PWM Methods

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   Application and power converter topologies are still expanding thanks to improvements in semiconductor technology, which offer higher voltage and current rating as well as better switching characteristics. On the other hand, the main advantages of modern power electronic converters such as: high efficiency, low weight and small dimensions, fast operation and high power densities are being achieved trough the use of the so called switch mode operation, in which power semiconductor devices are controlled in ON/OFF fashion. This leads to different types of Pulse Width Modulation (PWM), which is basic energy processing technique applied in power converter systems. In modern converters, PWM is high-speed process ranging - depending on a rated power - from a few kHz (motor control) up to several MHz (resonant converters for power supply).
Fig.1 presents three-phase voltage source PWM converter, which is the most popular power conversion circuit used in industry. This topology can work in two modes:
- inverter - when energy, of adjusted amplitude and frequency, is converted from DC side to AC side. This mode is used in variable speed drives and AC power supply including uninterruptible power supply (UPS),
- rectifier - when energy of mains (50 Hz or 60Hz) is converted from AC side to DC side. This mode has application in power supply with Unity Power Factor (UPF).
Fig.1Three-phase voltage source PWM converter.

Performance significantly depends on control methods and type of modulation. Therefore the PWM converter, should perform some general demands like:
- wide range of linear operation,
- minimal number of (frequency) switching to keep low switching losses in power components,
- low content of higher harmonics in voltage and current, because they produce additional losses and noise   in load,
- elimination of low frequency harmonics (in case of motors it generates torque pulsation),
- operation in overmodulation region including square wave.