1. Dr. Unnikrishnan P.C. Professor, EEE
EE305 Power Electronics
Dr. Unnikrishnan P.C.
Professor, EEE

2. Sinusoidal Pulse Width Modulation

3. Sinusoidal Pulse Width Modulation

4. 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

5. Switching Schemes for SPWM
Two types of switching schemes
Unipolar Switching Scheme
Bipolar Switching Scheme

6. 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

7. Bipolar Switching-Generation of Output Pulses

8. 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.

9. 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

10. 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

11. Unipolar Switching Scheme for SPWM

12. Unipolar Switching Scheme for SPWM

13. 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.

14. 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.

15. Unipolar Switching Scheme :