pwm inverter


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pwm inverter

  1. 1. Design a 200 Watt, 150 Vrms PWM Bipolar Inverter 1 3 4hafzam@yahoo.comAbstract-This paper is concentrate on design procedure which including designrequirement, calculation respect to design, circuit building and simulation result. Thesimulation in this assiment by using MULTISIM software and it is base on componentsthat availale in market. I. INTRODUCTIONPower inverters are circuits used to convert Direct Current (DC) to Alternating current (AC).The input of the inverter may come from a DC source or from rectified AC input. There aretwo main categories for switch mode voltage source inverters: square wave and pulse widthmodulated (PWM). Square wave inverters are the simplest to implement. The simplicity ofthe square wave inverter comes along with the disadvantage of harmonics close to thefundamental frequency. PWM inverters function by comparing a sinusoidal control signal atthe desired output frequency with a triangular carrier signal at switching the frequency. Theharmonics of PWM inverters are located at multiples of the carrier signal frequency which istypically in the kHz range. This simply means the output waveform of PWM appears moresinusoidal than a square wave inverter. Also, higher frequency harmonics are easier to filterthan harmonics near the fundamental frequency.The requirement of this asignment is to design full-bridge inverter that can produce an acoutput voltage of 150 Vrms with frequency of 50 Hz. The output power is 200 Watt. Themodulation index (mi) is 1.0 and the frequency of the carrier is 5 kHz. The PWM modulationtype is the bipolar based PWM. This paper highlight:  The design of bipolar PWM  The THD voltage before filtering  The low-pass filter design 1
  2. 2. II. METHODOLOGYTo make the flow ease and simple, we discribe this design methodology by refering to theblock diagram in Figure 1 Sine wave generator comparator amplifier Inverter / PE buffer Inverter Triangle wave generator Figure 1: PWM inverter block diagramTwo main block in designing the PWM inverter are the PWM generator and the basictopology of the square wave inverter. The main differ is the PWM inverter use PWM wave asa switching signal instead of using square wave. The PWM generator is discribed in the dashbox in the Figure 1 above while the PE inverter use the same topology of square wave inverteras Figure 2. Figure 2: Inverter topology 2
  3. 3. Sine wave and triangle wave is feed to comparator for comparison to produce PWM signal.This signal then amplified by emplifier. Since the T1, T2 apply the same signal and T3, T4switching signal inverse from T1, T1, thus the digital inverter/buffer is used to perform theoperation. This inverter/buffer output then feed to the T1, T2 and T3, T4 inverter. III CALCULATION AND DESIGNThe requirement of this design as follow: i) ac output voltage of 150 Vrms with frequency of 50 Hz. ii) output power is 200 Watt iii) modulation index (mi) is 1.0 iv) frequency of the carrier, fc is 5 kHz.i) output voltage, Vo = 150 Vrms Vds drop is ≈ 30V due to characteristic of the mosfet (refer mosfet datasheet), thus Vdc need to increased to suit the Vo = 150Vrms 150 = √2 = 212.12 dc Therefore, 212.12 + 30 = 242.12 dc 242.12 Vdc need to be feed to Vdd (see figure 3) and resulting the PWM inverter output as Figure 15. The modulation frequency, fm is 50Hz applied.ii) The output power, Po = 200W Po = 3
  4. 4. Since Vo = 212.12, thus 212.12 200 = = 225iii) modulation index (mi) is 1.0 Vmodulation, Vm = 10 Vpic and Vcarrier Vc = 10 Vpic mi = mi = 1All the calculated value then applied to the circuit below by using MultiSim software. Themosfet type 2SK2978 was selected which has the characteristic as in mosfet datasheet. (seemosfet datasheet attached) Figure 3: PWM inverter (full circuit) 4
  5. 5. V1 = Vin when mi = 1Calculation for THDV  Vout (t )  Vn sin( nwt ) n 1 T 2 Vnk  V (t ) sin( nwt ) d ( wt ) 0Performing the integration 2Vdc Vnk  Cosnk  Cosnk  1  2Cosn(k  k )  , mf = = , = = 100When mf >10, or so, the harmonics can be normalised as shown in the Figure 4. It is desirableto have mf as large as possible and the 100 mf is quite good. This will push the harmonic athigher frequencies on the spectrum. Thus filtering requirement is reduced. Figure 4: Frequency spectrum for bipolar PWM with mi = 1 5
  6. 6. Each courier frequency spectrum. The harmonic amplitudes are a function of mi=1 THDv= −1 150 = −1 0.6 212.12 THDv  62%Although the voltage THD improvement is not significant, but the current THD will improvegreatly because the load normally has some current filtering effect.By additional of filter; the largest harmonic (see Figure 4) can be eliminated. A simplemethod reducing inverter output harmonic uses the LC low-pass passive filters. Second orderfilter is represented by LC circuit, where the inductor is shunt with the Equations belowexpress LC filter transfer function as derived from voltage-divider rule (Anca, 2009)Use of LC-filter lowers cost and losses of the inverter system. Equation below is for filterinductance. The maximum ripple current was chosen to be 5%-20% (typical value ofmaximum ripple current is 20% of rated current (Wang, 2003)).Filter capacitance is determinable by the reactive power absorbed in the filter capacitor;Equation below defines it. being the reactive power factor, its value was selected to be lessthan 5%(Wang, 2003). 6
