Subject : Power Electronics
Unit : DC to AC Converters
A 1-Phase Sine-PWM Inverter Of H-Bridge Topology
A 1-Phase Sine-PWM Inverter of H-Bridge Topology:
Fig: Sine-PWM waveforms for single-phase H-Bridge inverter
- In the square-wave inverter the switches conduct continuously for 180^{0 }in each output cycle whereas in PWM inverter large number of switching takes place in each output cycle. The half bridge sine-PWM inverter employing only one leg has already been described in the previous section. The full bridge inverter employs one additional leg but the control signals of the half bridge circuit may still be employed for switches of the other leg.
- As in the square-wave inverter (Lesson-34) the diagonal switches of the two legs may be turned on together to produce a load voltage that has double the magnitude of individual pole voltage. The PWM signals for the high and low level switches of one leg (obtained by sine-triangle comparison) may again be used for low and high level switches, respectively, of the other leg.
- Alternately (also, preferably), the modulating waveform for the other leg may be inverted (keeping the carrier waveform same). The two inverted modulating waveforms are then compared with the same carrier waveform using two different comparators. The comparator outputs, one for each leg, are then used to switch the high and low level switches as in the half bridge circuit.
- The relevant waveforms uses two inverted sine waves as modulating signals for the two legs of the inverter. For better visibility the ratio between the carrier and modulating wave frequencies has been assumed equal to ‘eight’ (normally carrier frequency is much higher) and circuit waveforms for only part of the modulating wave cycle has been shown.
- In the figure, the blue colored modulating wave is used for pole-A of the inverter and the green colored for pole-B. The corresponding pole voltages (V_{AO}, V_{BO}) and the load voltage (V_{AB}) are also shown in the figure.
- The scheme, using two inverted modulating waves, has the following advantages over the one that uses single modulating wave and employs simultaneous switching of the diagonal switches of the two legs:-
- Overall harmonic distortion of the load voltage waveform is reduced and
- The frequency of the ripple voltage in the load waveform doubles.
- Both these points may be verified by mere inspection of the load voltage waveform. In case of single modulating wave, the instantaneous load voltage has double the amplitude of pole-A voltage and thus the harmonic distortion of the load voltage and pole voltage remains same.
- It may be noted that the instantaneous magnitude of load voltage, in this case, has two levels ( 0.5E_{dc }and -0.5E_{dc}). In the alternate scheme, using two inverted modulating waves, the load voltage has double the number of pulses per carrier time period, thus doubling the ripple frequency. Now, higher the frequency of unwanted ripple-voltage, easier it is to filter out the ripple current. Also, the load voltage now has three levels ( 0.5E_{dc}, zero, and -0.5E_{dc}).
- Presence of zero duration reduces the rms magnitude of the overall load voltage (fundamental component along with harmonics), while keeping the magnitude of fundamental component of load voltage same as in the previous case (the rms of the overall load voltage for the two-level waveform equals E_{dc}). Thus the overall distortion of the load voltage waveform is less.