generation of pulse width modulation
Contents
1. Untitled 20 Jurnal Teknologi 34 D Jun 2001 1 12 Universiti Teknologi Malaysia GENERATION OF PULSE WIDTH MODULATION PWM SIGNALS FOR THREE PHASE INVERTER USING A SINGLE CHIP MICROCONTROLLER ZAINAL SALAM amp KHOSRU MOHAMMAD SALIM Abstract This paper describes the use of a general purpose microcontroller to generate the pulse width modulation PWM pulses for a three phase inverter The main feature of the work is the simplicity of the hardware only a fixed point microcontroller with its associated internal peripheral is required This result in an extremely simple low cost and reliable system A labora tory inverter prototype using a Siemens 80C167 microcontroller is developed and typical results are presented Key word Microcontroller inverter pulse width modulation power electronics renewable en ergy Abstrak Artikel ini menerangkan penggunaan pengawal mikro serbaguna untuk menjana denyut denyut modulasi lebar jalur untuk penyongsang tiga fasa Kelebihan utama teknik ini adalah keringkasan perkakasan hanya sebuah pengawal mikro dengan peralatan dalaman sahaja diperlukan Hasilnya adalah sebuah sistem yang ringkas murah dan dapat dipercayai Setelah prototaip penyongsang menggunakan pengawal mikro Siemens 80C167 telah dibangunkan dan beberapa hasil ujian diberikan Kata kunci Pengawal mikro penyongsong denyut modulasi lebar jalur elektronik kuasa 1 0 INTRODUCTION Recent developments in very large2. 17 11 Pz Untitled 20 10 ZAINAL SALAM amp KHOSRU MOHAMMAD SALIM Stopped q 1999 02 08 22 58 12 S0us dj S50usz div EA a PA EREN PEE A H PN TTA A E ET A E NORMEZIMS TE SE E Figure 9 Three phase PWM signals at different time scale Stepped N 1999702709 12 31 24 2ms div 2 ms div HORM 500kS s Figure 10 Line to line voltage of sinusoidal PWM waveform Vertical scale 240 V div N 1999 02 09 14 22 06 2ms div Stopped 2ms div g HORM S00kS fs ac Figure 11 Line to line voltage of harmonic injection sinusoidal PWM Vertical scale 240 V div 10 02 16 2007 17 11 Boe Sh 6B Ss l Lees T GENERATION OF PULSE WIDTH MODULATION PWM SIGNALS FOR THREE PHASE 11 Stopped q 1999 02 09 13 53 44 Ims div vette Tecesbenenlemerbennnbenenbennsi io LY 2 mem oe 3c REPETIE BOREL eee EEN PEE Rise T ELTETE EE ENSPE AARTEEN R PETE Figure 12 Line to line voltage and FFT of regular sampling PWM at unity modulation index Stepped 4 1999702709 14 05 09 2ms div re Benenbenenbenenbenssieenienmmie u OMS Pdi gt Figure 13 Line to line voltage and FFT of harmonic injection PWM at unity modulation index ponent of the harmonic injection PWM is bigger than that of the sinusoidal PWM This is consistent with results presented in 3 4 Untitled 20 11 02 16 2
3. CC1 and CC2 are chosen between OFF05 to 5 02 16 2007 17 11 Untitled 20 6 ZAINAL SALAM amp KHOSRU MOHAMMAD SALIM PEC PEC source source pointer pointer phase 1 phase 1 PEC PEC source source pointer pointer phase 2 phase 2 PEC PEC source source pointer pointer phase 3 phase 3 Figure 4 Regular sampling look up Figure 5 Harmonic injection sine look up OxFFFF When TIMER 0 starts to count three CAPCOM interrupts occur due to the matches within one PWM period However the occurrence of the interrupts are at different instants specified by the values of CAPCOM registers These inter rupts are set to trigger the PEC channels When CAPCOM interrupts occur data from the PEC source pointers are trans ferred to the destination pointers through the PEC channels In one PWM period three new data are transferred from the look up table to three different PWM regis ters After each transfer the source pointer is incremented by one and shifts to the next data location while the destination pointer remains fixed to the PWM registers In a full length of 360 of the look up table all the source pointers are separated by 120 from each other to ensure the three phase PWM are correctly generated 6 02 16 2007 17 11 Untitled 20 GENERATION OF PULSE WIDTH MODULATION PWM SIGNALS FOR THREE PHASE 7 oie Bet ee ae Se ee ee he E ee FFFFH CAPCOM Timer T0 cco oe CCI C02 Reload Interrupt Requests C10 C1112 T
