spwm generator based on fpga for high switching …here an fpga based spwm generator is implemented,...

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Int. J. Elec&Electr.Eng&Telecoms. 2015 M Angelin Ponrani and R Vel Murugan, 2015

SPWM GENERATOR BASED ON FPGA FOR HIGHSWITCHING FREQUENCY DC/AC INVERTERS

M Angelin Ponrani1* and R Vel Murugan1

*Corresponding Author: M Angelin Ponrani,[email protected]

The digital implementation of Sinusoidal Pulse Width Modulation (SPWM) generators havedominated over their counterparts based on analog circuits. Here an FPGA based SPWMgenerator is implemented, which is capable to support the high switching frequencies morethan 1 MHZ, thus it is capable to support the high switching frequency requirements of modernsingle-phase dc/ac power converters. This design occupies a small fraction of a medium sizedFPGA and, thus can be integrate in larger designs. In addition to that, it has a flexible architecturethat can be implemented to a variety of single-phase dc/ac inverter applications. This experimentalresults confirm that compared to the past-proposed microcontrollers and DSPs generation unitdesigns, the SPWM generator exhibits much faster frequency, lower power consumption, andhigher accuracy of generating the desired SPWM waveform.

Keywords: SPWM, DC, AC, FPGA, DSP

INTRODUCTIONThe dc/ac converters (inverters) are the majorpower electronic conversion units inrenewable energy production, motor drive,and uninterruptible power supply applications.The Sinusoidal Pulse Width Modulation(SPWM) technique is widely employed inorder to adjust the dc/ac inverter outputvoltage amplitude and frequency to thedesired value. In this inquest, the powerconverter switch MOSFET is set to the ON orOFF state according to the result of thecomparison between a high-frequency,

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Int. J. Elec&Electr.Eng&Telecoms. 2015

1 Department of EIE, Sethu Institute of Technology, Kariapatti, India.

constant-amplitude triangular wave (carrier)with two low-frequency (e.g., 50 Hz) referencesine waves of adjustable amplitude and/orfrequency. A dc/ac inverter comprised of fourcascaded Z-source inverter modules, whichhave been built using Gallium Nitride devicesoperating at high switching Frequency.Compared to the microcontroller and DSP-integrated circuits, FPGAs have theadvantage of flexibility in case of changesand they enable the reduction of the executiontime of the dc/ac inverter control algorithmdue to their capability to integrate digital

Research Paper

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hardware with high speed and parallelprocessing features. The triangular wave isimplemented in the form of an up-downcounter. The SPWM control signals areproduced by comparing the correspondingvalues of the sinusoidal and carrier digitalsignals. The SPWM pulse train is producedby comparing the sinusoidal and triangularsignals generated according to the DirectDigital Synthesis (DDS) technique. Thecomparison is performed using a high-speedanalog comparator. The regular-sampledPWM technique presented in targets toreduce the amount of computation timerequired in order to facilitate the generationof higher switching frequencies online and inreal time.

In this technique, the pulse width iscalculated once and used over N consecutiveswitching edges of the SPWM wave pulses.Then, a new sample of the reference sine waveis acquired. Consequently, the number ofcalculations required to produce the completeSPWM waveform is N times less than in theconventional SPWM generation methods. Inthis architecture the values of both thereference sine and triangular waves are storedin the FPGA device Block RAMs (BRAMs) inorder to exploit their one-clock cycle accesstime, thus providing a much higher switchingfrequency capability.

RELATED WORKThere are different types of methodologiesfound in architecture to obtain high switchingfrequency. However it is important to reachthe high switching frequency. SPWMgenerator is used to adjust the dc/ac inverteroutput voltage and frequency. It can be

adapted in various single phase invertermainly to increase switching frequency mostlyrecently developed reduced common voltagePWM methods under one umbrella, isestablished. They program the pulse patternsof various high performance applications.However, this will not modulate the severalpulses per half cycle. So if we make use ofSinusoidal Pulse Width Modulation (SPWM)the width of each pulse is varied proportionalto the amplitude of a sine wave evaluated atthe center of the same pulse. DPWMarchitecture Reduce the exceedance of clockfrequency requirements but the disadvantagein this paper is that the Clock frequency mayexceeds the operating frequency (Hava andCetin, 2011).

The possibility of Adjustable switchingfrequency can be done by Full bridge HF LCLresonant inverters switching is achieved for allswitches for the whole power range. With thephase shift modulated as sinusoidal, a full-wave rectified output current synchronized withthe utility line is obtained. So here we makeuse of the dc/ac inverter as major powerconversion production units which can beimplemented in renewable energy sources,motor drives and uninterruptable power supplyapplications but it has only fixed switchingfrequency (Patel and Madawala, 2009).

