Introduction to sine PWM
Introduction to sine PWM:
Fig: A schematic circuit for comparison of Modulating and Carrier signals
- In Sine-PWM inverter the widths of the pole-voltage pulses, over the output cycle, vary in a sinusoidal manner.
- The scheme involves comparison of a high frequency triangular carrier voltage with a sinusoidal modulating signal that represents the desired fundamental component of the pole voltage waveform.
- The peak magnitude of the modulating signal should remain limited to the peak magnitude of the carrier signal. The comparator output is then used to control the high side and low side switches of the particular pole.
- The figure above shows an op-amp based comparator output along with representative sinusoidal and triangular signals as inputs. In the comparator shown, the triangular and sinusoidal signals are fed to the inverting and the non-inverting input terminals respectively and the comparator output magnitudes for high and low levels are assumed to be VCC and -VCC.
- The comparator output signal ‘Q’ is used to turn-on the high side and low side switches of the inverter pole. When ‘Q’ is high, upper (high side) switch of the particular pole is turned on and when ‘Q’ is low the lower switch is turned on.
- The pole voltage, thus obtained is a replica of the comparator output voltage. When ‘Q’= VCC, the pole voltage (measured with respect to the mid potential point of the dc supply) is 0.5Edc and when ‘Q’= (-)VCC, the pole voltage becomes (-0.5)Edc.
- The input dc voltage to the inverter (Edc) has been assumed to be of constant magnitude. Thus, on a normalized scale, the harmonic contents in the comparator output voltage and the pole voltage waveforms are identical.