4. scale integration VLSI programmable devices have influenced the design of power electronics systems in ways which are incon ceivable before Proliferation of power electronics systems based on microprocessor uP microcontroller uC digital signal processor DSP and field programmable gate array FPGA signalled the numbered days of analogue systems It is well accepted that the use of these advanced integrated circuits has significantly improve system performance The scenario is expected to be irreversible as more powerful and faster chips enter the market The main advantage of programmable devices is that it offers a considerable degree of flexibility in the overall system design For example in an embedded microcontroller system complex functions can be implemented using software and 1 amp 2 Power Electronics Research Group Faculty of Electrical Engineering Universiti Teknologi Ma laysia 81310 Skudai Johor Darul Takzim Malaysia Email zainals fke utm my 1 02 16 2007 17 11 i TT o E 2 ZAINAL SALAM amp KHOSRU MOHAMMAD SALIM hardware with execution speed comparable to analogue circuits The flexibility of software allows functions to be altered as many times as required with minimal changes in hardware Furthermore suitable hardware parts can be replaced by software which may results in reduced cost and increased reliability For microcontrollers the current trend is to pack sophisticated functions onto a sing
5. 007 17 11 Pas Untitled 20 12 ZAINAL SALAM amp KHOSRU MOHAMMAD SALIM 6 0 CONCLUSION In this paper the use of a low cost microcontroller i e SAB80C167 to generate the PWM pulses for a three phase inverter is described The main feature of the system is its simplicity this is primarily due to the minimum hardware required The pe ripherals of 80C167 such as PWM module CAMCOM and PEC are described and their functions in generating PWM signals are explained Using the look up table approach two PWM generation schemes namely regular sampling and harmonic injection method are implemented Several typical results are presented and they are found to agree well with other established works 1 2 3 4 5 6 REFERENCES Zainal Salam Khosru Mohammad Salim and Faridah Taha 2000 Design and Development of a Three Phase 5kW Power Conditioning Unit for Fuel cell Application Journal Institution of Engineers Malay sia 61 2 71 77 SAB 80C166 83C167 16 bit CMOS Single chip microcontrollers for embedded control applications User s Manual Siemens AG 1996 Bowes S R 1975 New sinusoidal pulse width modulated inverter Proc TEE 122 11 1279 1285 Bowes S R and Mount M J 1981 Microprocessor control of PWM inverters JEE proc Pt B 128 6 293 305 Siemens GmBH 1995 MCB 167 prototype board with Siemens C167 CPU User s Guide 5 95 Kiel Elec tronics Kiel GmBH 1992
6. 01C10C11 C12 T01 C10 C11 C12 T01 C10 C11 C12 T01 Ate AAA tase A A PWM module 1 for phase 1 PWM pulses of phase 1 PWM module 2 for phase 2 PWM pulses of phase 2 PWM module 3 for phase 3 PWM pulses of phase 3 Figure 6 Generation of three phase PWM signals LEGEND for Figure 6 CCx x 1 2 3 Content of Compare Register x Clx Interrupt Request for Compare Register x match s value of TO TO Interrupt Request for Timer0 when overflows Timer overflows means cycle is ended TOREL Content of Reload Register of Timer0 This value is adjusted in such way that the time cycle of Timer0 is equal to PWM time set in the PWM module P 1 P 2 P 3 are the pulse width values stored in the look up table starting at 00 of the modulating sinewave those generate the phase l of the 3 phase sinusoidal PWM signals P 66 P 67 P 68 are the pulse width values stored in the look up table starting at 120 of the modulating sinewave those generate the phase 2 of the 3 phase sinusoidal PWM signals P 132 P 133 P 134 are the pulse width values stored in the look up table stating at 240 of the modulating sinewave those generate the phase 3 of the 3 phase sinusoidal PWM signals 7 i 02 16 2007 17 11 E 8 ZAINAL SALAM amp KHOSRU MOHAMMAD SALIM 4 0 IMPLEMENTATION 4 1 Software The flowchart of the PWM generation program is shown in Figure 7 First initialisation of all variables and p
7. C766 Professional Developers Kit Kiel Software GmbH 12 02 16 2007 17 11
8. ariation in the modulation index or ratio The capture compare CAPCOM unit can be effectively used to update the values in pulse width registers The C167 provides two almost identical CAPCOM units It can be programmed to capture the content of a timer on a specific internal or external event Alterna tively it can be used to compare a specific timer content with a value in the CAPCOM register known as the compare register and modify the output signal when a match is met Once met an interrupt request is generated and it can be used to toggle an output port or jump elsewhere in the program to execute a specific task Using the compare mode the CAPCOM unit can be configured to generate pulse timing sequences with minimum software intervention 2 3 