  7. 7. Equation below defines the resonance frequency of the LC filter circuit (Khaled, 2007):Since the MultiSim cannot provide the current waveform, thus the value of LC cant becalculated properly and the THDi correction cannot be obtained. IV SIMULATION RESULT AND DISCUSSION Figure 5: PWM generatorThe PWM signal is produced by comparing sine wave and triangle wave (5 Hz and 5 kHz asrequired in this design). This can be done by using LM311H comparator (refer Figure 6). 7
  8. 8. Figure 6: Comparator input with natural sampling (m=1, fc = 5kHz, fm = 50Hz) Figure 7: Virtual Comparator outputSince the Comparator LM311H output change between 12V and 30V (refer Figure 8 andLM311H datasheet), we need to change the Vmin from 12V to 0V, thus the higher value then12V need as a comparison value. Figure 8: Comparator LM311H output 8
  9. 9. 20V has been selected for this design feed to the 741 op-amp VS+ input (refer Figure 5) andthe ap-amp output generated as Figure 9 (also refer 741 op-amp datasheet). Figure 9: 741 Op-amp outputSince the switching freq for Q1, Q3 is the same and Q2, Q4 is inverse from Q1, Q3, thus weuse the inverter/buffer ic 4041BP to perform and the resulting output (switching signal) asFigure 10 and Figure 11. Figure 10: Digital 4041BP inverter/buffer output – switching signal Q1 and Q2 9
  10. 10. Figure 11: Digital 4041BP inverter/buffer output – switching signal Q3 and Q4This PWM signal then feed to the inverter at point IO1 and IO2 as Figure 12. Figure 12: PWM inverterThe PWM inverter output is produced as Figure 13, Figure 14 and Figure 15; each taken fromdifferent point (XSC6, XSC 7 and XSC8). 10
  11. 11. Figure 13: output point XSC6 Figure 14: output point XSC7Inverter output Vo = VabVds drop is ≈ 30V due to characteristic of the mosfet (refer mosfet datasheet), thus Vdc need toincreased to suit the Vo = 150Vrms242.12 Vdc need to be feed to Vdd as calculated in DESIGN AND CALCULATION sectionpreviously (see Figure 3) and resulting the PWM inverter output in Figure 15 as required inthis design. 11
  12. 12. Figure 15: PWM inverter output point XSC8Since the MultiSim software cannot generate the current waveform, thus the currentwaveform was obtained by theoretically and the output should be as Figure below (blue line). Figure 16: PWM inverter voltage and current output V CONCLUSIONAfter flow through all the process, its can be conclude that designing the PWM converter wasnot easy as square wave converter. Instead of square wave switching signal, we need togenerate PWM signal. PWM switching signal can be genarate by comparing sinewave andtrianglewave via comparator. In this design, natural sampling was used. One major limitationwith natural sampling PWM is the difficulty of its implementation in a digital modulationsystem, because the intersection between the reference waveform and the triangular waveformis defined by a transcendental equation and is complex to calculate. An analogue circuit 12
  13. 13. possesses the advantages of a low cost with a fast dynamic response, but suffers from acomplex circuitry to generate complex PWM, limited function ability and difficulty toperform in circuit modifications (Mekhlief , 1999). To overcome this limitation the modernpopular alternative is to implement the modulation system using a regular sampling PWMstrategy. This technique was introduced to provide a more flexible way of designing thesystem. The system offers simple circuitry, software control and flexibility in adaptation tovarious applications. The two most common regular sampling techniques are regularsymmetrical and asymmetrical sampling (Ledwich, 1991).Simulation with an actual components parameter usually will cause waveform deviationcomparing to the ideal parameter. Thus the modification from the original topology isnecessary. The operation and characteristic of the ideal and the actual components parameterneed to understand. Waveform deviation that we obtained from LM311H comparator outputfrom the actual signal that we want push us to feed the signal via 741 op-amp to obtained theright signal. Since the switching signal for Q1, Q2 and Q3, Q4 inverse each other, it isnecassary to feed this output signal of 741 op-amp through the digital inverter/buffer 4041BP.In this simulation we took the sinewave and the squarewave direct from the signal generatorprovided by MultiSim but for the real design, the both signal should be obtained by a circuitdesign or generated by special components that available in market such as crystal oscillatorto generate sinewane and 555 timer circuit with some additional circuit to generatetrianglewave which not discuss in this paper.On the fullwave inverter side is just the same as original squarewave inverter topology. Thechalenging on this side is to find the write mosfet that available is MultiSim software andfinally the n-channel mosfet by Hitachi 2SK2978 was selected. Since we use the MultiSimsoftware text book edition, even its provide the actual component parameter but not all of thecomponent available is the sistem. For cormesial circuit design, it is necessary to buy theexpensive solfware (e.g. professional edition) that provide more actual components parameter. 13
  14. 14. REFERENCE[1] Daniel W. Hart, Introduction to Power Electronics, Prentice Hall International Inc, 2003.[2] Robert W. Erickson, “DC-DC Power Converters”, Department of Electrical and Computer Engineering, University of Colorado[3] Rashid, M.H., Power Electronics, Circuits, Devices and Applications, Pearson/Prentice Hall, 2004[4] Mohan, Undeland & Robbins, Power Electronics – Converters, Applications and Design, 2nd edition, John Wiley & Sons, 2003.[5] Datasheet download at 14
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