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Int. J. Elec&Electr.Eng&Telecoms. 2015 Priya Seema Miranda and Sathisha K, 2015

COMPARATIVE STUDY OF SINGLE PHASEINVERTER-BASED ON UNIPOLAR PWM

TECHNIQUES

Priya Seema Miranda1* and Sathisha K1

*Corresponding Author: Priya Seema Miranda,priyamiranda666@gmail.com

Performance of a single phase unipolar PWM inverter is compared based on circuit configurations.A part of main switches are connected to high frequency arm and the remaining switches to lowfrequency arm. All main switches of high frequency arm operate at Zero Voltage Switching(ZVS) turn on and all the main switches of low frequency arm operate at 50 Hz to reduce switchinglosses. The main purpose of using Unipolar PWM inverter is to reduce output voltage harmonics.

Keywords: Soft-switching, Unipolar Pulse width Modulation (PWM) inverter, Zero VoltageSwitching (ZVS), Electromagnetic Interference (EMI)

INTRODUCTIONAs the demands for high-quality powersources increases, a pulse-width modulated(PWM) inverter has been used as a keyelement for a high performance powerconversion system such as UninterruptiblePower Supplies (UPS), medical equipmentand communication systems [2]. The DC toAC converters based on PWM technology issuperior when compared to other invertersdesigned using conventional technologies.

PWM or Pulse width Modulation isnecessary to maintain the output voltage of theinverter at the rated voltage irrespective of theoutput load. In a conventional inverter the output

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1 Department of Electrical and Electronics, St. Joseph Engineering College, Vamanjoor, Mangalore, India.

voltage changes as the changes in the loadoccur. To nullify the effect caused by thechanging loads, the PWM inverter is used tocorrect the output voltage according to thevalue of the load connected at the output. Thiscan be achieved by changing the width of theswitching frequency. The AC voltage at theoutput depends on the width of the switchingpulse.

PWM inverters require high-speedswitching devices to achieve high performancein term of dynamic response, size, and weight.This results in switching stresses on the powerdevices, switching loss, and generation ofelectromagnetic interference (EMI). To

Research Paper

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Int. J. Elec&Electr.Eng&Telecoms. 2015 Priya Seema Miranda and Sathisha K, 2015

overcome these problems soft switchingtechniques have been introduced [3]-[6].

In the proposed inverter all main switchesof high-frequency arm operate at Zero-Voltage-Switching (ZVS) turn-on and all themain switches of low frequency arm operateat 50 Hz to reduce switching losses. The mainpurpose of using Unipolar PWM scheme is toreduce the output voltage Harmonics. TheSimulink model of proposed inverter issimulated in Matlab.

PROPOSED INVERTERSystem ConfigurationThe block diagram of proposed inverter isshown in Figure 1.

Single Phase Unipolar PWMInverterInverters are those which convert DC into AC.The source can be either current source orvoltage source corresponding to a CurrentSource Inverter (CSI) or a Voltage SourceInverter (VSI) respectively [7].

There are two different types of voltagesource inverter

• Half Bridge topology

• Full Bridge topology

Full bridge topology consists of 4 switchingdevices connected in the form of bridge. Pulsewidth modulation techniques provide currentand voltage shaping modifications underspecif ic needs of applications. PWMtechniques also minimize switching stresspresent on the switching devices [8].

INVERTERDCSUPPLY

PWMSwitchingscheme

Output

Figure 1: Block Diagram

Power circuit consists of DC input, fullbridge inverter and PWM switching scheme.

The inverter consists of arm A and B arm.The B arm is switched at Low Frequency toachieve low switching losses and the A arm isswitched at high Frequency for the better outputregulation. Proposed scheme uses Unipolarswitching scheme. The soft-switching of themain switches can be achieved using ZVStechnique.

The circuit configuration for Unipolar PWMinverter is shown in Figures 2 and 3.

Figure 2: Configuration 1

Figure 3: Configuration 2

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Int. J. Elec&Electr.Eng&Telecoms. 2015 Priya Seema Miranda and Sathisha K, 2015

PWM signal is usually generated by comparingsinusoidal waveform with triangular waveform.

PWM inverters are two types

• Unipolar inverter

• Bipolar inverter

The Unipolar inverter always requires tworeference signals (Vref+ and Vref–) which mustbe same magnitude and frequency but are outof phase. This reference wave is thencompared with triangular wave [8].

In the Unipolar switching scheme the outputvoltage changes from positive voltage (+V) tozero during positive half cycle or from Zero tonegative voltage (–V) during negative halfcycle, Whereas in Bipolar switching Schemethe output voltage switches between positivevoltage and negative voltage levels [1].

The Unipolar switching scheme has theadvantage of doubling the switching frequencycompared to bipolar switching scheme. As aresult the output harmonics is reduced inunipolar switching scheme than bipolarswitching scheme [7].Efficiency of unipolarswitched inverter is better compared to bipolarswitched device.