Peripheral Event Controller PEC For high frequency PWM operation very fast data transfer from the CPU to a pulse width register is required This can be achieved using the Peri pheral Event Control ler PEC When an interrupt request from a CAPCOM unit occurs the PEC re sponses to the request by moving a byte or word to the destination i e to the pulse width register within a single CPU cycle The result is that the pulse width register is updated almost immediately The CI67 PEC provides 8 channels which moves a single byte or word between two locations in memory This is the fastest possible interrupt response available for data transfer In most cases it is sufficient to service any per
9. aves is chosen to be 490 Thus the value of the sinewave at each sampling point can be written as 360 2 i 1 1 2 198 Where 7 0 1 2 up to 197 The exact number that need to be loaded into the pulse width register must therefore equal to Equation 1 multiplied by the modula tion index The latter varies form 0 to 1 Figure 3 shows the graphical presentation of the data in the look up table for several angles of the sinewave y otan Sampling angle Modulating sinewave 0 90 2 722 455 636 818 0 1 81 3 64 5 45 7 27 9 09 Figure 3 Position of sampling angle For harmonic injection sinusoidal PWM the modulating wave equation is 360 2 i 1 3 360 2 i 1 padli Oran Calne 2 198 2 198 9 360 2 i 1 0 27 sin Che 2 2 198 Where 7 0 1 2 up to 197 Formulation of regular sampling and harmonic injection PWM look up tables are shown in Figures 4 and 5 respectively Figure 6 shows the details for the pulse generation using an internal timer CAMCOM and PWM module and PEC The PWM frequency is set to 10 KHz A specific time of the C167 ie TIMER 0 is used as to generate this frequency using its overflow interrupt To obtain the correct timing the upper and lower limits of TIMER 0 is set to OFFF and OFF05 respectively The CAPCOM unit is configured to gene rate a single interrupt signal when a match is found The compare values in the CAPCOM compare registers e CC0
10. eripheral ports take place These include the setting of the switching frequency configuring the CAPCOM compare values initialising the PEC channels and setting all the interrupt priority levels Next the look up table is pre pared Once the main program is executed the system goes into an infinite interrupt driven loop In this loop the pulse width registers are updated This is achieved by multiplying the basic data in the sinewave table with the current modulation index The loop can be terminated by resetting the reset switch of the hardware boar START y Initialise variables y Initialise ports CAPCOM PWM module PEC and set all interrupts y Initialise sine look up table y p New Modulation Index Yy Modify data in the look up table No ee END Figure 7 Flowchart for generation program 4 2 Hardware The MCB 167 microcontroller evaluation board is used to develop debug and exe cute C167 application programs The board manufactured by Kiel electronics is 8 02 16 2007 17 11 Pas Untitled 20 GENERATION OF PULSE WIDTH MODULATION PWM SIGNALS FOR THREE PHASE 9 based on the C167 CPU with two memory devices each of 512 kilobytes in size 5 A wire wrap field is available for additional application hardware construction on board For communication with other computer systems an RS232 serial lin
11. ipheral request 3 0 PWM GENERATION SCHEME EXAMPLE Using facilities provided by the above mentioned peripherals two PWM types for a three phase inverter namely regular sampling PWM and third harmonics injection PWM are implemented Both schemes are well established and are widely reported in literature for example 3 4 The basic approach of signal generation is using the look up table method If the switching frequency is selected to be 10 KHz the modulation ratio i e the frequency ratio between the carrier wave and the modulating wave is 200 However it should be noted that the number of PWM pulses in a full modulating period should be divisible by three This is to avoid asynchronous PWM in a three phase system to occur The nearest number to 200 which is divisible by three is 198 Hence for a complete modulating cycle 198 data are required for the look up table Each data contains a numerical value which corresponds to the amplitude of the sinewave at a particular angle The numerical peak value of the sinewave is set to 4 02 16 2007 17 11 Pas Untitled 20 GENERATION OF PULSE WIDTH MODULATION PWM SIGNALS FOR THREE PHASE 5 500 To avoid over modulation i e both positive and negative peak of sinewave should be within the magnitude of the triangular wave the maximum amplitude numerical value of the modulating wave would be slightly less than the peak For convenience the maximum value of the modulating sinew
