Brief Description Of Some Popular PWM Techniques
Brief description of some popular PWM techniques:
Fig: 1-phase half bridge VSI for CCPWM control
- Some PWM techniques, the important ones are: SINE-PWM technique, Space Vector based PWM technique, Hysteresis current controller based PWM technique etc.
- Some of the PWM techniques can be realized using analog circuits alone; some others are more easily realized with the help of digital processors like microprocessor, Digital signal processor (DSP) or Personal Computer (PC), whereas some other PWM controllers could be a hybrid between analog and digital circuits. For example, the selective harmonic elimination technique described above requires numerical solutions of the transcendental equations for arriving at the required notch angles.
- These transcendental equations are solved off-line and the information regarding notch angles (switching instances) is stored in digital memory, like EPROM. It may be realized that the notch instances may not occur at regular time intervals.
- Similarly fundamental output voltage requirement may not remain fixed for all output frequencies and hence the transcendental equations will be different for different output frequencies.
- If the switching frequency is kept constant, there will be more notch angles (per quarter cycle) at low output frequencies and less number of notches at higher frequencies. Thus the set of notch angles for one frequency may be different from the notch angles at some other frequency.
- For satisfactory implementation of this technique, generally the desired output frequency range is divided in few discrete frequencies. For example, it may be desired to output a 3-phase balanced voltage in the frequency range of 5 Hz to 50 Hz with the constraint that the ratio between output voltage magnitude and output frequency should remain fixed to some predetermined value.
- Under this situation the output voltage range may be discretized in steps of, say, 1Hz. Thus the available output may vary from 5 Hz to 50 Hz through the following discrete values of intermediate frequencies: 6 Hz, 7 Hz, 8 Hz, …, 49 Hz. The desired magnitudes of output voltage for all these discrete frequencies is found out and accordingly the notch angles are calculated to eliminate as many unwanted harmonics as possible (keeping in mind the constraint on switching frequency).
- Now switching information for successive output frequencies may be stored in successive memory blocks. For each of these output frequencies, it may be convenient to discretize one complete output cycle time interval in small steps (say, in steps of 10 microseconds) and the inverter switching word (as described below) at these successive time intervals are then stored in the successive memory locations.
- The switching word combines the switching information for all three legs (all six switches) of the inverter and may be obtained in the form of a six bit binary word, each bit corresponding to one particular switch. When a particular bit value is ‘1’ that particular switch may require being turned-on. Similarly ‘0’ bit value may correspond to turn-off command of the switch.