For a full bridge inverter the root meansquare (RMS) output voltage can be foundfrom

Vo = (2/To 1/2 ...(1)

Vo = Vs ...(2)

Design of Output FilterThe ripples in PWM output voltage is filteredusing a low pass filter. The cut-off frequency ofoutput filter can be obtained by selecting thevalues of C and L.

fc = 1/(2** ) ...(3)

By selecting C = 25 µF and L = 1 mH thecut-off frequency Obtained is fc = 1 kHz.

SOFT-SWITCHINGHard Switching Occurs when there is overlapbetween the voltage and current when theSwitching device is turned on and off. Becauseof this, overlap energy losses increases andas a result electromagnetic interference (EMI)is generated. These problems can beovercome using Soft Switching refer Figures4 and 5.

Switching transitions occur when devicevoltage or current is Zero. Soft switching hasfollowing advantages

• Reduced Switching losses

• Reduced switch stress

Figure 4: Hard Switching

Figure 5: Soft Switching

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Int. J. Elec&Electr.Eng&Telecoms. 2015 Priya Seema Miranda and Sathisha K, 2015

• Low EMI

• Easier Thermal management

SIMULATION RESULTThe Unipolar PWM inverter is simulated usingMATLAB Simulik and the following results arenoted below for different circuit configuration.The parameters for the proposed inverter areas follows:

Input Voltage Vdc = 12 V

Output Voltage Vo = 10 V

Filter Inductor L = 1 mH

Filter Capacitor C = 25 µF

Configuration 1: This configuration is shownin Figure 2. Before the main switches of highfrequency arm are turned on (S3 and S4) drainvoltages have been decreased to zero. Hencethese switches are turned on at ZVS. Thesimulation result is shown in Figure 6 and

Figure 7. From the result it is found thatswitching losses for the inverter proposed inconfiguration 1are reduced.

Configuration 2: This configuration is shownin Figure 2. It consists of switches S1, S2, S3,

Performance Configuration 1 Configuration 2Parameter

THD Reduced (10.03%) More (41.08%)

Switching loss Less More

EMI Low High

Table 1: Comparative Study

Figure 6: Simulation of Inverter Proposedin Configuration 1

Figure 7: Simulation Result of InverterProposed in Configuration 1

Figure 8: Simulation of Inverter Proposedin Configuration 2

Figure 9: Simulation Result of InverterProposed in Configuration 2

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Int. J. Elec&Electr.Eng&Telecoms. 2015 Priya Seema Miranda and Sathisha K, 2015

S4 and output LC filter which forms aconventional hard switching PWM inverter.The simulation result is shown in Figure 8 andFigure 9.

Harmonic Analysis of Configuration 1 andConfiguration 2: FFT analysis is performedto determine the total harmonic distortion forconfiguration 1 and configuration 2. It is shownin Figure 10 and Figure 11. Total HarmonicDistortion (THD) is a measure of closeness inshape between a waveform and itsfundamental component. The Comparison isshown in Table 1.

THD = ...(4)

CONCLUSIONSingle phase Unipolar PWM inverter has beencompared using different configuration.Configuration 1 is a soft-switching inverterconsists of high frequency arm and lowfrequency arm. All the main switches of highfrequency arm operate at ZVS turn on.Configuration 2 is an conventional hardswitching PWM inverter. Unipolar PWMscheme is used to reduce output voltageharmonics. The single phase inverter issimulated in both the configurations and resultsare obtained according to the parameters andit was found that performance is better in softswitching inverter compared to conventionalhard switching PWM inverter.

REFERENCES1. Abdel-Rahim N M and Quaicoe J E

(1996), “Analysis and Design of a MultipleFeedback Loop Control Strategy forSingle-Phase Voltage-Source UPSInverters”, IEEE Transactions on PowerElectronics, Vol. 11, No. 4, pp. 532-541.

2. Baharuddin Bin Ismail (2008), “Designand Development of Unipolar SPWMSwitching Pulses for Single Phase FullBridge Inverter Application”, May.

3. Divan M (1989), “The Resonant DC LinkConverter—A New Concept in StaticPower Conversion”, IEEE Trans. IndustryApplications, Vol. 25, pp. 317-325.

4. Lai J S and Bose B K (1990), “HighFrequency Quasi Resonant dc VoltageNotching Inverter for ac Motor Drives”,Conf. Rec. IEEE-IAS Annu. Meeting,pp. 1202-1207.

Figure 10: FFT Analysis of Configuration 1

Figure 11: FFT Analysis of Configuration 2

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5. Michael James McCarty (2010),“Determining the Optimum OperatingParameters of a Unipolar PWM Inverter”,May.

6. Mohan N, Undeland T M and Robbins WP (1989), Power Electronics: Converters,Applications and Design, John Wiley &Sons.

7. Venkataramanan G and Divan M (1992),“Control of Pulse Width ModulatedResonant dc Link Inverter”, Conf. Rec.IEEE-IAS Annu. Meeting, pp. 737-743.

8. Venkataramanan G and Divan M (1993),“Pulse Width Modulation with Resonant dcLink Converters”, IEEE Trans. IndustryApplications, Vol. 29, pp. 113-120.