International Journal of Engineering and Techniques - Volume 2 Issue 5, Sep – Oct 2016

ISSN: 2395-1303 http://www.ijetjournal.org Page 88

A Buck-Boost AC-AC Converter

Topology Eliminating Commutation Problem with Multiple Mode of Operations

Mr. Harikrishnan U1, Dr. Bos Mathew Jos2, Mr.Thomas P Rajan3 1,2,3( Department of EEE, MA College of Engineering, M.G University, India)

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.

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

RESEARCH ARTICLE OPEN ACCESS

International Journal of Engineering and Techniques

ISSN: 2395-1303

Figure 1: Schematic of Non Inverting bucktype

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 Cin and Co unidirectional current switches can be realized by series combination of power MOSFETs with externalfast 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

International Journal of Engineering and Techniques - Volume 2 Issue 5, Sep –

1303 http://www.ijetjournal.org

Non Inverting buck-boost

converter

The switches do not have any overlap or deadhowever, practically there exists a smalldead-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 differentbecome 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 deadintentionally added in switching signals tooverlap time, and then bulky and lossy RC snubbers are used to protect switching.

CONVERTER

AC Converter Topology

Figure 3.1 shows the circuit topology of AC converter consisting of six unidirectional

current flowing bidirectional voltage blocking L , and two input and

Co . The six current switches can be realized by

series combination of power MOSFETs with externa lfast recovery diodes, as shown in Figure 3.1. In this

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 inThis converter can operate as traditional nonbuck 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 using only six switches, it can combine the functionality of eight switches non-boost converter shown in Fig. 1.1 and four switches inverting buck-boost converter shown in Fig 1.2. Therefore ,it can be used as non-inverting buckconverter to replace the traditional inverting buckboost converter in various ac-applications For its as DVR, the non-boost mode can be used to compensate voltage sags(which occurs more often), and inverting buckboost mode for voltage swells (whichoften). The current and voltage stresses of the components in the converter are calculatedsection, 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

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switches do not have any overlap or dead-time; however, practically there exists a small overlap or

time owing to different time delays of gating of switching devices. During

the overlap time between complementary switches, circuit of voltage source (or capacitor)

occurs, or two capacitors with different voltages become in parallel to each other; both of which results

may damage the switching time, there is no current path

, or two inductors tage 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

eliminated. For high power applications, it can either use six reverse blocking

IGBTs) , or six IGBTs (without body diode) with external fast recovery diodes in series. This converter can operate as traditional non-inverting

converters with voltage gain of D and ) , respectively, and also as inverting buck-converter with voltage gain of ( D / (1 - D ) . By

using only six switches, it can combine the -inverting buck-

Fig. 1.1 and four switches boost converter shown in Fig 1.2.

inverting buck-boost converter to replace the traditional inverting buck-

-ac conversion -inverting buck-

boost mode can be used to compensate voltage sags(which occurs more often), and inverting buck-boost mode for voltage swells (which occurs less

The current and voltage stresses of the calculated and In this

section, the design parameters of inductor and based on maximum

values of their stresses for its operation Non Inverting Buck Mode

International Journal of Engineering and Techniques - Volume 2 Issue 5, Sep – Oct 2016

ISSN: 2395-1303 http://www.ijetjournal.org Page 90

Figure 4: Non Inverting Buck mode

[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 .

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 vin across it. Therefore, no current flows through switch S1 during this interval, as shown in Fig.6 (a). Applying KVL, we get VL = Vin - Vo. 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,

VL = -VO

Figure 6. Operation during negative half of input ac voltage (a)During (D) (b) During (1-D)

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

VO/Vi= 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 non-inverting boost mode operation and key waveforms are shown in below figure 7.

Figure 7: Non Inverting Boost mode a. Positive Half Cycle

For vin > 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

International Journal of Engineering and Techniques

ISSN: 2395-1303

inverse output voltage vo across it. Therefore, no current flows through switch S6during this interval, as shown in Fig. 8a). Applying KVL, we get

VL = Vin During (1-D)T interval as shown in Fig. 3.6(b), switch 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,

VL = Vin - VO

Figure 8: Operation during positive half of input ac voltage (a)During (D) (b) During

b. Negative Half Cycle For Vin < 0 , switches S1,S4,S6 are always turn on while switches S2 S3 , are alwaysS6 becomes high frequency switch. The operation for vin < 0 is same as explained for only difference is that now the operation of switch S6 is same as that of S5 (for vin > 0 ), and viceversa. The equivalent circuits during cycle are shown in Fig. 3.7(a) and (b). By applying volt-sec balance condition onfrom above Equations, the voltage gain in this boost mode is given by

11

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 acconverter.

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

International Journal of Engineering and Techniques - Volume 2 Issue 5, Sep –

1303 http://www.ijetjournal.org

across it. Therefore, no current flows through switch S6during this interval, as

a). Applying KVL, we get D)T interval as shown in Fig. 3.6(b), switch

S5 is turned off while S6conducts in this interval as its series diode becomes forward biased due to free-

action of inductor L current. Energy stored interval.

: Operation during positive half of input ac voltage (a)During (D) (b) During (1-D)

0 , switches S1,S4,S6 are always turn always turn off, and

S6 becomes high frequency switch. The operation explained for vin > 0 , with

only difference is that now the operation of switch 0 ), and vice-

versa. The equivalent circuits during this half-cycle are shown in Fig. 3.7(a) and (b). By

sec balance condition on inductor L , the voltage gain in this

It can be concluded that the voltage gain of the converter in this operation mode

inverting boost ac-ac

SIMULATION RESULTS The simulation was done in MATLAB/SIMULINK 2014a. The simulation model andresult for the two stage configuration is shown. A constant output

age 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

Boost operations,withswitching = 50 Khz and PWM Generation of switching

signals involves switching pulses for converter switches S1to S6. For each configuration, the switching pulses for switches changes according tothe

configuration requirement.The PWM switching frequency is chosen to be 25 Khzfor PWM Switched Switches and 50hz for other 4 switches. Table 2: Simulation Parameters

Table 3: Prototype Parameters

Figure 8: Gate signal to converter switches (a) S(b) S2 S4 (c) S5 (d) S6

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configuration requirement.The PWM switching for PWM Switched

Switches and 50hz for other 4 switches. : Simulation Parameters

: Prototype Parameters

switches (a) S1 S3

International Journal of Engineering and Techniques - Volume 2 Issue 5, Sep – Oct 2016

ISSN: 2395-1303 http://www.ijetjournal.org Page 92

Figure 9: a) Voltage and Current stress across switch S6 (b) Voltage and Current stress across switch S4

Figure 10: Output voltage and current & Input Voltage and current of Buck Mode

Figure 11 : Output voltage and current & Input voltage and current of Boost mode

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 S5 & S6 and other switches S1- S4 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 S1 & S2 and other switches S3- S6 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

verification. Experimentally obtained value is 12.18 V which is nearly equals to the calculated value.

Figure 12:Hardware Prototype

Figure 13: PWM Gating Signal

Figure 14: Gating Pulse at 50Hz

Figure 15: The output voltage of the converter (Boost mode)

International Journal of Engineering and Techniques - Volume 2 Issue 5, Sep – Oct 2016

ISSN: 2395-1303 http://www.ijetjournal.org Page 93

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

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 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.

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