lot in completing the paper successfully. I am deeply indebted to assistant Prof. Thomas P Rajan, for his guidance, patience and encouragement will be a guiding spirit in all endeavours of my future. REFERENCES [1]. Hafiz Furqan Ahmed, Honnyong Cha,Ashraf Ali Khan, Heung-Geun Kim,"A Novel Buck-Boost AC-AC Converter With Both Inverting And Non-inverting Operations and Without Commutation Problem ", IEEE transaction on Power electronics,, Vol.31, Issue: 6, June [2]. J. Cheng, T. Sun, and A. Wang, Highly compact ac-ac converter achieving a high voltage transfer ratio, IEEE Trans. Ind. Electron., vol.4, Apr [3]. H. Sarnago, O. Lucia, Direct ac-ac resonant boost converter for efficient domestic induction heating applications, IEEE Trans. Power Electron., vol. 29, Mar [4]. C. Liu, B. Wu, N. R. Zargari, D. Xu and J. Wang, A novel three-phase three-leg ac-ac converter using nine IGBTs, IEEE Trans. Power Electron., vol. 24, no.5, pp , May [5].. M.-K. Nguyen, Y.-C. Lim, and Y.-J. Kim, A modified single-phase quasi-z-source ac- ac converter, IEEE Tran. Power Electron.,vol.27,no. 1, Jan [6]. J. Salmon, A. M. Knight, and J. Ewanchuk, Single-phase multilevel PWM inverter topologies using coupled inductors, IEEE Trans. Power Electron., vol. 24, no.5, pp , May [7]. R. Cardenas, C. Juri, R. Pena, P. Wheeler, and J. Clare, The application of resonant controllers to four-leg matrix converters feeding unbalanced or nonlinear loads, IEEE Trans. Power Electron., vol. 27, no. 3, pp , Mar [8]. R. Moghe, R. P. Kandula, A. Iyer, and D. Divan, Losses in medium-voltage megawatt-rated direct ac/ac power electronics converters, IEEE Trans. PowerElectron., vol. 30, no. 7, pp , Jul [9]. P. Sun, C. Liu, J.-S. Lai, C.-L. Chen, and N. Kees, Three-phase dual-buck inverter with unified pulse width modulation, IEEE Trans. Power Electron., vol. 27,no. 3, pp , Mar [10]. M. K. Nguyen,Y. G. Jung, andy.c. Lim, Voltage swell/sag compensation with single-phase Z-source AC/AC converter, Proc. EPE, 2009, P.1-P.8 ISSN: Page 93