Multi bit binary signals (for generation ofsinusoidal pulses) has the possibility ofreduced Inverter output filter size (due toreduction in harmonics distortion) and so costis low in case of improved size filter it failed toretain its capacity (Li and Bhat, 2012).

The bit stream representation has theadvantage of having the characteristics of aclassical pulse width modulation signal and

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hence can be used to drive high powerswitching devices with minimal conditioning.The practical implementation lead to totalharmonic distortion measures rangingbetween 0.75% and 4.92%.But this leads tohigh cost and the filter size is large in size.Increasing switching frequency of the triangularwave results in a reduction of the dc/ac inverteroutput filter size and cost (Navarro et al., 2012).

Advantages of digital control in powerelectronics have led to an increasing use ofDigital Pulse Width Modulators (DPWM). Butif we make use of DPWM architecture thenthe clock frequency requirements may exceedthe operational limits when the powerconverter switching frequency is increased. Inorder to reduce the exceedance of clockfrequency requirements here we implementingthe FPGA based SPWM generator unit (Leiand Tina-Yu, 2011).

A phase modulated high frequency isolatedDC/AC converter is used for a PMSG-basedgrid-connected wind generation application.With the single phase shift modulated assinusoidal, a 120 Hz rectified output current isobtained. A Control strategy for a single phaseseries connected inverter with a micro grid isused to connect the ac load not only to regulatethe load voltage under load disturbances butalso to control the load power drawn from themicro grid. Use of control strategy alsofacilitates a specific power of active power flowto the irrespective of the micro grid voltagecondition. The rest of the load power issupplied to the micro grid (Hayashi et al.,2007).

DESIGN METHODOLOGYArchitecture of SPWM Generator

Clock Generator SubsystemThe “Clock generator” subsystem takes asinput the FPGA input clock and produces anew clock signal used by the digital circuits ofthe SPWM generator, such that the desiredSPWM switching frequency fc specified by thedesigner/user is generated. A two-state FiniteState Machine (FSM) is initially used to set theinput clock frequency fclk to fclk/2 and then aDigital Clock Manager module adapts thisfrequency to the desired value.

Modulation Index SubsystemThe “Modulation index” subsystem is used toconvert the floating-point modulation index M,which is input in the SPWM generation systemto the corresponding value in fixed-pointarithmetic. The floating point value producedis then converted into a fixed-point valueranging from 0 to 255, via a float-to-fixed pointconversion unit, thus producing the “Index”output of the “Modulation index” subsystem.

Sine-Carrier SubsystemThe “Sine-Carrier” subsystem consists of thecontrol unit, two BRAMs, which contain

Figure 1: Block Diagram of SPWMGenerator

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samples of the sinusoidal and triangular (i.e.,carrier) waves and two multiplexers thatproduce the two constant-amplitudereference sine waves used for the productionof the SPWM output signals. The BRAMs ofthe sine wave and carrier operate as LUTs.Both the sinusoidal and triangular waves aresampled and quantized with the samesampling frequency fs using MATLAB. Inorder to minimize the utilization of the FPGAresources, only the values are stored in thecorresponding BRAM, while the values of thesine wave during the time interval /2 – 2are calculated by mirroring and inverting thevalues of the first quarter. The BRAM of thecarrier contains the values of a completeperiod of the reference triangular wave.Consecutive addresses of both memoriesare generated in every clock cycle by thecontrol unit. The control unit also produces a“flag” signal, which is responsible for theretrieval of the sine-wave values during thetime interval – 2 of the reference sine-wave period. The BRAM of the constant-amplitude sinusoidal wave is scanned up anddown four times, since this memory containsonly the values during the first quarter of thesinusoidal-wave period. The value of the “flag”signal determines the up or down directionof consecutive accesses performed forretrieving the data stored in the BRAMmemory of the reference sine wave. While“flag” is set to 0, the multiplexer “MUX1”outputs the data read from the constant-amplitude sinusoidal memory. Otherwise,“MUX1” outputs the values corresponding tothe second half-cycle (i.e., during – 2) ofthe constant-amplitude sinusoidal wave. Themultiplexer “MUX2” is used for the productionof the second constant-amplitude reference

sine wave, which operates with a 180° phasedifference compared with the one analyzedabove.

Adjustable Amplitude SineSubsystemThe “Adjustable amplitude sine” subsystemtakes as input the constant-amplitudereference sinusoidal values produced by the“Sine-Carrier” subsystem and generatessinusoidal digital signal Yawith amplitudeadjustable according to the value of themodulation index M which is an input in theSPWM generation system. The value of Ya isin the range 0-255 Index is the output of the“Modulation Index” subsystem.