12. k is provided For 166 167 microcontroller applications the Kiel C166 Cross Compiler 6 offers a way to program in C which truly matches assembly programming in terms of code efficiency and speed The Kiel C166 is a dedicated 166 167 C compiler that generates extremely fast and compact code The Kiel C166 Compiler implements the ANSI standard for the C language 5 0 RESULTS Oscillograms of PWM waveforms regular sampling and third harmonic injection PWM using techniques described in the previous sections are presented The re sults can be validated by established works such as 3 4 Stopped q 1999702708 23 05 04 1C Figure 8 Three phase PWM gate signals at 10 KHz Vertical scale 2 0 V div Figures 8 shows the gate signals of the three phases for the regular sampling PWM Figure 9 shows the same signal but at different time scale shown for clarity It can be seen that the PWM gate signals are successfully generated using the pro posed method Figure 10 shows the line to line voltage waveform of regular sam pling sinusoidal PWM signals at unity modulation index and supply voltage of 240 V Figure 11 shows the line to line voltage waveform of harmonic injection sinusoi dal PWM under the same condition The Fast Fourier Transform of both sinusoidal and harmonic injection PWM waveforms at unity modulation index are shown in Figure 12 and 13 respectively It is clear from the results that the fundamental com 9 02 16 2007
13. le chip It is common for microcontrollers to be equipped with a wide range of on chip peripherals such as analogue to digital converter timers interrupt control lers networking facilities etc In power electronics applications these peripherals are very useful in realtime systems such as control and waveforms generation This is the concept will that will be highlighted in this paper the authors will demonstrate the use a l6 bit general purpose microcontroller in generating a three phase PWM pulses for a DC to AC inverter It will be shown that using this approach power electronics systems can be designed with minimum external circuitry Besides the above mentioned task the same microcontroller is also used for inverter control purpose as described elsewhere by the authors 1 2 0 SIEMENS 80C167 MICROCONTROLLER This work demonstrates the use of the Seimen s SAB 80CI67CR microcontroller to generate PWM pulses for a three phase inverter system for fuel cell application SAB 80C167CR XTAL Interupt Controller Figure 1 Block diagram of Cl67 Microcontroller Untitled 20 2 02 16 2007 17 11 Pas Boe Sh d l a GENERATION OF PULSE WIDTH MODULATION PWM SIGNALS FOR THREE PHASE 3 Figure 1 shows the block diagram of the core CPU with its associated internal pe ripherals Details on the IC can be found in 2 The main peripherals that will be utilised in the waveform generations are briefly described below For c
14. onvenience the SAB 80C167 is abbreviated as C167 2 1 PWM Module The C167 allows the generation of four independent PWM signals Each signal is designated to a particular programmable output channel The frequency range at 20 MHz CPU clock is from 4 8 Hz to 10 MHz for edge aligned and 2 4 Hz to 5 MHz for centre aligned PWM respectively Figure 2 illustrates the output of a centre aligned PWM The width of the pulse is determined by the content of the pulse width register PWx where x denotes a particular PWM channel In a centre aligned PWM the value in the pulse width register effects both edges of the output signal symmetrically whereas in the edge aligned one of the edges is fixed The minimum pulse width depend on the resolution of the PWM timer PTx For 20 MHz CPU clock the resolution is 50 ns The period of the pulse can be adjusted by changing the content the pulse period register PPx In the example shown below the period is set to 7 2 50 ns which turns out to be 700 ns Period 1 PWx 1 PWx 2 PWx 3 PWx 4 PWx 5 PWx 6 PWx 7 A rN LSR Change count LSR direction Figure 2 Centre aligned PWM operation Untitled 20 3 02 16 2007 17 11 iPS Untitled 20 4 ZAINAL SALAM amp KHOSRU MOHAMMAD SALIM 2 2 The Capture and Compare CAPCOM Unit In PWM the pulse width ie the numerical value of pulse width register need to be changed dynamically with respect to the v
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