Power Electronics Circuits
In sinusoidal PWM for an inverter, which of the following correctly describes how the switches are turned on and off?
- AThe switches change state only once per half cycle of the desired output frequency
- BWithout a carrier, the on and off states of the switches are chosen at random so that the average of many pulses approaches a sine wave
- CThe desired sine wave (reference) is compared with a triangular wave (carrier), and the upper switch is on while the reference is larger
Show answer and explanation
Answer: C. The desired sine wave (reference) is compared with a triangular wave (carrier), and the upper switch is on while the reference is larger
Sinusoidal PWM compares a sine-wave reference at the desired output frequency with a triangular carrier of much higher frequency. Turning on the upper switch while the reference is larger than the carrier and the lower switch while it is smaller gives a voltage whose pulse widths follow the sine wave. When this voltage is smoothed, for example by the inductance of the motor windings, a sinusoidal component proportional to the reference remains. Switching only once per half cycle is called a square-wave inverter, whose output contains many low-order harmonics.
- A. This is how a square-wave inverter works, and its output contains many low-order harmonics such as the 5th and 7th.
- B. Random switching cannot control the magnitude and frequency of the output sine wave accurately.
- C. Correct. Comparing the reference with the carrier makes the pulse widths follow the sine wave.