Comparison SubsystemThe “Comparison” subsystem implements thecomparison between the high frequencyconstant-amplitude triangular wave (carrier)with the two low-frequency reference sinewaves, using two comparators. The controlsignals, Ta– Tb+, and Tb– of the single-phasedc/ac inverter power switches are generatedfrom the outputs of the correspondingcomparators of this subsystem, thus formingthe SPWM wave at the dc/ac inverter outputterminals. Ta+ and Tb+ are the outputs of thecomparators in the “Comparison subsystem”.

IMPLEMENTATION OF SPWMGENERATOR UNITMATLAB SystemThe MATLAB system consists of these mainparts:

Desktop Tools and DevelopmentEnvironment: This part of MATLAB is the setof tools and facilities that help you use andbecome more productive with MATLAB files

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and functions. Many of these usages aregraphical user interfaces. It includes: browsersfor viewing help, the workspace, and folders.Mathematical Function Library: This library isa vast collection of computational algorithmsranging from elementary functions, likeaddition, sine, cosine, and complex arithmetic,to more advanced functions like matrix inverse,matrix Eigen values, and fast Fouriertransforms.

The Language: The MATLAB language is ahigh-level matrix/array language with controlflow, data structures, functions, input/output,and many other programming features. It allowsboth “programming in the small” to rapidlycreate quick programs you do not intend toreuse. You can also do “programming in thelarge” to create complex application programsintended for reuse.

Graphics: MATLAB has extensive facilitiesfor displaying vectors and matrices as graphs.It contains high-level functions for bi-dimensional and three-dimensional datavisualization, image processing and graphics.It also includes low-level functions that allowsto fully customize the appearance of graphicsand to build complete graphical user interfaceson MATLAB applications.

External Interfaces: The interfaces libraryallows you to write C/C++ and FORTRANprograms that interacts with MATLAB whichincludes facilities for calling routines (dynamiclinking), for calling MATLAB as a computationalengine, and for retrieving and writing MAT-files.

ModelsimThe Modelsim advanced code coveragecapabilities provide valuable metrics forsystematic verification. Information is stored

in the Unified Coverage Database, which isused to manage and collect all coverageinformation in a highly efficient database.Coverage materials that analyze codecoverage data, such as test ranking andmerging, are available. Coverage results canbe viewed by post-simulation else after amerge of multiple simulation runs. Codecoverage metrics can be depicted by instanceor by design unit, giving flexibility in managingcoverage data.

RESULTS AND DISCUSSION

Figure 2: SPWM Generation Unit

Figure 3: Output of SPWM Generator Unit

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CONCLUSIONThe SPWM principle is widely used in dc/acinverters in energy conversion and motor driveapplications. The past proposed SPWMgenerators have been designed to operate atlow switching frequencies, while this have ahigher switching frequencies.

Here an FPGA-based SPWM generatorhas been presented, which is capable tooperate at switching frequencies more than 1MHz, thus it is able to support the highswitching frequency requirements of modernsingle-phase dc/ac inverters.

SPWM generator exhibits much rapidswitching frequency, lower power

Figure 4: Output of dc/ac Inverter consumption, and higher accuracy ofgenerating the desire SPWM waveform.

REFERENCES1. Hava A M and Cetin N O (2011), “A

Generalized Scalar PWM Approach withEasy Implementation Features for Three-Phase, Three-Wire Voltage Inverters”,IEEE Trans. Power Electron, Vol. 26,No. 5, pp. 1385-1395.

2. Hayashi Y, Takao K, Shimizu T and OhashiH (2007), “High Power Density DesignMethodology”.

3. Lei L and Tina-Yu W (2011), “Applicationof Sinusoidal Pulse Width ModulationAlgorithm in the Grid-ConnectedPhotovoltaic System”.

4. Li X and Bhat A K S (2012), “A Utility-Interfaced Phase-Modulated HighFrequency Isolated Dual LCL DC/ACConverter”, IEEE Trans. Ind. Electron,Vol. 59, No. 2, pp. 1008-1019.

5. Navarro D, Lucić O, Barragán L A,Artigas J I, Urriza I and Jimeńnez O(2012), “Synchronous FPGA-BasedHigh-Resolution Implementations ofDigital Pulse-Width Modulator”, IEEETrans. Power Electron, Vol. 27, No. 5,pp. 2515-2525.

6. Patel N D and Madawala U K (2009), “ABit-Stream-Based PWM Technique forSine-Wave Generation”, IEEE Trans. Ind.Electron., Vol. 56, No. 7, pp. 2530-2539.

spwm generator based on fpga for high switching …here an fpga based spwm generator is implemented,...

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