Dr. Unnikrishnan P.C. Professor, EEE EE305 Power Electronics Dr. Unnikrishnan P.C. Professor, EEE
Sinusoidal Pulse Width Modulation
Sinusoidal Pulse Width Modulation
Sinusoidal Pulse Width Modulation (SPWM) Reference signal is a sine wave Whenever the reference signal is greater than the carrier signal, a high pulse is produced by a comparator Width of the pulses not constant unlike in other cases. By adjusting ma the output voltage can be varied By changing mf, the switching frequency can be varied and harmonics in the output voltage can be controlled
Switching Schemes for SPWM Two types of switching schemes Unipolar Switching Scheme Bipolar Switching Scheme
Full Bridge Inverter for Bipolar Switching Here two switches are simultaneously operated When Vr > Vc, S1 and S2 are turned ON and VO = +VS When Vr < Vc, S3 and S4 are turned ON and VO = -VS
Bipolar Switching-Generation of Output Pulses
Bipolar Switching-Harmonic Spectrum The magnitude of the harmonics are a function of the modulation index ma, as width of the pulse depends on it The first harmonic components are at frequencies about mf.
Unipolar Switching Scheme for SPWM Operation of the switches When vr >vc S1 is turned ON and when vr<vc S4 is turned ON When -vr >vc S3 is turned ON and when -vr<vc S2 is turned ON
Unipolar Switching Scheme for SPWM The load voltage changes from +Vs to 0 in the first half cycle and from 0 to –Vs in the second half cycle- unipolar switching scheme The amplitude of the output voltage can be controled by varying the modulation index ma
Unipolar Switching Scheme for SPWM
Unipolar Switching Scheme for SPWM
Harmonic Spectrum The fundamental component is proportional to the amplitude modulation index The first predominant harmonics appears around 2mf as against around mf in bipolar switching.
Advantages of Unipolar Switching Higher harmonics can easily be filtered Less stress experienced by the switches as the switching takes from +Vs to Zero and from Zero to –Vs as against +Vs to –Vs in the bipolar scheme.
Unipolar Switching Scheme :