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1. 1. ELIMINATION OF DEAD TIME IN PWM CONTROLLED INVERTERS Presented by: Priyambada priyadarshini sahoo Reg.No:0901106039 Branch: Electrical Engineering
2. 2. OUT LINE 1.what is dead time. 2.effect of dead time3.principle of dead time elimination4.implementation method5.conclusion
3. 3. WHAT IS DEAD TIME• To avoid shoot through in pwm controlled vsi a blank time is introduced• In this period both upper and lower switches in a phase leg are off• So that short circuit can be avoided and switches are not damaged due to high current
4. 4. single phase half bridge vsi
5. 5. Dead time varies with 1.devices 2.output current 3.temperaturewhich makes the compensation less effective at low outputCurrent & low frequency
6. 6. EFFECT OF DEAD TIMEone leg of the inverter single phase full bridge inverter
7. 7. Considering one leg of the inverter the effect of blanking time is given in below figure.
8. 8. Comparing the ideal waveform of VAN without blanking timeto actual waveform with blank time the difference betweenideal & actual output voltage is Vϵ=(VAN)ideal-(VAN)actualBy averaging Vϵ over one time period Ts change in outputvoltage due to t∆(drop is taken positive)In leg B of the inverter recognizing that iA= -iB
9. 9. Since Vo=VAN-VBN & io=iA the instantaneous average valueof the voltage difference that is the average value during oneperiod of the idealized waveform minus the actual waveformis
10. 10. Plot of instantaneous average effect of blank time on sinusoidalvalue Vo as a function of Vref output
11. 11. PRINCIPLE OF DEAD TIME ELIMINATIONA generic phase leg of VSIs.
12. 12. •Current flowing out of the phase leg is considered as positive here• Dead time is not required for p or N switch cells because both the cells are configured with a controllable switch in series with a uncontrollable diode•Gate control signal is selected to gate on or gate off upper device Kp or lower device Kn only
13. 13. CONTROL SCHEMESDead-time elimination control schemes
14. 14. • Determination of load current direction is key for dead time elimination• It can be detected by operating status of switches & their anti parallel diodes instead of expensive current sensors• Gate signal level is for determination of operating status of switches• Diode-conducting detection(DCD) circuit is for determination of status of anti parallel diode
15. 15. •If D1 is ON the comparator o/p is low, D0 light up otherwise itis OFF Diode-conducting detection(DCD) circuit
16. 16. SIMULATION RESULTS H-bridge voltage source inverter
17. 17. •Load is 8mH inductor & 2.4 ohm resistor•Vdc is 250v•Inverter is controlled by unipolar sinusoidal pwm•Switching frequency 10KHz•Fundamental frequency of o/p voltage is set to 60Hz
18. 18. Simulated output current waveforms with MI=0.2
19. 19. Comparison of o/p current with 2 usec dead time & without dead time
20. 20. IMPLEMENTATION METHODS
21. 21. •Two IGBT modules with a load of 8mH &2.4 ohmsresistor•Four DCD ckts to detect anti-parallel diodesDap,Dan,Dbp & Dbn diode•Output signals of diodes Cap,Can,Cbp,Cbn are fed backto a complex programmable logic device(CPLD)•DSP sends two PWM signals Sa & Sb CPLD•gate signals are achieved by optic electrical interface unit
22. 22. conclusion•Compared to conventional PWM control with dead timethis method reduces output distortion•Regains rms value•Low cost DCD circuits ,simple logic & flexibleimplementation•Avoids using expensive current sensors•Attractive option for VSI applications
23. 23. REFERNCES1.http://www.ee.bgu.ac.il/~pedesign/Graduate_problem_pape rs/papers2007/PWM_Deadtime.pdf2.Power electronics converters, applications & design Ned Mohan Tore M. Undeland William P. Robbins3.Power electronics Dr. P.S. Bimbhra4.Power electronics Principles & applications Joseph Vithayathil5.Power electronics M.H.Rashid
24. 24. THANK YOU